Evaluation of Highly Thermally Mature Shale-Gas Reservoirs in Complex Structural Parts of the Sichuan Basin

Journal of Earth Science, Vol. 24, No. 6, p. 863–873, December 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0384-4

Tonglou Guo* (郭彤楼) SINOPEC Exploration Southern Company, Chengdu 610041, China

ABSTRACT: Successful exploration and development of shale-gas in the United States and Canada suggest a new solution to the energy problem in China. The Longmaxi (龙马溪 ) Formation in the Si- chuan (四川) Basin is regarded as a strong potential play for shale-gas with the following significant features: (1) complex structural types caused by multiphase tectonic superposition and reconstruction; (2) varied slippage processes that enhance porosity and permeability; (3) high thermal maturation of organic matter (Ro>2.5%); (4) high brittle mineral contents; (5) high and constant thicknesses of shale horizons within the formation. Evaluation of shale-gas prospects in this area should consider not only hydrocarbon parameters, but also preservation conditions and structural stability. Data from several new exploration wells in the Sichuan Basin indicate that tectonically induced net-shaped fractures ef- fectively enhance shale reservoir properties. Structural types providing favorable storage conditions for shale-gas are described and evaluated. The high-yielding shale gas reservoir shares the same characteristics of conventional gas reservoirs except for its consubstantial source rock and reservoir in South China. KEY WORDS: Sichuan Basin, Longmaxi Formation, high thermal maturity, shale-gas.

INTRODUCTION al., 2013; Hao and Zou, 2013; Jia et al., 2012; Lin and Industry reports suggest that marine shale is the Wang, 2012; Li et al., 2009; Pan and Huang, 2009; most important future shale-gas resource play in South Wang L S et al., 2009; Wang S J et al., 2009; Ross and China because of its characteristics of stable distribu- Bustin, 2008; Zhang et al., 2008; Daniel et al., 2007; tion, high organic content and high thermal maturity, Jarvie et al., 2007; Liu et al., 2006) on the edges of quite similar to conditions in North America. Such basins exposed to air. In the centers of basins, shale is shale mainly occurs in the Yangtze Platform (Hao et cut by faults caused by complex tectonic activity and this creates difficulties for shale-gas exploration. This study was supported by the Shale Gas Specific Program Southern China has two significant marine shale from Ministry of Land and Resources of China (No. formations: the Lower Silurian Longmaxi Formation 2009GYXQ15-06) (Fm) and the Upper Ordovician Wufeng Fm There *Corresponding author: [email protected] have been many studies of the organic matter content, © China University of Geosciences and Springer-Verlag Berlin organic matter type, sedimentary setting, thermal ma- Heidelberg 2013 turity, and distribution of thickness in Longmaxi Fm shale (Chen and Guo, 2012; Huang et al., 2012; Liang Manuscript received April 22, 2013. C et al., 2012; Tao et al., 2012; Chen et al., 2011; Manuscript accepted August 19, 2013. Dong et al., 2009; Liang D G et al., 2009a, b, 2008),

864 Tonglou Guo but most concentrated on single characteristics of GEOLOGICAL SETTING shale and did not say much about shale-gas accumula- The Yangtze Craton has undergone Post- tion, which limits their value for further shale-gas ex- Cambrian Caledonian, Hercynian, Indosinian, Yan- ploration. The present study draws on analysis of ex- shanian, and Himalayan earth movements which have ploration wells JY1, YY1, PY1 (Fig. 1) to identify given rise to complex tectonic settings. Take eastern controlling factors for shale-gas accumulation on Sichuan Basin for example. The Chuanzhong pa- highly thermally mature shale of the Longmaxi Fm in leouplift formed in Caledonian movement is slope in a complex tectonic setting. The ultimate goal is to research area. And Cambrian, Ordovician and Silurian provide a comprehensive detailed understanding of depositions are continuous while Devonian and Lower the shale in the area, and guidelines for wider explora- Carboniferous sediments are absent. Hercynian tion of potential shale-gas resources in southern Chi- movements resulted in the cessation of deposition at na. the top of the Lower Permian Maokou Fm. Indosinian In this article, the Upper Ordovician Wufeng Fm movements caused an important tectonic change be- and Longmaxi Fm is assigned as the Longmaxi Fm for cause it ended marine deposition history and initiated simplified reason, since both of them are continuous the present tectonic framework (Zhu, 1986). A typical deposition with thickness of 3–7 m and share similar feature of the Yanshanian movements was gravity sedimentary characteristics. sliding of basement and cap rocks. For example dur- ing this period in east Sichuan, the rocks of Xuefeng

Figure 1. The Sichuan Basin. (a) Tectonic subdivisions; (b) geological map of study area SW of Chongqing; (c) representative borehole logs.

