Characteristics of Lacustrine Organic-Rich Shale: a Case Study of the Chang 7 Member, Triassic Yanchang Formation, Ordos Basin, China*

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Journal of Natural Gas Geoscience xx (2016) 1e13 http://www.keaipublishing.com/jnggs Original research paper Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China*

Chuang Er a,b,*, Yangyang Li c, Jingzhou Zhao a,b, Rui Wang a, Zhikun Wei a

a School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an, 710065, China b Shaanxi Key Lab of Petroleum Accumulation Geology, Xi'an Shiyou University, Xi'an, 710065, China c C&C Reservoirs, Houston, TX, 77036, USA

Received 6 December 2015; revised 3 February 2016 Available online ▪▪▪

Abstract

This paper presents a case study of lacustrine organic-rich shale from the Yanchang Formation, Ordos Basin, China. Sedimentary and geochemical analyses were performed on shale acquired from the Chang 7 member through its outcrop, core and thin section data indicate that depositional environments have a great impact on the distribution and quality of shale reservoirs. Shale in the Chang 7 member was interpreted to have been deposited mainly in delta-front and deep-lake environments. Delta-front shale is 2e10 m thick (average ~5 m), laterally discontinuous and commonly interbedded with equally thick layers of sandstone. In contrast, deep-lake shale is 5e74 m thick (average ~21 m), laterally continuous and locally interbedded with thin layers of sandstone. Delta-front shale was mostly massive without obvious sedimentary structures while deep-lake shale was characterized by abundant horizontal laminations. More importantly, delta-front shale primarily contains Type Ⅱ1 and Ⅲ kerogen, with TOC commonly less than 2%. In comparison, deep-lake shale primarily contains Type Ⅰ and Ⅱ1 kerogen, with TOC mostly higher than 2%. Based on the analysis and comparison between shales deposited in two types of environments, it is suggested that deep- lake shales in the Chang 7 member were favorable exploration target for hydrocarbons. Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Organic-rich shale; Shale oil and gas; Depositional environment; Shale distribution; Yanchang Formation

1. Introduction China). Compared to marine shales, lacustrine shales have mixed kerogen types, wider ranges of TOC, and generally less Sedimentary research of shale has been greatly focused on continuous both laterally and vertically, which makes it more those of marine origin [1e3]. Studies on lacustrine shales are challenging for hydrocarbon exploration and development. still rare even though non-marine shales have become Consequently, there is a demand from the petroleum industry increasingly important in hydrocarbon exploration (e.g., for robust sedimentary research on lacustrine shale. The Triassic Yanchang Formation in the Ordos Basin was deposited in a lake environment. It was subdivided into 10 e * This is English translational work of an article originally published in members, Chang 1 10 in ascending order. The Chang 7 Natural Gas Geoscience (in Chinese). The original article can be found at:10. member, which was deposited during the largest lake trans- 11764/j.issn.1672-1926.2015.05.0823. gression, contains the thickest and most continuous dark shale * Corresponding author. School of Earth Sciences and Engineering, Xi'an and mudstone in the Yanchang Formation. The Zhangjiatan Shiyou University, Xi'an, 710065, China. shale represented the maximum transgression [4,5]. When the E-mail address: [email protected] (C. Er). Yanchang Formation was deposited, the basin was character- Peer review under responsibility of Editorial Ofﬁce of Journal of Natural Gas e Geoscience. ized by a long axis of NW SE, with slopes that were gentle in http://dx.doi.org/10.1016/j.jnggs.2016.04.001 2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, Petro- China. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 2 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

Fig. 1. Features of the Zhangjiatan shale of Chang 7 member in the Niejiahe field profile. the NE and steep in the SW [6,7]. Previous researchers have lake turbidites [8e10]. Deep lake facies include organic-rich identified several sedimentary facies in the Chang 7 member shale, dark mudstone, sandy mudstone, muddy sandstone, including lacustrine delta, semi-deep to deep lake, and deep- and sandstone [11,12].

Fig. 2. Features of the Zhangjiatan shale of Chang 7 member in the Tangnihe ﬁeld proﬁle.

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 3

Fig. 3. Feature of the Zhangjiatan shale of Chang 7 member in the Xufengchuan ﬁeld proﬁle.

