Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China

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1 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number pp Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Tieya Jing 1, Jinchuan Zhang 1*, Junli Mao 2, Wanjun Li 1 and Shengling Jiang 1 1 School of Energy Resources, China University of Geosciences (Beijing), Beijing , China 2 Research Institutes of Exploration and Development of Liaohe Oil Company, Panjin , China *Corresponding author. zhangjc@cugb.edu.cn (Received 11 August 2013; accepted 19 January 2014) Abstract Recently, the accumulation of shale gas and shale oil in the rifted lacustrine basin of China has garnered increasing interest. In this paper, the shale located in the Shahejie Formation of the Liaohe western depression of the Bohai Bay Basin was selected as the focus of a comprehensive evaluation of the geological controls of shale gas and shale oil accumulation. (1) In a rifted lacustrine basin, the subsidence rate of the stratigraphy is rapid, which results in a massive sedimentation of organic-rich shale. The shale that developed in the deep and semi-deep lacustrine facies is characterized by a high concentration of organic matter, with an average total organic carbon (TOC) content over 2.0% being measured. The TOC consists primarily of type I-II 1 kerogen, which contains abundant sapropelic materials and has relatively low thermal maturity, ranging from 0.4 to 0.9%R o, due to the shallow burial depth and the young deposition epoch. (2) Various pore-fractures in the core samples and erosion occurring in the calcareous and dolomitic shales were observed, which provide storage space for oil-gas accumulations. Furthermore, the mechanisms of the accumulation of shale gas and shale oil in the study area were established according to analyses of the geochemistry characteristics and the sedimentary environment, as well as a thermal pressure simulation experiment. Overall, the accumulation model of shale oil-gas is characterized by upper oil and lower gas. In addition, oil-gas resources in siltstone, dolomite and limestone, which are adjacent to or interbedded with organic-rich shale, are also important targets in shale reservoirs. Keywords: Rifted lacustrine basin, Liaohe western depression, Shale gas, Shale oil, Geological controls, Mechanism

2 504 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China 1. INTRODUCTION Marine shale strata in North America, such as the Mississippian Barnett shale in the Fort Worth Basin, the Devonian Marcellus shale in the Appalachian Basin, the Upper Jurassic Haynesville shale in eastern Texas and the Devonian Mississippian Bakken shale in the Williston Basin, have produced great amounts of shale gas and shale oil (Curtis, 2002; Martini, et al., 2003; Montgomery et al., 2005; Jarvie et al., 2007; Martineau, 2007; Hammes et al., 2011; Sonnenberg et al., 2009; Nie et al., 2009b). The successful development of oil-gas resources in the North American shales is drawing considerable attention from petroleum geologists and officials in China who want to research and develop shale gas and shale oil. According to data from the EIA (2011) and the Ministry of Land and Resources, P.R.C. (2012), the shale gas resources potential in mainland China, except the Qinghai-Tibet area, is (23-25) m 3. A number of wells targeting marine shale were drilled in the Sichuan Basin and the residual basins in the Upper Yangtze area and revealed favorable geological conditions for shale gas accumulation. Numerous studies have been performed, including tectonic evolution, sedimentary environment, reservoir characteristics and gas content, to obtain the accumulation geological conditions and resource potential of the shale gas in the Longmaxi and Niutitang shales in these areas (Zhou et al., 2010, 2011; Zhang et al., 2008a, 2008b, 2009, 2012a; Li et al., 2009b, 2011; Liu et al., 2013; Ding et al., 2012). However, the commercial production of shale gas and shale oil in China has not been achieved to date. China was the cradle of the terrestrial hydrocarbon generation theory, which made a considerable contribution to the world petroleum industry and theory. Compared with the marine shale formations, studies on shale gas and shale oil in continental facies are relatively new. In China, rifted lacustrine basins, such as the Songliao Basin, the Bohai Bay Basin and the Ordos Basin, are rich in petroleum resources and have already been comprehensively explored. Previously, the shales in these basins were usually considered to be the source rocks of conventional oil and gas. Recently, especially in the last three years, shale gas and shale oil have attracted increasing attention, and the shale formation has come to be regarded as a reservoir. Xu et al. (2011) and Wang et al. (2010) analyzed the geological conditions of the shale gas in the Liaohe eastern depression and considered the Shahejie Formation to be favorable for shale gas accumulation because of the high organic abundance, the moderate thermal maturity and the humus kerogen. Zhang et al. (2012c) demonstrated that organic matter abundance was critical for shale gas and shale oil accumulations in the Bonan sub-sag of the Jiyang depression in the Bohai Bay Basin. Zhang et al. (2012b) studied the possibility of shale gas accumulation in the Jiyang depression and concluded that the shale in the third member of the Shahejie Formation (S 3 ) and the marine shale in America are similar. Chen et al. (2011) determined that shale oil had the characteristics of high Gammy, high neutron, high acoustic, low density and low resistivity in logging responses and that brittle minerals, such as quartz and feldspar, would be beneficial to fracture stimulation. The Liaohe western depression in the Bohai Bay Basin is one of the most important regions for oil production in the Liaohe Oilfield. While there has been vast exploration of conventional hydrocarbon, the exploration for unconventional shale gas and shale