Evaluation of Highly Thermally Mature Shale-Gas Reservoirs in Complex Structural Parts of the Sichuan Basin 865

Mountain slipped into the Sichuan Basin giving rise to SHALE-GAS ACCUMULATION CHARACTE- parallel fold and fault zones in west Hunan and Hubei, RISTICS drastically affecting petroleum accumulation and pre- Thickness and Lithology of Shale servation. Himalayan movements resulted in stress There are three depositional centers in Longmaxi field adjustments that may be driving further hydro- Fm. Shale (TOC>0.5%): the first in the north area of carbon remigration and accumulation up to the present Wanyuan reaching a maximum thickness of 133 m; (Liu et al., 2006). the second the Linchuan-Fuling area, maximum Thus the characteristics of the first three tectonic thickness 120 m; the third in the Zigong- Luz- movements were uplift and erosion that could have hou-Yibin area, maximum thickness 120 m (Fig. 2). retarded generation of source rocks in the Silurian The depositional environment changed from Basin. The last two tectonic movements not only deep-water continental shelf to shallow-water conti- caused uplift and erosion but also involved compres- nental shelf between the Changning and Well sional deformation that gave rise to complex fold and YY1-Well PY1 areas, and the thickness of Longmaxi fault combinations. For example, Yanshanian move- Fm in reaches 320 m at outcrop in Shizhuqiliao and ments might produce tilted and horizontal strata in gets up to 308 m in Shuanghe. The thickness is 405 m Silurian mudstone, and following Himalayan move- in Well PY1, 266 m in JY1 and about 148 m in DS1 ments might result in the appearance of high angle (Fig. 3). fractures (Zhang et al., 2008). The upper part of Longmaxi Fm is mainly composed of mudstone, the middle is interbedded

Hanzhong

Ankang

80 N Guangyuan 60

Wanyuan Bazhong Langzhong 0 60 km Mianyang

Deyang 20 Nanchong Chengdu 40 Wanxian Qionglai 60 Huaying 80 Lichuan Enshi

Ziyang 100 Ya’an 0 Hechuan

100 80 60 Well YY1 40 20 20 Well JY1 Leshan Neijiang 40 60 Chongqing Fuling 80 Yongchuan Well PY1 Zigong Nanchuan Well Y101 100 Yibin Luzhou 120 Well DS1

80 60 40 Well L1 20 Tongren Zunyi

Figure 2. Isopach map of thickness in Longmaxi Formation, Sichuan Basin.

866 Tonglou Guo

Shuanghe DS1 JY1 PY1 Qiliao

TOC TOC TOC TOC TOC

Depth Depth Depth

Depth

Strata 0 5 Strata 0 5 Strata 0 5 Strata 0 5 Strata 0 5 (m) (%) (m) (%) (m) (%) (m) (%) (m) (%)

Lithology

Lithology Lithology

Lithology 2 160 1 760 20 1 400 20 2 200 1 800 60 1 440 60 2 240 1 840 100 1 480 100 2 280 1 880