2. Dark shale depositional environments horizontal laminations and is presumed to be deposited in a deep lake environment. A single layer of the black shale was Outcrop and core data analysis suggest that dark shale in as thick as 35e130 cm (Fig. 1d). There were 10 thin layers of the Chang 7 member was deposited in delta front, prodelta, sandstone in total with a thickness ranging 4e20 cm. The and semi-deep to deep lake environments. Since there was a single layer black shales were not homogeneous. They are very subtle difference between prodelta, semi-deep, and deep characterized by mm- to cm-scale variations with thin layers lake shales, they're not differentiated in the paper, and are of shale interbedded with tuff and sandstone (Fig. 1c). These treated together as deep lake shales. variations are understood to be a result of combined high- frequency lake-level fluctuation, volcanic eruption, and 2.1. Outcrop data event-triggered turbidite deposition. The mm- to cm-scale sandstone layers were more often laterally continuous and Three outcrop cross sections are presented in this paper, could be traced and correlated between cross sections in namely, Niejiahe, Tangnihe, and Xuefengchuan, which are Fig. 1a and b. These sandstones are interpreted to be deep-lake located tectonically on the Weibei Uplift and will be examined turbidites. in the following section. 2.1.2. Tangnihe cross section 2.1.1. Niejiahe cross section The Tangnihe cross section is exposed in the Tangnihe The Niejiahe cross-section is located in the Yaoqu town, village, Yijun county, Tongchuan city (N3511048.700; Yaozhou district, Tongchuan city (N359057.400; E1093048.800), located southeast of the Banjiegou cross sec- E10851030.200). It is ~35 m long and 6.9 m high (Fig. 1a and tion. The outcrop exposure is heavily weathered. Different b). The Chang 7 member exposed at this location consists of from rocks in the Niejiahe outcrop, strata at the Tangnihe primarily black shale and dark mudstone, which are inter- outcrop dip 60 toward NNE. In a 20 m vertical succession, 11 bedded with thin layers of dark gray fine-grained sandstone depositional cycles were identified (Fig. 2). Each was char- and brown tuff. Black shale is characterized by abundant acterized by the black shale from the bottom upward changing

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 4 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

Fig. 4. Sedimentary microfacies of Chang 7 member of Well Jingtan 913 as well as study of dark mudstone that is located at the bottom of Chang 7 member. to dark gray muddy siltstone, which is assumed to reﬂect high- major normal fault is observed, dipping 220 with an angle of frequency lake-level ﬂuctuation in a deep-lake environment. A 56. A subsidiary fault dips 198 with an angle of 55 (Fig. 2).

Fig. 5. Core features of dark mudstone of Chang 7 member in Well Jingtan 913. (a) 1936.21 m, dark gray mudstone; (b) 1936.83 m, dark gray mudstone; (c) 1937.17 m, dark gray mudstone interbedded with pelitic siltstone, which is oil-soaked sandstone; (d) 1937.44 m, dark gray mudstone with horizontal bedding and carbon nodules.

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 5

Fig. 6. Core features of dark shale that developed in the deep lacustrine. (a) Well Wu 100, Chang 73, black shale with calcareous strip, thin tuff layers; (b) Well Wu 100, Chang 73, black shale with potato-shaped sandstone; (c) Well Zhang 22, Chang 73, black shale with sandstone dyke.