3 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number oil is only beginning. To date, the geological conditions and accumulation mechanisms for shale gas and shale oil were not clear. Thus, in this paper, the shale gas and shale oil found in the Shahejie Formation in the Liaohe western depression was investigated. The geological conditions for the accumulation of shale gas and shale oil in the depression were investigated through core observation, experimental analyses and theory research combined with production data. Moreover, the accumulation mechanisms of shale gas and shale oil were summarized for a rifted lacustrine basin. This study contributes to the theory regarding unconventional shale gas and shale oil and is useful as a reference for other regions with similar geological conditions. 2. GEOLOGICAL SETTINGS 2.1. Tectonic features The Liaohe subbasin consists of three uplifts and three depressions and is located in the northeastern part of the Baohai Bay Basin. Geotectonically, this subbasin is located in the northeastern part of the North China plate and belongs to a continental rift valley subbasin that is developing cenozoic formations with a pre-mesozoic complex basement. The subbasin underwent magma upwarping, faulting and depressing in sequence. The study area in this paper (i.e., the Liaohe western depression) is the largest depression in the Liaohe subbasin with an area of 2560 km 2 and is bounded by the western uplift in the west and the central uplift in the east. In the western depression, six small-scale sags (Niuxintuo, Taian, Chenjia, Panshan, Qingshui and Yuanyanggou) were developed from the north to the south (Xu et al., 1993; Liao et al., 1996; Chen et al., 1997; Zhang et al., 2002) (Fig. 1a). By geological structural analysis, the depression consists of a gentle slope belt, a sag area and a steep slope belt from northwest to southeast (Fig. 1b). Variation in the Eogene formation exists between north and south, and the differences are observed in the aspects of the thickness of the formation and the property of the main faults, especially the Taian- Dawa fault. Faults found in the northern areas are reverse faults, while those found in the south are normal faults. The formation is much thicker in the southern region than in the northern region. In the process of tectonic evolution, the movement magnitude of the Taian-Dawa fault is stronger southward as the strata becomes younger, resulting in the southeastward transfer of the subsidence and sedimentation center (Leng et al., 2008; Li et al., 2007). The transfer of the sedimentation center primarily affects the thickness and distribution of the Shahejie Formation. The sedimentary environment during different periods and the geochemistry of the Shahejie shale were also controlled by the evolution.

4 506 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Figure 1a. Structural location of the Liaohe western depression. The blue line in the top left corner square is the range of the Bohai Bay Basin, and the black horizontal lines in the bottom-left represent sea area. Figure 1b. The stratigraphic cross-section. The depression is divided into gentle slope belt, sag region and steep slope belts. Each geological unit had different sedimentary patterns and was controlled by accommodations and materials source Stratigraphy and sedimentary Formations developed unevenly in the study area during the Palaeozoic, Mesozoic and Cenozoic. The Eogene was the predominant sedimentation period, in which the Fangshenpao, Shahejie and Dongying Formations were formed. Shales in the Shahejie Formation are widely developed, and the overall thickness is greater than 1000 m, which was previously considered to be the main source rock for conventional

5 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 2. Stratigraphic column and tectonic movement of the Shahejie Formation in the study area. hydrocarbon (Wu, 1993; Chen et al., 1997; Wang, et al., 2002; Li et al., 2007; Sun et al., 2008) (Fig. 2). At the end of the Mesozoic, magma intrusion resulted in the thinning of the crust and created huge tensile stress, which developed the main faults that control the evolution of the depression. In the Fangshenpao period, the basalt formation was formed by volcanic eruptions that developed along large faults (Liao et al., 1996; Shan et al., 2005). When the magma eruption slowed, the crust subsided rapidly with the increasing intensity of faulting. Half-grabens that trend southeast and overlap northwest under normal faulting conditions were developed in the study area, and many rifted sags throughout the study area provided excellent conditions for organicrich shale in the 4 th member of the Shahejie (S 4 ) period. In the northern region, coastal and shallow lacustrine facies were developed, while mud platform and lime platform facies were formed in the middle areas. At the end of

