Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 39, Issue 6, December 2012 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2012, 39(6): 700–711.

RESEARCH PAPER

Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China KUANG Lichun1,*, TANG Yong1, LEI Dewen1, CHANG Qiusheng1, OUYANG Min1, HOU Lianhua2, LIU Deguang1 1. PetroChina Xinjiang Oilfield Company, Karamay 834000, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: The features and exploration potential of the Permian tight oil in the Junggar Basin were analyzed and evaluated using outcrop, core and geochemical data etc. The Junggar Basin in the Early-Mid Permian is a saline lacustrine basin after the residual sea is closed, a set of hybrid sedimentation of deep-lake dark mudstone and dolomitic rock is developed, and the high-quality mudstone source rocks and the dolomite mudstone are alternated. High quality source rocks in mature stage are next to tight dolomitic rock reservoirs closely and provide good conditions for tight oil accumulation of proximal source type. Tight oil reservoirs are mainly distributed in the centre and slope region of the lake basin, and two types sweetspots of “dissolved pore” and “fracture-pore” exist locally. The enrichment of tight oil is controlled by the distribution of effective source rocks and dolomitic rocks, and the tight oil occurs in the entire strata vertically and spreads across large continuous areas horizontally. The Junggar Basin has four Permian tight oil distribution areas, Fencheng Formation in the Mahu sag, Lucaogou Formation in the Jimusaer sag, Pidiquan Formation in the Shazhang-Shishugou sag, and Lucaogou Formation in the Bogeda piedmont. A number of wells obtained oil flow in these areas, suggesting great resource potential and favorable targets for future exploration. Key words: Junggar Basin; Permian; saltwater lake; dolomitic rock; tight oil; resource potential

Introduction Tight oil refers to crude oil accumulating in sandstone or carbonate rock with matrix permeability less than 0.2×103 m2 under overburden formation pressure or with air permeability less than 2×103 m2. There is generally no natural productivity in single wells or natural productivity is less than the lower limit of commercial oil and gas flow; whereas commercial yield may be reached through technologies such as fracturing, horizontal well, multilateral well, etc [1]. Compared with conventional hydrocarbon reservoirs [2], tight oil reservoirs have the following features: their source rocks may also act as reservoir rocks [3], they are distributed continuously, and there is no clear trap boundary, which means tight oil is in an open system [4-7]. Rich in tight oil resources, China has promising prospects for tight oil [8-9]. In recent years much progress has been made in tight oil exploration and development in the Upper Triassic Yanchang Formation in the Ordos Basin, the Permian Fengcheng Formation in the Mahu Sag in the Junggar Basin, the Permian Lucaogou Formation in the Jimusar Sag, the Permian Pingdiquan Formation in the Shazhang-Shishugou Sag, Fuyu

oil layers and Qingshankou Formation in the Songliao Basin, Sha1 Member dolomite and Sha3 Member argillaceous dolomite in the Qikou Sag in the Bohai Bay Basin, Shuixigou Group at the south slope of the Qiudong Sag in the Tuha Basin, etc. By the end of 2011, prospective areas of 15×104 km2 had been preliminarily ascertained with resources of 7.5×108 t in the Ordos, Junggar, Songliao, Sichuan, Bohai Bay and Qaidam Basins. It is anticipated that onshore petroliferous basins in China contain a huge amount of tight oil resources, about 80×108100×108 t [1012]. In December 2011, the project-43 in the national major hydrocarbon projects and PetroChina Changqing Oilfield Company jointly held a “seminar on China tight oil exploration and resources potential”, which reached the following conclusions: (1) tight oil and shale oil are two different crude types and tight oil mainly includes tight sand oil, tight carbonate oil, etc.; (2) tight oil accumulation in China has the following features: depositing in the sedimentary system of continental lake basin and the sedimentary environment of saltwater lake; alternation with good source rocks, high in organic carbon content and medium in maturity; diverse in reservoir rocks,

Received date: 29 Aug. 2012; Revised date: 07 Sep. 2012. * Corresponding author. E-mail: [email protected] Copyright © 2012, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

including dolomite, sandstone, etc., sandstone changes quickly in lateral direction and is mostly composed of thin interbeds; relatively small area and scale; (3) there are 10 basic criteria for tight oil evaluation, i.e. porosity and permeability, matrix pore type (organic/inorganic), fluid quality and flowability, reservoir pressure, fracturing capacity (brittleness, mineral content, natural fracture), source rock TOC and Ro, reservoir thickness, structural complexity, buried depth, ground fracturing conditions (water, factors of environment protection, well site, etc.); (4) China has huge tight oil resources potential and should accelerate the development of tight oil in the Chang6 to Chang7 Members in the Ordos Basin, the Permian in the Junggar Basin, and the Jurassic in the Sichuan Basin. The exploration and research of tight oil in the Permian dolomitic rock in the Junggar Basin is just at its beginning and we still have no idea about its generation, controlling factors and distribution. This paper analyzes the features of Permian tight oil in the Junggar Basin and evaluates its exploration potential carefully in hope of facilitating tight oil research and exploration.

