Carbonate Fracture-Cavity Reservoir

Chapter 6

Carbonate Fracture-Cavity Reservoir Chapter Outline Section 1. Origins and Identification of Carbonate FractureCavity Reservoir 192 1. General Characteristics of Fracture-Cavity Reservoirs 192 2. Main Controlling Factors for Fracture-Cavity Reservoir Development 193 (1) Control of Lithology on Fracture-Cavity Reservoir Development 193 (2) Control of Karstification on Reservoir Quality 193 A. Paleokarst Types 193 B. Paleokarst Zonation 195 (3) Control of Paleogeomorphology on a Karsted Reservoir 196 (4) Control of Paleofaults and Fractures on the Karsted Reservoir 196 (5) Impact of Fracture-Cavity Filling on a Karsted Reservoir197 3. Identification of a Fracture-Cavity Reservoir 197 (1) Paleokarst Identified in Outcropa 197 (2) Paleokarst Evidence in Drilling and Logging 197 (3) Paleokarst Evidence Observed in Core 197 (4) Paleokarst Evidence in Wireline-Log Display 197 (5) Seismic Evidence of Paleokarst 198 (6) Paleokarst Evidence in Thin Section 198 (7) Geochemical Characteristics 198 4. Characteristics and Distribution of Karsted Carbonate Facture-Cavity Reservoirs in China 199 (1) Karsted Cambrian-Ordovician Reservoirs in the Tabei Area of Tarim Basin 199 (2) Karsted Cambrian-Ordovician Reservoirs in the Bachu and Tazhong Areas of Tarim Basin 199 (3) The Karsted Ordovician Reservoir of the Majiagou Formation in the Ordos Basin 200 (4) Karsted Sinian Reservoir in the Weiyuan Gasfield of Sichuan Basin 200 Section 2. The Fracture-Cavity System and Units of Carbonate Reservoir 1. Concept of Fracture-Cavity System and Unit 2. Principles and Methods for Classification of Fracture-Cavity Unit (1) Principles for Classification of Fracture-Cavity Unit (2) Determination of Fracture-Cavity Unit Boundary (3) Division of Fracture-Cavity Units A. Reservoir Pressure Drop Method B. Well Interference Method C. Fluid Property Method

201 201 201 201 201 201 201 202 202

Unconventional Petroleum Geology. http://dx.doi.org/10.1016/B978-0-12-397162-3.00006-2 Copyright Ó 2013 Petroleum Industry Press. Published by Elsevier Inc. All rights reserved.

D. Tracer Identification Method E. Constant-Volume Body Method 3. Types of Fracture-Cavity Units 4. Significance of Fracture-Cavity Unit Classification

202 202 202 202

Section 3. Migration Mechanism and Enrichment Factors of Hydrocarbon Accumulations in Carbonate Fracture-Cavity Reservoirs 203 1. Oil/Gas Production Characteristics in Different Types of Fracture-Cavity Reservoirs 204 (1) Isolated-Cavity-Type Oil and Gas Reservoir 204 (2) Connected Fracture-Cavity Type Oil and Gas Reservoir 204 2. Oil/Gas Conduction System 205 (1) Fault and Fracture Conduction System 206 (2) Unconformity Conduction System 206 (3) Connected Sand-Body Conduction System 206 (4) Combined Conduction System 206 3. Hydrocarbon Migration and Accumulation Mechanism 206 4. Factors That Control Oil/Gas Enrichment in Fracture-Cavity Reservoirs 208 (1) Long-Term Exposed Paleo-Uplift Controlled the Development of Quality Reservoir 208 (2) Quality Reservoir Controlled the Oil/Gas Enrichment in Fracture-Cavity Fields 209 (3) Superimposition of Reservoirs with Multiple Origins Is the Basis for Large-Scale Distribution of Oil/Gas 210 (4) Long-Term Successively Uplifted Slope Area, Overlapping and Pinchout Area, and Areas with Frequent Lithological Bariations Are Favorable Places for Oil and Gas Accumulation 210 Section 4. Exploration and Development Technologies for Carbonate Fracture-Cavity Oil and Gas Accumulations 1. Lithologic and Paleo-Geomorphologic Analysis 2. Seismic Prediction Technology 3. Physical Modeling 4. Reservoir Sculpturing Technique 5. Direct Hydrocarbon Detection 6. Reservoir Acid-Fracturing Technique 7. Development Technology of Horizontal Wells 8. Stimulation Technology in Horizontal Wells 9. Water Injection and Oil Displacement Technology

210 211 211 211 211 212 212 212 213 213

Section 5. Exploration Potential and Direction of Carbonate Fracture-Cavity Hydrocarbon Resources 214

191

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1. Paleo-Uplifts and Surrounding Slope Areas 2. Large Stratigraphic Unconformity Reservoirs 3. DeepeUltra-Deep Reservoirs

214 214 216

Section 6. Case Study 1. Tectonic Evolution of Lunnan Low Uplift 2. Sedimentary Facies and Reservoir-Caprock Assemblage 3. Reservoir Characteristics of Lunnan Low Uplift (1) Vertical Distribution of Reservoirs

216 216 216 217 217

The carbonate fracture-cavity reservoirs are widely distributed in the world. Statistics show that 20%e30% of recoverable hydrocarbon resources are related to unconformity surfaces (Flugel, 2004). And many reservoirs are related to paleokarsts, such as carbonate weathered crust reservoirs at Top Majiagou-5 Member of Lower Ordovician in Ordos Basin, China (Dai et al., 1997); gas reservoirs of Sinian, Carboniferous, and Lower Permian in Sichuan Basin; Mesoproterozoic and Neoproterozoic reservoirs of Renqiu oil field in Bohai Bay Basin; and Ordovician reservoirs of Tabei, Tazhong, and Bachu-Maigaiti areas in Tarim Basin. Among those reservoirs, the Ordovician fracture-cavity reservoirs in the Yingmaili-HalahatangTahe-Lungu area in Tabei, Tarim Basin are the most representative, by showing sizable and highly variable distribution. This chapter focuses on the carbonate fracturecavity reservoir, that is, the hydrocarbon reservoir with fractures and cavities as the main pore network.

SECTION 1. ORIGINS AND IDENTIFICATION OF CARBONATE FRACTURE-CAVITY RESERVOIR The carbonate fracture-cavity reservoir contains both fractures and dissolved pores and cavities, which are affected by original lithology, structure, and the karstification process. Studies on carbonate fracture-cavity reservoirs mainly include: (1) description of pore network and reservoir types for carbonate reservoirs; (2) study on sedimentary processes; (3) control of structural evolution on the karsted fracture-cavity system; (4) identification techniques of fracture-cavity reservoir, such as core analysis, mud log, and electrical logging; (5) prediction of fracture-cavity reservoirs, including geophysical and structural methods; and (6) geologic modeling for fracturecavity reservoirs. It is significant for exploration and development of carbonate oiland gas fields to conduct both microscopic and macroscopic studies on carbonate pore network and to understand both paleo-geomorphology and the history of paleokarsted formations.

(2) Horizontal Distribution of Reservoirs 4. Hydrocarbons Distribution 5. Main Controlling Factors for High Hydrocarbon Productivity and Accumulation (1) Sufficient Hydrocarbon Sources (2) Development of Layer-Like Fracture-Cavity System (3) Enrichment at Structural Highs (4) Caprocks Overlain Paleokarsts References

217 217 218 219 219 219 219 220

1. GENERAL CHARACTERISTICS OF FRACTURE-CAVITY RESERVOIRS The main pore network of the fracture-cavity reservoir is composed of big and small dissolved cavities, fractures, and dissolved pores. Large cavities constitute the major pore network, while matrix pores are underdeveloped, and fractures mainly play the role of communication. Generally, dissolved pores with diameter more than 5e15 mm are deemed to be dissolved cavities, and continuously extended cavities are called the dissolved cavity system or cavity system (Ford, 1998). Based on industrial standards, the pore network of fracture-cavity reservoirs can be further classified into big pore, midpore, small pore, micropore, huge cavity, big cavity, midcavity, small cavity, big fracture, midfracture, small fracture, and microfracture (Table 6-1). The carbonate pore network can be further divided intothe macroscopic fracture-cavity pore network and the microscopic pore-fracture pore network. The macroscopic fracture-cavity pore network includes the cavities and fractures observed in core, and the large dissolved cavities evidenced by bleed-off, blowout, and lost circulation in drilling (including large dissolved cavities in logging data interpretation). For example, from the drilling results of Well Lunnan West LG15, the thickness of Ordovician is 20.5 m, and the cumulative bleed-off thickness in the cavity development zone is 2.09 m; Well LG432 is 59 m away from weathered crust, and a large dissolved cavity locates at 5645e5720 m depth, which is filled by gray-green argillaceous siltstone and limy siltstone. Vertically, large dissolved cavities were usually developed in underflow karst zone at 50e140 m from the top of weathered crust; horizontally, they were developed in the paleokarst slope zone. The microscopic pore-fracture pore network includes pores with diameter less than 2 mm and microfractures with a width less than 1 mm under cast slice analysis and SEM observation. Micropores include intercrystal pore, intercrystal dissolved pore, and intragranular pore. Microfractures include structural fracture, pressolved fracture, and dissolved fracture.

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Carbonate Fracture-Cavity Reservoir

193

TABLE 6-1 Classification of Carbonate Pores, Cavities, and Fractures Pore

Cavity

Fracture

Type

Diameter (mm)

Type

Diameter (mm)

Type

Width (mm)

Big pore

0.5e2

Huge cavity


1000

Huge fracture


100

Mid pore

0.25e0.5

Big cavity

100e1000

Big fracture

10e100

Small pore

0.01e0.25

Mid cavity

20e100

Mid fracture

1e10

Micro pore

＜0.01

Small cavity

2e20

Small fracture

0.1e1

Micro fracture

＜0.1

The pore network combination types include fracture type, fracture/cavity type, small cavity type, and cavity type. In fracture-type reservoirs, factures constitute both pore network and filtration passage, presenting the feature of low porosity and high permeability. In fracture/cavity type reservoirs, the pore network is mainly composed of pores and cavities, while the filtration passage is composed mainly of fractures. This type of reservoir has low porosity but high permeability, presenting good quality and obtaining high oil flow in tests. In the small-cavity-type reservoir, the pore network is mainly pores and small cavities, which can hardly get oil flowing without fracture communication. In the cavity-type reservoir, the pore network is mainly composed of unfilled or half-filled large cavities, such as sinkholes and sack-like cavities in the surface karst zone, as well as beadlike dissolved cavities along fractures.

2. MAIN CONTROLLING FACTORS FOR FRACTURE-CAVITY RESERVOIR DEVELOPMENT Fracture-cavity reservoirs, with variable pore network, are formed by complex controlling factors, which can be classified into internal causes and external causes. Internal causes mainly refer to lithology and physical property. External causes include climate condition, breaking strength, paleogeomorphology, paleodrainage, vegetation, and length of exposure, among which climate condition is the main controlling factor (Yuan et al., 1987; Ford and Williams, 1989; James and Choquette, 1988).

(1) Control of Lithology on Fracture-Cavity Reservoir Development Favorable sedimentary facies constitute the basis for reservoir development. The dissolubility of rocks depends on their composition, rock fabric, and physical-chemical properties. In general, limestone is more dissoluble than

dolomite; among different types of limestones, bioherm limestone, grainstone, and micrite are more dissoluble than argillaceous limestone. As long as rock composition has an impact on dissolubility, coarse-grained rocks have intergranular pores and good connectivity, where dissoluble water can diffuse and leach along intergranular pores and disperse into whole rocks, resulting in spatial dissolution. Rocks with primary pores (such as reef limestone) are also under strong dissolution.

(2) Control of Karstification on Reservoir Quality As a diagenetic facies (Esteban and Klappa, 1983), karst refers to the destruction and transformation actions of CO2bearing surface water and underground water on dissoluble rocks (such as dissolution, leaching, erosion, transportation and deposition). Iintegrated hydrologic and geomorphic phenomena resulted therefrom, when carbonate rocks (including evaporates) are exposed to the atmospheric water diagenetic environment, which includes both chemical and physical processes. Wright (1982) defined paleokarst as “karstic features formed at one time, and subsequently buried by younger formations” (page 83 to 84). In general, paleokarst refers to the karst at geologic times; however, there is no consensus on e whether the time is before Cenozoic or before Quaternary.