Dark shale in the Tangnihe cross section is presumed to have member at this well location is assumed to be delta-front been deposited in a deep-lake environment. interdistributary bay deposits. Sandstones are thick (Fig. 4) and are characterized by a tank- or bell-shaped log responses. 2.1.3. Xuefengchuan cross section These sandstones are interpreted to be delta-front distributary The Xuefengchuan cross-section is located in the Hancheng channel deposits. The Zhangjiatan shale between 1936 and city (N35390; E11080). The Chang 7 member exposed at this 1938.2 m TVD, which can be correlated regionally, is rela- outcrop consists of fine-grained sandstone, gray to dark gray tively thin and light in color at this location (Fig. 5). Core data mudstone, and thin coal seam (Fig. 3aec). Mudstone layers of the Zhangjiatan shale shows that it primarily consists of range 10e100 cm in thickness and are commonly 10e20 cm. dark gray mudstone, which is interbedded with thin muddy Sandstone layers range 20e100 cm in thickness and are siltstone and silty mudstone. The mudstone is mostly massive. commonly 40e100 cm (Fig. 3d). Sedimentary structures Silty mudstone shows horizontal lamination and wavy lami- observed from this outcrop include trough cross stratification, nation suggesting a subaqueous interdistributary bay pillow, and loading structures (Fig. 3aec). The rock succes- environment. sion at the Xuefengchuan cross section has thicker sandstone layers and higher N:G ratio than the strata at Niejiahe and 2.2.2. Deep-lake dark shale Tangnihe cross sections. This indicated a much shallow Cores from two wells, namely, Well Wu 100 and Well depositional environment. This section is understood to be Zhang 22, located in the central part of the basin were delta front deposits based on the regional context. examined. The Chang 7 member in these two wells consists primarily of back shale and mudstone, which are locally 2.2. Core data interbedded with thin tuff layers. String- and potato-shaped calcareous sandstone and sandstone dyke are locally Analysis of cores shows that shale in the Chang 7 member observed (Fig. 6). Tuff layers indicate occasional volcanic consists of laminated shale, massive mudstone, silty mudstone, eruptions. Patchy sandstones are interpreted to be turbidites and muddy siltstone. They are primarily gray and black in brought about by events such as earthquake or volcanic color. Sedimentary structures include horizontal lamination, eruption. At Wu-100 well, there is a 40 m-thick dark shale wavy lamination, and massive structure. These indicated between 1884 and 1932.5 m. It consists predominantly dark overall static water environment. shale with interbedded thin layers of tuff, siltstone, and massive fine-grained sandstone. Sandstone is interpreted to be 2.2.1. Delta-front shale turbidite or sandy debris flow deposits (Fig. 7). The Chang 7 member is buried at 1863.05e1938.36 m TVD at the Well Jingtan 913, which is located in the south of 2.2.3. Lateral facies distribution the Jingbian County. It consists of gray fine-grained sandstone, When the Chang 7 member was deposited, semi-deep to siltstone, muddy fine-grained sandstone, and dark gray deep lake environment covered a large area from the west of mudstone. Mudstone layers are thin (1e2 m) and are char- Dingbian-Wuqi-Zhidan-Ganquan-Fuxian-Luochuan-Yijun- acterized by finger-shaped log responses. Shale in the Chang 7 Xunyi-Changwu to the east of Jingchuan-Zhenyuan-Huanxian

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 6 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

Fig. 7. Columnar section of dark shale in Chang 7 member of Well Jingtan 100.

(Fig. 8). The long axis, oriented NWeSE, of the basin, whereas the overall N:G is not higher than 0.3. Thick and controlled the regional distribution of the shale (Fig. 9). Deep- vertically continuous shale and shale with thin and less num- lake dark shale is concentrated in the central part of the basin ber of sandstone layers are favorable exploration targets for and oriented in an NWeSE direction while delta-front shale is shale gas. Consequently, it is important to characterize the primarily distributed in the NE margin of the lake between vertical distribution of shale and its relationship with sand- distributary channels (Figs. 8 and 9). The Chang 73 sandstone stone layers. Shale in the Chang 7 member was deposited in submember contains dark shale of up to 70 m, representing the delta-front and semi-deep to deep lake environments. Vertical maximum transgression of the lake (Fig. 8). distribution characteristics of these two types of shales are different and are discussed as follows. 3. Vertical distribution characteristics 3.1. Delta-front dark shale Exploration targets of shale gas in a marine depositional environment are commonly at least of 30 m for each single Delta-front dark shale is commonly interbedded with layer thickness [13e18]. Single sandstone layer that is inter- sandstone layers. Dark shale layers are relatively thin and bedded with the shale reservoir is commonly less than 3 m, typically less than 11 m. Sandstone layers, which are abundant

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 7

Fig. 8. Sedimentary facies of Chang 73 submember shale (modiﬁed from Ref. [5]). in the delta-front environment, are commonly equal in thick- discontinuous. The Chang 7 member in the Well Li 57, which ness or are locally thicker than shale layers. is located in the west of the Huachi county is thought to have Chang 7 member in the Well Pan 3321, which is located in been deposited in a deep lake environment (Fig. 11). The the east of the Ansai county is buried in 960.74e1026.16 m Chang 73 submember consists of mainly black shale, carbo- TVD, is understood to have been deposited in a lacustrine naceous mudstone, and black mudstone. The dark shale in the delta-front environment. It was subdivided into three sub- Chang 73 is thick (up to 25 m) and vertically continuous with members: Chang 71, Chang 72, and Chang 73. The Chang 71 high TOC. The Chang 71 and Chang 72 submembers consist of and Chang 72 submembers consist of predominantly thick primarily dark shale of comparatively lower TOC with a single sandstone with thin layers of shale and silty mudstone. The layer thickness ranging from 9.2 to 22 m. Interbedded turbidite Chang 73 submember at the bottom of Chang 7 member sandstone layers are 2.2e7.5 m. consists of ~9.7 thick dark shales. The TOC is less than 2% for both measured and calculated results (Fig. 10). 3.3. Statistical analysis