6 508 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China the S 4 period, the Chenjia sag was the center of subsidence and sedimentation because of structural movement, which increased the thickness of the strata and enlarged the areas of the paleolake, resulting in the deep lacustrine environment that was beneficial for dark shale deposition. The S 4 strata underlain by the Fangshenpao Formation in the angular unconformity were primary products when the depression began to form, which was distributed unevenly throughout the entire area. Sandstone and dolomitic limestone interbedded with shales were developed in the Niuxintuo area and were overlain upward by the shale formation. In the Gaosheng region, thin oil shale, dolomite, lime shale and dolomitic shale were widely developed because of the influence of the adjacent southern paleohigh that caused still water environments. In the southern region, conglomerate, sandstone and shale were alternately developed due to the input of abundant material sources. After the regression of the S 4 period, another subtle transgression was begun in the S 3 period. Subsidence continued and enlarged the sedimentation extent. The sedimentation of the S 3 formation began in the process of sag deepening and lasted for more than 2 Ma yr, developing the thickest strata in the Shahejie Formation. At the beginning of the S 3 period (S 33 ), the areas of deep or semi-deep lacustrine facies enlarged gradually, and mounts of dark shale were accumulated. Later, the northern part of the depression uplifted, which made the water become shallow, and the sedimentation center eventually moved to the southeast. At the middle of the S 3 period (S 32 ), the depression had the deepest water, and the sedimentation center had already moved to the Qingshui sag. In the end of the S 3 period (S 31 ), regression occurred in the study area, which made the water shallow, and the strata were eroded in the northern and western parts of the depression. In this period, semi-deep or deep lacustrine facies existed only in the southeastern depressions, such as the Qingshui sag. Another tectonic cycle began in the second member of the Shahejie (S 2 ) period. Flluvial, fan delta and lacustrine facies were developed during this period, and the area of the paleolake shrank. The northern part became a continental environment, and deposition happened only in the southeast regions, such as the Qingshui sag. The areas of semi-deep or deep lacustrine facies in the first member of the Shahejie (S 1 ) period were larger than those in the S 2 period and mainly located in the Panshan, Qingshui and Yangyuangou sags, developing a large thickness of shale strata in the depression. The overlain Dongying formation consists mainly of shale, minor sandstone and conglomerate. Generally, the relationships between the S 3 and S 4, the S 3 and S 2, and the S 1 and Dongying Formation consist of false conformity in the sag areas and angular conformity in the slope belts, where onlap phenomenon is common. This phenomenon can be recognized easily on the seismic reflection profile. 3. ANALYTIC METHODS To analyze the types of kerogen in the Shahejie shales, experimental analyses of organic elements (carbon, oxygen and hydrogen) were conducted on 149 cuttings following the GB/T standard. Experiments using Rock-Eval were also performed on 43 shale samples to analyze their kerogen type and thermal maturity. Indicators characterizing organic matter abundance mainly include total organic

