Genesis and evolution of Lower Cambrian Longwangmiao Formation reservoirs, Sichuan Basin, SW China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 2, April 2015 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(2): 175–184.

RESEARCH PAPER

Genesis and evolution of Lower Cambrian Longwangmiao Formation reservoirs, Sichuan Basin, SW China ZHOU Jingao1,2, XU Chunchun3, YAO Genshun1,2, YANG Guang3, ZHANG Jianyong1,2, HAO Yi1,2, WANG Fang1, PAN Liyin1,2, GU Mingfeng1, LI Wenzheng1,* 1. PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China; 2. CNPC Key Laboratory of Carbonate Reservoirs, Hangzhou 310023, China; 3. PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China

Abstract: Based on observation of outcrops, cores and thin sections and analysis of logging data and experiment, the features, main controlling factors, evolution and distribution of the Longwangmiao Formation reservoirs in the Lower Cambrian, Sichuan Basin, are examined carefully and the distribution of favorable reservoirs is predicted. Mostly fracture-pore type, the Longwangmiao Formation reservoirs are dominantly comprised of residual dolarenite, oolitic dolomite and crystal dolomite, with dissolution cavities and dissolution pores as the main storage space, an average porosity of 4.28%, and average reservoir thickness of 36 m. The formation of the reservoirs is controlled by grain shoal facies, parasyngenetic dissolution and parasyngenetic dolomitization. The reservoirs have experienced four evolution stages, the period of pore formation laid the foundation for storage space types and physical conditions of the reservoirs; the reservoir physical properties were improved further in the hypergene karstification period; the porosities were decreased by minerals filling during the hydrothermal period; and the reservoirs became denser in the burial dissolution and asphaltic filling period. Based on the main controlling factors of Longwangmiao reservoirs, the high geomorphology area between Huayingshan fault and Longquanshan fault is predicted as the most favorable reservoir zone. Exploration breakthroughs will possibly be made in Guang’an-Nanchong-Jiange area. Key words: Sichuan Basin; Lower Cambrian Longwangmiao Formation; reservoir type; reservoir controlling factor; reservoir evolution mode; reservoir prediction

Introduction In recent years, the largest integral single gas field of the Lower Cambrian Longwangmiao Formation with proved reserves of 4 403×108 m3 was found in Sichuan Basin, China, and the main production is from Longwangmiao Formation grain dolomite. Based on research of lithofacies paleogeography, observation of typical outcrops, core samples, especially thin slices, and the analysis of logging and experiment data, the types, main controlling factors, evolution stage of the Longwangmiao Formation reservoir were investigated and its favorable distribution scope was predicted in this paper, in the hope of finding the right direction for further exploration.

1.

Background of reservoir development

The Longwangmiao Formation (called Qingxudong Formation in Southeastern Sichuan and Northern Guizhou, and Shilongdong Formation in Northeastern Chongqiang and Western Hunan) is composed of dolomite, limestone and sand-shale,

and gypsum interlayers. Vertically, it is a third-order sequence, in which the transgressive system tract (TST) is thin, made up of argillaceous limestone, argillaceous dolomite and nodular limestone; the highstand systme tract (HST) commonly consists of 3-4 secondary shallowing-upward sequences, and main lithology is crystalline dolomite, grain dolomite and gypsum salt or micrite, grain limestone interbeds; the reservoir occurs in HST and mainly in the upper part of secondary cycle[1]. On the plane, west of Qiyueshan Fault is inner gentle slope facies, the middle gentle slope facies is between Qiyueshan and Dayong Fault, and east of Dayong Fault is the outer slope-basin facies (Fig. 1). The inner gentle slope facies can be divided into three subfacies: (1) mixed tidal flat, in the east of Longquanshan Fault, and mainly made up of mudstone, argillaceous dolomite and siltstone; (2) grain shoal-interbank sea, between Longquanshan Fault and Huayingshan Fault, and mainly made up of grain dolomite, crystal dolomite and dolomicrite; (3) grain shoal-lagoon, from Huayingshan Fault to Qiyueshan Fault, and mainly made up of grain dolomite,