A. Paleokarst Types Although scholars do not agree on karst classification (Bathurst, 1975; Longman, 1980; Tucker and Wright, 1990; Palmer, 1991), generally three categories can be distinguished: (1) penecontemporaneous karst, (2) epigenetic karst, and (3) buried karst (Table 6-2). Epigenetic karst is highly affected by structural unconformity and paleostructure, presenting a complex pore/cavity-fracture network with apparent vertical

194

TABLE 6-2 Genetic Classification and Characteristics of Paleokarst Category

Type

Diagenetic Diagenetic stage environment

Corrosive fluid source

Water circulation drive force of karst

Water circulation condition of karst

Main controlling factors

Penecontemporaneous Fresh water dissolution Penecontem karst poraneous

Atmospheric

Atmosphere

Gravity

Open

Sedimentary environment and sea level change

Epigenetic karst

Atmospheric

Atmosphere

Gravity

Open

Unconformity surface, eroded datum

Gravity, pressure

partially-open

Confined water circulation condition, regional ground water level

Buoyancy

Sealed

Organic acid concentration

Exposed weathered crust karst

Epigenetic

Bedding karst

Buried karst

Organic acid dissolution

Early diagenetic～ Buried Late diagenetic

Hydrocarbon fluid

Deep thermal fluid Density drive

Sealed

Deep thermal fluid circulation condition

Pressolved water dissolution

Overlying strata

Pressure, gravity

Sealed

Acid substance content in underlying strata

Mixed water dissolution

Multiple sources

Density, gravity

Relatively sealed

Water circulation condition

Unconventional Petroleum Geology

Deep thermal fluid dissolution

Chapter | 6

Carbonate Fracture-Cavity Reservoir

zonation and developing such typical features as calcareous crust, paleosoil, bauxite, light pink calcite crystal, dissolved ditch, pit, sinkhole; crescent, overhanging, and fibrous filtration sand or cement; breccia and mechanical flow deposits related to underground river. Buried karst is mainly controlled by faults and deep fluid, which often develop such minerals as heterotactic ferrodolomite, fluorite, sphalerite, magnetic pyrite, and collapsed structures, fissures, and irregular breccia massifs. Based on the study of paleokarst reservoirs in the Tabei area, Tarim Basin, the main Ordovician paleokarst is interlayer karst, buried hill plus bedding karst, which can be divided into Tahe-Lunnan type and Halahatang type. Among them, Tahe-Lunnan type was developed at a structural high location, with steep slope and strong hydrodynamics, forming typical karst; Halahatang type is at a structural low location, with gentle terrain and weak hydrodynamics, resulting in interlayer karst and buried hill plus bedding karst (Figure 6-1).

B. Paleokarst Zonation Carbonate karst system shows vertical zoning, which consists of a surface karst zone, a seepage karst zone, and an underflow karst zone from top to bottom. Each karst zone has characteristic features, showing certain horizontal distribution patterns and layer-like architecture. A surface karst zone is normally developed near weathered paleokarst and above a downward seepage zone, with thickness less than 50 m. Affected by meteoric fresh water near the surface, it involves surface colluviation, biological denudation, and certain depositional processes. The karst pattern is dominated by surface runoff of fresh water. Karst products include some dissolved ditch, cavity, fracture, lowland, funnel, and sinkhole formed by flushing and dissolving of surface runoff (with high CO2 content and strong dissolubility) generated from fresh water, and the Late strong uplift Typical karst type

Penecontemporaneous intermission Exposed shallow reef-bank type

195

deposits are mainly surface eluvium and cavity wall colluvium. Surface deposits are mainly brown-red oxidized sediments, including bauxitic and collapsed breccia. The reservoir is mainly composed of fractures and dissolved pores and cavities, with a large effective pore network, which is one of the most favorable targets for current exploration because of well-developed fractures and good connectivity. In the drilling process, well kick, drilling-bit fall, and lost circulation often occur. In Well Lungu 15, for example, three intervals were totally bled off at 5736e5750 m, with total thickness of 2.09 m. A seepage karst zone is located between the surface karst zone and maximum ground water level, with a thickness of 30e120 m and 150 m at the thickest. It is dominated by leaching and dissolving actions with downward filtration of the surface water system or downward seepage along early fractures, and it is dominated by vertical karst action. The distribution is related to the strength of karstification, location at structure, and ground water level. In the seepage karst zone, small, middle, or large dissolved cavities and fractures in the shape of bottleneck, calabash, sack, and bead are developed. The bottom of the cavities often extend toward karst lowland, until cavities are interconnected to form a huge fracture-cavity pore network. Cavities and dissolved fractures are mostly distributed vertically, with relatively limited lateral extent. Only local calcite cements in dissolved fractures and sand-mud fillings in a few dissolved pores and cavities can be seen. If the dissolved cavities cannot bear the pressure of upper and surrounding rocks, collapsed cavities at the top of a buried hill could be developed. This karst zone is also one of the most favorable intervals for exploration. For the wells drilled in Lungu west area of the Tarim Basin, the thickness of seepage zone is from 12.3 m to 119 m, and is generally within 120 m. An underflow karst zone is located near the underground water level, with a thickness of 50e80 m. In general, Early dissolution slightly eroded platform margin type

Contemporaneous unexposed Carbonate platform type

Mean high tide level Mean low tide level

FIGURE 6-1 Paleokarst distribution pattern in the Tabei area, Tarim Basin.

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Unconventional Petroleum Geology

the depth of open structural fractures with a certain aperture is the bottom of underflow karst zone. In this zone, underground water is very active. Water flow mostly shows horizontal movement and is under the CaCO3 unsaturation state. Therefore, extensive dissolution exists, which dissolves calcite and aragonite into pores and cavities, and then further into small, medium, and large dissolved cavities (underground river). Where structural fractures are developed, water mostly flows along the strike of structural fractures; dissolved pores and cavities are interconnected to form a huge reservoir body. Because water flows horizontally, the mud and sand brought in from the surface can easily form sand-mud sediments at lowlands of caves and areas with gentle water flow, even with good bedding. Some caves are fully or partially filled with sand and mud. Due to the continuous flushing and dissolving of underground water, dissolved cavities would extend continuously and collapsed rocks would form at bottom of caves. This karst zone is also a favorable target for exploration.

(3) Control of Paleogeomorphology on a Karsted Reservoir Paleogeomorphology plays an important role in the development of karsted reservoirs. Karst paleogeomorphology can be divided into three types: (1) karst highland, (2) karst slope, and (3) karst lowland. In the karst highland type, surface and seepage karst zones are developed, which are dominated by water drainage and characterized by a large thickness and significant cave-filling. In the karst slope type, karst is moderately developed, an

Paleo-fluvial channel river distribution at top of Yijianfang Formation

underflow zone often develops into underground river; and karst conduits are less filled, which are favorable for pore space development and are a main target of oil and gas exploration. In the karst lowland type, dissolution is extensive, underflow karst is significantly filled and collapsed, and karst lowland cavities have high waterbearing possibilities. The paleodrainage system in the karst area includes surface and underground rivers, whose development is affected by faulting and lithology; laterally dissolved cavities are often developed along the main stream of the surface water system. For example, the Ordovician karst system in the Tabei area of the Tarim Basin developed two-stage paleochannels. The first-stage channel was developed at lowland by a short time exposure after deposition of the Yijianfang Formation, which is a highly sinuous meandering river, with an elevation difference between upstream and downstream of less than 15 m, presenting a gentle paleogeomorphologic background. The second-stage channel was developed by the short-time exposure of the Sangtamu Formation at the end of the Ordovician, with a high sinuosity, a fixed widthto-depth ratio, an absence of lateral migration, and an upstream-downstream elevation difference of less than 6 m on a gentle structure (Figure 6-2).

(4) Control of Paleofaults and Fractures on the Karsted Reservoir Faults and structural fractures determine the permeability and its orientation in host rocks, which controls the flow path and direction of surface runoff and underground

Paleo-fluvial channel distribution at top of Sangtamu Formation

FIGURE 6-2 Seismic imaging of Ordovician paleodrainage system in the Tabei area, Tarim Basin.

Chapter | 6

Carbonate Fracture-Cavity Reservoir

water flow. Therefore, the karsted reservoirs distribute along fault and fracture zones. Usually, karsted reservoirs are more developed in zones of dense faults and fractures, as well as hinge and crossing points of faults. For example, faults were developed in three stages in the Ha-6 Block of the Tabei area, Tarim Basin. This fault system was dominated by conjugated shear faults, with connectivity being improved by late echelon faults cutting through early X-shaped faults. There are abundant highangle structural fractures, oblique fractures, and microfissures; AND multiscale fractures were interconnected to form complex, network-like migration pathways that promoted further formation and modification of karsted zones.

(5) Impact of Fracture-Cavity Filling on a Karsted Reservoir The filling of fractures and cavities highly affects the pore network for oil and gas. Generally, in a surface karst zone, fractures, pores, and cavities are relatively developed, with less-filled void, high horizontal connectivity, and best storage capacity. In a seepage karst zone, fractures are more developed than dissolved fractures and cavities; however, as a zone under long-term downward seepage of surface water, this zone is poorly filled and has some effective storage space. In the seepage karst zone, surface water after filtration mainly shows horizontal flow. Following the orientation of fractures and drainage, water often flows toward a certain direction. Therefore, this zone often develops huge, near-horizontal pores and cavities with transverse connectivity, that is, underground rivers. Mud and sand carried from the surface would easily deposit in these cavities, to form fully filled and half-filled pores and cavities, with good storage capacity.

197

bauxite, pyrite, or limonite, and overlying brecciola, collapsed breccia, interstitial breccia, limy siltstone, and argillaceous siltstone associated with an erosion surface.

(2) Paleokarst Evidence in Drilling and Logging In a karsted interval, drilling often encounters speed-up, bleed-off, bit bouncing, lost circulation and blowout, oil bloom and film in mud pit, and drill cuttings with fluorescent displays frequently showing oil trace. Automorphichypautomorphic calcite crystals are often seen in drillcutting samples. Gas survey indicates apparent signs of oil and gas, with significantly increased total hydrocarbon, heavy hydrocarbon, and hydrocarbon components. Many wells in Lungu and Tahe oil fields were bled off. For example, a total of four intervals were bled off in Well Lungu-102, with a total thickness of 15.64 m. Multiple wells in the Lungu west and Lungu-7 well area have a similar situation (Figure 6-3).

(3) Paleokarst Evidence Observed in Core In core observation, a paleokarst system can be indicated by: (1) small dissolved pores and cavities kept open or being filled by calcite or a sand-mud substance; (2) small dissolved pores and cavities with mauve or brownyellowish inner walls, which are mostly filled or half-filled by mud, and show bottleneck, calabash or bead shapes; (3) breccia, such as collapsed breccia and interstitial breccia; (4) bedded mud and sand sediments within the cavities that mostly constitute karst conduit system; (5) autogenetic minerals in caves, such as giant-crystal calcite and stalactite; and (6) high-angle dissolved fractures filled with red, gray-greenish mud or calcite.

3. IDENTIFICATION OF A FRACTURECAVITY RESERVOIR

(4) Paleokarst Evidence in Wireline-Log Display

Paleokarst can be identified from both macroscopic and microscopic aspects. The macroscopic aspect includes outcrop, drilling, mud log, core, logging, seismic, and production response, while the microscopic aspect includes thin section, carbon and oxygen isotope, trace element, and fluid inclusion.

The wireline-logging responses of paleokarst are generally represented by three highs and two lows: (1) high gamma ray; (2) high transit time; (3) high neutron porosity; (4) low resistivity; and (5) low bulk density (Zhang and Liu, 2009). In large dissolved cavities, the gamma ray value increases with mud content; deep and shallow dual-lateral and microlateral resistivity values are low with difference; holes are severely enlarged; and neutral, density and acoustic curves vary greatly. Small dissolved pores and cavities present leopard-shaped irregular black dots in microresistivity image logging (EMI or FMI), while large dissolved cavities present all black pads in EMI or FMI images (Figure 6-4).

(1) Paleokarst Identified in Outcropa Karsted carbonate rocks show apparent macrofeatures due to long-term weather denudation and leaching impacts, characterized by such features as long-term depositional hiatus, weathered eluvium such as bauxitic mudstone,

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Unconventional Petroleum Geology

FIGURE 6-3 Bleed-offs in drilling process.

(5) Seismic Evidence of Paleokarst Because the existence of a fracture-cavity zone increases the absorption decay of seismic waves, the dissolved cave system usually presents “bead-like” features on seismic profile (Figure 6-5). Other seismic features, such as lowered frequency, weakened amplitude, random reflection, weak reflection, beaded reflection (discontinuous events or complex waves), and reduced velocity (up to 20%), all indicate development of the dissolved cavity system.

(6) Paleokarst Evidence in Thin Section The microfeatures of penecontemporaneous karst include the following: (1) in high-energy grainstone of bank facies, within the primary intergranular pores only the first-stage fibrous rim calcite cements are dissolved, and late-stage granular calcite or coarse grained calcite are well preserved; (2) selective dissolution forms intragranular dissolved pores, moldic pores, intergranular pores, micrite envelopes, and the like; (3) intergranular dissolved pores

are filled by seepage siltstones; and (4) overhanging or crescent calcite cements are developed. The microfeatures of buried karst include: (1) ferrocalcite, ferrodolomite, and heteromorphic dolomite filled in pore, cavity. and fracture are dissolved into intergranular and intragranular pores and cavities; (2) dissolution expands along early suture lines to form pressolved fractures and dissolved micropores or unfilled fractures; (3) big intergranular pores or intergranular dissolved pores exist among middle-coarse grained dolomite crystals in dense array; and (4) there are thermal fluid minerals such as fluorite and flint (Wang et al., 2008).