7 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number content (TOC), chloroform bitumen A ( A ) and total hydrocarbon content (HC). In this study, more than 1000 sets of data on TOC and A were mostly provided by the Research Institute of Exploration and Development of Liaohe Oil Company. The vitrinite reflectance (R o ) and pyrolysis peak temperature (T max ) that can reflect the thermal maturity of shales were also from the Liaohe Oilfield. Mineral compositions and clay components analyses were performed for 40 shale cores with the aid of an X-ray diffraction spectrum. Macroscopically, shale core samples from wells were observed to identify fractures and karst caves. Microscopically, pores of nanometer scale were studied under a scanning electron microscope (SEM). The porosity and permeability of the Shahejie shales were measured using the methods of regular porosimetry and pulsed permeability measurement by the SY/T standard. One shale sample from well SG 165 was used for the thermal pressure simulation experiment at the Experimental Research Center of Wuxi Research Institute of Petroleum Geology of SINOPEC to simulate the processes of generation and expulsion of hydrocarbon in the Shahejie shale. The simulation products at seven temperatures (250 C, 300 C, 320 C, 330 C, 350 C, 370 C and 400 C) were collected and analyzed. In addition, 20 shale samples containing oil content were measured based on the Chinese Oil and Gas Industry Standard SH/T (1992). 4. RESULTS AND DISCUSSION 4.1. Geochemistry characteristics It is important to analyze geochemistry parameters, such as the type and abundance of organic matter and thermal maturation, when examining shale gas and shale oil accumulations (Zhang et al, 2004; 2008b; Hill, 2007). Shale gas and shale oil can exist respectively or symbiotically, and the existence model is controlled by such factors as kerogen type, thermal maturity, formation temperature and formation pressure, (Lu et al., 1995; Li et al, 2011; Chen et al., 2011). Jarvie et al. (2007) found that the total amount of organic matter and the thermal maturation of shale were important parameters in the production of shale gas. Curtis (2002) demonstrated that the distribution of shale gas was defined by the organic carbon content, kerogen type and thermal maturity. Hill and Nelson (2000) indicated that the accumulation of shale gas was influenced by the organic carbon content, thermal maturity, shale thickness and gas content. Therefore, the geochemical conditions of shales play an important role in the generation and accumulation of shale gas and shale oil, which determine the resource potential and the latter exploitation of the shale oil-gas Types and abundance of organic matter Organic matter type is a critical parameter for determining the hydrocarbon products in different thermal maturities. Based on experimental analyses of the organic elements, the type of organic matter was determined according to the relationship of H/C and O/C. The results showed that (1) type II 1 is the main kerogen type of organic matter; (2) type I and type II 2 are also well-developed; (3) Type III is seldom developed (Fig. 3). Furthermore, results from the Rock-Eval also reached the same 3 conclusions concerning kerogen types. The deposition rate was rapid during the S 3

8 510 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China and S 4 periods, which created the deep water column and restricted water flow. Thus, a semi-deep or deep lacustrine environment is beneficial for sapropelic kerogen development due to the massive plankton reservation. When entering into the depression period, the water column became shallow due to the tectonic uplift, creating favorable conditions for the accumulation of humic kerogen at the slope belts. In the sag areas, however, types I-II 1 were also developed locally. As the strata become younger, the proportion of humic kerogen increases gradually. Figure 3. Types of organic matter in the shale from the study area. Abundant organic matter forms the foundation for shale gas and shale oil accumulations. After a volcano eruption, the shale that developed in the transgression environment had high organic matter abundance (Fig. 4a, Fig. 4b). In the study area, the TOC content of the shale from the S 4 formations ranges mainly from 1 to 4% with an average of 2.55%. The range of A is to 2.63% (averaging 0.27%), indicating a high potential for hydrocarbon generation. The HC in the shale varies widely from 1 to 50 mg/g but primarily ranges from 5 to 10 mg/g. In the early S 3 period, shale developed widely under the regressive systems tract and accumulated organic-rich materials. Overall, the TOC content of the shale is between 1% and 4%, while small amounts of shale with a TOC content of more than 4% were also present in the deep lacustrine facies. In the S 33 formations, more than 200 sets of data on TOC and A were collected. The TOC ranges from <0.1 to 8.5%, of which 60% falls in the range of 1 to 3% with an average of 1.61%, and the content of A ranges from % (averaging 0.15%). The TOC content (462 samples) of the S 32 formations ranges from <0.1->9.5% (averaging 1.84%), with most values ranging

9 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number from 1 to 3%, while A ranges from to 0.72% (averaging 0.132%) according to 256 samples. For the S 31 shales, the organic matter abundance varies considerably with an average TOC content of 1.63% and an average A content of 0.144%. Figure 4. Values and relationships of (a) TOC content and (b) A content with buried depth in the study area. The average values of the TOC are 1.72% and 1.74% in the S 2 and S 1 shales, respectively, with most values being observed between 0.5% and 2.5%. The average values of A are 0.14% and 0.15% in the S 2 and S 1 shales, respectively. The lateral distribution of organic matter abundance is significant for the exploration of shale gas and shale oil and can indicate the favorable areas for exploration. Contour lines of the TOC content in each member of the Shahejie Formation were plotted. Most samples of the S 4 shales have a TOC larger than 2%, and shale with a TOC of > 5% is present primarily in the Chenjia and Panshan sags. In addition, a TOC of > 2% is present in the Chenjia and Niuxintuo sags and the western Qingshui sag because of the favorable conditions for the accumulation and preservation of organic matter (Fig. 5a). The rich organic matter in the S 33 formation was accumulated in the Chenjia sag, where the TOC is mostly greater than 3.5%. The TOC values of shale in the Gaosheng region and the Qingshui sag are also more than