Received date: 29 May 2014; Revised date: 28 Dec. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2011ZX05004-002) and PetroChina Exploration and Production Major Project (2012ZD01-02-03). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

ZHOU Jingao et al. / Petroleum Exploration and Development, 2015, 42(2): 175–184

Fig. 1. Lithofacies paleogeography of Lower Cambrian Longwangmiao Formation in Sichuan Basin. ① Longmenshan Fault; ② Longquanshan Fault; ③ Huangyingshan Fault; ④ Qiyueshan Fault; ⑤ Dayong Fault; ⑥ Wuxi-Tiexi Fault; ⑦ Xuanhan-Wanzhou Fault; ⑧ Leshan-Yibin Fault; ⑨ Anpingdian Fault; ⑩ Moxi-Gaoshiti Fault.

dolomicrite, and gypsum salt rock. Exploration practices and study show that the grain shoal in inner gentle slope is the favorable reservoir facies belt, where the major reservoir type is grain shoal-dolostone fracture–vug type. The main study area in this paper is located in Anyue-Nanchong area (Fig. 1).

2.

Lithological features of the reservoir

The Longwangmiao reservoir mainly consists of dolarenite, oolitic dolomite and crystal dolomite (including porphyritic silt crystalline dolomite), and its main characteristics are shown in Fig. 2. 2.1.

Dolarenite

Occurring in medium-thick beds, dolarenite is mainly made up of arene (over 65% of the total grain) (Fig 2a, 2b). Medium-well sorted, medium rounded, the dolarenite often contains oolite, bioclasts, occasional terrigenous quartz, with 1-2 stages of intergranular dolomite cement, and rich remnant intergranular pores and intergranular dissolution pores. 2.2.

Oolitic dolomite

Oolitic dolomite is more than 60% grains, mainly ooids (60%−80% of the total grains), and some dolarenite and bioclasts. The ooids usually with fine-silt crystal dolomite or quartz as core, are round-semiround, 0.4−1.0 mm in grain size, and there are 1-2 stages of dolomite cement between the ooid grains (Fig. 2c).

2.3.

Crystal dolomite

Crystal dolomite includes and porphyritic silt crystalline dolomite. fine-silt crystalline dolomite is mainly composed of fine crystalline dolomite and silty dolomite, fine-silt crystalline dolomite is damaged in original structure due to domomitization, with some particle phantom remained and most of the granular structure completely disappeared. The dolomite, in hypautomorphic-allotriomorphic shape, is rich in intracrystalline pores (Fig. 2d). Porphyritic silt crystalline dolomite has a porphyritic structure of black and white in disorder distribution on macroscope (Fig. 2e), while on microscope, the areas rich in intergranular pores are filled with asphalt, appearing as dark spots (Fig. 2f), and the areas with underdeveloped pores are little or not filled by asphalt (dolomite mosatic contact), appearing as white patches. The above three kinds of rock constitute the main rock types of Longwangmiao Formation grain shoal and the important carrier of the reservoir. The former two kinds of rock have abundant intergranular pores, dissolved pores and vugs, while crystalline dolostone mainly has intercrystalline pores, and a small amount of dissolved pores and vugs.

3. 3.1.

The reservoir space Types of reservoir space

According to the types of reservoir space and physical properties of Longwangmiao Formation, the characteristics of the reservoir space were examined. The reservoir space of