140 Formation Longmaxi 1 520 140

2 320 1 920 180 180 Longmaxi Formation Longmaxi Longmaxi Formation Longmaxi

2 360 1 960 Formation Longmaxi 220 220

2 400 2 000

260 Formation Longmaxi 260

2 040 300

2 080

2 120

Figure 3. S-E stratigraphic correlation of the Longmaxi Formation, Sichuan Basin. argillaceous siltstone and siltstone, the lower is stone (116 m), grayish-black silty mudstone with in- medium-bedded shale, carbonaceous shale, mudstone, tercalated muddy siltstone (61 m), and carbonaceous carbonaceous mudstone, siliceous shale interbedded siliceous shale (89 m). Organic geochemistry shows with silty mudstone and thin argillaceous limestone, that TOC of shale is always greater than 0.5% and the and the basal bed of the Longmaxi Fm is black shale average 2.54%. The lowest 38 m of shale has espe- with a high organic matter content and abundant cially high TOC greater than 2% average 3.5% (Fig. graptolite fossils (Fig. 3). 3). Ro value ranges from 2.2% to 3.6%. The organic kerogen types are type I (Fig. 4). Organic Geochemical Characteristics of Shale In Well DS1 the Longmaxi Fm is 98 m thick The highest organic matter content is usually in consisting of grey-black lime mudstone with TOC the basal part of the Longmaxi Fm. The organic matter from 0.06% to 0.90% (average 0.55%), 33 m of content gradually decreases as the formation sand grayish-black mudstone with TOC from 0.64% to content increases. Total organic content (TOC) and 1.90% (average 1.14%), and 24 m black carbonaceous shale thickness change sharply, especially in those mudstone whose shale component is described below. shales in which TOC is more than 2% (Table 1). Wang Ro ranges from 3.14% to 3.38% with organic kero- S J et al. (2009) concluded that the hydrocarbon in gens of types I and II1 (Fu and Qing, 2008). Ordovician rocks of the Upper Yangtze Craton is in an In Well CX1, 153 m continuous black shale core over-mature stage because average vitrinite reflec- sample are recovered from lower Longmaxi Fm. The tance (Ro) was >2.5%, in the Luzhou-Yibin-Zigong TOC of Shale is from 1% to 3% (average 2%) in the area, average Ro was 1.8%; in the Fuling-Shizhu area, lowest 110 m. Black shale with high organic matter is it was 3.2% to 3.8%. concentrated near the base of the Longmaxi Fm We found organic-rich shale in the basal part of (110–153 m) and has TOC greater than 2% (average the Longmaxi Fm in wells JY1, DS1 and CX1. In JY1, 6.0%), (Ro) is more than 3.0% (average value 3.2%) the Longmaxi Fm consisted of grey argillaceous sand- (Wang S J et al., 2009).

Evaluation of Highly Thermally Mature Shale-Gas Reservoirs in Complex Structural Parts of the Sichuan Basin 867

Table 1 Parameters of Longmaxi shale in exploration wells

Well JY1 CX1① DS1 PY1 YY1② Parameters

33.9–80.3 21.2–29.8 2–18 34.3–57.6 26.3–54.2 Quartz (48) (25.3) (9.86) (44.2) (36.2) Mineral Carbonate 0–34.5 7–35.5 5–30 5–24.5 0–17.9 content rocks (11) (18.2) (9.85) (12) (6.2) (%) Clay min- 22.6–51.8 26.5–59.5 17.2–45.6 25–32.7 17.4–53.2 eral (33) (48.3) (25.6) (29.9) (37.7) Thick- Thickness 266 >153 148 405 >320 ness (m) TOC>2% 38 >80 34.5 23 >176 Average TOC>2 (%) 3.5 6 2.5 3.2 3.7 Ro (%) 2.2–3.06 2.81–3.11 3.14–3.38 2.3–2.8 1.62–2.26

Notes: (1) ① (Fu and Qing, 2008), Longmaxi Formation not drilled; (2) ② (Wang S J et al., 2009), real thickness not calculated from raw data and Longmaxi Formation not drilled; (3) brittle mineral does not include feldspar; (4) “( )” average values.

Figuer 4. Kerogen of JY1. (a) 2 339.33 m, organic matter showing as alga body and sapropel amorphous body; (b) 2 349.23 m, organic matter showing as alga body and sapropel amorphous body; (c) 2 349.23 m, organic matter showing as alga body and sapropel amorphous body. (a), (b), (c) amorphous kerogen including algal remains and sapropel.

Clay Mineral Components 0 100 The mineral component has a major influence on 10 90 JY1 the physical properties of mudstone and is associated 20 80 with fracturing. In Well JY1, clay minerals provide the 30 70 highest content ranging from 16.6% to 62.8% but 40 60 mostly between 35% and 45%; the brittle mineral 50 50 content is from 55% to 65% including quartz, potash 60 40 feldspar, plagioclase, calcite, dolomite, and little iron 70 30 pyrite and hematite. Quartz is 44.4% of the brittle 80 20 minerals, feldspar 8.3%, calcite 5.9%, dolomite 3.8%, 90 10 and accessory pyrite (Fig. 5). Silicified graptolites and 100 0 0 10 20 30 40 50 60 70 80 90 100 radiolarian fossils are found in the bottom of Long- Carbonate Quartz+feldspar+pyrite maxi Fm and may be one cause of its high organic matter content (Fig. 6). Figure 5. Mineral plot of shale in JY1.