3.2. Semi-deep to deep lake shale Delta-front shale is relatively thin, ranging from 2.1 to 11 m in thickness (average 5.43) (Table 1). Interbedded sandstone Shale deposited in semi-deep to deep lake environment is layers were usually of equal thickness or locally thicker than commonly thick and vertically continuous. Sandstone layers, shale layers, ranging from 2 to 10 m in thickness (average which are presumed to be turbidities, are thin and laterally 5.8 m). Semi-deep to deep lake shale is much thicker, ranging

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 8 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

Fig. 9. Isopach map of Chang 73 submember shale. from 4.85 to 74.61 m in thickness (average 21 m). It may or high dip angles could have played a negative role in terms of may not contain sandstone layers internally. In case it does, hydrocarbon preservation. However, primary depositional interbedded sandstone layers are thin ranging from 1 to 3 m stratifications increase permeability and enhance the effec- (average 2 m). Sandstone layers that are overlying or under- tiveness of later reservoir simulation. lying the continuous shale layer are 3.3e31 m in thickness Previous studies identified several different types of lith- (average ~12). It can be seen that single layer shale deposited ofacies in shale reservoirs in the Yanchang Formation in semi-deep to a deep environment is much thicker in average including massive mudstone, laminated mudstone, massive than that deposited in a delta-front environment, which makes silty mudstone, graded silty mudstone, carbonaceous lami- it a favorable target for shale gas. nated silty mudstone, and silty mudstone with composite laminations [19]. Core samples of black shale, sandstone, and 4. Microstructures tuff layers are characterized by laminations (Fig. 12a) that show grain alignment in a certain direction when examined in Study of conventional sandstone reservoir indicates that thin sections (Fig. 12b). In comparison, core samples of dark sedimentary structures have a great impact on reservoir quality gray mudstone and silty mudstone are characterized by (e.g., permeability). Dark shale is characterized by extremely massive structure and are interbedded with string-shaped low permeability and it commonly required fracturing to sandstones (Fig. 12c). In thin sections, no obvious grain produce efficiently. Primary fractures, especially those with alignment could be seen in these mudstones (Fig. 12d). Shale

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 9

Fig. 10. Vertical overlaying features of dark shale of Chang 7 member in Well Pan 3321. deposited in semi-deep to deep lake environment is commonly 5.1. Kerogen type characterized by abundant laminations, which contribute to higher permeability [20]. On the contrary, laminations are not Analysis of kerogen types for shale in the Chang 7 member well developed in delta-front shale, thus resulting in relatively is based on data from core and outcrop samples as well as low permeability. results from previous studies. TmaxeIH plots were made to determine kerogen types in this paper (Fig. 13). 5. Organic geochemical characteristics Outcrop samples from the Niejiahe, Banjiegou, and Tang- nihe cross sections, which are understood to have been Organic matter richness, kerogen type, and maturity are key deposited in a deep-lake environment, primarily contains type components in source rocks' characterization, among which ⅠeⅡ1 kerogen. Outcrop samples from the Zhangjiatan and the ﬁrst two are closely related to the depositional environ- Xuefengchuan cross sections, which are presumed to have ment. In shallow lake environment (e.g., delta front), where been deposited in shallow-lake delta-front environment, pri- sediment supply and oxygen content are high, organic matter marily contains type Ⅱ2eⅢ kerogen with a mix of type Ⅰ mostly originated from higher plants and has a low preserva- kerogen (Fig. 13a). tion potential, yielding primarily humic substance [21,22].In Core analysis showed that the kerogen type in shale varies opposition, organic matter in semi-deep to deep lake envi- greatly between different regions. Shale in the Zhidan, Ansai, ronment, where sediment supply and oxygen are relatively and Ganquan area, which was located in a deep-lake environ- low, primarily originated from algae and cyanobacteria and ment when the Yanchang Formation was deposited, primarily has a high preservation potential, yielding predominantly contains type Ⅰ and Ⅱ1 kerogen (Fig. 13b). Shale in the Zichang sapropelic substance. area primarily contains Type Ⅱ2 kerogen. Shale in the Xifeng

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 10 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

Fig. 11. Vertical overlaying features of dark shale of Chang 7 member in Well Li 57.

area primarily has type Ⅱ2eⅢ kerogen (Fig. 13b). Both Zichang 5.2. TOC and Xifeng areas were located in the delta-front environment during the Yanchang Period. It is obvious from the core analysis TOC is much higher in shales deposited in semi-deep to deep that shales deposited in semi-deep to deep lake environments lake environments than shales deposited in the delta-front primarily contains type Ⅰ and Ⅱ1 kerogen. In comparison, shales environment (Figs. 10 and 11). TOC of dark shale in the Zhi- deposited in the delta-front environments primarily contain dan area, which was located in a deep-lake environment, is much Type Ⅱ2, but with a mix of both type Ⅰ and Ⅲ kerogen. higher than shale in the Zichang, Ansai, and Xifeng, which were