(7) Geochemical Characteristics When carbon and oxygen isotopes vary in different diagenetic environments, such as seepage-underflow, mixed water, and dissolved cavity, geochemical studies on carbonate reservoir are often adopted for identification of these environments, such as studies of trace elements and fluid inclusion. The calcite crystals, as cements of karst fractures and cavities formed under buried karst action,

FIGURE 6-4 FMI characteristics of paleokarst-related pores, cavities, and fractures.

Chapter | 6

LG100

Carbonate Fracture-Cavity Reservoir

LN54−1

LN54

LG100−6

199

LG100−11

LG100−10

LG101

LG16−2

LG18

FIGURE 6-5 Bead-like responses to paleokarst caves on a seismic profile.

normally have a high homogenization temperature of inclusion (generally higher than 90 oC).

4. CHARACTERISTICS AND DISTRIBUTION OF KARSTED CARBONATE FACTURE-CAVITY RESERVOIRS IN CHINA Generally, fracture-cavity reservoirs in Chinese oil and gas fields are characterized by that the following features: (1) paleokarst has apparent vertical zonation, with a welldeveloped surface karst zone, vertical seepage zone, and horizontal underflow zone; (2) storage space is mainly composed of half-filled or unfilled residual large dissolved cavities and dissolved pore-cavity-facture system; (3) highquality reservoir is dominated by fracture-dissolved pore and cavity-large cavity, which constitutes the most important reservoirs and main pay zones for high and steady production of large oil and gas fields; (4) reservoirs are greatly controlled by paleokarst landforms and fault fractures, and karst slope and faulted areas are the most favorable areas for reservoir development; (5) secondary pores resulting from buried organic dissolution are also important, whose development tends to coincide with hydrocarbon formation, evolution, and migration; and (6) multistage overlapping and modification of surface karst and buried organic dissolution constitutes the best combination for generation of paleokarst reservoirs (Chen, 2004).

(1) Karsted Cambrian-Ordovician Reservoirs in the Tabei Area of Tarim Basin The Tabei area belongs to a residual paleohigh, which experienced multistage overlapping and modification of tectonic movements in the Caledonian-Himalayan period,

and Paleozoic karsted reservoirs are widely distributed. In Yaha and Yingmai-32 well areas around the axis of adjacent composite anticlines, there are buried hill reservoirs of the Indosinian-Yanshan period; toward the south. Hercynian, Early Hercynian, and Late Caledonian karst reservoirs were also developed; and at the surrounding slope area of paleohigh overlain by mudstone of Sangtamu Formation, Upper Ordovician, multistage deep bedding underflow karst reservoirs were developed in Ordovician carbonate sequences. The bedding karst reservoirs are characterized by large cavity size, low filling extent, and good connectivity. For example, in Well Lungu-35, drilling results show that the dissolved cavities are as high as 31 m, with the top 6 m being hollow (Figure 6-6) (Zhang and Liu, 2009). In general, the three categories of Ordovician carbonate reservoirs in theTahe oil field constitute five types of reservoirs with different combinations: fault/cavity, fracture/cavity, cavity/fracture, fracture/reef (bank) pore, and fracture (Gu and Zhou, 2001).

(2) Karsted Cambrian-Ordovician Reservoirs in the Bachu and Tazhong Areas of Tarim Basin In Tarim Basin, paleokarst reservoirs were also widely developed in the Cambrian-Ordovician Formation of the Bachu and Tazhong areas, where three types of paleokarst reservoirs were developed in five stages: end of Early Caledonian (Top of Cambrian) (2) early Middle Celedonian (Top of Penglaiba Formation), (3) Middle-stage (Top of Yingshan Formation) interbedded karsted reservoir, (4) Middle Celedonian late-stage (Lianglitage Formation) reef/bank karsted reservoir, (5) Late Caledonian and Early Hercynian buried hill reservoir. Among them, the interbedded karsted reservoirs in the third stage are widely

200

Unconventional Petroleum Geology

FIGURE 6-6 Types and distribution of Karsted reservoirs in the Tabei area, Tarim Basin.

distributed in the Bachu and Tazhong areas, with an exploration area of over 5  104 km2; reef/bank karsted reservoirs of the Lianglitage Formation were mainly developed along No. I platform margin zone, which changes to a normal buried hill reservoir toward the broad intraplatform area, where Lianglitage Formation shows disconformity or small-angle unconformity contact with “black quilt” of overlying Sangtamu Formation, with depositional hiatus of about 2 Ma. The buried hill reservoirs formed in the second stage were widely developed in the Hetianhe Gasfield-Maigaiti Slope, especially in the Tazhong main horst belt and the broad southern area.

(3) The Karsted Ordovician Reservoir of the Majiagou Formation in the Ordos Basin The main part of the Ordos Basin covers an area of about 25  104 km2, with the Majiagou Formation of MidOrdovician distributed in an area of nearly 20  104 km2. The Majiagou Formation is divided into six lithologic intervals from top down, among which Ma6 Member in top was mostly denuded and top of M5 Member was partially absent. In the middle and eastern part of the basin, M5 Member is further divided into 10 subunits from top down 1 (Ma15 to M10 5 ). In the gypsum-bearing dolomite flat of Ma5 to 4 M5, the pore-cavity microcrystal dolomite containing gypsum nodules and mottles constitutes the main target zone of exploration and the pay zone of Jingbian gasfield. Ma15 is the most important subunit, where the single-layer thickness of dolomite is 3e5 m, and reservoirs are connected and in stable distribution, with mean effective thickness of gas pay 2.40 m and a distribution area of up to 4  104 km2.

Between the end of Ordovician to pre-deposition of the Upper Carboniferous Benxi Formation, the Late Caledonian-Early Hercynian movement led to an overall uplift of the Ordos Basin, exposing the basin for 150 Ma. As a result, widely distributed karsted reservoirs were formed in the Ordos Basin. In the main area of the basin, the karsted zone is about 30e80 m thick, which can be divided into a surface karst eluvium zone, a vertical seepage karst zone, and a horizontal underflow karst zone among others. In the vertical seepage karst zone, meteoric water runoff rapidly flew and denuded downward and vertically along fractures, to form the reservoir intervals dominated by fracture type and cavity/fracture type rocks. In the horizontal underflow karst zone, as the horizontal water flow was controlled by a pressure gradient to form laminar flow, unsaturated underground water flew actively near the groundwater level, with horizontal karst developed. Meanwhile, due to strong dissolution by soluble minerals such as anhydrite (nodule) and salt, underground water containing rich SO2 4 was generated, which reinforced karstification against carbonate rocks. As a result, reservoir bodies dominated by fracture/dissolved pore and cavity type pores were developed, and the storage-seepage system with interconnected cavities and fractures constitutes the most important natural gas reservoir interval of Ma15.

(4) Karsted Sinian Reservoir in the Weiyuan Gasfield of Sichuan Basin Based on statistics for 61 gas wells in the Weiyuan Gasfield, paleokarst reservoirs are mainly distributed in two intervals in 12e23 m and in 43e80 m below the top

Chapter | 6

Carbonate Fracture-Cavity Reservoir

erosional surface of Sinian. The paleokarsted dolomite in the Sinian Dengying Formation belongs to the overlapped and modified product of multistage karstification. Paleokarst has apparent vertical zonation, and the weathered eluvium zone and seepage-underflow karst zone were well developed. The eluvium zone is composed of weathered eluvial breccia, ferrite mudstone, and bauxitic mudstone, with thickness of 3e3.5 m. The seepage karst zone mainly developed the cavities and fractures consisting of fractures, dissolved fractures, karst feeders, beaded karst pore-cavities, and sinkholes in vertical and high-angle distribution. Such cavities and fractures were mostly filled and partiallyfilled by mud, seepage siltstone, grainy dolomite, and breccia, which belong to cavity/fracture type or fracturetype reservoirs. The underflow karst zone is featured by multisets of near-horizontal dissolved pore and cavity zones and cavity zones, which developed multiple reservoir types such as fracture/cavity type, fracture/small cavity type, cavity/fracture type, and fracture type.

SECTION 2. THE FRACTURE-CAVITY SYSTEM AND UNITS OF CARBONATE RESERVOIR Greatly different from clastic reservoir, the carbonate reservoir has a pore network and flow path dominated by dissolved pores, cavities, and fractures of various origins, and is strongly heterogeneous, irregular, and multiscale in nature. Therefore, classification and evaluation for carbonate fracture-cavity system and unit is very important.

1. CONCEPT OF FRACTURE-CAVITY SYSTEM AND UNIT A fracture-cavity system refers to a karsted fracture-cavity development zone or fracture-cavity assembly composed of interconnected pores, cavities, and fractures. The spatial distribution of the fracture-cavity system is controlled by fault, fracture, paleogeomorphology, and the paleodrainage system, which often present complex architectures such as a branch-like conduit cavity and a networked fracturecavity. The systems are separated by intact host rocks, and the boundary of the fracture-cavity system represents the karstification and related fracture boundary. The fracturecavity hydrocarbon accumulation is featured by a “unified temperature-pressure system, large oil-bearing area, and enriched fracture-cavity reservoir body,” and the fault conduit system controls oil and gas distribution of the fracture-cavity system. A fracture-cavity unit refers to a hydrodynamic unit with unified pressure system and hydrodynamic system composed of one or more dissolved cavities with N

201

interconnected fracture network, with the pore network and seepage path dominated by pores, cavities, and fractures and separated by a relatively compact or low-permeability barrier (body). The same fracture-cavity unit with consistent pressure (pressure connectivity) and similar fluid property can act as a relatively independent fluid flow unit and basic oil-gas development unit.

2. PRINCIPLES AND METHODS FOR CLASSIFICATION OF FRACTURE-CAVITY UNIT (1) Principles for Classification of Fracture-Cavity Unit Based on studies of fracture-cavity systems, we propose the following classification principles by referring to various dynamic and static data: 1) Vertically, thick, dense barrier beds exist between single-well production zones; if produced fluid properties and production characteristics vary greatly between different zones, they belong to different fracture-cavity units. 2) In the same karsted residual-hill structure, the areas with similar seismic amplitude or seismic wave characteristics and with the same hydrodynamic system belong to the same fracture-cavity unit; among different fracture-cavity units, segmentation is apparent, and storage-seepage distribution is highly different. 3) Within a fracture-cavity unit, reservoir properties are similar, with the same hydrodynamic condition, which is unrelated to the reservoir bodies outside of the unit.

(2) Determination of Fracture-Cavity Unit Boundary The fracture-cavity unit boundary is determined as follows: 1) Natural boundaries of current karsted landforms, such as karst gully, fault scarp, and karst lowland. 2) Drainage radius or impermeable boundary of fully sealed reservoir in well test interpretation. 3) Range of elliptic, beaded, and banded amplitude anomalies. 4) Boundary of waveform in a 3D visualization of the reservoirs.

(3) Division of Fracture-Cavity Units A. Reservoir Pressure Drop Method The well groups with consistent pressure drop or pressure trends can be classified into the same fracture-cavity unit.

202

For example, Well LN44 was shut downon April 4, 2008, due to high water cut. Top of Ordovician in a sidetracked wellbore (LN44C) is 43 m higher than in the original well, with slightly lower crude oil gravity and a slightly higher gas-oil ratio. On January 20, 2008, Well LN44C was put into production, whose initial production index is consistent with Well LN44, and the water cut suddenly grew to 40% from zero after 65 days of production. Later, the pressure coefficient of both wells was stablized at 1.0. Both dynamic and static data indicated that the two wells belonged to the same fracture-cavity unit.

B. Well Interference Method In the development process, a well group with interwell interference can be divided into the same fracture-cavity unit. A decision as to whether they belong to the same fracture-cavity unit can be made based on the information of interference between adjacent wells related to acid fracturing of new wells, water flooding of old wells, water shutoff, and change of choke sizes. For example, in the LG100 well area of the Sangnan West region in Lungu oil field, based on the well interference behavior, we grouped six wells (Well LG100-6, LG100-9, LG100-10, LG100-11, LN54, and LN54-1) into the same fracturecavity unit.

C. Fluid Property Method In carbonate fracture-cavity reservoirs, fluid property change is complex. Both vertically and horizontally, oilegasewater property shows strong heterogeneity. Therefore, we can determine the connectivity of reservoirs by the fluid heterogeneity inside the reservoirs. For instance, Ordovician crude oil in Well Xinken-9 of Tabei Halahatang shows a density of 0.8302 g/cm3, while the crude oil in the Sidetracked Well Xinken-9C shows a density of 0.8091 g/cm3, with quite different crude oil components. Therefore, we can determine that these two wells belong to different fracture-cavity units.

D. Tracer Identification Method The interwell tracer monitoring technique is to inject watersoluble tracers into the water-injection well, then take water samples from surrounding monitor wells, and analyze the tracer concentration in samples. By drafting a curve of the adjacent-well tracer concentration versus time, we can determine the connectivity between the reservoirs. If the wells are connected, they belong to the same fracturecavity unit.