10 512 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Figure 5. Distributions of TOC in (a) S 4, (b) S 33, (c) S 32, (d) S 31, (e) S 2 and (f) S 1 shales in the study area. 3% locally (Fig. 5b). The Chenjia sag also has the highest TOC in the S 32 formation with a value of >4% in certain places (Fig. 5c). In the S 31 period, the Qingshui sag is

11 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number the center of distribution of the TOC. Outward from the sags, the TOC decreases slightly (Fig. 5d). The TOC distributions in the S 1 and S 2 formations are similar throughout the depression. Two organic abundance high zones developed in the Qingshui sag and southeastern Panshan. The TOC is relatively low over the whole region, but values over 3% exist locally, such as those at the center of the Qingshui sag (Figs. 5e and 5f). From the TOC contours (Figs. 5a-5f), it can be observed that the zones with high TOC transfer southeastward as the layer becomes younger, which is consistent with the sedimentary center change. From the S 4 to the S 1 formation, the transfer of subsidence resulting from structural movements controlled by primary faults directly influenced the variety of organic matter abundance. Generally, the high abundance of organic matter was observed in semi-deep or deep lacustrine facies. In addition, areas with minor values were also developed in the underwater distributary inter-channel and lagoonal facies Thermal maturation Experimental data on the thermal maturity of the Shahejie shales show that R o mainly ranges from 0.4 to 0.9%, and T max is between 420 C and 450 C (Fig. 6). The thermal maturity is largely determined by the buried depth. The shallow buried depth in the S 4 formations results in low R o. The averages of R o and T max are 0.42% and 440?, Figure 6. Relationship of R o and T max with burial depth of shale in the study area

12 514 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China respectively. There are also several shale samples with R o values of more than 0.5% (maximum value is 0.66%). Commercial light oil was produced in the dolomitic formation in the Gaosheng regions where the R o of the source rock is less than 0.5%; therefore, thermal maturity may be not the decisive parameter for oil production in the study area. Although the S 3 shales are younger, the overall thermal maturity is higher than the S 4 shales because of the deeper buried depth that resulted from the transfer of the centers of subsidence. Within the S 3 formation, the thermal maturity gradually decreases upward from the S 3 3 to the S 3 1 formation. R o is 0.4 to 0.7% (averaging 0.53%) in the S 33 shales. In certain places where the buried depth is >5000 m, R o exceeds 1.2%. T max ranges from C, with an average of 437 C. The average R o of the S 32 is 0.59%, and the range of T max varies intensively between 435 C and 445 C with an average of 438 C. R o in the S 3 1 formation ranges between 0.32% and 1.08%, and the mean value is 0.53%. T max ranges from 418 to 488 C with a mean of 437 C. R o in the S 2 shales ranges from 0.31% at a depth of 790 m to 1.1% at a depth of 3709 m and has an average of 0.52%. T max varies in a range of 419 to 468 C with an average of 435 C. The S 1 formation has relatively low thermal maturity because of the shallow buried depth, and R o mostly ranges from 0.3 to 0.7% with an average of 0.43%. The average T max is C and is lower than that of the S 2 formation. In such places as the Qingshui sag, where the buried depth is large, R o is relatively high. (The highest value is 0.89%.) Large tectonic movement did not happen through the Shahejie period in the study area, and the formation did not undergo massive uplift and erosion. Therefore, the thermal maturity is related to the burial depth. The thermal maturity, reflected by R o and T max, increases slowly within the depth of 3400 m. When the depth exceeds 3400 m, thermal maturity increases rapidly with burial depth (Fig. 6). Furthermore, a relationship between depth and R o is concluded for the study area. R o values of 0.5%, 0.7% and 1.2% correspond to depths of 2500 to 2600 m, 3400 to 3600 m and 4300 m, respectively. The thermal maturity of organic matter is low because of its shallow burial depth, and R o values increase eastward in the S 4 shales from %. In the younger S 3 formation, the thermal maturity increases due to the increase in the burial depth. A high level of thermal maturity is found in the Chenjia, Qingshui and other sags. For most of shale in the S 3 formation, R o values are between 0.3% and 1% and increase from northwest to southeast (Fig. 7). Shale in the S 1 and S 2 formations has low thermal maturity, but that maturity increases in the Qingshui sag where R o can reach 0.89%.