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Fig. 2. Characteristics of reservoir lithology and reservoir space of Lower Cambrian Longwangmiao Formation grain shoal in Sichuan Basin. (a) Well Moxi13, 4 607.54 m, Longwangmiao Formation, brown gray dolarenite dolomite, dissolved pores and vugs are in irregular circle-ellipse shape, 2-10 mm in diameter, with a small amount of dolomite and asphalt fillings, plane porosity of 14%; (b) Well Moxi13, 4 615.09 m, Longwangmiao Formation, dolarenite, 0.3-3.0 mm in grain size, mainly ellipse in shape, partially dissolved, with thick rim fibrous dolomite cement between grains and residual intergranular pores, casting thin section, single polarization; (c) Well Gaoshi6, 4 546.23 m, oolitic dolomite, ooids wih the 0.8-1.0 mm in diameter, round, layering structure remained in part of the ooids, dolosparite cementation, and rich residual intergranular pores, and a small amount of asphalt filling, blue casting thin section, single polarization; (d) Well Moxi13, 4 613.15 m, Longwangmiao Formation, fine crystalline dolomite, 0.1-0.2 mm in grain size, hypautomorphic or allotriomorphic shape, with rich intergranular pores and dissolution pores, asphalt filling, and plane porosity of 9%, blue cast thin section, single polarization; (e) Well Gaoshi10, 4 632.66 m, Longwangmiao Formation, porphyritic dolomite with irregular grey and dark brown spots, the formation of dark spots may be related to biological disturbance, dissolution pores and vugs are abundant; (f) Well Moxi12, 4 646.50 m, Longwangmiao Formation, porphyritic dolomite, composed of hypautomorphic or allotriomorphic fine dolomite, rich in intracrystalline pores, the pores filled by asphalt are shown as dark spots, blue cast thin section, single polarization; (g) Well Moxi17, 4 625.60 m, Longwangmiao Formation, silt crystalline dolomite, cracks and sutures forming a network; (h) Well Moxi13, 4 578.73 m, Longwangmiao Formation, silt crystalline dolomite, pores are connected by fractures, blue cast thin section, single polarization.

Longwangmiao Formation is divided into four types based on genesis. 3.1.1.

Dissolution pores and vugs

With long axis of 0.2 to12.0 mm, mainly 4-8 mm, dissolution pores and vugs, more abundant in all the three kinds of reservoir rocks of Longwangmiao Formation mentioned above, are the main reservoir space (Fig. 2a, 2d, 2e). Intrusive mercury curve shows ‘low platform’ feature, indicating low mercury injection pressure and large mercury injection volume. Moreover, dissolution pores have obvious responses on imaging logging, shown as black porphyritic spots on orange background. Penecontemporaneous dissolution, supergene karst and burial dissolution can all give rise to dissolution pores, but different geneses would result in different characteristics of pores: (1) Dissolution pores caused by penecontemporaneous dissolution generally occur in grain shoal along bedding, and their size is related to the particle size, that is to say, the pores

in the sandy dolarenite are bigger than those in the silt-fine crystalline dolarenite, showing the characteristics of selective dissolution of fabric (Fig. 3); microscopically, this kind of pores has relatively smooth side wall, hydrothermal filling and asphalt, but no mud or vadose silt inside (Fig. 2b, 2c). (2) Supergene karstification mainly gives birth to fissures and caves, which is distributed in the upper Longwangmiao Formation or along the Caledonian fault zone on macroscope, are usually filled with shale, pyrite or vadose silt, and sometimes associated with karst breccia. (3) Pores created by burial dissolution are mainly distributed along micro-fractures in bead-string shape, and dissolution of early hydrothermal dolomite filling pores can be seen microscopically. It is very difficult to accurately distinguish the genesis of dissolution pores, as the actual pores observed are mostly the comprehensive results of the above 3 kinds of dissolution. Even so, based on observation of a lot of field outcrops, core samples and slices under microscope, the author believes penecontemporaneous dissolution plays a dominant role in

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pore generation, and supergene karstification and burial dissolution mainly reform the reservoir along the early formed pore zone. The dissolution of hydrothermal dolomite filling in pores is weak, and mud and pyrite fillings are not found in most pores, indicating pores formed by superimposed reformation are limited in quantity.

tures related with fault, sutures and reticular fractures related with diagenesis are commonly seen in core samples. High angle fractures on core samples are usually 30−100 cm in extension, with dolomite filling inside (Fig. 2g). Sutures and reticular fractures, small in aperture and some with expansion dissolution, are commonly impregnated by asphalt (Fig. 2h).

3.1.2.

3.2.