E. Constant-Volume Body Method A constant-volume body indicates an independent fracturecavity unit with limited volume. The constant-volume body

Unconventional Petroleum Geology

method is mainly based on the assumption that formation pressure and oil production decline rapidly, and water flooding could normally be used to improve the formation energy of the constant-volume body at the late production period. Based on the above methods, reservoirs in the Halahatang Block of Tarim Basin can be divided into four types of reservoir units, that is, water-free constant-volume, purewater constant-volume, oil-water constant-volume, and communicating-water units. For each of these units, we built an AVO oilewater identification model, and performed quantitative seismic sculpturing, with good application results. Water-free constant-volume type is dominant, which offers the best development results with a small producible oil reserve.

3. TYPES OF FRACTURE-CAVITY UNITS Fracture-cavity units can be classified and evaluated from the aspects of fracture-cavity unit energy, reservoir geometry and size. Energy determines the development characteristics of different fracture-cavity units. The same fracture-cavity system should have the same hydrodynamic system, and the discrepancy of different types of fracturecavity units is mainly reflected in their reservoir size and natural drive energy, especially the strength of bottom water energy. Based on the geometry, size, and connectivity, the fracture-cavity units can be divided into three types: isolated unit, large fracture-cavity unit, and small fracturecavity unit. (Table 6-3; Figure 6-7). Production test results indicate that a large fracture-cavity unit is featured by high productivity, gentle energy change, and rapidly declined oilegas production after occurrence of bottom water; and the energy and fluid production of an isolated fracturecavity unit shows gradual decline trend.

4. SIGNIFICANCE OF FRACTURE-CAVITY UNIT CLASSIFICATION The fracture-cavity unit is a further subdivision of the reservoir body in the fracture-cavity system, and is the minimum unit of carbonate fracture-cavity reservoir. Detection and evaluation of the fracture-cavity units help us to: 1) Understand the heterogeneity of the karsted reservoir more objectively and to deepen recognition of the development geologic features for varied fracturecavity reservoir bodies, facilitating oilegas development with improved understanding of oilegasewater movement. 2) Depict the distribution of networked fracture-cavity units and compartments, and guide the preparation of

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Carbonate Fracture-Cavity Reservoir

203

TABLE 6-3 Fracture-Cavity Reservoir in Halahatang Area of Tarim Basin Type of fracture-cavity reservoir

Main characteristics

Isolated fracture-cavity unit

(1) Isolated, strong seismic reflection (2) Formation and production test results indicate high energy and fluid production in initial period and gradual decline in late period

Well Ha-7, Well Ha-12

Connected large fracture-cavity unit

(1) A series of strong seismic reflections (2) Formation and production test results indicate high energy and fluid production, and change occurred under natural production condition

Well Ha-13, Well Ha-11, Well Ha 11-2

Connected small fracture-cavity unit

(1) Indistinct strong reflections (2) Formation and production test results indicate low energy. Acid fracturing can achieve good effects

Pattern map

Example

Well Ha 12-2

development and adjustment plans, as well as the selection of oil-well production techniques. 3) Establish a development plan for each type of fracturecavity units, and conduct well pattern deployment and index adjustment. 4) Provide new thoughts for studies of the water-flooding technique and hydrocarbon displacement with other injection materials. If some unit or block in an oil field has continuous oil pay zones, we can adopt the highly efficient water flooding development mode, to maintain high production with improved ultimate oil recovery. For areas with poor connectivity, we can adopt sidetracked branch holes and additional new wells to improve oil recovery. 5) Understand the control of fracture-cavity units on the distribution pattern of remaining oil and adopt different production measures based on different types of flow units.

SECTION 3. MIGRATION MECHANISM AND ENRICHMENT FACTORS OF HYDROCARBON ACCUMULATIONS IN CARBONATE FRACTURE-CAVITY RESERVOIRS The research on the mechanism of oil/gas migration and accumulation mainly includes oil/gas source identification, migration direction, timing, conduction system, and accumulation mechanism. Among these, oil/gas source identification, migration, and accumulation are the core of study. Two conduction systems, unconformity surface and fracture, can effectively communicate with hydrocarbon sources, which is the precondition for large-area oil/gas accumulation in the fracture-cavity carbonate system. In this section, the oil and gas conduction system, migration mechanism, and trapping models of the fracture-cavity carbonate reservoirs will be discussed.

204

Unconventional Petroleum Geology

h (m)

Isolated fracture-cavity unit Well Ha 11−1

Connected multiple large fracture-cavity units Well Ha 11 Well Ha 11−2

3000

4000

Impedance body Well Ha 11−1

Well Ha 11

Well Ha 11−2

h (m) 6400 6450 6500 6550 6600 6650 6700 6750 6800 FIGURE 6-7 Geophysical response characteristics of various fracture-cavity bodies.

1. OIL/GAS PRODUCTION CHARACTERISTICS IN DIFFERENT TYPES OF FRACTURE-CAVITY RESERVOIRS Based on the connectivity of fractures and cavities, the fracture-cavity oil/gas reservoir can be classified as an isolated cavity type and a connected fracture-cavity type. For the isolated cavity type, the isolated cavity is the reservoir, with unified temperature and pressure system and fluid property. The oil/water contact is evident; the bottom water is developed; and oil/gas production is subject to the cavity size with obvious constant-volume characteristics. The connected fracture-cavity type features multiple fracture-cavity systems with variable connectivity. The same fracture-cavity system has the same fluid property and unified oil/gas/water contact. Different fracture-cavity systems may vary in those aspects. During the oil/gas recovery, additional fracture-cavity bodies will supply oil and gas. The oil/gas production is unstable, with various results of water flooding. Many complex phenomena, such as production volatility and unpredictable oil and water production, may take place.

separated cavity units are formed. For example, Well Ha-7 is located in the northwest of the Halahatang region in the western slope belt of Lunnan Uplift. Based on the characteristics of reservoir and fluid property as well as analysis of the production test data, Well Ha-7 is a typical cavity unit with constant volume. Results of the reservoir prediction and fracture-cavity characterization suggested that Well Ha-7 is an isolated cavity system, far away from other fractures or cavities in the area with poor connectivity (Figure 6-8). Oil/gas property varies greatly in different wells. For example, Well Ha-7 produces heavy oil, while Well Ha-11 in the southwest produces normal oil, indicating that they are not connected. From the production tests in many wells, we observed no water in Well Ha-7, but a big water flooding in Well Ha-9 and water-free, high, and stable production in Well Ha-11 located in the structural low, which indicates that different wells have different conditions of edge and bottom water. Formation water in different wells is not connected; the production test in Well Ha-7 shows rapid decline in oil pressure and obvious production depletion, which means it is an oil accumulation unit with constant volume.

(1) Isolated-Cavity-Type Oil and Gas Reservoir

(2) Connected Fracture-Cavity Type Oil and Gas Reservoir

In the fracture-cavity type oil/gas reservoirs of Ordovician in the Lunnan, Halahatang, and Tazhong North Slope of the Tarim Basin, large cavities provide major pore space. Due to long-term, deep burial and diagenesis, the majority of pathways between cavities have been collapsed, filled, and cemented. Thus, the connectivity is poor, and relatively

During the development of the karsted fracture-cavity reservoir, due to the connected nature of some underground channels, fractures, and faults, many facture-cavity reservoir bodies are connected. Although at a later stage they would be collapsed and filled, some facture-cavity bodies remain, forming

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Carbonate Fracture-Cavity Reservoir

205

FIGURE 6-8 Carbonate fracture-cavity visualization and cavity-type accumulation unit profile.

interconnected multifracture-cavity systems. Sometimes, under a certain differential pressure, the isolated fracturecavity bodies would interconnect for unknown reasons, forming unified multifracture-cavity oil/gas reservoirs. Although it is difficult to identify the connectivity of fractures and cavities, some methods do exist, for example, production test, interference test, and tracing agent application. Many wells such as the Wells Zhonggu-162, Lungu101, and Lungu-15 in the Tarim Basin developed connected fracture-cavity type oil/gas reservoirs. For example, Well Zhongu-162 is a weakly vaporized oil accumulation unit in a multifracture-cavity system. On the fracture-cavity visualization map (Figure 6-9), we can see there are many sets of connected fracture-cavity bodies developed near Well Zhonggu-162; from the production test curve, such phenomena as rising oil pressure, declining gas/oil ratio, and increasing oil production occurred without any measures taken during the midterm production period. After analysis, it was concluded that some other reservoir bodies were connected during production due to

differential pressure, leading to more oil/gas supply and increase of oil/gas production and rise of oil pressure. The nearby fracture-cavity bodies in the region could form an interconnected reservoir unit under some conditions, becoming the connected fracture-cavity oil accumulation unit (Figure 6-9).

2. OIL/GAS CONDUCTION SYSTEM As the bridge and link connecting source rock and reservoir during the oil/gas accumulation process, the conduction system is not only a key control factor for oil/gas accumulation, but also a key condition for systematized and dynamic oil/gas accumulation (Magoon and Dow, 1994). The oil/gas conduction system is subject to many factors such as the basin structure, sedimentary and diagenetic evolution, and fluid activity (fluid potential and pressure), which are related to complex changes of conduction property and capability in time and space (Hao et al., 2000). Generally, a conduction system refers to the all-path

FIGURE 6-9 Fracture-cavity visualization map and profile showing multifracture-cavity-type oil accumulation unit in Ordovician in Well Zhonggu-162.

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network, by which oil/gas bypassed from source rock to traps, including faults, fractures, unconformity surfaces, connected sand bodies, and their combinations (Fu et al., 2001).

(1) Fault and Fracture Conduction System The fault and fracture conduction system is the oil/gas migration pathway formed by fault opening. At the same time, large amounts of associated fractures can also significantly improve the reservoir property of carbonate reservoirs, forming a dissolved pore-dissolved cavityfracture system. The more the fractures develop, the larger the percolation space is, and the more favorable for the migration of oil/gas. For example, the faults and fractures developed extensively in the Ordovician, Lunnan area of Tabei Uplift, typically together with folding structures. Among them, the faults extending to the source rock of mid-lower Cambrian are the effective migration pathway of oil/gas. The oil and gas in Lunnan region were mainly from the marine source rocks in the Paleozoic, and the karsted carbonate rocks connected to the source rocks through the faults became the targets for oil/gas accumulation. For example, areas surrounding Well Yingmai-2, Well S86eS67eS65eT401 and Well T402eS78 in Tahe region developed northeastern-trending factures, forming the oil/gas migration and accumulation belt in the mid-lower Ordovician from Area7eArea-6eArea-4 in Tahe (Gu et al., 2007). The faults also had some reforming and damaging impact on some oil/gas reservoirs in Paleozoic.

(2) Unconformity Conduction System Tectonic movements in multiple periods formed multiple unconformities. Large-scale dissolved pore, cavity, and fracture systems were formed within a certain depth range below unconformity surfaces, becoming the major pore network for fracture-cavity carbonate rock. At the same time, the unconformity surfaces are important pathways for lateral migration of oil/gas, provided that sealed caprock exists above the unconformities. For example, after long-term exposure, weathering and erosion in the Caledonian-Early Hercynian, mid-lower Ordovician in LungueTahe oil field formed extensively distributed weathering crust. In the surface eluvium with good storage-seepage conditions, weathered fractured breccias, and semiweathered layers, the karsted network system composed of dissolved pores, cavities, and factures connected by fractures and satures is the most important conduction system in the LungueTahe oil field. In particular, the large-amount of oil supply in source rock area during the late Hercynian, combined with leaky caprock conditions in Tahe region, led to the current distribution of

Unconventional Petroleum Geology

heavy oil in Ordovician, implying the importance of unconformity and karsted conduction system to the forming of LungueTahe oil field (Gu et al., 2007).

(3) Connected Sand-Body Conduction System In the connected sand-body conduction system, connected pores are the pathway of oil/gas migration. For example, in Lunnangu-Tahe, Tabei region, such a conduction system mainly developed in sandstones of the Carboniferous Kalashayi Formation and Triassic (Cheng, 2004). Kalashayi Formation features multiple layers of sandstone that are thin and with great lateral variation; the sand body distribution in the Triassic is relatively stable with minor lateral changes, which, combined with faults and unconformities, became important conduction system for oil/gas migration.

(4) Combined Conduction System Regional faults, unconformity surfaces, karsted fracturecavity systems, sand bodies, and fractures could create combined pathways for oil/gas migration, which is an important condition to form large-scale composite oil/gas accumulations. For example, oil/gas pools in Triassic, Carboniferous, and Ordovician in Lunnan, Tabei region, resulted from multiple periods of hydrocarbon generation, filling and injection, and adjustment that formed multilayer, composite oil/gas accumulations. The process shows the effect of the composite conduction system. Before the gas invasion at the late Himalayan movement, the faulted horst in Sangtamu and the fractures and faults in Lunnan Faulted Belt connected the sand bodies in Triassic with the Paleo-pools in Ordovician, which led to migration of oil/gas toward the overlying Carboniferous and Triassic. Further migration within Carboniferous and Triassic also took place. During the gas invasion at the late Himalayan movement, the cracked natural gas with high-dryness coefficient was charged into the fracture-cavity carbonate reservoirs in Ordovician along the strike-slip faults in Eastern Lungu. Due to the forming of a high-pressure unit in Carboniferous, the faults closed in Carboniferous, and the natural gas cracked at the later stage could only migrate along a conduction system of the faults and unconformity surfaces. The migration pathway is the fracture-cavity carbonate rocks in Ordovician in the Tasangmu faulted belt.