13 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 7. Contour of R o in S 33 shale in the study area Reservoir characteristics Mineral matter As mentioned above, shale in the Shahejie Formation was developed in a lacustrine environment. The content of clay is higher than that in marine shales, such as the Longmaxi in southern China and the Barnett in the Fort Worth Basin, and the contents of brittle minerals, such as quartz and feldspar, are relatively low. The results of experiments on mineral composition are presented in Table 1. Quartz and clay are the two most prominent minerals in the Shahejie shales. In the S 4 formation, the average content of clay is 58.4%, the sum content of quartz, K-feldspar and plagioclase is between 16.4% and 50.1% and the average carbonate content is 9.2%. Carbonate minerals are primarily derived from dolomitic or calcareous shales, which developed in the Gaosheng region. The average content of clay in the S 3 formation is 48.3%, and the sum content of quartz, K-feldspar and plagioclase ranges from 30.5 to 56.9% (averaging 42.4%). Quartz content increases slightly from 19.2% in the S 4 formation to 31.2% in the S 3 formation. Pyrites and analcidite are also present. Shale that developed in the S 1 and S 2 formations have an average clay content of 54.4%, and the average content of quartz is 21.3%. Small amounts of carbonate, pyrite and feldspar are also present in the upper Shahejie Formation.

14 516 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Table 1. Mineral compositions of shale in the Shahejie Formation in the study area. Quartz Member Clay (%) (%) K-feldspar (%) Plagioclase (%) Calcite (%) Dolomite (%) Siderite (%) Diallogite (%) Pyrite (%) Analcidite (%) S /(58.4) /(19.2) /(3.1) /(5.7) /(3.1) 0-28 /(3.8) /(2.3) /(0.8) /(2.9) /(0.8) S3 3 /(50.9) /(24.4) /(4.9) /(11.4) /(1.6) /(1.6) /(0.6) /(0.5) /(3.6) /(0.45) S3 2 /(46.7) /(34.0) /(2.2) /(7.0) 0-22 /(5.8) 0-10 /(2.1) 0-5 /(0.9) /(0.24) /(0.9) / S / / / / / S /(47.8) /(25.2) 0-11 /(3.7) /(10.5) /(1.4) /(6.1) / /(1.8) / /(3.6) S /(70.8) /(11.6) 0-3 /(1.5) /(5.5) /(1.4) /(2.0) /(0.35) /(1.0) / /(5.9)

15 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Clay compositions vary quickly with the change in the sedimentary process of shale. Clay compositions in the S 4 shales mostly consist of illite, smectite, illite-smectitemixed layer (I/S) and small amounts of kaolinite and chlorite. Smectite or I/S is usually over 50%. The illite varies largely from 5 to 91.4% (averaging 32.91%). Clay compositions in the S 33 formation shale consist of I/S (averaging 70.4%), illite (averaging 23.25%) and minor chlorite and kaolinite. In the S 32 formation shale, I/S or illite is predominant. In addition, minor smectite, kaolinite and chlorite are present. In the S 31 shales, the content of smectite varies largely, and I/S or illite is the dominant clay mineral. When entering into the S 1 and S 2 periods, the dominant minerals are illite or smectite, and the proportion of I/S decreased with an average of 23.77%. The relative proportion of clay minerals is a function of the burial depth. As the depth increases, smectite will convert into illite in the diagenetic process; therefore, the variety of clay minerals can reflect the diagenetic stage, and even the thermal maturity, of shales Pore-fracture system As unconventional oil-gas resources, shale is not only a source rock but also a reservoir. Thus, a pore-fracture system is significant for the accumulation and exploitation of shale gas and shale oil (Curtis, 2002; Zhang et al., 2004; Zhou et al., 2011; Li et al., 2011). Macroscopically, fractures and karst caves in shales can be observed from the core samples, which could provide storage space for oil and gas accumulations. Many fractures were produced due to the differential compaction or structural stress. Moreover, most fractures are cemented by calcite or quartz (Fig. 8). Shale in the northern Chenjia sag developed many karst caves because of the dissolution of carbonate. Figure 8. Pore-fracture developed in shale cores in the study area. (a) Bedding fissure developed in Well Lei37 at a depth of 2814 m; (b) structure fracture was filled by calcite in Well L97 at a depth of m; (c) solution pores developed dolomitic mudstone in the Lei37 at a depth of m; (d) structure fracture developed in Well Sh202 at a depth of m.