Intergranular pores

As secondary storage space, intergranular pores occurring in grain dolomite, were formed in the sedimentary period of grain shoal, in irregular polygon shape, and 0.02−0.08 mm in size. They usually have some fibrous or bladed dolomite rim cements, sometimes, the two stage filling of fibrous and grain dolomite cements. The intergranular pores seen at present are generally residual intergranular pores after cementation (Fig. 2b, 2c). 3.1.3.

Intercrystalline pore

Intracrystalline pores, another kind of secondary storage space, occur in crystal dolomite. They may be formed by transformation of pores earlier created (Intergranular pores and penecontemporaneous dissolution pores) by dolomitization. The pore size, generally 0.003−0.004 mm, is closely related to crystal size, and in triangular or irregular polygon shape, these pores are often partially filled by asphalt. 3.1.4.

Fracture

Although limited in storage capacity, fractures can greatly improve the permeability of the reservoir. High angle frac-

Fig. 3.

Physical properties of the reservoir space

Features of reservoir physical properties were figured out by three ways in this study: analysis of core plugs, full diameter core samples and logging interpretation. The analysis of core plugs indicates that grain shoal reservoirs of Longwangmiao Formation have a porosity of 2.01% to 18.48%, 4.28% on average (Fig. 4a), and a permeability of (0.000 1−248)×10−3 μm2, 0.096×10−3 μm2 on average (Fig. 4b). Samples 2% to 4%, 4%−8% and more than 8% in porosity account for 54.55%, 39.54% and 5.91% of the total samples repectively. The samples with the permeability of less than 0.01×10−3 μm2, (0.01−1.00)×10−3 µm2 and more than 1×10−3 µm2 account for 46.55%, 46.95%, and 6.4% of the total samples respectively. Analysis of full diameter core shows the porosity is 2.02%−10.92%, 4.81% on average (Fig. 4c), and the permeability is (0.01−78.50)×10−3 µm2, 4.75×10−3 µm2 on average (Fig. 4d). Samples with the porosity of 2%−4%, 4%−8% and more than 8% account for 37.8%, 53.54%, and 8.66% of the total samples respectively. While samples with the permeability of (0.01−1.00)×10−3 µm2 and more than 1.0×10−3 µm2 account for 58.33% and 41.67% of the total samples respectively.

Relationship between dissolution pores and particle size of grain dolomite.

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Fig. 4.

Physical property features of grain shoal reservoirs of Lower Cambrian Longwangmiao Formation, Sichuan Basin.

There is a good correlation between measured porosity and logging porosity, indicating it is feasible to use logging porosity to characterize reservoir physical properties (Fig. 4e), thus the problem of evaluation towards non-cored section reservoir can be solved. Statistics of logging physical property of 18 wells show the frequency of Longwangmiao Formation reservoirs 4%−8% in porosity is 60%, indicating relatively good physical properties on the whole (Fig. 4f).

4.

Reservoir type

Through comprehensive analysis of lithology and physical property, according to three main factors: microfacies, lithology and pore type, Longwangmiao Formation reservoirs can be divided into two types, grain shoal dolostone fracture-pore reservoir and grain shoal dolostone fracture-vug reservoir, and the latter is the main type. The porosity of fracture-vug reservoirs is generally greater than 4%, dissolution pores and vugs (70%−80% of total pore volume) as main storage space, and intergranular pores, intracrystalline pores and fractures account for only about 20%−30%. Their intrusive mercury curves have a “dual platform” feature (Fig. 5), with most of the initial mercury injection pressure of less than 0.3Mpa. The mercury saturation was 70%−80% in the low platform of less than 5 MPa injection pressure, while pores in the high platform account for 10%−20%, indicating dual porosity medium, and absolute dominance of large pores and large throats. Mostly 2%−4% in porosity, main reservoir space of fracture-pore reservoirs are intergranular pores and intercrystalline pores (about 60% of total pore volume), and dissolution pores, vugs and fractures (about 40%) as secondary reservoir space. Their intrusive mercury curves show monoclinic characteristic of low slope, with the initial mercury injection pressure of 3MPa on average; moreover, as the pressure increases, the amount of mercury injection rises fast, mercury injection saturation reaches about 60%, when the pressure is 30 MPa, indicating that the small pores and small throats