3. HYDROCARBON MIGRATION AND ACCUMULATION MECHANISM A fracture-cavity carbonate oil pool is a continuous medium composed of matrix, fracture, and cavity. Fractures and the

Chapter | 6

Carbonate Fracture-Cavity Reservoir

cavities connected by fractures communicated with source rock; they are both pore network and flow channel; the pores and cavities connected by fractures are characterized by tube flow. In a fracture system, the oil/gas seepage follows Darcy’s flow. The seepage capability of matrix system is very low, featuring non-Darcy’s flow. In fracture-cavity carbonate reservoirs, the pore network is mainly composed of cavities and fractures that act mostly as flow channel. Cavities and fractures are distributed randomly, with the feature of a crystal framework-like oil pool. The dynamic size of fractures and connected cavities is large; the fluid flow can be regarded as tube flow. Due to low permeability of matrix, the fluid in the matrix follows non-Darcy’s law. Large fractures and cavities coexist with small fractures and small cavities in a fracture-cavity reservoir. And medium demonstrates strong discontinuity. The shape and size of the space for fluid flow vary greatly; the mode of fluid flow can be either the linear flow in small fractures and cavities or the nonlinear flow in large fractures and cavities. In addition, mixed flow modes combining the above two flows exist. With regard to flow laws in carbonate reservoir, the majority of discussions have been made on the basis of continuous medium theory or by replacing a discontinuous medium with a flow system of equivalent continuous medium. So the reservoir could be modeled as a pore-cavity double-medium system, pore-fracture-cavity triplemedium system, or multimedium system (Figure 6-10). In doing so, the flow inside the reservoir is considered as seepage flow completely. Based on the features of fracture-cavity reservoir systems, we suggest a fracture-cavity hydrocarbon accumulation mechanismdthat is, a physical flow model with large-scale flow in cavities connecting seepage flow in fractures, or facture-cavity connection flow model (Figure 6-11), or tubeeseepage connection flow model. This coupling model combing seepage flow and tube flow not only reflects fluid flow in a large fracture and cavity system, but also reflects fluid flow in matrix and isolated pores and cavities. This model considers a fracture-cavity reservoir system as a geological model with a unified and continuous medium. For example, the cavities can be assumed as columns, and they are connected by fractures featuring seepage flow. One fracture-cavity system can be taken as a web-like physical model. The fluid flow in the cavities can be approximated by irregular tube flowdthat is, the fluid flow in the column, which is a relatively simple flow in hydrodynamics. A cavity is the major pore space and can be viewed as a tube channel. The flow in the cavity can be taken as tube flow, and fluid is deemed as incompressible viscous fluid. Fracture is the major seepage flow channel, connecting cavities. But fracture also has a certain ability to serve as a reservoir. The flow in the fracture can be seen as linear flow. The combination of cavity units and fracture units

207

thus constitutes a fracture-cavity unit. The seepage capability in tight matrix is very low. Due to special accumulation conditions, the heterogeneity of fracture and cavity is significant, with complicated fluid flow. Due to the large size of fractures and cavities, the fluid flow can be seen as tube flow. However, if the fractures are tiny and the matrix is tight, the pore size is very small, and the fluid flow will follow Darcy’s law or non-Darcy’s law. As “seepage flow” in matrix and “tube flow” (cavity flow and turbulent flow) in fractures and cavities coexist in a fracture-cavity reservoir, the current reservoir fluid dynamics theory cannot effectively describe the fluid flow. The heterogeneity of fracture-cavity carbonate reservoirs results in complicated oil/gas migration, accumulation, and distribution (Figure 6-12). For example, Lungu Uplift experienced strong uplifting erosion in late Caledonian and Hercynian as well as structural superimposition and reformation since the Indo-Chinese epoch. Lunnan Uplift and surrounding areas were the target of oil/gas migration for a long time, experiencing three oil/gas accumulation cycles of first-order fluctuation, which were characterized by damaging, alternating, and enrichment, respectively. The cavity system in the Lunnan region experienced three development stages. The extent and connectivity of fracturecavity systems are important factors in oil/gas enrichment in weathering crust. The densely developed fractures and small faults connected cavities, forming oil/gas enrichment area. The isolated cavities do not have oil/gas sources; wells drilled into such cavities produce only water. The leakage area at the top of the horst is water saturated. Due to the poor sealing condition of the caprock in the structural highs in adjacent slope, heavy oil is distributed in the strong leakage area. In the structural low areas of the slope and platform, the oil/gas filling and charging at late stages formed the accumulations of light oil and condensate oil. The eroded remnants of mid- and upper Ordovician is the favorable area to look for a primary carbonate oil/gas reservoir formed at the early stage. The area toward sag along the slope around Lunnan Lower Uplift is the favorable carbonate oil/gas enrichment area. The carbonate reservoir in the Tahe oil field is mainly cavities. The size of cavity and fracture that contributes to the production is greater than 300 mm; the fracture width after acid fracturing is usually 1 to 8 mm. Judged by the flow patterns, the fluid flow in the fracture-cavity reservoir in the Tahe oil field is mainly Darcy flow, which could be approximated as irregular tube flow. And the fluid flow in cavities and fractures with size less than 300 mm can be considered as seepage flow. For the fracture-cavity carbonate oil/gas reservoir, the accurate prediction of carbonate fracture-cavity distribution is a precondition for the discovery of oil and gas, and precise identification of fractures and small faults is the key to improve the successful ratio for drilling.

208

Unconventional Petroleum Geology

Lower member of Kalashayi Formation, Carboniferous

Bachu Formation, Carboniferous Middle and Upper Ordovician

Trap Hydrocarbon source rock Conduction system Lower Ordovician FIGURE 6-10 Profile of fracture-cavity carbonate reservoir in Ordovician, South Slope, Akekule Uplift (from Tarim Oilfield Company, 2007).

4. FACTORS THAT CONTROL OIL/GAS ENRICHMENT IN FRACTURE-CAVITY RESERVOIRS The marine sedimentary basins in China are characterized by old geologic ages, long thermal evolutionary history of organic matter, high maturity, deeply buried reservoirs, strong heterogeneity of reservoirs, complicated reservoir distribution, strong adjustment, severe modification, and damage at the later stage. The fracture-cavity oil/gas refers to oil/gas stored in fractures and cavities under the effect of karstification. The heterogeneity of reservoirs is extremely strong; the matrix porosity is usually less than 1.2%; the permeability is usually less than 0.5  103 mm2. The distribution of oil/gas is mainly controlled by a series of fractures and cavity bodies. In separate fracture/cavity bodies, there are unified temperature and pressure systems

FIGURE 6-11 Hydrocarbon accumulation mechanisms in a carbonate fracture-cavity reservoir.

and unified oil/gas/water contacts. For example, the oil/gas distribution area of weathering crust in buried hill in the LunnaneTahe oil field and the large-scale condensate oil/gas distribution in the Yingshan Formation, north slope of Tazhong, and are both controlled by a series of superimposed and connected cavities and fractures.

(1) Long-Term Exposed Paleo-Uplift Controlled the Development of Quality Reservoir The distribution and development of karsted fracture-cavity reservoirs are controlled by paleogeomorphology. In different geomorphologic units, the karstification and reservoir development differ and the oil/gas enrichment varies. The karst platforms are high in paleopaleotopographic maps with severe formation corrosion. The karstification mainly led to the development of vertical caverns, which were the major area for water supply. The overlying caprock is relatively thin, preventing oil/gas from accumulating. Karst basin and valleys are located in the convergence and discharge area with severe filling of reservoir pore space, leading to a poor preservation of pore network. The karst terrace is located in the gentle transitional belt between karst platform and karst basin, with good hydrodynamic conditions. Strong water supply and discharge resulted in intense karstification, developing reservoirs with a good pore network. The paleo-uplifts in Shaanganning, Sichuan, and Tarim basins experienced weathering and corrosion in 140 Ma, 120 Ma, and 77e32 Ma, respectively, and the formed weathering crusts constituted good reservoirs. Taking paleo-uplift in Central Sichuan as an example, before Permian, the geomorphology of the paleo-uplift was paraplain, and the percentage of weathering crust with carbonate rock as the base rock was 90.32%e96.52%,

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Carbonate Fracture-Cavity Reservoir

209

Lunnan Oilfield T Common oil Heavy oil belt

Lungu 32 O1 Gas Lunnan 30

T1 Condensate gas pool

Lungu 38

Lungu 1 O1 High-wax oil Lungu 2

Lunnan 621 Lungu 39

T Common oil

O1 Oil & Gas

Lundong 1

Lungu 63

O1 Condensate gas Sangtamu Oilfield T Common oil

C Oil & Gas

Lungu 391

Lunnan 631 O1 Gas/High-wax oil eld ilfi O ng do qu g fan Jie

T Common oil

Lungudong condensate oil/gas belt

O1 Gas

T/J Common oil

Lunxi Oilfield

Lungu 35 T Common oil Lungu 34

Oil rim

Tahe Oilfield Common oil belt Jilake Gasfield Heavy oil belt

Conventional black oil pool

Condensate gas pool

Oil/gas pool

Gas pool with oil rim

Transitional belt with gas

Fault

FIGURE 6-12 Distribution of oil/gas of different properties in the Lunnan region (from Zhang et al., 2001).

which belongs to the karstified weathering crust. The percentage of weathering crust with mudstone and sandstone as base rock was 25%, which belongs to residual weathering crust; the weathering crust of carbonate rock features strong eluviation and leaching effect and weak residual effect. Therefore, it tends to form karsted fracturecavity zones. Shaanganning paleo-uplift has the similar phenomenon. The distribution of carbonate reservoir in CambrianeOrdovician in Tarim Platform and Basin was mainly subject to the late weathering corrosion and paleokarstification. Due to long-term exposure of Ordovician paleo-uplifts in Lunnan, Tazhong, and Southeastern Bachu, the reservoir conditions are good. On the contrary, in areas situated in structural lows in paleo-slope, such as Well Yangwu-2 in Manjiaer Sag and Well He-3 in East Bachu, the reservoir conditions are poor. In addition, successive, long-term paleo-uplift usually formed multiple sets of quality reservoirs. The existence of multiple sets of quality reservoirs in Ordovician, Carboniferous, Triassic, and Jurassic in the Lunnan region are closely related to the long-term uplifting settings.

(2) Quality Reservoir Controlled the Oil/ Gas Enrichment in Fracture-Cavity Fields It is the fracture-cavity bodies, not local structure, that controlled the enrichment of oil and gas in weathering crust. For example, the Ordovician in the South margin of the Tabei Uplift is mainly platform-facies limestone, with very few preserved primary pores. The reservoir is mainly

karstified fracture-cavity bodies. Many wells were drilled into large-scale fracture-cavity systems. Bleed-off, kick, and lost circulation occurred in the drilling of more than 20 wells in the Lunnan region, with great changes among wells. In plain view, karstified fracture-cavities distribute in strips and blocks. Reservoir developed mostly in the slope area with many large-size caverns with little filling. A series of fracture-cavity zones are superimposed and connected. Only wells drilled into large cavities can acquire high commercial flow, such as in Well Sha-48, Well Lungu15, Well Lungu-42, Well Lungu-701, and Well Aiding-4. Low or no production in wells like Well Lunnan-15 is mainly related to underdevelopment of reservoir. The exploration practice in LunnaneTahe buried hill indicated that only when wells were drilled into large cavities or fractures that connect to large cavities can high and stable production be obtained. The extent of fractures and cavities determines the productivity of the Ordovician reservoir, and the distribution of quality reservoir controls the oil/gas enrichment. The fracture-cavity system led to uneven oil/gas accumulation. The drilling of Ordovician in Lunnan indicated that a favorable reservoir was distributed within the top 200 m of weathering crust in buried hill. Oil/gas distribution is subject to the spatial extent of paleokarst system and fracture system. Oil/gas shows a layer-like distribution in large scale, yet due to the strong heterogeneity of karsted reservoir, the fracture-cavity system is isolated in the tight limestone. The relatively independent one or more paleocave systems formed a relatively independent oil/gas pool

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with relatively unified oil/gas/water contact and a unified pressure and temperature system. Due to the lack of a structural trap or stratigraphic barrier, the independence and connectivity of a fracture-cavity system have only a relative meaning. During different geologic times, under the different boundary conditions, a connected oil/gas reservoir could be separated into multiple isolated oil/gas reservoirs. On the other hand, relatively independent fracture-cavity systems could be reconnected and the distribution of oil and gas could be readjusted. As a result, during the production of oil and gas, the connection status of different cavity-fracture systems would lead to changes in oil, water, and gas properties and periodic variations in production. A relatively isolated fracture-cavity system formed a constant volume body. The initial oil and gas production was high, but the water flooding came quickly, so the production was limited. However, the connected multiple fracture-cavity systems constitute a large-scale reservoir, with stable or slowly declined oil and gas production and steadily rising water cut. In terms of multiple fracture-cavity systems with poor connectivity, connections could be realized under a certain differential pressure, resulting in periodic changes in oil/gas production. For example, after production from one fracturecavity system declines, another system may kick in, providing oil and gas, leading to highly fluctuated productions as well as huge changes in water cut.