868 Tonglou Guo

Porosity and Permeability of Shale cates that the bottom of Well JY1 has high gas content One hundred and fifty nine samples from Well ranged from 0.89 to 5.19 m3/t and average value is JY1 were examined. Porosity ranged from 1.17% to 2.96 m3/t (Figs. 10 and 11). 7.22%, average 4.52%. Permeability ranged from 0.001 5 to 81.35 md and average 0.32 md (Figs. 7 and FACTORS CONTROLLING SHALE-GAS AC- 8). This data is similar to the Barnett shale in U.S. CUMULATION (Chalmers et al., 2012; Loucks et al., 2009) which As previous mentioned, a complex tectonic evo- suggests that the Longmaxi Fm shale may have com- lution has caused difficulties for shale-gas exploration parable shale gas potential. especially by influencing tectonically controlled Porosity and permeability increased from the top shale-gas accumulation that must be taken into ac- to bottom with an obvious change at 2 377 m, where count before exploration. This topic will be discussed not only does TOC content become higher, but also in the following text. porosity and permeability become better with numer- ous pores in Longmaxi Fm shale. It has been further Importance of Structure Style confirmed that effective reservoir space is increased All shale of the Longmaxi Fm occurs in the with organic carbon loss after pyrolysis (Daniel et al, Yangtze Craton between the Xuefeng uplift and Qinl- 2007). SEM reveals nano pores in the size range 3.8 to ing-Dabieshan area; including the Sichuan Basin, 36 nm; and also some small dissolved pores and inter- Jianghan Basin and Subei Basin that all have complex granular pores (Fig. 9). Some net fractures are ob- tectonic evolution histories of folding, faulting, uplift served in the bottom of Well JY1, which may improve and erosion. Structural styles are indicated by anti- the shale permeability and promote the adsorption clines, synclines, monoclines and fault-blocks. Some capacity of shale. Furthermore, the in-situ test indi- geologists consider that the Sichuan Basin may have

Figure 6. Fossils photos of shale in JY1. (a) 2 389.31 m, graptolites in gray-black shale; (b) 2 389.31 m, ra- diolarians in carbonaceous mudstone; (c) 2 402.55 m, silica sponge spicules in carbonaceous mudstone.

80 60 64.8 50 60 44.7 40 33.3 40 34.6 30 22.0 20

Frequency (%) Frequency 20 Frequency (%) Frequency 10 0.6 0 0 0-2 2-5 >5 0.001-0.1 0.1-10 10-100 Porosity (%) Permeability (md) Figure 7. Porosity histogram of Longmaxi Fm in Figure 8. Permeability histogram of Longmaxi Fm Well JY1. in Well JY1.

Evaluation of Highly Thermally Mature Shale-Gas Reservoirs in Complex Structural Parts of the Sichuan Basin 869

Figure 9. SEM images of Longmaxi Fm Shale from Well JY1. (a) 2 381.91 m, organic pores; (b) 2 335.3 m, intergranular pores; (c) 2 340.82 m, intragranular dissolution pores.

2 378 in turn better than simple anticlines. Among monoc- 2 384 lines, a monocline with an overlying uniformity is

2 391 better than one in a continuous stratal sequence. Among all types of structures the presence of fault 2 397 blocks is bad for shale-gas preservation.