Fig. 5. Intrusive mercury curve characteristics of grain shoal reservoirs of Lower Cambrian Longwangmiao Formation in Sichuan Basin.

dominate reservoir space; when the pressure is 100 MPa, the final mercury injection saturation reaches 80%−90%, indicating there is a certain amount of micropores. The study shows Longwangmiao Formation reservoirs in Moxi-Gaoshiti area are mainly with fracture-vug type, and reservoir type, closely related to single well production. Fracture-vug reservoirs usually can produce tens to one million cubic meters of gas one day, while fracture-pore reservoirs can produce only thousands to tens of thousands of cubic meters of gas one day.

5.

Reservoir formation and evolution

Taking the major gas reservoirs, fracture-vug type reservoirs as study object, the formation and evolution of Longwangmiao reservoirs are discussed. Sedimentary facies and diagenetic sequence show that the formation of this kind of reservoir is affected by the grain shoals, penecontemporaneous dissolution and penecontemporaneous dolomitization, and the later karstification and burial dissolution improve the reservoir quality to some extent. The reservoirs have experienced

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four stages of evolution: pore formation, supergene karstification, hydrothermal mineral filling, and burial dissolution and asphalt filling. 5.1.

Factors controlling the reservoir formation

The reservoir formation is controlled by sedimentary facies, penecontemporaneous dissolution, penecontemporaneous dolomitization, supergene karstification and burial dissolution. 5.1.1.

Grain shoal

The reservoirs only occur in the grain shoal subfacies. Field outcrops and drilling data in the Weiyuan-Gaoshiti-Moxi-Nanchong area have proved that the Longwangmiao Formation reservoirs develop only in the grain shoal subfacies of the inner slope, and other subfacies, such as mixed tidal flat, interbank sea and lagoon, dense in lithology, and unable to form reservoir (Fig. 6). The grain shoal can be further divided into three microfacies: shoal body, shoal edge and shoal flat[2], and the reservoirs mainly develop in the shoal body microfacies. Examining the relationship between the core microfacies and the reservoir in Moxi-Gaoshiti region shows that shoal body microfacies are most developed in dissolved pores and vugs, and the pore or vug size is positively correlated with the grain size; the shoal edge microfacies mainly has residual intergranular pores and a small amount of dissolved pores and vugs; while the shoal flat microfacies has no pore and vug, but some reticular fractures resulted from dissolution (Fig. 3). 5.1.2.

Penecontemporaneous dissolution

Aragonite provides favorable condition for penecontemporaneous dissolution. This area was in aragonite sea stage in the early Cambrian[3], so part of arene and bioclasts constituting the shoal body are aragonite, and the early fiber or blade shape seawater cement is mainly composed of aragonite. Aragonite is easily soluble in fresh water and even sea water, which lays the foundation for penecontemporaneous dissolution. Fig. 2b shows the dissolution of fiber-shape cement in the intergranular pores is associated with penecontemporaneous dissolution.

Fig. 6. Average porosity histogram of sedimentary subfacies of Longwangmiao Formation.

The penecontemporaneous meteoric fresh-water leaching and dissolution due to sea-level fall is the key factor causing large amounts of dissolved pores and vugs in the Longwangmiao Formation. The outcrops and drilling data in the Gaoshiti-Moxi region show there were 3-4 times of sea level fall during the deposition of the Longwangmiao Formation, correspondingly, there are 3-4 sets of pore-vug-type reservoirs on the vertical section[2]. The drop of the sea-level caused the exposure of shoal body. Thanks to large area and long duration of exposure, the shoal body located at the ancient high topographic area is abundant in dissolved pores and vugs and good in physical properties; while the shoal body and edge located in the ancient low topographic are are not rich in dissolution pores because of lack in or no exposure, together with strong cementation results in poor porosity and permeability. According to the data from the core samples and thin sections, penecontemporaneous dissolution can increase the porosity by 5%−15%. 5.1.3.