1

Overview of study area

A set of deep lacustrine dark mudstone (as major source rocks) and dolomitic rock (as major tight oil reservoirs) are distributed widely in the Middle and Lower Permian Series in the Junggar Basin, i.e. Pingdiquan Formation (P2p) and Lucaogou Formation (P2l) in the east and Fengcheng Formation (P1f) in the west [13]. Dark mudstone and dolomitic rock laminate alternately and are superimposed laterally by each other [1415]. The dolomitic rock in the Junggar Basin mainly occurs in three major Permian foreland sags, that is Fengcheng Formation mainly in the Mahu Sag at the Hala'alat Mountain-Zaire Mountain front sag, Lucaogou Formation in the Bogda Mountain front sag and Pingdiquan Formation in the Kelameili Mountain front sag. The dolomitic rock developed vertically in different sections, i.e. the Feng3 and Feng1 Members in the Fengcheng Formation, the Middle and Upper Members in the Lucaogou Formation and Ping1 Member in the Pingdiquan Formation. Exploration of the Permian dolomitic mudstone in the Junggar Basin started in Fengcheng area in the northwest and Shazhang area in the east in 1981 [16], targeting conventional structural hydrocarbon reservoirs. The proved reserves of Fengcheng porous-fractured oil reservoirs controlled by anticlinal structure are 895×104 t. Initial daily oil production of individual wells in Feng3 well field ranges from 6.5 to 123.0 t, 17.9 t on average. In 1984 four oil reservoirs in Huoshaoshan Oilfield, i.e. Shadong1, Shadong2, Huo11 and Huonan8, were discovered in succession in porous-fractured reservoir beds of Ping2 Member in Shazhang area with the cumulative proved reserves of 7 660×104 t. Oil flow has been tapped from Ping1 tight dolomitic mudstone in Well Huobei1 and Shadong1 with the Permian dolomitic rock as reservoir rocks in massive bottom-water structural reservoirs [1718]; but limited by the geo-

logic understanding and technologies at the time, tight oil layers had not drawn enough attention. In recent years oil and gas shows were found in thick layers in some exploratory wells aiming at the Wutonggou and Lucaogou Formations in the Jimusar Sag; in 2010 acid fracturing with coiled tubing in Well Ji23 at the interval 2 3092 386 m in the Lucaogou Formation gave a daily oil yield of 1.96 m3, confirming the oiliness of tight Lucaogou reservoir. Based on the understanding of tight oil, Jimusar Sag, north Shazhang fault-fold zone and west Mahu slope were selected in 2011 as a breach in tight oil exploration; Well Ji25, Huobei2 and Fengnan7 were drilled, which yielded commercial oil flow of 18.16 t/d, 14.39 t/d, and 12.3 t/d respectively after moderate stimulation [19] .

2

Features of tight oil in lacustrine dolomitic rock

The Permian dolomitic rock as tight oil reservoirs in the Junggar Basin came into being due to the joint work of a continuously subsided lacustrine environment, wide spread high-quality and mature source rocks, nano-scale pore throats and alternate source-reservoir rocks or close source-reservoir contact. 2.1 Sedimentary environments of wide and continuously subsiding saltwater lake basin The Middle and Lower Permian dolomitic rock in the Junggar Basin formed in a salty lake basin after relict sea close-off [2021]. Under the influence of ancient landform at the depositional stages, there were some large deposition and subsiding centers in the Early Permian Epoch, which include three major sags, i.e. Mahu, Shazhang and Jimusar. The tight dolomitic reservoir is mainly composed of a set of diamictite after dolomitization at the penecontemporaneous stages of salty lakes. Dolomitic rock and clastic rocks act as the complementation of each other in the plane and are distributed generally in lake-basin centers and slopes (Figure 1). 2.1.1

Forming environment of dolomitic rock

Dolomitic rock formed in a salty lake basin with layers rich in salts and alkaline minerals, e.g. Fengcheng Formation in Fengnan5 well field contains the combination of evaporates, such as dissolvable salts and alkaline minerals nahcolite, sodium carbonate, and calcium carbonate, etc. (Figure 2a and 2b), indicating a briny alkaline sedimentary environment at depositional stages. There are also many marks of salty lake in Pingdiquan and Lucaogou Formation (Figure 2c and 2d), e.g. gypsum crystal casts in Lucaogou dolomitic rock in Jimusar, rod-shaped gypsum crystals in Pingdiquan dolomitic reservoir rocks, ancient cod and fish scale fossils in saltwater lakes discovered in Pingdiquan sections in Laoshangou area, etc. In addition high B/Ga values (5.721.9) in Pingdiquan Formation also suggests the deposition of a salty lake basin. 2.1.2