(3) Superimposition of Reservoirs with Multiple Origins Is the Basis for LargeScale Distribution of Oil/Gas The carbonate reservoirs in China experienced multiple tectonic uplifts and exposures, as well as multiple karstifications, leading to different paleokarst geomorphologies of weathering crust and differences in reservoir characteristics. The secondary pores of multiple types led to the heterogeneity of reservoirs. Because of the multiple dissolution and fracturing effects, the carbonate reservoir has multiple rock fabrics, forming complicated secondary pores with multiple origins and complex spatial distribution, leading to the strong heterogeneity of carbonate reservoir. Paleokarst carbonate reservoirs tend to be superimposed vertically with a layer-like horizontal distribution in a large area. For example, the vertical layering of a fracture-cavity system in the Lunnan buried hill is clearly seen. Although there are significant differences in the quantity and depth range of caverns between wells, some multistory karsted caverns have been identified. Laterally, karsted fractures and cavities appear in stripped and blocky patterns. A series of fracture-cavity zones are superimposed and connected, forming more than 5000 km2 paleokarst reservoir. The distribution of weathering crust reservoir in Yingshan

Unconventional Petroleum Geology

Formation, Tazhong, is similar to that in the Lunnan area. The multistory and blocky features are even more obvious. The weathering crust reservoir of the Yingshan Formation, lower Ordovician, is mainly developed in the vertical seepage zone and horizontal undercurrent area within 200 m below the buried hill, with a total area of over 6000 km2 in the north slope of the Tazhong region.

(4) Long-Term Successively Uplifted Slope Area, Overlapping and Pinchout Area, and Areas with Frequent Lithological Bariations Are Favorable Places for Oil and Gas Accumulation Fracture-cavity oil and gas reservoir are mainly related to large unconformity surfaces and paleo-uplifts. Due to the successive uplifting in tectonic movements, paleo-uplifts serve as the targets of long-term migration of oil and gas; therefore, there are often rich oil and gas accumulations. Besides source rock, reservoir, and caprock conditions, other factors also affect the forming of fracture-cavity reservoir and oil/gas accumulation, including the timing of uplifting, the stability of structure at the later stage, the scale of paleo-uplift, and the coupling oil/gas charging and accumulation. The earlier a paleo-uplift formed, the longer it developed, the more stable the structure at the later stage, the larger the scale of the paleo-uplift, and the more favorable for the oil/gas accumulation, preservation, and enrichment. Typically, the structural highs of paleo-uplifts were severely altered in the later-stage tectonic movements. The old oil and gas pools were modified and damaged, forming secondary oil/gas accumulations. If the tectonic movements were too strong, there would be no oil and gas generated or preserved. In structural lows of the uplifts and the slope of paleo-uplifts, tectonic activities at the later stages were relatively weak, promoting the generation and preservation of primary oil and gas, or coexistence of largescale accumulations of primary oil and gas and small- and medium-scale accumulations of secondary oil and gas. Karsted reservoirs were superimposed on the slope of the paleo-uplifts. Oil and gas were distributed along the unconformity surfaces, forming large layer-like oil and gas fields (Figure 6-13).

SECTION 4. EXPLORATION AND DEVELOPMENT TECHNOLOGIES FOR CARBONATE FRACTURE-CAVITY OIL AND GAS ACCUMULATIONS Exploration and development of carbonate fracture-cavity oil and gas involve seismic data acquisition, processing, interpretation, and indoor modeling. It requires

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211

FIGURE 6-13 South-north reservoir profile of Tarim Basin (from Tarim Oilfield Company, 2010).

comprehensive analysis on seismic, wireline logs, drilling data, core data, and structural evolution, and production performance by applying such techniques as lithologic and paleo-geomorphologic analysis, full 3D structural interpretation, fine-scale fault interpretation and 3D visualization, physical modeling of fracture-cavity reservoir, seismic detection of carbonate fracture-cavity reservoir, direct hydrocarbon detection, reservoir acid fracturing, horizontal well development, staged acid fracturing stimulation in horizontal well and water injection, and oil displacement.

1. LITHOLOGIC AND PALEOGEOMORPHOLOGIC ANALYSIS Guided by sequence stratigraphy, reservoir geology, structural geology, and sedimentary geology, computers can be used as a tool to do sequence recognition, stratigraphic correlation, single-well facies analysis, sectional facies analysis, macro- (such as core observation) and micro(such as thin section, geochemistry) analyses of reservoir characteristics, statistical analysis on reservoir physical properties, so as to establish the reservoir models of different origins based on their stratigraphic correlation, reservoir types and forming mechanism, sedimentary facies, paleo-geomorphology, and paleo-drainage systems.

2. SEISMIC PREDICTION TECHNOLOGY Based on the full 3D seismic data volume, we can analyze seismic attributes such as amplitude and frequency from points and lines to surfaces to realize a detailed 3D visualization. Adopted techniques include precise horizon calibration, coherency and dip analysis, 3D interpretation and spatial visualization. New 3D seismic processing workflow provides accurate basic data for reservoir prediction with unified grid, static correction, seismic record (polarity, time difference, amplitude, frequency, and

waveform), velocity model, and stack/migration. With such processing, the quality of seismic data is improved. The surfaces of weathering crusts in major targets were finely characterized to identify the structures previously at the interfaces of individual 3D seismic cubes. The resolution, S/N ratio, and fidelity of data have been effectively improved, so as to provide reliable basic data for subsequent seismic data interpretation, reservoir prediction, seismic inversion, and overall evaluation.

3. PHYSICAL MODELING In 1977, the Seismic Acoustic Lab at the University of Houston in the United States created the water-tank physical models. In 1985, Nanjing Petroleum Prospect Research Institute and Tongji University also built large-scale automatic physical modeling and observation systems with a water tank. However, the water-tank modeling has some defects, as it cannot correctly simulate the onshore seismic survey process. It can only record P-wave, not S-wave and transformed S-wave. In order to overcome these shortcomings, Columbia University, Exxon, the University of Houston, and China University of Petroleum developed solid geophysical models in the 1980s. Research on the physical simulation technology of carbonate fracturecavity reservoir has begun, although a systematic study is still lacking. The numerical simulation technology, along with the improvement of algorism and development of computer technology, has evolved from acoustic raytracing into wave equation simulation. As a result, simulation accuracy and speed have been improved greatly.

4. RESERVOIR SCULPTURING TECHNIQUE The identification of carbonate fracture-cavity reservoirs could begin with analysis of seismic attributes, drilling,

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mud logging, wireline logs, core. and thin sections. The pre-drilling identification of fracture-cavity reservoirs mainly relies on seismic survey. The fine reservoir calibration and forward modeling can be utilized to identify geophysical responses of the reservoir, to conduct sensitivity analysis of seismic attributes, and to identify and extract effective seismic attributes. Based on geologicalorigin analysis and the reservoir geological model, the identification of paleokarst caves, pores, and fractures can be conducted. Multi-attribute analysis seeks to extract different attributes from the data in a time window along a horizon, so as to get maps or 3D displays of the attributes and to conduct comprehensive geological analysis. From multi-attribute cross-plot analysis, it is concluded that coherency, frequency-dependent amplitudes, and acoustic impedance are sensitive attributes that are appropriate for predicting carbonate fracture-cavity reservoirs. The attribute extraction technique could be subdivided into RMS amplitude, amplitude variability, frequency decomposition, coherency along horizon, impedance and seismic facies at limestone top, and so on. The key is to use a proper time window and an accurate interpretation horizon. The extraction of seismic attributes is a matured and commonly used technology. However, the quantitative description of cavity and fluid prediction is still very difficult. In recent years, quantitative sculpturing technology of the fracture-cavity reservoirs has made great progress. For example, utilizing the well-controlled high-fidelity prestack time migration processing, reservoir imaging has improved in the Tarim oil field . The trace gather data, in particular, have laid a solid basis for quantitative reservoir description and pre-stack oil/gas detection. Through highaccuracy pre-stack depth migration processing, the improper placement of “bead string” has been effectively solved so as to provide robust support for accurate identification of fracture-cavity locations. On the basis of wellseismic modeling, quantitative relationships between seismic responses and fracture-cavity conditions are established for a quantitative calculation of pore space of fracture-cavity units. The 3D sculpturing and quantitative description of fracture-cavity have played an important role in determining drilling locations. In the last two years, more than 98% of the wells have penetrated the reservoir (Figure 6-14).

5. DIRECT HYDROCARBON DETECTION Hydrocarbon detection in carbonate fracture-cavity reservoirs is a very difficult but hotspot in research. Currently available techniques include pre-stack AVO trace gather and frequency absorption. In terms of frequency absorption technology, high-production wells have demonstrated declined dominant frequency, rapid attenuation in high

Unconventional Petroleum Geology

frequency, and energy increase in low frequency; dry wells with mudstone filling have shown higher energy, high frequency, and low absorption; fault zones have been characterized by low energy, low frequency, and high absorption. Pre-stack AVO trace gather is to use the variation of amplitude along with offset (incident angle) to identify the fluid type in cavity. In general, the amplitude in oil wells will increase along with the offset, and the amplitude in water wells will decrease along with the offset.

6. RESERVOIR ACID-FRACTURING TECHNIQUE Carbonate fracture-cavity reservoir features strong heterogeneity and low permeability and no pore space in matrix. The seepage channel of oil and gas is mainly fractures. Most of the oil wells do not have productivity after well completion. Only after acid-fracturing stimulation can acidized fractures of a certain length and high-conductivity be formed to connect oil/gas seepage channels and oil pore network, supporting the normal production and keeping long-term high and stable productivity. History shows that the technology of acid-fracturing stimulation can help release formation energy, dramatically increase oil and gas well productivity, and greatly promote the economic benefits of oilfield development. It has become an indispensable key technology for the development of carbonate fracture-cavity oil/gas reservoirs.

7. DEVELOPMENT TECHNOLOGY OF HORIZONTAL WELLS At places where many carbonate cavities developed, in order to drill more fracture-cavity units to increase the production of individual wells, horizontal wells are usually drilled. The track of horizontal wells is usually perpendicular to strike direction of fractures, Thus the probability that the drilling hole could go through the fracture-cavity zone will increase greatly, benefiting the development of facture-cavity reservoirs. Problems commonly encountered in the drilling and development of horizontal wells include the following. (1) When drilling-bit fall and fluid loss take place while penetrating a large-scale cavity reservoir, the mud circulation cannot be established. It is therefore impossible to drill into other cavities as planned. In most cases it could only be put into production directly. (2) When drilling into several fracture-cavity units in the horizontal interval of a horizontal well, if one fracture-cavity unit produces water, water flooding in the whole system would occur and reserves in other fracture-cavity units could not be effectively recovered. Therefore, if the

Chapter | 6

Carbonate Fracture-Cavity Reservoir

213

FIGURE 6-14 Fracture-cavity sculpture map for well Zhonggu-11 in Tazhong area (from Tarim Oilfield Company, 2000).

reservoir and fluid prediction and quantitative description of fracture-cavity system cannot meet the design requirement for horizontal wells, it is not proper to conduct large-scale horizontal well development. During well deployment, the vertical well plus sidetracks shall be adopted as priority (Lv et al., 2006).

8. STIMULATION TECHNOLOGY IN HORIZONTAL WELLS Staged stimulation in horizontal wells is an important technology to increase production in the worlddthat is, to adopt specialized tools to divide horizontal well interval into relatively independent segments to selectively conduct acidizing stimulation. For example, acid-fracturing stimulation in horizontal wells in the Tarim oil field began in 2005. The technology was used in Well Tazhong 62e7H in 2008, resulting in a high daily oil production of 220 m3 and high daily gas production of 20  104 m3. Compared with conventional acid fracturing, this technology has many apparent advantages. It can form relatively independent artificial fracture systems and better utilize reservoir intervals with different physical properties

to fully excavate the productivity of horizontal wells for maximal increase of production in individual wells. Practices show that after the staged stimulation of a horizontal interval, average productivity increased by 3.8 times compared with that in vertical wells in the same block, and it nearly doubled compared with conventional acid fracturing in the horizontal well. After years of research, adoption of horizontal well development gradually became an important measure for establishing high-production wells, high-production well groups, and high-production blocks in carbonate reservoirs in the Tazhong region. Furthermore, staged acid fracturing technology in the horizontal well has played an important role in improving development efficiency and extending the service life of individual wells.

9. WATER INJECTION AND OIL DISPLACEMENT TECHNOLOGY Natural energy declines greatly after the carbonate fracture-cavity reservoir is put into production, with a low elastic recovery ratio. For example, for the individual wells with constant-volume cavity as reservoir, annual

214

production declined by 30% to 90% in the Tahe oil field (Tu, 2008). The water injection and oil displacement are an important approach to increase recovery. Oil wells in carbonate fracture-cavity units make the best use of natural energy for development before water injection and oil displacement. At the later stage, only when the formation pressure cannot maintain normal production with a pumping unit could water injection and oil displacement be adopted. The constant-volume oil cavities should be the favorable target. By water injection, formation energy is supplied, formation pressure is restored. By use of principle of gravitational differentiation, during the well shut-in, oil is displaced by water and the oil/water contact is raised. The injected water enters into small fractures adjacent to oil wells to displace the remaining oil, which is difficult to recover. The oil well undergoes the cycle of “water injectionewell shut-ineoil production,” so as to conduct injection and production circulation. After several cycles of water injection and oil displacement, the oil recovery ratio could gradually be enhanced (Rong et al., 2008).