Penecontemporaneous dolomitization

There are two main modes of penecontemporaneous dolomitization associated with evaporite rocks: evaporative pumping dolomitization[4] and reflux dolomitization[5]. Surface seawater or pore water constantly concentrates and becomes salty constantly due to strong evaporation in dry and hot climate, to form brine with high Mg2+/Ca2+ ratio, causing dolomitization of the surrounding sediments (i.e. evaporative pumping dolomitization). When penetrating down and flowing toward sea, the excess brine could give rise to dolomitization of sediments along high porosity and permeability channels it passes (i.e. reflux dolomitization). The depositional stage of Longwangmiao Formation had the geological environment for penecontemporaneous dolomitization: dry and hot climate or intense evaporation and adjacent to the gypsum-salt lake (Fig. 1). In addition, the dolomite of the Longwangmiao Formation has a low order degree, and similar characteristics of the C, O, and Sr isotope with sea water in the same period, also indicating that the dolomite was formed in the penecontemporaneous period. Penecontemporaneous dolomitization benefited the preservation of the early pores. Previous researchers mostly focused on the porosity-increasing effect[6] or porosity-decreasing effect[7−8] due to the dolomitization, but for thick widespread dolomite retaining the original fabric, there aren not enough evidences to prove either the porosity-increasing or porosity-decreasing effect of dolomitization. Dolomitization may only replaces limestone into dolomite, for example, after penecontemporaneous dolomitization, the grainstone with penecontemporaneous dissolved pores in the Longwangmiao Formation transformed into grain dolostone with dissolved pores, and the tight micrite after transforming into dolomicrite remains tight. However, pore-preservation effect of dolomitization cannot be ignored, because the dolomitization increases the rock intension and ability to resist pressure and dissolution,

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and effectively restrain the precipitation of late cement, the pores can be preserved for long time. In the Tieshanpo-Luojiazhai region of the Sichuan Basin, the preservation of intergranular and intragranular dissolved pores in the oolitic dolomite of the Feixianguan Formation is closely related to the penecontemporaneous dolomitization[9]. 5.1.4.

Supergene karstification and burial dissolution

The intergranular pores formed during sedimentary period and dissolved pores and vugs formed in penecontemporaneous period are the paramount reservoir space of Longwangmiao reservoirs, which after multi-stage cementation and filling, have decreased to certain extent, meanwhile, multi-stage dissolution (supergene karstification in later Longwangmiao sedimentary period and late Caledonian, burial dissolution in Indosinian and Yanshan period) have worked to improve the reservoir quality. The karstification in the late Longwangmiao sedimentary period mainly took place at the ancient topographic highs, such as Weiyuan-Moxi-Nanchong area in Sichuan and Shizhu, Pengshui, Nanchuan in eastern Chongqing and Xishui area in Guizhou, and the author has found the Longwangmiao Formation in Yudong and Nanjiang of north Sichuan unconformably contacts with the overlying Gaotai Formation. The karstification is weaker in the middle Sichuan basin, in penecontemporaneous period, dissolved pores were transformed slightly, meanwhile small solution channels and fractures were produced, for example, solution channels filled with seeping dolomitic silt can be seen in 3 688 m in Well Moxi32 and 4 620 m in Well Moxi13; while gypsum breccia formed by evaporite dissolution occurs in eastern Chongqing[9]. Caledonian karstification is related to the uplifting and exposure of Leshan – Longnüsi paleo-uplift[10−12], during which the strata above the Sinian at the core of paleo-uplift was eroded away, and those in Gaoshiti-Moxi area were locally eroded and mostly preserved. Drilling data suggests that the karstification in this period has little reformation to pores created before, but it could give rise to large fracture caves along the fracture, for example a cave system about 6 m high was found in Well Gaoshi 17, which are filled by Lower Permian carbonaceous mudstone and pyrite. Results of log interpretation show that the pyrite filling significantly reduces from the denuded zone to the unexposed area, suggesting karstification weakened rapidly. There may be two stages of burial dissolution: organic dissolution related to oil charge[13] and the TSR (thermochemical sulfate reduction) related dissolution[14]. The primary evidence is the dissolution of hydrothermal dolomite filling the wall of early dissolved pores and caves, and a small amount of bead-string dissolution pores along microfractures. Above dissolution occurs mainly along pore zones formed earlier, superimposing on the early pores and caves, and increasing porosity to some extent. Grain shoal and penecontemporaneous dissolution are the

main factors controlling the reservoir formation, and penecontemporaneous dolomitization, supergene karstification and burial dissolution have played constructive roles in the reservoir formation. 5.2.