 701 

Genetic types of dolomitic rock

The Middle and Lower Permian dolomitic rock in the

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

Fig. 1

Fig. 2

Middle and Lower Permian lacustrine dolomitic rock area in the Junggar Basin

Typical sedimentary features of the Permian salty lake-basin in the Junggar Basin

Junggar Basin formed in salty lake basin after relict sea close-off and main sedimentary sources around the lake-basin included carboniferous huge intermediate and basic volcanic

rocks and volcanoclastic rocks, which results in high content of intermediate and basic volcanic rock debris and intermediate and basic plagioclase in clastic rocks in lake-basin and

 702 

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

also provides a large quantity of calcium and magnesium ions. Magnesium ions in dark minerals, e.g. pyroxene and amphibole, and vitreous in intermediate and basic volcanic rocks are liable to run off under supergene conditions, which is in favor of increasing the proportion of magnesium and calcium ion deposits in dolomitic rock. In addition, relict seawater and staged transgression were apt to increase lake water salinity and promote the generation of dolomite. Dolomites in the Middle and Lower Permian dolomitic rock in the Junggar Basin are mainly of cotemporaneous and penecontemporaneous origin. There are also burial diagenetic dolomites and hydrothermal epidiagenetic dolomites of the Permian in some regions. Dolomites in the Middle and Lower Permian dolomitic

Fig. 3

rock in the Junggar Basin are mainly micro-laminar carbonate (aragonite and high-magnesian calcite) of contemporaneous and penecontemporaneous origin, which changed into dolomite after reconstruction at diagenetic stages. This genetic type of dolomitic rock has plenty of laminas and is rich in organic matter; dolomite crystals inside are very tiny and mainly micrite dolomites, indicating that they came into being at an early stage. A small amount of dolomites changes into silty dolomite crystals after recrystallization. Laminas can be universally seen; laminas in fine-grained terrigenous debris and those in micrite dolomites interlayer are alternated (Figure 3a and 3b). Terrigenous debris mainly includes mud-level and silt-level minerals formed in low energy environment. Laminas are apt to deform due to differential compaction in the

Micrographs of the Permian dolomitic rock in the Junggar Basin

 703 

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

process of consolidation and diagenesis; fluid in rocks may break through dolomitic laminas through drainage channels to form water escape structures owing to drainage from compaction in some dolomitic rock. A great number of lacustrine fossils can be found in outcrops, cores and thin sections, i.e. bivalve fossils, fossils of ancient cod once living in saltwater lakes, and phosphorus fishbone and fish scale fossils in dolomitic and limy rocks (Figure 3c and 3d). Burial diagenetic dolomites: dolomite dispersively distributed in rocks in a style of automorphic and semi-automorphic silty-fine crystals at the later stage of burial diagenesis, which can be seen in the Fengcheng mudstone in Well Fengnan1 (Figure 3e). Hydrothermal epidiagenetic dolomites (Figure 3f): they are found in the Fengcheng Formation in Fengcheng1 well field and Fengnan area. Mostly in semi-automorphic and allotriomorphic fine-medium crystals, this kind of dolomites usually formed along and around fractures and drainage channels and usually associate with such saline minerals as silica, borium, sodium, albite, barite, analcime, sodium carbonate and calcium carbonate. Hydrothermal activities might happen at the late depositional stage of Fengcheng Formation in Wuxia area[17], which may relate to the generation of basic volcanic rocks discovered in Feng3 Member in Ke80 well field. Hydrothermal fluid might move along Wunan fault zone and affected Fengcheng Formation around faults. High content of sodium, magnesium, calcium and borium ions in hydrothermal fluid at the late depositional stage of Fengcheng could lead to intense dolomitization; lithification of sodium borate, calcium sodium carbonate, analcite and barite; and also obvious rise in salinity of formation water in Fengcheng Formation around faults. Hydrothermal activities along Wunan faults at the late depositional stage of Fengcheng might result in a reducing environment which was favorable for the preservation of organic matter in source rocks. Geotemperature buildup from hydro-

Fig. 4

thermal activities made source rocks arrive at the peak of hydrocarbon generation and expulsion as soon as possible, conducive to hydrocarbon generation. In addition, hydrothermal activities could also improve oil and gas conversion rate in source rocks so as to produce more oil and gas from the Fengcheng Formation. Hydrothermal activities might influence the Wunan fault zone and its surrounding regions and Fengcheng Formation and Xiazijie Formation. 2.1.3