Depth (m) Depth 2 403 Structure style in the Sichuan Basin, from the in- 2 406 terior basin to the edge of the basin changes from typ-

2 412 ical ejective folds to trough-like folds. Key wells YY1 and PY1 are located from ejective fold to trough like 3 Gas content (g/cm ) folds transition zone. The Qiyueshan fault bounded, Figure 10. Gas content against burial depth in Well Well JY1 of interior basin is only exposed Permian. JY1. Silurian and Cambrian have been exposed in core of anticline outer of basin. Take well YY1 for instance, it 6 yx=.0 905 8 -. 0 336 2 is in the core of an asymmetrical anticline dips of 2 5 R =0.567 5 3 35°–68° in the NW limb and 22°–45° in the SE limb. 4 Middle and Lower Silurian strata was exposed on the 3 core of anticline, there were lots of fracture (Zhang et

2 al., 2010) (Fig. 12). This is classic shallow-level fo- reland deformation. A type area is the Zagros Moun- Gas content (g/cm content Gas ) 1 tains in Iran. The YY1 anticline appears to overlie a 0 0 1 2 3 4 5 6 thrust ramp, also characteristic. For being intense re- TOC (%) constructed, whether the anticline or syncline in outer Figure 11. Correlation between TOC and gas con- of basin, objective layer directly exposed on the sur- tent in Well JY1. face or outcropped in the monoclinic form preserved conditions were apparent damage (e.g., Well PY1). better accumulation conditions for shale-gas than the The good progress of shale gas exploration and Subei and Jianghan basins because of its stable tec- development is made in JY1 well area, Yang 101 well tonic background (SINOPEC internal information area (Fig. 1), the common feature of these two areas 2013). Anticlines provide better conditions for are located within the basin, has anticline background. shale-gas preservation than other structures. Among anticlines, simple anticlines are better than single fault Bedding Plane Slip within Reservoirs anticlines which are in turn better than double fault Flexural slip along bedding surfaces in the anticlines. Within larger synclines, double fault anti- Longmaxi Fm shale caused a rise in local porosity and clines are better than single fault anticlines which are permeability to values almost equal to that of the tight

870 Tonglou Guo

Evaluation of Highly Thermally Mature Shale-Gas Reservoirs in Complex Structural Parts of the Sichuan Basin 871

the Longmaxi Fm, which acted as a regional ductile layer or horizon of décollement. Strong stress trans- mitted from the Xuefeng Mountains also resulted in high angle fractures structure (Fig. 14) and these two movement periods have created many reticular cracks structures suitable for shale-gas accumulation.

Roofs and Floors of Shale-Gas Reservoirs Figure 13. Bedding parallel polished slip surface Favorable conditions in the roof and floor strata with slickensides in Longmaxi Fm shale (after Guo of shale reservoirs are very important for shale-gas and Liu, 2013; core diameter 10 cm). accumulation because they not only prevent gas from escaping, but provide bedding parallel fracture surfaces. The underlying stratum of the Longmaxi Fm is

the Jiancaogou Fm Q3j composed of tight carbonate 45–50 m thick. In Well DS1 this underlying stratum has porosity from 0.61% to 1.66% (average 1.01%) and permeability 0.005 8 to 0.109 2 md (average 0.020 1 md), characteristic of tight carbonate with ultra-low porosity and permeability. Regionally, It is widely distributed regionally and no water and gas Figure 14. Net fractures perpendicular to bedding have been discovered. The overlying stratum is mud- in Longmaxi Fm shale (after Guo and Liu, 2013; stone of the Upper Longmaxi Fm S1l which has low core diameter 10 cm). organic matter content, low porosity and permeability. Drilling mud escape occurred at seven levels in Well sandstone of the Xujiahe Fm. In Well JY1 it has JY1 (five in the Hanjiadian Fm and two in the Xiao- created high angle fractures with mirror smooth frac- heba Fm) none at all in the Longmaxi Fm (Fig. 15). ture surfaces, slickensides and steps (Fig. 13). Bed- ding plane slip has occurred since Indosinian times in

Underground structural zone Outcrop zone

x S1 : compact muddy siltstone l JY1 S1 : grey and black- grey mudstone

l S1 : shale gas strata j O3 : compact limestone

Muddy siltstone Limestone Mudstone Shale gas strata Strata boundary Fault Top boundary of quality shale

Figure 15. Roof S1l and floor Q3j of the shale-gas reservoir in the Longmaxi Fm.