16 518 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Microscopically, matrix pores, intergranular pores, intragranular pores, organic pores and microcracks were well-developed (Fig. 9). The diameter of those pores ranges from 0.1 to 10 µm. The porosity of the shale is low and ranges from 0.8 to 18.1%, averaging 5.81%, while the permeability varies greatly from to md (averaging md) due to the existence of fractures. Figure 9. Microscope photographs of shale in the study area. (a) Microcrack developed in Well L97 at a depth of m; (b) microcrack developed in Well Lei36 at a depth of m; (c) matrix pores in Well SG165 at a depth of 3005 m; (d) intragranular pores in Well Lei37 at a depth of m; (e) organic pores in Well Lei36 at a depth of m; (f) organic pores and matrix fracture in Well Lei37 at a depth of m Accumulation mechanism Oil content is a significant parameter for shale oil exploration and exploitation. The range of oil content in the shale developed in the S4 and S33 formations is 0.14 to 8.07% with an average of 1.39%. Furthermore, the factors influencing the oil content of shale, such as TOC and Ro, were investigated. The relationships between the oil yield and

17 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Figure 10. Relationships between oil content and (a) TOC content and (b) R o. other geological factors have also been studied. Oil content increases with an increase in organic matter abundance. A positive relationship exists between oil content and TOC (Fig. 10a). The oil content increases initially and later decreases exponentially with increasing thermal maturity due to the pyrolysis of kerogen and the cracking of pre-existing shale oil (Fig. 10b). The sample used in the thermal pressure simulation experiment was selected from Well SG 165 because of its high TOC content (2.38%), low thermal maturity (R o, 0.445%; T max, 431 C) and sapropel kerogen (type II 1 ). In the processes of hydrocarbon generation and expulsion, the hydrocarbon generation is greatest at a temperature of 350 C and has a total hydrocarbon of kg/t. Next, gas began to be generated at 330 C and increased quickly as the simulation temperate increased continuously. At the beginning of oil generation, oil was detained into the shale reservoir; therefore, little oil could be expelled to the outside. As generation continued, the quantity of expulsion oil increased slowly up to 360 C when there was the largest generation capacity. Next, the quantity of oil generation and expulsion decreased due to entering the gas generation window (Fig. 11). The different kerogen types will have different products when thermal maturity increases, resulting in a regular distribution in profile and on the plane. In the study area, sapropel organic matter developed under the semi-deep or deep lacustrine environment, while in the slope belts, it changes to kerogen of types II 2 -III, as developed under the fan delta facies. For the Shahejie shales, the kerogen changes to humic upward, and from the sedimentary center to the depression edge, the type of kerogen gradually changes from type I-II 1 to humic. Generally, sapropel organic matter will generate oil when the Ro is between 0.4% and 1.2% according to the exploration practice and hydrocarbon generation model, while type II 2 -III kerogen will primarily produce gas in this thermal period. When the Ro is >1.2%, the oil generated from the sapropel kerogen will crack into gas, while humic kerogen generates gas all of the time. Integrating with the sedimentary environment, geochemistry characteristics and hydrocarbon generation simulation, the accumulation mechanism of shale gas and shale oil in the study area is established based on the structural profile, which has the

18 520 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China Figure 11. (a) Hydrocarbon generation curve and (b) oil generation curve for shale sample from Well SG 165. typical characteristics of a rifted lacustrine basin (Fig. 12). The depression can be divided into gentle slope belts in the western areas, sag regions in the central region and steep slope belts in the eastern areas. Normal faults developed widely with a northwest orientation. The kerogen is of type I-II 1 in the sag regions and is buried at a large depth. When the burial depth is less than 4300 m, corresponding to the Ro of 1.2%, the organic matter can generate oil in the shale formation, and shale oil will accumulate easily. In certain places where the Ro is over 1.2%, the pre-existing oil will crack into gas and form a shale gas accumulation. When the organic matter becomes Figure 12. Accumulation mechanism of shale gas and shale oil in the Shahejie Formation.

19 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number humic upwards and laterally, the products of kerogen will primarily be gas. However, when kerogen of type I-II 1 develops in the slope belt, shale oil can also be accumulated in the shale formation Combined with accumulation distribution characteristics of shale gas and shale oil in rifted lacustrine basin and practical geological conditions, the distribution of shale gas and shale oil in the study area on the planar is observed (Fig. 13). Shale oil developed prominently across the study area, while shale gas exists on the western side of the Yuanyanggou sag, where the thermal maturity is over 1.2%. Between those two types of accumulations, shale oil and gas co-exist belts are developed. Figure 13. Distribution of shale gas and shale oil in the Liaohe western depression Discussion Compared to marine shales, the area in the rifted lacustrine basin is relatively small, and the sedimentary facies varied quickly, resulting in the diversity of the types and the abundance of organic matter. This diversity leads to a challenge in the exploration of shale gas and shale oil in the rifted lacustrine basin. Furthermore, clay content is much higher, and brittle mineral content, such as quartz and feldspar, is lower, than successfully developed marine shales in North America, which presents a new challenge for hydraulic stimulation in regard to shale gas and shale oil exploitation. When thermal maturity is low, shale oil should be the main target, while shale gas will develop when the thermal maturity is high, and the burial depth is large. The sweet spots for shale gas and shale oil should be focused not only on the fracture zones in shale formation but also on the thin sandstone intervals, limestone and dolomite that