Dolomitic rock distribution

Different from marine carbonate rocks in the Qingshui platform, the Permian dolomitic rock in a salty lake in the Junggar Basin are jointly controlled by sources, mechanical deposition and chemical deposition. Very complicated in lithologic composition, most are transitional rocks, from dolomitic sandstone to dolomite with increased content of dolomitic components and decreased terrigenous debris. Micrite limy mass depositing in contemporaneous periods further crystallized, after dolomitization in penecontemporaneous periods, into such little dolomite crystals as micrite-microlite dolomites concentrating in laminas and thin layers. Dolomitic rock is distributed widely in lake-basin center and along slope zone (Figure 4). Three major Permian sags in the Junggar Basin have large area of thick dolomitic rock: Fengcheng dolomitic rock in Mahu Sag at the west has an average thickness of 1 200 m and an area of 2 770 km2; Lucaogou dolomitic rock in the Jimusar Sag at the east has an average thickness of 300 m and an area of 900 km2; Pingdiquan dolomitic rock in Shazhang area at Kelameili Mountain front has an average thickness of 260 m and an area of 700 km2. 2.2 Nano-scale pore throats in tight lacustrine dolomitic rock Similar to tight oil reservoirs in other basins

Depositional model of the Permian Fengcheng dolomitic rock in the Mahu Sag

 704 

[22]

, the Per-

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

mian dolomitic reservoirs in the Junggar Basin are also very tight [23] with pore throat radius mostly less than 1 m and tight core samples accounting for 85% of the total. 2.2.1

Lithology of dolomitic reservoir rocks

The tight Permian lacustrine dolomitic rock is a set of diamictite jointly influenced by mechanical compaction, chemical deposition and local biogenic effects, volcanic actions and deep hydrothermal fluid activities. In the process of lacustrine dolomitic rock deposition, hydrodynamic force was usually weak, which led to mud-level, silt-level and fine-level clastic deposits in rocks. Affected by dolomitization in penecontemporaneous periods, micrite-microlite dolomites widely spread in rocks which are mostly mixed silt, fine sand and dolomitic diamictite. The growth of dolomitic reservoirs were controlled by sedimentary environments, distance to sedimentary sources, lake water salinity, etc., which may explain why there are so many differences among the Permian dolomitic rocks in the Junggar Basin [24]. At Fengcheng depositional stage in Mahu area, the lake water was high in salinity; dolomitic reservoir rocks formed in coastal areas with sufficient continental fresh water supply and strong hydrodynamic force (Figure 5a); argillaceous rocks, usually containing sodium (calcium) carbonate, concentrated in offshore areas with small impact of fresh water and high water salinity (Figure 5b) and some very soluble alkaline carbonate minerals such as nahcolite might exist in local intervals. Dolomitic diamictite is composed of

Fig. 5

dolomitic sandy mudstone and anisomerous dolomitic sandstone sandwiched with micrite dolomites, medium to coarse sands and gravels. At the Lucaogou depositional stage in the Jimusar Sag, dolomitic rocks, basically including silt-level extremely fine dolomitic sandstone, silty dolomite (Figure 5c) and mixture of micrite-powder crystal dolomites, silt sands, powered debris and extremely fine sands, came into being especially in the middle and upper intervals because the sedimentary environment was suitable for dolomitization in penecontemporaneous periods. At the Pingdiquan depositional stage in Shazhang province, the salinity of lake water dropped down, especially in the Ping2 and Ping3 Members, and dolomites in sandstone and siltstone were not developed; during the deposition of lower part of Ping1 Member, dolomites mixed with muddy composition, and muddy dolomite and dolomitic mudstone developed due to transgression and dolomitization in penecontemporaneous periods (Figure 5d). 2.2.2

Reservoir space types in dolomitic reservoir rocks

Apart from a small amount of micron-scale and millimeter-scale residual intergranular pores and dissolved pores with relatively large throats, the tight Permian dolomitic reservoir rocks contain a large amount of nano-scale pores and microfractures which can be observed with SEM. Nano-scale pores usually have a diameter of 100750 nm and are mainly composed of intercrystalline pores, intercrystalline dissolved pores and microfractures (Figure 6) between such minute minerals as quartz, dolomite, feldspar, illite, etc.

Microscopic lithologic features of the Permian dolomitic reservoir rocks in the Junggar Basin