872 Tonglou Guo

CONCLUSIONS and Material Distribution of the Upper Superimposed Favorable structural and preservation conditions Yangtze basin. Science Press, Beijing. 113–118 (in Chi- are the main controlling factors for shale-gas accumu- nese) lation in shale with high thermal maturity in complex Chen, S. B., Zhu, Y. M., Wang, H. Y., et al., 2011. Characteris- structural. Shale in the Longmaxi Formation of the tics and Significance of Mineral Compositions of Lower Sichuan Basin has favorable thickness, TOC, and brit- Silurian Longmaxi Formation Shale Gas Reservoir in the tle mineral content compared with shale from stable Southern Margin of Sichuan Basin. Acta Petrolei Sinica, tectonic environments in the USA, further work needs 32(5): 775–782 (in Chinese with English Abstract) to be done in future to bring the reservoirs into pro- Daniel, M. J., Ronald, J., Tim, E. R., et al., 2007. Unconven- duction. tional Shale Gas Systems: The Mississippian Barnett 1. Interaction between tectonic evolution, struc- Shale of North Central Texasas One Model for Thermo- tural style and high hydrocarbon maturity must be genic Shale Gas Assessment. AAPG Bulletin, 91: 475–499 investigated fully to assess potential targets in this Dong, D. Z., Cheng, K. M., Wang, S. Q., et al., 2009. An Eval- complex evolutionary setting. uation Method of Shale Gas Resource and Its Application 2. The roof and floor layers of shale gas reser- in the Sichuan Basin. Natural Gas Industry, 29(5): 33–39 voirs must be the first focus because they are very (in Chinese with English Abstract) important for shale-gas accumulation. Fu, X. D., Qing, J. Z., 2008. Evaluation on Excellent Marine 3. Although the thickness, TOC, and brittle min- Hydrocarbon Source Layers in Southeast Area of the Si- eral content are important for shale-gas exploration chuan Basin—An Example from Well D-1. Petroleum there are already a large number of studies have al- Geology and Experiment, 30(6): 621–628 (in Chinese with ready been carried out and these be a secondary focus English Abstract) of research. Guo, T. L., Liu, R. B., 2013. Implications from Marine Shale 4. Reservoir of complex tectonic zone and high Gas Exploration Breakthrough in Complicated Structural evolution degree shale strata has the characteristics of Area at High Thermal Stage: Taking Longmaxi Formation conventional gas reservoir, in addition to the hydro- in Well JY1 as an Example. Natural Gas Geoscience, carbon source rock, reservoir are same one. 24(4): 643–651 (in Chinese with English Abstract) Shale gas was brought into production in the US Hao, F., Zou, H. Y., 2013. Cause of Shale Gas Geochemical after a long tough effort. It has made a major break- Anomalies and Mechanisms for Gas Enrichment and through in shale gas in Jiaoshiba area. Because tec- Depletion in High-Maturity Shales. Marine and Petroleum tonic evolution, surface conditions and geographic Geology, 44: 1–12 environment are more complex in China we must not Hao, F., Zou, H. Y., Lu, Y. C., 2013. Mechanisms of Shale Gas expect shale-gas exploration to be successful quickly. Storage: Implications for Shale Gas Exploration China. We should also extend fundamental research, tech- AAPG Bulletin, 97(8): 1325–1346, nological development and environmental protection doi:10.1306/02141312091 to ensure that commercial shale-gas exploration and Huang, J. L., Zou, C. N., Li, J. Z., et al., 2012. Shale Gas Gen- development will be “green”. eration and Potential of the Lower Cambrian Qiongzhusi Formation in the Southern Sichuan Basin, China. Petro- REFERENCES CITED leum Exploration and Development, 39(1): 69–75 Chalmers, G. R., Bustin, M., Power, I. M., 2012. Characteriza- Jarvie, D. M., Hill, R. J., Ruble, T. E., et al., 2007. Unconven- tion of Gas Shale Pore Systems by Porosimetry, Pycno- tional Shale-Gas Systems: The Mississippian Barnett metry, Surface Area, and Field Mission Scanning Electron Shale of North-Central Texas as One Model for Thermo- Microscopy/Transmission Electron Microscopy Image genic Shale-Gas Assessment. AAPG Bulletin, 91(4): Analyses: Examples from the Barnett, Woodford, Haynes- 475–499 ville, Marcellus, and Doig Units. AAPG Bulletin, 96(6): Jia, C. Z., Zhang, M., Zhang, Y. F., 2012. Unconventional Hy- 1099–1119 drocarbon Resources in China and the Prospect of Explo- Chen, H. D., Guo, T. L., 2012. The Sedimentary Filling Process ration and Development. Petroleum Exploration and De-

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