SECTION 5. EXPLORATION POTENTIAL AND DIRECTION OF CARBONATE FRACTURE-CAVITY HYDROCARBON RESOURCES Marine basins are widely distributed in China. Marine sediments are important both aerially and volumetrically. The Lower Paleozoic is dominated by Carbonate paleokarst reservoirs, with widespread interlayer and intralayer stratoid beds (Table 6-4) and multistage unconformity surfaces developed, providing a critical geologic foundation for the formation of large and medium-sized stratigraphic unconformity pools. Deep bedding karst is a key element controlling the preservation of large-scale, deep, effective reservoirs, and large oil/gas fields can be discovered in deep formations.

1. PALEO-UPLIFTS AND SURROUNDING SLOPE AREAS Paleo-uplifts in old marine basins formed earlier and extended for a long time, and they are always the favorable destinations for oil and gas migration. The uplifts are sizable, generally in the (1e4)  104 km2 range, serving as major targets of exploration in marine formations. The uplifts are characterized by good source-reservoircaprock assemblage and multiphase accumulation. In late stages of uplifting, tectonic activities were very intense, contributing to the accumulation and adjustment of hydrocarbons.

Unconventional Petroleum Geology

Depending on the intensity and evolution of Craton tectonic activities, large uplifts played different roles in controlling hydrocarbons. Among several Paleozoic cratonic basins in China, Tarim Basin is the most active one, Ordos and Huabei basins are the most stable, while Shangyangzi Basin in Sichuan is somewhere in between. In Tarim Basin, most of the marine oil/gas fields are located in the three major uplifts and their slopes. In the Sichuan Basin, marine gas enriches in Himalayantime structures locally under the background of paleo-uplift. In Qingyang Uplift, Ordos Basin, the paleo-uplift does not control the enrichment of natural gas directly. In the periphery of Hetianhe Gasfield, three exploration domainsdpaleo-weathering crust, platform-margin bank, and nappe structuredwere formed due to the migration of a paleo-uplift. Early accumulations are inclined northward, while late accumulations are inclined southward. Accumulation conditions were different in Madong, Mabei, and Manan areas. In Manan, which is located in the northeast extension of Hetianhe Paleo-uplift, before Carboniferous sedimentation, the structure was weathered and leached, and the Sangtamu Formation of Ordovician was corroded. Certain grain limestones in Lianglitage platform-margin facies were preserved and later were overlain by lower mudstone Member of Carboniferous Formation. It is predicted through seismic inversion that Ordovician weathering crust reservoirs are developed in this area, along with compression-torsion faults connecting to Cambrian formation, which are beneficial for hydrocarbon accumulation.

2. LARGE STRATIGRAPHIC UNCONFORMITY RESERVOIRS Paleo-uplifts and paleo-slopes are favorable zones for forming large- and medium-sized unconformity pools, including the truncated pools under the unconformity surfaces and the overlapping pools above the unconformity surfaces. Forming the pools is controlled not only by unconformity type, trap forming time, and the hydrocarbon migration period, accumulation. and preservation conditions, but also by development of source-reservoir-caprock assemblage, development of faults, and structural deformation. Quality of caprock is directly related to the forming of unconformity pools, while the property and heterogeneity of the reservoir directly contribute to the reserves and productivity. Unconformity (surface) can be both constructive and destructive for hydrocarbon accumulation. It is constructive because: (1) it provides good channels for oil and gas migration; (2) it improves the capability of reservoir under the unconformity surface; and (3) there are numerous unconformity traps above and under an unconformity

Chapter | 6

Carbonate Fracture-Cavity Reservoir

215

TABLE 6-4 Distribution Characteristics of Paleokarst Reservoirs in Marine Basins, China

Lithology

Reservoir Thickness (m)

Size (104km2)

Sedimentary facies

Favorable microfacies

Pore-making process

Dolomite

3~8

5~8

Restricted e evaporative marine platform facies

Dolomite flat, grain bank

Dolomitization and paleokarst

Carboniferous, East Sichuan

Dolomite

3.5~29

3.5~4

Tidal flat

Depression margin bank and sand flat facies

Dolomitization and paleokarst

Majiagou Formation, Jingbian

Ma-5 Member

Dolomite

1~4

3~4

Basin-marginal gypsum-dolomite flat

Anhydrite concretion dolomite flat

Weathering crust-type karst

Ordovician, Tabei

Lianglitage Formation

Limestone

5~50

4.15

Gentle-slope platform margin, intra-platform reef bank

Bioclastic bank, sand bank

Paleokarst

Yijianfang Formation

Limestone

10~50

3.5

Gentle-slope platform margin, intra-platform reef flat

Bio-clastic bank, sand-clastic bank

Paleokarst

Yingshan Formation

Limestone

50~100

0.8

Open platform

Grain-calstic bank

Paleokarst

Lianglitage Formation

Limestone

80~150

1.2

Rimmed platformmargin reef bank

Reef bank

Paleokarst

Yingshan Formation

Limestone

150~250

0.6

Open platform

Grain-calstic bank

Paleokarst

Penglaiba Formation

Dolomite

50~200

0.3

Open platform

Grain-calstic bank

Dolomitization and paleokarst

Example Leikoupo Formation, Sichuan

Ordovician, Tazhong

Lei-4 Member

surface. It is destructive because erosion of the caprock leads to significant dispersion of hydrocarbons. Even in the situation of local destruction, the crude oil may be oxidized and water flushed, leading to dispersion of light components and preservation of heavy components, finally forming pools of heavy oil or asphalt that is difficult to flow. Additionally, the unconformity surface, along with the faults connecting to the paleo-surface, may enable the hydrocarbons to migrate along the unconformity surface and fault surfaces and to escape out of the ground, which destroys the reservoir to a certain extent. Large- and medium-sized stratigraphic unconformity reservoirs in three major basins in China are worthy of exploration. In Tarim Basin, for example, the unconformity reservoirs are diversified and widespread. Along the south margin of Tabei Uplift, where Ordovician paleokarsts were developed, three sets of reservoirs in the Lianglitage, Yijianfang, and Yingshan formations are 70e250 m thick, covering an area of 3.5  104 km2. In the Tazhong area,

where Ordovician reef bank and paleokarst were developed, three sets of reservoirs in Lianglitage, Yingshan, and Penglai formations are 100e400 m thick, covering an area of 1.2  104 km2. In the Ordovician paleokarst zone in Maigaiti Slope, three phases of paleokarsts were developed in an area 0.8  104 km2. In the periphery of Hetianhe, weathering crust-type karsted reservoir is observed in the Ordovician buried hill, with an area of 0.9  104 km2. In the Ordovician paleokarst zone along the periphery of Jingbian Gasfield in Ordos Basin, Ma-5 member dolomite expands in a radiant manner, with a favorable area of 1.0  104 km2. Weathering crust-type paleokarst reservoirs were developed in the Leikoupo weathering crust zone in the Sichuan Basin, where discovery has been made in exploration, with a favorable area of 1.2  104 km2. In the broad SinianeLower Paleozoic paleokarst slope in Sichuan Basin, karsted dolomite reservoirs were developed, with a favorable area of 8.5  104 km2. In the buried hills in Bohai Bay Basin, the hydrocarbon-rich sags

216

show good accumulation conditions, with a favorable area of 0.5  104 km2.

3. DEEPeULTRA-DEEP RESERVOIRS As oil and gas exploration expands, it is imperative to work on deeper targets. In the United States, Jaiy-Felder Gasfield was discovered in Cambrian-Ordovician carbonates at 8088 m, where secondary pores, cavities, and fractures were developed considerably, with a porosity of 25% and a permeability of 1020  103 mm3 (Wu and Xian, 2006). Moreover, in the Western Interior Basin, Mills-Lanch Gasfield in Anadarko Depression was found in the Lower Ordovician carbonate at 7663e8083 m. In Tarim Basin in China, Well TS1 encountered Upper Sinian granodiorites at 7100 m, above which dolomite cavities/fractures were well developed and both oil and gas were observed in geologic logging, coring, and gas logging. Well LD1 produced 28.61 m3 oil from the Ordovician reservoir at 6785e6805 m, and low-yield gas was observed at 7141e7180 m. Commercial oil and gas flow was discovered in the Ordovician reservoir at nearly 7000 m in several wells in the Halahatang area. In the Tabei area, abundant hydrocarbons have been found in Ordovician and younger reservoirs. The lower part of the Tabei area showed a lithology much similar to Cambrian source rocks, marking them as the prospective replacement targets in exploration. Well TS1 of Sinopec revealed good oil and gas shows in Cambrian, with fulvous liquid hydrocarbons in Upper Cambrian dolomite karstcavity reservoirs at 8400 m under160  C and 80 MPa, which were proved as highly matured light oil or condensate, since their crude oil maturity ranges in 1.08%e1.2% as converted by methylphenanthrene. In a test of Lower Ordovician-Upper Cambrian at 6800e7538 m, a small amount of gas was produced, which contained mainly hydrocarbon gas (97%). The dryness coefficient is 0.97, and the component of methane isotope is e37.9&, while the corresponding gas source rock Ro is 1.65%e1.91%, showing higher gas maturity than crude oil. Therefore, it is a typical highly matured oil-type dry gas. The Cambrian reservoir of Well TS1 is located along the Lunnan platform margin, where intercrystalline pore, intercrystalline dissolved pore, and broken (dissolved) fractures are major pore networks in dolomite reservoirs. The core test for the well shows that the porosity is 0.6%e 9.1% and the permeability is (0.001e34.4)  103 mm3. The wireline log interpretation involved 44 reservoirs of 641 m, including 7 Type-I reservoirs (66 m), which have fracture/cavity porosity of 4.5%e10.4% and are mainly revealed in Middle Cambrian; 9 Type-II reservoirs (127 m), which have cavity porosity of 3%e5.7% and are mainly developed in Lower Cambrian, Middle Cambrian, and bottom of Upper Cambrian; and 28 Type-III reservoirs

Unconventional Petroleum Geology

(456.5 m), which have cavity or fracture porosity of 0.63%e5% and are mainly developed in Upper Cambrian. The above data in Well TS1 indicate that deep Cambrian strata have the conditions for hydrocarbon migration and accumulation.

SECTION 6. CASE STUDY

1. TECTONIC EVOLUTION OF LUNNAN LOW UPLIFT Tabei Uplift is a paleo-uplift derived in the Paleozoic era for a long term. It experienced three development periods: (1) initiating in Caledonian, (2) pattern setting in Hercynian-Indosinian, and (3) subsiding in Yanshan-Himalaya. Many structures and nonstructural traps were formed on the uplift and the slope area. Lunnan Uplift is a remnant Paleozoic paleo-uplift, which experienced multiphase structural evolution and showed different shapes and deformation features in different phases as the structural stress field changed. Lunnan buried hill was reformed by three major tectonic movements (early Hercynian, middle and late Hercynian-Indosinian, and early Yanshan-Himalaya) and was corroded seriously in the process, directly impacting the formation and reformation of buried-hill reservoirs. Consequently, three grade top windows and three pinchout lines were generated (Pan et al., 2001). The early Hercynian movement in Late DevonianeEarly Carboniferous raised and corroded the Ordovician greatly for a long time, making the most intensive impacts on the formation, evolution and distribution of MiddleeLower Ordovician fractures, pores/cavities, and large caves (Xu et al., 2005; Liu et al., 2008).

2. SEDIMENTARY FACIES AND RESERVOIR-CAPROCK ASSEMBLAGE Local Ordovician buried hill formations consist of, from top to bottom, Sangtamu Formation (Q3s), Lianglitage Formation (Q3l), and Tumuxiuke Formation (Q3t) of Upper Ordovician, Yijianfang Formation (Q2y) of Middle Ordovician, Yingshan Formation (Q1-2y) of MiddleeLower Ordovician, and Penglaiba Formation (Q1p) of Lower Ordovician. Yingshan Formation is subdivided into upper member (sand-clastic limestone, Q1-2y1) and lower member (dolomitic sand-clastic limestone, Q1-2y2); Penglaiba Formation is subdivided into two members: (1) dolomitic limestone (Q1p1) and (2) micritic limestone (Q1p2). In Lianglitage Formation, Liang-1eLiang-4 Members are absent, and only Liang-5 Member remains; the formation thickness tends to reduce from north to south in both the Tahe sub-salt area (Hou, 2006) and Lungu East area.