Table 1 Thickness characteristics of dark shale and sandstone in different sedimentary environments. Sedimentary Delta front Semi deep to deep lacustrine environment Shale and mudstone/ m Sandstone/ m Shale and mudstone/ m Sandstone (interbedded)/ m Sandstone (overlying and underlying)/ m Thickness/m min. 2.1 2 4.85 1.06 3.3 max. 11 10 74.61 3 31 ave. 5.43 5.8 21 2.05 11.94

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 11

Fig. 12. Sedimentary structures and micro-bedding structures of dark shales of different lithologies. (a) Well Cheng 96, 2065.3 m, C73, black shale with sandy and tufaceous lamina; (b) Well Cheng 96, 2065.3 m, C73, massive bedding and lamellar structure in thin section; (c) Well Cheng 96, 2064.3 m, C73, black mudstone with sandy and tufaceous lamina; (d) Well Cheng 96, 2064.3 m, C73, sandy laminas distributed continuously or discretely; (e) Well Cheng 96, 2050.65 m, C73, dark gray mudstone; (f) Well Cheng 96, 2050.65 m, C73, massive bedding and no obvious grain alignment; (g) Well Cheng 96, 2034.09 m, C73, gray silty mudstone; (h) Well Cheng 96, 2034.09 m, C73, massive bedding and no obvious grain alignment. located in a delta-front environment when the Chang 7 member (Fig. 15). TOC is low (<2%) in shales deposited in the delta- deposited (Fig. 14). An SWeNE correlated TOC proﬁle through front environments and increases in semi-deep to deep lake Well Zhen 61 and Well Pan 3323 with data from well logs and environments (>2%). From SW to NE, depositional facies analyses of rock samples showed its lateral and vertical varia- change laterally from delta-front to semi-deep to the deep lake tions along with the change of depositional environments and then back to delta front environments, which correspond

Fig. 13. Organic types in Chang 7 member of Yanchang Formation that was conﬁrmed by TmaxeIH diagram. (a) Field data; (b) Core data.

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL 12 C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13

with thick sandstone layers. Dark shale deposited in semi-deep and deep lake environment is thick and vertically continuous with less and thin sandstone and tuff layers. Semi-deep to deep-lake shale is characterized by abundant sedimentary stratiﬁcations which contribute favorably to higher permeability. TOC is higher in deep lake shale than in delta-front shales. Therefore, semi-deep to deep lake shale is the prefer- able exploration target for shale gas.

Conﬂict of interest

We declare that we do not have any commercial or asso- ciative interest that represents a conﬂict of interest in connection with the work submitted.

Foundation item

Supported by Project of Natural Science Foundation of Shaanxi Province, China (2013JQ5003); National Science & Fig. 14. TOC distribution features of Chang 7 member in different areas.

Fig. 15. Comparison of TOC between semi-deep lacustrine-deep lacustrine and delta front.

with a low-high-low trend in TOC (Fig. 15). Vertically, TOC is Technology Special Project (2011ZX05018001-004); Post- the highest in the Chang 73 submember, which is recognized to doctoral Science Foundation of Xi'an Shiyou University, be a maximum ﬂooding surface. China (YS29031219).

6. Conclusions References The dark shales in the Chang 7 member of the Triassic Yanchang Formation were deposited is lacustrine delta-front [1] J.J. Hickey, B. Henk, Lithofacies summary of the Mississippian Barnett Shale, Mitchell 2 T.P. Sims well, Wise County, Texas, AAPG Bull. 91 (4) and semi-deep to deep lake environments. The quality and (2007) 437e443. distribution of dark shales are greatly inﬂuenced by deposi- [2] R.G. Loucks, S.C. Ruppel, Mississippian Barnett Shale: Lithofacies tional environments. Dark shale deposited in a delta-front and depositional setting of a deep-water shale-gas succession in the environment is relatively thin and commonly interbedded Fort Worth Basin, Texas, AAPG Bull. 91 (4) (2007) 579e601.

Please cite this article in press as: C. Er, et al., Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China, Journal of Natural Gas Geoscience (2016), http://dx.doi.org/10.1016/j.jnggs.2016.04.001 + MODEL C. Er et al. / Journal of Natural Gas Geoscience xx (2016) 1e13 13

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