Scholarly article on topic 'Discussion on the exploration & development prospect of shale gas in the Sichuan Basin'

Discussion on the exploration & development prospect of shale gas in the Sichuan Basin Academic research paper on "Earth and related environmental sciences"

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{"Sichuan Basin" / "Shale gas" / "Exploration and development" / "New progress" / "Enrichment condition" / Prospect / Challenge / "Annual gas output"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Dazhong Dong, Shikui Gao, Jinliang Huang, Quanzhong Guan, Shufang Wang, et al.

Abstract The Sichuan Basin, a hotspot and one of the most successful areas for shale gas exploration and development, can largely reflect and have a big say in the future prospect of shale gas in China. Through an overall review on the progress in shale gas exploration and development in the Sichuan Basin, we obtained the following findings: (1) the Sichuan Basin has experienced the marine and terrestrial depositional evolution, resulting in the deposition of three types of organic-matter-rich shales (i.e. marine, transitional, and terrestrial), and the occurrence of six sets of favorable shale gas enrichment strata (i.e. the Sinian Doushantuo Fm, the Cambrian Qiongzhusi Fm, the Ordovician Wufeng–Silurian Longmaxi Fm, the Permian Longtan Fm, the Triassic Xujiahe Fm, and the Jurassic Zhiliujing Fm); (2) the five key elements for shale gas accumulation in the Wufeng-Longmaxi Fm are deep-water shelf facies, greater thickness of organic-rich shales, moderate thermal evolution, abundant structural fractures, reservoir overpressure; and (3) the exploration and development of shale gas in this basin still confronts two major challenges, namely, uncertain sweet spots and potential prospect of shale gas, and the immature technologies in the development of shale gas resources at a depth of more than 3500 m. In conclusion, shale gas has been discovered in the Jurassic, Triassic and Cambrian, and preliminary industrial-scale gas has been produced in the Ordovician-Silurian Fm in the Sichuan Basin, indicating a promising prospect there; commercial shale gas can be produced there with an estimated annual gas output of 30–60 billion m3; and shale gas exploration and production experiences in this basin will provide valuable theoretical and technical support for commercial shale gas development in China.

Academic research paper on topic "Discussion on the exploration & development prospect of shale gas in the Sichuan Basin"

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Natural Gas Industry B 2 (2015) 9-23

Research article

Natural Gas Industry B

www.elsevier.com/locate/ngib

Discussion on the exploration & development prospect of shale gas

in the Sichuan Basin

Dong Dazhongab*, Gao Shikuic, Huang Jinlianga,b, Guan Quanzhonga,b, Wang Shufang

Wang Yuman

a PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China b National Energy Shale Gas R&D (Experiment) Center, Langfang, Hebei 065007, China c China University of Geosciences (Beijing), Beijing 100083, China

Abstract

The Sichuan Basin, a hotspot and one of the most successful areas for shale gas exploration and development, can largely reflect and have a big say in the future prospect of shale gas in China. Through an overall review on the progress in shale gas exploration and development in the Sichuan Basin, we obtained the following findings: (1) the Sichuan Basin has experienced the marine and terrestrial depositional evolution, resulting in the deposition of three types of organic-matter-rich shales (i.e. marine, transitional, and terrestrial), and the occurrence of six sets of favorable shale gas enrichment strata (i.e. the Sinian Doushantuo Fm, the Cambrian Qiongzhusi Fm, the Ordovician Wufeng—Silurian Longmaxi Fm, the Permian Longtan Fm, the Triassic Xujiahe Fm, and the Jurassic Zhiliujing Fm); (2) the five key elements for shale gas accumulation in the Wufeng-Longmaxi Fm are deep-water shelf facies, greater thickness of organic-rich shales, moderate thermal evolution, abundant structural fractures, reservoir overpressure; and (3) the exploration and development of shale gas in this basin still confronts two major challenges, namely, uncertain sweet spots and potential prospect of shale gas, and the immature technologies in the development of shale gas resources at a depth of more than 3500 m. In conclusion, shale gas has been discovered in the Jurassic, Triassic and Cambrian, and preliminary industrial-scale gas has been produced in the Ordovician-Silurian Fm in the Sichuan Basin, indicating a promising prospect there; commercial shale gas can be produced there with an estimated annual gas output of 30—60 billion m3; and shale gas exploration and production experiences in this basin will provide valuable theoretical and technical support for commercial shale gas development in China.

© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Sichuan Basin; Shale gas; Exploration and development; New progress; Enrichment condition; Prospect; Challenge; Annual gas output

1. Introduction

The exploration and development of shale gas has a long history of about 200 years since the first discovery of shale gas in North America in 1821. However, the rapid development has just been seen in the recent decade. According to EIA, in 2013, the shale gas production in North America was more than 350 x 109 m3 [1], with more than 320 x 109 m3 in the U.S, and approximately 30 x 109 m3 in Canada. China is also

* Corresponding author. E-mail address: ddz@petrochina.com.cn (Dong DZ). Peer review under responsibility of Sichuan Petroleum Administration.

abundant in shale gas resources, with a geologic resource volume of 134.4 x 1012 m3, and a technologically recoverable resource volume of 25.08 x 1012 m3, according to the prediction of the Ministry of Land and Resources in 2012 [2], indicating a broad development prospect of shale gas in China. To promote the exploration and development of shale gas in China, a pilot test of shale gas exploration and development was carried out first in the Sichuan Basin due to the two unique advantages of the Sichuan Basin — abundant organic-rich shales and great potential of shale gas resources [3,4].

The Sichuan Basin has always been the area with the greatest potential of natural gas in China, where there are both conventional gas fields such as Puguang gas field discovered in

http://dx.doi.org/10.1016/j.ngib.2015.02.002

2352-8540/© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

2003 and Anyue-Longwangmiao gas field discovered in 2012, and abundant unconventional natural gas resources such as commercial tight gas in eastern Sichuan and western Sichuan. Shale gas was discovered in the Qiongzhusi (Qz) Fm in Well Wei 5 in 1966, and then in the Wufeng-Longmaxi (WL) Fm in Well Yang 63 in the 1980s. After the introduction of the new concept of shale gas from North America in 2005, a strategic breakthrough of shale gas exploration and development was made in Well Wei 201 with the discovery of the Qz and WL Fms shale gas reservoirs, which marked a new historic beginning and brought about a craze of shale gas exploration and development in China. Four years later, the first large-scale shale gas field of 100 bcm was discovered in Jiaoshiba, Fuling, in 2014 [5] with a productivity of 5 x 109 m3 per year, marking a historic turning-point of commercial production of shale gas in the Sichuan Basin, with it comes a big boom of shale gas exploration and development in China presently [6-8].

Shale gas exploration and development in the Sichuan Basin will have a great influence to some degree on the future shale gas development in China. For speeding up the process of shale gas exploration and development in China, the sedimentary evolution and characteristics of organic-rich shales in the Sichuan Basin are analyzed, and the enrichment conditions and key control factors of marine shale gas are elucidated. The main challenges the exploration and development of shale gas confronts are also pointed out. And the development prospect of shale gas in the Sichuan Basin is predicted in the paper as well based on an overall review on the latest progress of shale gas exploration and development there. Hopefully, this paper

will provide valuable theoretical and technical support for commercial shale gas exploitation in China.

2. Geologic setting

The Sichuan Basin, covering Sichuan Province and Chongqing city, is bordered by Micang Mountain and Daba Mountain in the north, Daliang Mountain and Lou Mountain in the south, Longmen Mountain and Qionglai Mountain in the west, and Qiyao Mountain in the east, with a total area of approximately 190 x 103 km2 (Fig. 1). After over 6 decades of unremitting exploration and development, 115 gas fields have been discovered, and 110 gas fields have been developed in the Sichuan Basin. The production of natural gas there in 2013 was 24.3 x 109 m3, accounting for 20% of the total in China, demonstrating an indisputable place of the basin in natural gas production of China.

The Sichuan Basin is an important primary structural unit in the west of Yangtze platform and a large ancient superimposed sedimentary basin, with Presinian metamorphic and igneous rocks as its basement. Influenced by Tethyan and Pacific structural domains, this basin has experienced two stages of sedimentary evolution: Sinian-Middle Triassic cratonic platform deposition, and Late Triassic-Cenozoic foreland basin deposition. Before Indo-China movement, as a part of Yangtze palaeo-ocean basin, the Sichuan Basin was dominantly affected by the development of Yangtze platform, abundant organic-rich shales deposited in half-deep to deep water shelf facies during Sinian, Cambrian, Ordovician, and Silurian. Influenced by Caledonian and Hercynian movements,

Fig. 1. Tectonic position and stratigraphic structure of the Sichuan Basin.

some shale layers were denuded in local areas including the basin margin and Leshan-Longnusi palaeohigh. Moreover, the uplifting in Sichuan Basin-northern Guizhou during Devonian and Carboniferous led to the absence of deposits of this period in a large area. After the early Indo-China movement, the Sichuan Basin turned into the sedimentary evolution stage of large inland depression-foreland basin, during which lacustrine and limnetic shales deposited. Then the basin completely experienced overall folded inversion during the Himalayan movement, forming the fold and fault system largely in southeast direction.