 705 

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

Fig. 6

Microscopic feature of permian dolomitic reservoir pore in the Junggar Basin

2.2.3 Physical property characteristics of dolomitic reservoir Three types of reservoirs, fracture-pore, intergranular and dissolution pore and intercrystalline pore, are developed in the Permian dolomitic rock of the Junggar Basin. Based on the physical property analysis data of 438 cores (in which the core samples from tight reservoir account for 85%), the porosity is divided into three classes: porosity of class I is more than 8%, class II 5%8%, and class III less than 5%. The physical property of dolomitic tight reservoir is different in different regions. The tight reservoir core samples account for about 86.2% in the cores taken from Lucaogou Formation of Jimsar region, in which the porosity of class I accounts for 35.6%, class II 19.6% and class III 31%. The tight reservoir core samples account for about 77.2% in the cores taken from Pingdiquan Formation of Shazhang region, in which the porosity of class I accounts for 19.3%, class II 19.3% and class III 38.6%. The tight reservoir core samples account for about 89.4% in the cores taken from the Fengcheng Formation of Mahu region, in which the porosity of class I accounts for 2.1%, class II 8.0% and class III 79.3%. 2.2.4 Physical property characteristics of “sweet spot” in the dolomitic tight reservoir “Sweet spots” in the tight oil reservoir are the key point of exploration and development at present[25]. Based on the analysis of data like core, well logging and well test, the Permian dolomitic rock “sweet spots” reservoir in the Junggar

Basin is defined as the tight oil reservoir interval with porosity of more than 6%, the “sweet spots” reservoir developmental area mainly consists of dissolved pore development type tight oil “sweet spots” and fracture-pore type “sweet spots”. The dissolved pore development type tight oil “sweet spots” mainly developed in the upper Lucaogou Formation dolomitic reservoir of the Jimsar Sag. It is indicated by the well tie fine calibration that the dissolved pore development zone is located in the vicinity of top unconformity of Lucaogou Formation, and its distribution is related to the unconformity. A great deal of marks reflecting the weathering, leaching and denudation and the fresh water exposed action (mainly exhibit as hypergenic weathering leaching and dissolution pore, dedolomitization calcite, geopetal structure, etc., Fig. 7) are seen in the core slices of several wells; dedolomitization occurs along the microfracture and periphery in the micritic limy dolomite of well Ji23 (Fig.7a); and dissolved vugs formed and geopetal structure is seen in the silty bearing dolomicrite due to weathering and leaching in well Ji25 (Fig. 7b). The matrix pores are underdeveloped in the fracture-pore type “sweet spots”, which mainly developed in the Pingdiquan Formation Member I dolomitic rock of the Zhangbei fault-fold zone and the Fengcheng Formation Member I of the Bai-Wu faulted zone; reaching some scale, they are mudstone and dolomitic rock tight reservoirs. 2.3 2.3.1

 706 

Widely spread high quality and mature source rocks Source rock characteristics

Widely spread high quality and mature source rock devel-

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

Fig. 7

Permian dissolution type reservoir characteristic indication in Junggar Basin

Table 1

Experimental analysis data of Permian source rock in the Junggar Basin Type of organic Organic carbon Chloroform bitumen “A” (S1+S2)/(mg·g1) matter content/% content/%

Ro/%

Evaluation result

II1

0.661.63

good

0.883.70

II1

0.541.21

good

5.20(71.00)

I-II1

0.851.40

good

Region

Formation

Jimsar Sag

Lucaogou

5.16(31.00)

0.730 0

20.98(30.00)

Kelameili Mountain front Sag

Pingdiquan

3.45(11.94)

0.189 8

Mahu Sag

Fengcheng

1.21(71.00)

0.348 3

Note: Values inside the parentheses are the maximum ones

oped in the Permian of the Junggar Basin. Composed of gray black mudstone and dolomitic mudstone, the source rock is high in the organic matter abundance, mainly types I and II, the major part has reached mature phase (Table 1), and it belongs to a source rock with fairly good hydrocarbon generation conditions. The organic matter abundance of source rock is somewhat different in different regions. For instance, the organic carbon content of the Lucaogou Formation in Jimsar Sag averages 5.16%, the chloroform bitumen “A” content averages 0.730%, and the hydrocarbon generation index (S1+S2) is 20.98 mg/g. The organic content in the Pingdiquan Formation of Kelameili mountain front sag is slightly higher than that in the Fengcheng Formation of Mahu Sag; the organic carbon content of the former is 0.32%11.94%, 3.45% on average, while the average of the latter is 1.21%; however, the soluble hydrocarbon content is apparently on the low side, the chloroform bitumen “A” content of the former averages

0.190%, while the latter averages 0.348%, and this is also shown by the hydrocarbon generation index. It is indicated by the thermal history simulation results that the Lucaogou Formation in Jimsar Sag began to enter the oil generation window at late stage of the Jurassic, and entered oil generation and expulsion peak from the Cretaceous to present; the Pingdiquan Formation in Shazhang fault-fold zone began to enter oil generation window in the Middle to Late Cretaceous, and entered oil generation and expulsion peak from Late Cretaceous to present; the Fengcheng Formation source rock in Mahu Sag began to discharge a great deal of oil in the Middle - Late Permian, reached a relatively high level in the Early and Middle Triassic to the early stage of the Early Jurassic, and then declined rapidly, but reached a new oil expulsion peak in the Early Cretaceous. Simultaneously, the buried depth played an apparent controlling role on the thermal evolution of source rock; the closer to the sag center, the