20 522 Geological controls and mechanism of shale gas and shale oil accumulations in Liaohe western depression, China are juxtaposed or interbedded with the organic-rich shale formation. In the primary exploration stage, hydrocarbon well appraisal and fracturing design integrating with well logging are necessary. After making the breakthrough on shale gas and shale oil in the most favorable regions, commercial exploitation is available. In addition, the research on geological conditions on shale gas and shale oil in the Shahejie Formation in the study area can offer a case for other rifted lacustrine basins. Through the primary appraisal, the S 4 and S 3 formations in the Chenjia and Panshan sags exhibit favorable geological conditions for shale gas and shale oil accumulations. The next detailed work in these areas should focus on the seismic and log response characteristics for shale gas and shale oil to identify sweet spots in shale formations. 5. CONCLUSIONS In the Shahejie shale formations, (1) type II 1 is the primary kerogen type of organic matter; kerogen of type I and type II 2 is also well-developed; and Type III is seldom developed. (2) The organic abundance of the shale is relatively high. The average TOC content is larger than 2% in most areas that decrease from subsidence center to basin margin. (3) The thermal maturity is low because of the young deposition epoch. The Ro ranges from 0.4 to 0.9% in most areas and increases slowly with increasing burial depth. (4) Intergranular, intragranular and organic pores, matrix pores, fractures and karst caves were developed in the shale, which offer storage space for oil-gas accumulations. (5) Sapropelic organic matter was developed in semi-deep or deep lacustrine facies in the center regions. When 0.4% < Ro <1.2%, shale oil was accumulated. When Ro was larger than 1.2%, shale gas was generated by cracking from pre-existing shale oil. In the slope belts and center region of the upper parts, humic kerogens (II 2 -III) were deposited and exhibited low thermal maturity. In these regions, shale gas should be the primary target for exploration. To date, shale oil is the most feasible and promising resource for unconventional oil-gas exploration and exploitation in rifted lacustrine basins (e.g., the Liaohe western depression). ACKNOWLEDGMENTS This research was supported by the National Natural Science Founds (Grant No ) and the Fundamental Research Fund for Central Universities (Grant No. 2011PY0214). We also thank Dr Tang Xuan, Dr. Li Junqian from China University of Geosciences and Mrs. Zhang Qin from RIPED of PetroChina for preparing the manuscript. REFERNCES Chen X., Wang M., Yan Y.X., Zhang X.W., Luo Y. and Zhang Y.H., Accumulation conditions for continental shale and gas in the Biyang Depression. Oil & Gas Geology 32(4), Chen Z.Y., Yu B.J., Zheng Z.Y., Li M.S., Gao Q.S. and Shi L.C., The genetic of multi-source natural gas in Liaohe sub-basin. Acta Sedimentologica Sinica 15(2), Curtis J.B., Fractured shale-gas system. AAPG Bulletin 86(11),

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23 ENERGY EXPLORATION & EXPLOITATION Volume 32 Number Zhang S.W., Wang Y.S., Zhang L.Y., Li Z., Zhu J.J., Gong J.Q. and Hao Y.Q., 2012c. Formation conditions of shale oil and gas in Bonan sub-sag, Jiyang Depression. Engineering Sciences 14(6), Zhang Z.W., Chen Z.Y. and Guo K.Y., Geology of Natural Gas Accumulation in Liaohe sub-basin. Geology Press, Beijing, pp Zhou C.N., Dong D.Z., Wang S.J., Li J.Z., Li J.Z., Wang Y.M., Li D.H. and Cheng K.M., Geological characteristics, and resource potential of shale gas in China. Petroleum Exploration and Development 37(6), Zhou C.N., Dong D.Z., Yang H., Wang Y.M., Huang J.L., Wang S.F. and Fu C.X., Conditions of shale gas accumulation and exploration practices in China. Natural Gas Industry 31(12),

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