Chapter | 6

Carbonate Fracture-Cavity Reservoir

In general, Ordovician formations in the Lunnan area experienced the evolution process of semirestricted platform facieseopen platform facieseplatform margin facieseplatform-margin slope faciesehybrid shallowwater continental shelf facies in a period from Penglaiba sedimentation in Early Ordovician to Sangtamu sedimentation in Late Ordovician. During the process, the seawater depth and hydrodynamic intensity changed, in the order of shallowedeepeshallow, and weakestrongeweak, respectively. Thus, an entire marine transgression and regression cycle was completed. There are four reservoir-caprock assemblages in Lunnan buried hill: (1) Triassic Ehuobulake mudstoneLower Ordovician assemblage, (2) Carboniferous Middle mudstone-Lower Ordovician assemblage, (3) Carboniferous Bottom conglomerate-Lower Ordovician assemblage, and (4) Upper Ordovician Sangtamu-Middle-Lower Ordovician assemblage.

3. RESERVOIR CHARACTERISTICS OF LUNNAN LOW UPLIFT Based on static and dynamic data (core, wireline log, 3D seismic, and well production) and the knowledge of modern karsts, we summarize patterns and controlling factors for paleokarst, and discuss the relationships between paleokarst and distribution of effective reservoirs, so as to guide oilfield exploration and development.

(1) Vertical Distribution of Reservoirs In surface karst zones and vertical seepage zones, the paleokarst fracture-cavity system is relatively developed, and fracture/cavity and dissolved cavity are major pore space; the paleokarst fracture-cavity system has good reservoir properties. Surface paleokarst reservoirs are the most dominant, with horizontal connectivity. Paleokarst is less developed in phreatic zones, and only small-sized dissolved cavities or karst channels are observed locally; the karsted reservoirs are mainly of fracture /cavity or cavity types. For example, in the Sangtamu SW block, the 10e35 ms RMS amplitude attribute on top of Ordovician shows (Figure 6-15) that the dissolved reservoirs in the lower interval are distributed in star-like and ribbon-like manner, suggesting a reservoir development poorer than in the upper interval.

(2) Horizontal Distribution of Reservoirs Ordovician carbonates in the Lunnan region contain three classes of reservoirs in two sets (Figure 6-16). The first set of reservoirs, mainly affected by middle Caledonian paleokarsts, was developed about upper 30 m

217

of Lianglitage Formation, with cavern, fracture/cavity, and small cavity as major pore network. They are mainly distributed to the east of the pinchout line of the Sangtamu Formation. The second set of reservoirs is divided into two classes. To the east of the pinchout line of Sangtamu Formation, Hercynian paleokarsts are absent, and the reservoirs, mainly affected by middle Caledonian and deeply buried paleokarsts, are distributed in a 50 m interval in the lower part of marlstones and upper part of Yijiangfang Formation, with fracture, fracture/cavity, and cavity as major pore space. There are also cavity reservoirs locally in the Lungu East Gasfield. To the west of the pinchout line of the Sangtamu Formation, the reservoirs were affected by multiphase paleokarsts, including Caledonian and Hercynian, and locally Caledonian paleokarsts might be corroded due to late tectonic activities. Generally, the reservoirs are distributed 150 m below the unconformity surface of Ordovician and up to 400 m in some wells. These reservoirs are mainly in Yijianfang and Yingshan formations, including mostly caverns, small cavities, fractures, and cavities/fractures, which are representative in the Lungu and Tahe oil fields.

4. HYDROCARBONS DISTRIBUTION The hydrocarbon properties in the study area are as follows: 1) In the process of late accumulation, Ordovician formations were intensively gas-invaded along the line of southern Lungu East strike-slip faults, Sangtamu fault horst, and Wells LG8 and LG2 area in Sangtamu SW and Western Zhongpingtai. The crude oil demonstrates a higher density, as medium oil or condensate oil; the crude oil has high paraffin content, low gum and asphaltene contents, high gas maturity, heavier d13C1, high dryness coefficient, low iC4/nC4 and iC5/ nC5, and high H2S and low N2. In contrast, in northern Lungu East, Lunnan fault horst zone and the central and east parts of Zhongpingtai, the gas invasion acted slightly; in Well LG7 area, most zones, except for Wells LG4 and LN1, were not affected or slightly affected by gas invasion. 2) Vertically, Carboniferous, Triassic, and Ordovician hydrocarbons differ significantly in properties. Carboniferous gas features a high dryness coefficient, low N2, and heavier d13C1; Triassic gas features low dryness coefficient, high N2, and lighter d13C1. In the central and eastern sections of Sangtamu fault thrust, Ordovician oil has much lower density than Triassic oil; in the west of Zhongpingtai, where Wells LG8 and LG801 are located, Carboniferous oil has lower density than both Ordovician and Triassic. Gas invasion in the late period tampered upon Triassic invisibly, indicating

218

Unconventional Petroleum Geology

High-production well FIGURE 6-15 Superimposed map of weak amplitude on top of the buried hills and strong amplitude within the buried hills in Lungu central slope. (Red/ yellow represents weak amplitude attribute at surface of the buried hills, and pink represents strong amplitude attribute inside the buried hills.)

that the properties of Triassic and Ordovician are controlled by different accumulation phases and processes. 3) Vertically, natural gas is generally dry in the upper section and wet in the lower section, which reveals that natural gas invasion occurred from bottom to top, that is, first to Ordovician, and then to Carboniferous and Triassic. Horizontally, natural gas represents as dry in the east section and wet in the west section, indicating that natural gas invasion occurred from the east to the west. During the gas invasion, original crude oil in the reservoir showed high paraffin content due to gas flushing.

0

10

20

30

5. MAIN CONTROLLING FACTORS FOR HIGH HYDROCARBON PRODUCTIVITY AND ACCUMULATION Several reservoirs have been discovered in Lunnan Low Uplift (Figure 6-16). They are mainly distributed from Paleozoic to Mesozoic and are represented by the overlaying of multiphase structural movements and multiphase accumulation. Therefore, they feature as composite reservoirs consisting of alternating and overlaying reservoirs with different properties and types. The reservoirs are generally distributed along the fault belts, and the hydrocarbon properties are distinct depending on zones.

40km uried tone b

rst

leoka

hill pa

s

Xinhe

Lungu 30 Lungu 32 Lunnan 1 Lungu 2 Lunnan 621 Caohu Lundong 1 Lungu 16 Lunnan 63 Lunnan 15

okarst ried hill pale u Dolomite bu Sangtam F boundary of t u Yingmaili o Aiding 4 t i o ch n Pin

Lungu 15

a orm

Yingmai 32

Lime

Donghetang

Yingmai 10

Ha 6

Lunnan 46

Yingmai 1

Yingmai 2

Lungu 34

Xiang 3

Dissolution boundary

Yingmai 4

Tahe 2 Hade 17 Yangwu 2

FIGURE 6-16 The distribution map of Ordovician weathered crust paleokarst reservoirs at the South Margin of Tabei Uplift.

Chapter | 6

Carbonate Fracture-Cavity Reservoir

Vertically, the reservoirs are diversified in different horizons; the reservoirs with varying properties are superposed; and the reservoirs are deeply buried, the hydrocarbons reserves are concentrated, the reservoirs feature complex phases, and oil and gas coexist in multiphase reservoirs.

(1) Sufficient Hydrocarbon Sources Two sets of source rocks contributed to the Lungu-Tahe oil/ gas fields: (1) Middle-Upper Ordovician and (2) MiddleLower Cambrian. Neighboring the hydrocarbon-generating sags is the basis for high hydrocarbons productivity and accumulation in carbonate fracture/cavity reservoirs. Ordovician-Cambrian, as the major source rocks, generated hydrocarbons for a prolonged period and drained hydrocarbons in multiple phases, providing reliable hydrocarbon sources for considerable and multiphase accumulation in different stratigraphic intervals in the Lunnan Low Uplift.

(2) Development of Layer-Like Fracture-Cavity System In general, Ordovician reservoirs in the Lungu oil field may be divided into two parts: (1) the area to the west of pinchout line of Sangtamu Formation, and (2) Lungu East area to the east of pinchout line of Sangtamu Formation. In the latter part, the reservoir properties are poor, and the hydrocarbon reserves are less abundant. The reservoirs are relatively developed in successful wells with good formation testing results, but are less developed in dry wells in general (Table 6-5).

219

TABLE 6-5 Analysis of Failure Wells in Lungu East Dry well

Target formation

Cause of failure

LG32

Yijianfang Formation þ Ying-1 Member

Dissolution not observed

LG351

Yijianfang Formation þ Ying-1 Member

Horizontal fractures developed, poor physical properties

LG36

Yijianfang Formation þ Yingshan Formation þ Penglaiba Formation

Horizontal fractures developed, poor physical properties

LG37

Yijianfang Formation þ Ying-1 Member

Fractures filled; no reservoir

LG392

Yijianfang Formation þ Ying-1 Member

Deviated from “beads”, polluted in acidizing; Ying-1 reservoirs underdeveloped

LG632

Yijianfang Formation þ Ying-1 Member

No Ying-1 reservoirs; Yijianfang reservoirs polluted

LN62

Yijianfang Formation þ Ying-1 Member

Pore/fractures filled; no favorable reservoirs

LN635

Lianglitage Formation þ Yijianfang Formation þ Ying-1 Member

Reef bank bodies and dissolutions not developed

LD1

Yijianfang Formation þ Ying-1 Member

Water produced in production testing

(4) Caprocks Overlain Paleokarsts (3) Enrichment at Structural Highs Reservoirs are developed in both large karst monadnocks and karst lowlands. For example, in Well LG102, which is located in a karst lowland, the wireline log interpretation revealed 17.2 m dissolved cavity reservoirs, and bleed-off occurred in four intervals during drilling, with accumulated bleed-off thickness of 15.64 m, which indicates that the well contains the most abundant cavity reservoirs in the area. Reservoir thickness is positively correlated to accumulative production (Figure 6-17), and the development of reservoirs is the most essential condition for production. However, given that the reservoirs are available, the existence of low-relief structures is a major controlling factor. In the Sangtamu SW area, the wells in high positions of large karst monadnock, such as Wells LN54, LG100-6, LG100-10, LG100-11, LG101, and LG101-2 , show high oil columns (Figure 6-18) and high accumulative production, which is typical in a hydrocarbon-rich area.

Well LG7 area is located high on Lunnan buried hill, with highly eroded and underdeveloped caprocks. For example, in Wells LG7-11, LG7-10, and LG7-15, Ehuobulake

FIGURE 6-17 Ordovician reservoir thickness versus accumulative production in the Sangtamu SW area. The thicknesses were interpreted from wireline logs.

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Unconventional Petroleum Geology

h (m) −4450

LN54−3

LN54−2

LN54−1

A

LN54 119m

8m

−4500

LG100−6 LG100−10 LG100−11 78m 24m 35m

LG101 LG101−2 LG16−1 37m

24m −4550

B

35m

−4500 8m

9m

−4550

−4600 −4650

h (m) −4450

−4600 Large karst monadock

Oil zone Water zone Dry layer

−4650

FIGURE 6-18 Profile of oil column across a large karst monadnock, Ordovician, Sangtamu SW area.

h (m) −4000

LN11−3

LN11−4

LG7−7

LG7−4

Karst lowland

LG7−5

LG7−1

33m 39m

−4300

8m

104m

67m

LN1

68m

163m

−4200

−4500

LG701

Karst slope

−4100

−4400

LG7−8

LG7−12

LG7−10

LG7−15

Karst highland 11m

122m

0m

h (m)

−4000 −4100 −4200 −4300

50m

−4400

Oil zone Water zone Dry layer

−4500

FIGURE 6-19 Oil/gas/water distribution in an area with underdeveloped caprocks (Well LG7 area).

mudstone caprocks of Carboniferous and Triassic are absent; in Wells LN1 and LG21, Carboniferous caprock is missing, and only Ehuobulake mudstone of Triassic is preserved locally, allowing Ordovician hydrocarbons escaping to overlying Triassic reservoirs. Wells LN11-3, LN11-4, and LG7-7 are located in karst lowlands, with high water energy. Oil and gas migrated toward karst slope under the buoyancy, leading to a high water column and water production in general. On karst slope, the vertical and lateral seal resulted from overlying Carboniferous and strong heterogeneity of reservoirs and promoted preservation of the hydrocarbons, forming high oil columns, as shown in Wells LG7-5, LG7-1, LG7-8, and LG701 (Figure 6-19). In the Lundongu East area, there are multiple, thick caprocks, such as the Sangtamu Formation of Ordovician and Triassic. The oil/gas/water distribution is mainly controlled by the intensity of gas invasion in the late period. Since the Lungu East strike-slip faults acted as channels for hydrocarbon migration in the late period, which connected to the source rocks in Manjiaer Sag in the southern part of Lungu East, the natural gas is abundant in the southern segment of Lungu East faults. Where the connectivity to the gas sources lessened, oil, gas, and water coexist. In Wells QM1 and QM2 in the northern part of Lungu East, the formations produce water in general due to the poor connectivity to the gas sources.

In summary, there are four main controlling factors for high productivity and accumulation in carbonate fracturecavity reservoirs in the Lunnan Low Uplift: (1) the abundance of the source rocks sets the foundation; (2) the development of reservoirs is the precondition for hydrocarbon accumulation; (3) the structural highs and large karst monadnocks are the most favorable for hydrocarbon accumulation; and (4) the existence of good-quality caprocks safeguards the enrichment of oil and gas.

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Chapter | 6

Carbonate Fracture-Cavity Reservoir

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