 707 

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

higher the oil/gas generation amount and charge strength, and the better the oiliness. 2.3.2

Distribution of source rocks

The high quality and mature Permian source rocks are widely distributed in the Junggar Basin. For example, the area of source rocks more than 200 m thick in the Lucaogou Formation amounts to 806 km2 in the Jimsar Sag. The source rock penetrated in well Ji5 is 280 m thick, mainly composed of dark gray and gray black mudstone and dolomitic mudstone. The thickness of source rock in the wells Ji17-Ji5 zone reaches more than 350 m. The source rocks are thicker and have higher organic matter abundance and maturity in the sag centre than in the marginal zone. The lithology of source rock in the Shazhang fault-fold zone and the Pingdiquan Formation in the north of the Shishugou Sag is similar to that of the Lucaogou Formation, and the area of source rocks more than 200 m thick is about 1726 km2. The area of the source rocks more than 100 m thick in the Fengcheng Formation is about 4500 km2 in the Mahu Sag, which are mainly distributed in the midwest of the Mahu Sag and in the vicinity of Wuxia faulted zone. Vertically, the source rock mostly concentrates in Member I of the Fengcheng Formation, with a wide thickness variation, generally from 30 m to 150 m. Except the vicinity of the faulted zone, the source rock maturity in the other regions is apparently higher than that in the Lucaogou Formation and Pingdiquan Formation, and highly mature Fengcheng Formation oil and gas has been discovered in the slope and sag areas. 2.4 Interbedded source rock and reservoir in close contact The Permian Lucaogou Formation, Pingdiquan Formation and Fengcheng Formation source rocks interbed with the fine grain dolomitic reservoirs, characterized by integrative source rock and reservoir, proximal reservoir-forming, and vertically oil-bearing as a whole (Fig. 8). The dolomite, silt and fine contents have a significant impact on the physical property of dolomitic reservoir: the higher the contents of dolomite and silt and fine, the better the physical property of dolomitic reservoir. Drilling revealed obvious oil and gas shows in the siltstone/packsand interval that has a higher dolomitic rock or dolomite contents, crude oil leakage was seen at the time of coring, the oil-bearing level is in apparent positive correlation to the dolomitic rock thickness and the contents of dolomite and silt and fine sand, but the oiliness is poorer in the pure shale interval or the interval with fairly high shale content. The Permian depositional stage in the Junggar Basin was in a large ramp structure setting, the dolomitic rock and source rock stacked in a large area, reservoirs are formed near the source rock, resulting in large area continuous distribution of tight oil. For instance, the Lucaogou Formation in the Jimsar Sag is in a slope structure setting, where both the source rock and the dolomitic tight reservoir are thick, with good lateral

continuity, stable distribution, no apparent trap boundary, source rock and reservoir as a whole, and large coverage area. Also developed in the centre of the lake basin, the tight reservoirs together with the source rocks formed an optimal source-reservoir-caprock combination in either space or time, so, the most enriched zone of tight oil is the sag centre and slope area (Fig. 9). The tight oil reservoir-forming condition is different for different regions. For instance, the structural setting of the Lucaogou Formation is simple in the Jimsar Sag, barely impacted by tectonic movement after the Yanshan orogeny, it has been all the time in a monoclinal structure shape. However, affected by the restricted conduit system, it belongs to an in-situ self-generation and self-storage type tight oil, therefore, the closer the reservoir is to the sag area, the better the oiliness is. Due to the impact of intense Yanshanian movement on the Pingdiquan Formation in the Zhangbei fault-fold zone Shishugou Sag, the present alternating uplift and sag tectonic feature is formed, the reservoirs are somewhat different in tight level at different structural locations, especially, different in fracture development degree, causing the crude oil to flow toward the position where the physical property is good and the fluid potential is low, and thus big difference in oil saturation. Meanwhile, since the reservoir is tight as a whole, there may occur poor oil-water differentiation, coexistence of oil and water, no unified oil/water contact and pressure system. Multiple stages of tectonic movements led to the big changes in the structure of the Fengcheng Formation in the Mahu slope, and in turn the variation in oil reservoir type from the faulted zone to the slope area, that is, the transformation gradually from conventional hydrocarbon reservoirs to tight oil reservoirs, with mostly tight oil in the slope area.

3 Exploration potential of tight oil in permian dolomitic rocks of the Junggar Basin 3.1 Tight oil in Lucaogou Formation dolomitic rocks of Jimsar Sag Situated in a monocline setting, with medium buried depth, the source rock is high in abundance and thick, and as a whole rich in dolomitic rocks with plenty of dissolution pores, forming good source rock and reservoir combination. Based on well-seismic calibration, correlation, tracing and interpretation, class I dolomitic reservoirs cover an area of 460 km2, and class II 594 km2. The intervals with hydrocarbon shows are generally 3 0003 500 m deep, 100200 m thick, and are distributed across the whole sag; and upper and lower “sweet spot” sections have been found. At present, commercial oil flow has been obtained from both the upper and the lower “sweet spot” sections of the Lucaogou Formation, which will be important tight oil exploration zones. The major problem in this region is the relatively heavy oil, which is a big challenge to the productivity of tight oil; however, on the whole, the high carbonate content in tight oil reservoir and high brittleness are conducive to large-scale stimulation, therefore, the exploration potential is large.