Reservoir evolution model

The research on diagenesis shows that the Longwangmiao Formation experienced the following diagenetic sequence (Fig 7): grain shoal deposition, submarine cementation, penecontemporaneous dissolution, penecontemporaneous dolomitization, supergene karstification, burial filling, supergene karstification, hydrothermal mineral filling, burial organic dissolution, hydrocarbon charging, hydrocarbon cracking and asphalt filling, natural gas charging, structural fractures, etc, which can be summarized into four stages: pore formation, supergene dissolution, burial hydrothermal filling, and burial dissolution and asphalt filling. Pore-forming mainly occurred in the sedimentary-penecontemporaneous period, laying the foundation for pore type and physical properties. This period mainly involved grain shoal deposition, submarine cementation, penecontemporaneous fresh water dissolution and penecontemporaneous dolomitization. The initial porosity of grain shoal deposits could be as high as 40%−50%[15−16], which reduced to about 30% after preliminary compaction. Fibrous cement of seawater in vadose zone and blade cement in phreatic zone generated in the subsequent submarine cementation made porosity decrease sharply to 5%−10%. During the penecontemporaneous period, large amounts of dissolved pores and dissolved caves caused by meteoric fresh water dissolution to submarine cement and some aragonite particles increased the porosity to 10%−25%. Associated and subsequent penecontemporaneous dolomitization could transform sediments or rocks into dolomite, and keep a large amount of original rock fabric, including intergranular pores, dissolved pores and dissolved caves formed in early stage. Penecontemporaneous dissolution and penecontemporaneous dolomitization are interconnected. In an overall drought climate, when meteoric water is supplied, aragonite particles or cement would dissolute and produce some vugs,,release a large amount of Mg2+, which will meet the need of Mg2+ in the process of penecontemporaneous dolomitization just right. At the same time, the evaporation and concentration of seawater led to the increase of Mg2+/Ca2+, dolomitization of sediments, and preservation of pores and vugs formed in early stage. During the long geological evolution, due to the periodical change of climate and sea-level, dissolution and dolomitization happen alternately, some periods dissolution in dominance, other periods dolomitization in dominance, in the end, a large number of pores are produced, and sediments are transformed into dolomite. After this stage of diagenetic evolution, the main pore types changed from the original intergranular pores to dissolved pores and caves, with intergranular pores dropping to a secondary position.

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Fig. 7.

Evolution model of grain shoal dolomite pore-vug reservoirs of Lower Cambrian Longwangmiao Formation in Sichuan Basin.

Supergene karstification stage happened in the Caledonian period has two phases of supergene karstification. The first phase taking place at the end of Longwangmiao Formation deposition, is related to the fall of regional sea level. This phase of karstification although wide in the range, is short in duration, so it was weak, and only enlarged early pores, and produced a small amount of dissolved fractures. The second phase supergene karstification taking place in the late Caledonian, is related to tectonic uplifting and formation of Leshan-Longnüsi paleo-uplift. Compared with the first phase supergene karstification, this stage may last to the early Permian, with longer duration. The karstification is stronger around the denudation zone of paleo-uplift and in the areas with developed fracture cutting to the surface, where large-scale fracture-cave systems can be formed, but the reservoir properties are not good due to serious mud filling in later stages. Observation of a large number of core samples and statistics on thin slice show that the newly-formed porosity is less than 5% during this stage, and the reservoir porosity could be increased to 12%−28%. Burial hydrothermal filling stage, roughly from the Middle to Late Permian, is closely related to the Emeishan volcanic action[17]. This period filling is shown as bright coarse dolo-

mite and euhedral quartz mineral filling in cracks and pores formed earlier, homogenization temperature of fluid inclusions in the fillings is 180−220 °C[18]. The filling is uneven: in some areas, filling is stronger, and most of the caves are partially or fully filled, resulting in significant reduction of porosity; in other areas, the filling is weaker, only a small amount of euhedral dolomite precipitated along the cave wall. Core samples from 12 wells show the filling effect reduced the porosity by 10% to 3%−18%. Burial dissolution and asphalt filling stage since Late Permian include two phases of dissolution. The first phase is associated with hydrocarbon charging, the second phase is related to TSR effect, the two are both weak, resulting in the porosity increase of less than 2%; while at the same time, with the cracking of hydrocarbons, a large amount of asphalt was generated, filling pores and throats, resulting in porosity reduction of 2%−5%. Therefore, the reservoir porosity tended to be decreased on the whole to 2%−16%. Because of the regional tectonic uplift at Himalayan period[19], micro-fractures were massively developed, which although has little effect on the reservoir porosity, greatly improves the reservoir permeability. The common high production of Longwangmiao Formation is closely related to the dissolved pores and dissolved

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caves connected by the structural fractures. 5.3.

Favorable reservoir prediction

According to the analysis of main factors controlling reservoir development, areas where grain shoals subfacies has experienced penecontemporaneous karstification and dolomitization, ie. the grain shoals experiencing penecontemporaneous exposure dissolution and dolomitization are most favorable for reservoir development. Therefore, favorable reservoir prediction can be converted to the forecast of ancient topographic highs: (1) Ancient topographic highs with shallow water and strong water power were conducive to the development of grain shoal; (2) When sea level fell, they were more likely to be exposed and subject to dissolution; (3) At the ancient topographic high, the water body was likely to concentrate and become salty under strong evaporation, laying the foundation for penecontemporaneous dolomitization. In a word, the grain shoals in ancient topographic highs meet the three main conditions for reservoir development. Based on the above analysis, the periphery region topographically high in ancient time are all favorable for reservoir development except the relatively low-lying area between Huayinshan Fault and Qiyueshan Fault (Fig. 1). Drilling data in Moxi-Gaoshiti area have revealed grain shoal is widely developed in the ancient topographic high area between Huayinshan Fault and Longquanshan Fault, with a thickness of 20−60 m, on average about 36 m. A well drilled in Nanchong encountered a 24 m thick reservoir recently, showing that the favorable reservoir may extend northward to Guang’an-Nanchong-Jiange area. In addition, Well Li1 and Dingshan1 in the east of Qiyueshan fault also revealed good reservoirs, suggesting that there are favorable reservoirs in this area too[20]. New breakthroughs are expected in these areas with the deepening of exploration.

6.

forming period laid the material basis for reservoir space types and physical property conditions. Supergene karstification and burial dissolution made some contributions to the improvement of reservoir physical properties. Hydrothermal mineral filling and asphalt filling are the main factors making reservoir quality worse. Based on the main controlling factors of the Longwangmiao reservoir, the favorable reservoir zones are ancient high topography areas between Huayingshan Fault and Longquanshan Fault, and breakthroughs are expected to make in the Guangan-Nanchong-Jiange area.

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Conclusions

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The Longwangmiao Formation reservoirs are grain shoal-dolostone fracture–vug type, made up of residual dolarenite, oolitic dolomite, and crystal dolomite; with vugs and dissolution pores as the main storage spce, residual intergranular pores, intercrystalline pores and fractures as the secondary storage space, these reservoirs have a porosity of 2%−8%, 4.28% on average, and a thickness of 20 m−60 m, 36 m on average. Shoal facies and penecontemporaneous dissolution are the main factors controlling the reservoir occurrence. Grain shoal, the basis of reservoir development, controls the phases and distribution of reservoir. Penecontemporaneous dissolution is the key factor affecting the formation of the main reservoir space. In addition, penecontemporaneous dolomitization plays a constructive role in the preservation of the pores formed earlier and generation of micro-fractures in late stage. The reservoirs experienced four evolution stages. The sedimentation and penecontemporaneous dissolution in pore− 183 −

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