 708 

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

Fig. 8 Comprehensive well logging evaluation chart of the Lucaogou Formation reservoir in the Jimsar Sag. dh—hole diameter; SP—spontaneous potential; GR—gamma ray; Ri—resistivity of intrusion zone; Rt—formation resistivity; —density; Ƹt—interval transit time; I N—neutron porosity; AI—acoustic impedance.

3.2 Tight oil in Pingdiquan Formation dolomitic rocks of Zhangbei fault-fold zone - Shishugou Sag Shallow in buried depth, high in source rock abundance, rich in fractures, the dolomitic reservoirs in Member Ping I in this area spread extensively, with good source rock and reservoir configuration. The area of class I dolomitic rocks in Pingdiquan Formation Member I in the Zhangbei fault-fold

zone  Shishugou sag is about 240 km2, the net thickness of dolomitic reservoir being 38 m, the buried depth being generally 2 0003 000 m. It has certain tight oil exploration potential, but also faces the problem of heaviness of oil. 3.3 Tight oil in Fengcheng Formation dolomitic rocks of west Mahu slope

 709 

High in source rock abundance, high in crude oil maturity,

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

Fig. 9

Distribution of favorable Permian Lucaogou Formation tight oil reservoirs in the Jimsar Sag

rich in fractures in the fault-fold zone, and dissolution pores, the Members I and III dolomitic reservoirs are thick and widely spread, with local abnormal pressure, and good source rock and reservoir configuration. Based on preliminary estimation, in Member Feng I at northwestern margin, the favorable dolomitic rocks cover an area of 1268 km2, in which class I dolomitic reservoir is 32 m in effective thickness and class II 65 m. While in Member Feng III at the northwest margin, the dolomitic rock prospective area covers 1 460 km2, where class I dolomitic reservoir is 28 m in effective thickness and class II 60 m, with general buried depth of 4 0005 000 m, showing relatively large tight oil exploration potential. Major problems in the sag include: due to big buried depth and effect of deposition and late diagenetic reformation, the matrix pores and dissolution pores in the tight oil reservoir vary widely laterally, and it is difficult to predict the prospective area. However, light in oil quality, rich in oil source, the area is apt to deliver high yield through large-scale reservoir stimulation. 3.4 Tight oil in Lucaogou Formation dolomitic rock and sandstone, Bogda mountain front The region has high abundance source rock, extensive dolomitic rocks, reservoirs rich in fractures, good source rock and reservoir combination, and shallow burial depth. Interbedded Lucaogou Formation dolomitic rock and siltstone in the Bogda mountain front, generally 600900 m thick, and widely spread, they meet the essential conditions for forming

tight oil accumulation. The area of Permian Lucaogou Formation dolomitic rock prospective area in Bogda mountain front is 482 km2, with a net thickness of 120 m, and a buried depth of 3003 000 m, revealing large exploration potential, meanwhile, it is also a favorable area for oil shale exploration.

4

Conclusions

A large ramp salty lacustine depositional setting occurred in the Early and Middle Permian in the Junggar Basin. High quality source rocks and tight dolomitic rocks are alternated and distributed extensively. Mature and close to reservoirs, the source rocks formed proximal source reservoirs. The tight oil in dolomitic rocks is characterized by continuous distribution in a large area. There exist four prospective areas, namely Lucaogou Formation in Jimsar Sag, Pingdiquan Formation in Zhangbei fault-fold zone  Shishugou sag, Fengcheng Formation in west Mahu slope and Lucaogou Formation in Bogda mountain front, with a great resource potential, which are the important tight oil exploration replacement area in the Junggar Basin. Relatively concentrated in vertical direction, the “sweet spots” of dolomitic reservoir are the major oil and gas pay zones at present.

References

 710 

[1]

Dou Hongen, Ma Shiying. Lessons learned from oil production of tight oil reservoirs in Bakken play. Oil Drilling & Pro-

KUANG Lichun et al. / Petroleum Exploration and Development, 2012, 39(6): 700–711

673–675.

duction Technology, 2012, 42(3): 120–124. [2] [3]

Zhang Houfu, Fang Chaoliang, Gao Xianzhi, et a1. Petroleum geology. Beijing: Petroleum Industry Press, 1999.

in Lucaogou Formation of Middle Permian in Santanghu Ba-

Zou Caineng, Yang Zhi, Tao Shizhen, et al. Nano-hydrocarbon

sin. Northwestern Geology, 2009, 42(2): 95–99.

and the accumulation in coexisting source and reservoir. PeLaw B E, Curtis J B. Introduction to unconventional petroSchmoker J W. Resource-assessment perspectives for uncon-

genesis and hydrocarbon enrichment of the Fengcheng Forma-

ventional gas systems. AAPG Bulletin, 2002, 86(11):

tion in the northwestern margin of Junggar Basin. Petroleum Exploration and Development, 2011, 38(6): 685–692.

1993–1999. [6] [7]

Masters J A. Deep basin gas trap, Western Canada. AAPG

bution of dolomite reservoir of Permian Fengcheng Formation

Levorsen A I. Geology of petroleum. 2nd Edition. San Fran-

in Wuerhe Area, Junggar Basin. Xinjiang Petroleum Geology, 2009, 30(6): 699–701.

Lin Senhu, Zou Caineng, Yuan Xuanjun, et al. Status quo of

[19] Editorial Department of the Xinjiang Petroleum Geology.

tight oil exploitation in the United States and its implication.

There is another discovery in Jimusaer Depression, Xinjiang Oilfield. Xinjiang Petroleum Geology, 2011, 32(6): 629.

Lithologic Reservoirs, 2011, 23(4): 25–30. [9]

[18] Guo Jian’gang, Zhao Xiaoli, Liu Wei, et al. Origin and distri-

Bulletin, 1979, 63(2): 152–181. cisco: W H Free man, 1967. [8]

Junggar Basin. Xinjiang Petroleum Geology, 1985, 6(3): 1–4. [17] Feng Youliang, Zhang Yijie, Wang Ruiju, et al. Dolomites

leum systems. AAPG Bulletin, 2002, 86(11): 1851–1852. [5]

[16] Luo Weigang, Dong Guanghua. Permian petroleum resource evaluation and its exploration prospection of the Wu-Xia Area,

troleum Exploration and Development, 2012, 39(1): 1–13. [4]

[15] Zhu Yushuang, Liu Yiqun, Zhou Dingwu. Origin of dolomite

Zou Caineng, Tao Shizhen, Yuan Xuanjun, et al. The forma-

[20] Hou Lianhua, Zou Caineng, Liu Lei, et al. Geologic essential

tion conditions and distribution characteristics of continuous

elements for hydrocarbon accumulation within Carboniferous

petroleum accumulations. Acta Petrolei Sinica, 2009, 30(3):

volcanic weathered crusts in northern Xinjiang, China. Acta Petrolei Sinica, 2012, 33(4): 533–540.

324–331. [10] Zhao Zhengzhang, Du Jinhu, Zou Caineng, et al. Geological

[21] Zou Caineng, Hou Lianhua, Tao Shizhen, et al. Hydrocarbon

exploration theory for large oil and gas provinces and its sig-

accumulation mechanism and structure of large-scale volcanic

nificance. Petroleum Exploration and Development, 2011,

weathering crust of the Carboniferous in northern Xinjiang,

38(5): 513–522.

China. Science in China: Earth Sciences, 2011, 41(11):

[11] Hou Qijun, Zhao Zhanyin, Huang Zhilong. Accumulation threshold and exploration potential of deep basin oil in the Songliao Basin. Petroleum Exploration and Development,

1602–1612. [22] Nelson P H. Pore-throat sizes in sandstones, tight sandstones, and shales. AAPG Bulletin, 2009, 93(3): 329–340. [23] Zou Caineng, Hou Lianhua, Kuang Lichun, et al. Genetic

2011, 38(5): 523–529. [12] Li Yuxi, Zhang Jinchuan. China’s unconventional oil and gas

mechanism of diagenesis-reservoir facies of the fan-controlled

resources types and potential. International Petroleum Eco-

Permo-Triassic in the western marginal area, Junggar Basin. Chinese Journal of Geology, 2007, 42(3): 587–601.

nomics, 2011(3): 61–67. [13] Liu Wenbin. Study on sedimentary environment of Fengcheng

[24] Wang Zhenya. Permian resource conditions and evaluation of

Formation at northwest margin of Junggar Basin. Acta Sedi-

the Junggar Basin. Xinjiang Petroleum Geology, 1986, 7(2):

mentologica Sinica, 1989, 7(1): 61–70.

61–66.

[14] Zhang Yijie, Qi Xuefeng, Cheng Xiansheng, et al. Approach to

[25] Pang Zhenglian, Zou Caineng, Tao Shizhen, et al. Formation,

sedimentary environment of Late Carboniferous-Permian in

distribution and resource evaluation of tight oil in China. En-

Junggar Basin. Xinjiang Petroleum Geology, 2007, 12(6):

gineering Science, 2012, 14(7): 60–67.

 711