The total thickness of sedimentary rocks in the Sichuan Basin ranges from 7000 m to 12000 m, in which the marine Sinian-Middle Triassic sedimentary rocks are 4000-7000 m thick; the Upper Triassic-Quaternary continental sedimentary rocks are 3000-5000 m thick [9-14]. The organic-rich shales in the Sichuan Basin are abundant, with six sets of regional organic-rich shale from bottom to top: the Upper Sinian Doushantuo Fm, the Lower Cambrian Qiongzhusi Fm, the Upper Ordovician Wufeng-Lower Silurian Longmaxi Fm, the Upper Permian Longtan Fm, the Upper Triassic Xujiahe Fm, and the Lower Jurassic Ziliujing Fm (Shaximiao Fm) (Fig. 1, Table 1). Among them, the Doushantuo Fm is 15-120 m in thickness, in which shale with TOC of over 2% is 10-70 m thick. The Qz Fm is 400-600 m thick in the south and southeast of the basin, and shale with TOC of more than 2% in it is 60-150 m thick. The WL Fm is 300-600 m thick in the south, northeast and north of the basin, in which shale with TOC of over 2% is 80-120 m thick. The Longtan Fm, composed of transitional coal-measure carbonaceous shale, ranging 20-125 m in thickness, contains 20-52 m thick shale with TOC of over 2%. The Xujiahe Fm limnetic coal-measure shale, 100e800 m thick, includes 25e60 m thick shale with TOC of over 2%. The Ziliujing Fm half-deep to deep lacustrine facies shale, ranges from 40 to 180 m in the central, northern, and eastern Sichuan Basin, in which shale with TOC of over 2% is 20e40 m thick. It is concluded from a number of studies that the six sets of organic-rich shale, big in thickness, stable in regional distribution, high in TOC and maturity (Ro > 1.0%), and mainly gas-generating, all have huge potential of shale gas resources. Among them, the WL Fm is the most practical system for the exploration and development of shale gas.

3. Latest progress of shale gas exploration and development in the Sichuan Basin

The Sichuan Basin, a pilot demo base of shale gas exploration and development in China [3,4], has experienced about 10 years and six stages of development with an earlier start than the rest place of China (Fig. 2). Learning from the successful experiences of shale gas exploration and development in the U.S, Cheng Keming, Dong Dazhong, and Li Xinjing et al. conducted the evaluation of enrichment conditions and resource prospect of shale gas in the Sichuan Basin in 2005, starting with the Paleozoic marine shales in Weiyuan area [15 -19]. In 2010, a breakthrough was first made in the WL and

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Fig. 2. Progress of shale gas exploration and development in the Sichuan Basin.

Qz Fms in an appraisal well — Wei 210. In 2014, scale productivity has been constructed in blocks such as Changning-Weiyuan and Jiaoshiba, realizing commercial production of shale gas. To date, shale gas has been discovered successively in Paleozoic marine shales in the blocks including Changning, Fushun-Yongchuan, North Zhaotong, Jiaoshiba, and Jianwei, and also in Mesozoic limnetic coal-measure shales and lacustrine shales in the blocks including Fuling, Jiannan, Yuanba, and Xinchang by leading petroleum enterprises CNPC and Sinopec and some local enterprises like Sichuan Energy Industry Investment Group and Chongqing Energy Investment Group etc. Four pilot test areas for marine shale gas exploration and development have been established: Changning-Weiyuan, Fushun-Yongchuan, Jiaoshiba, and North Zhaotong, with a preliminary productivity of 2.5 x 109 m3 per year and an estimated shale gas production being 1.2 x 109 in 2014. Both research and practice have proved that the Sichuan Basin, with abundant shale gas which can reach commercial production, has better prospect in shale gas exploitation than the rest place of China, making it the most favorable and most important area of shale gas exploration and development in China.

3.1. Commercial development of marine shale gas in the Lower Paleozoic WL Fm

The WL Fm shales, in a mainly half-deep to deep water shelf environment, with wide distribution, big thickness, high TOC, high thermal evolution (Ro), high brittleness, and well-developed pores and fractures, have superior shale gas formation and enrichment conditions. The WL Fm, 300—600 m thick, consists of three lithologic members from top to bottom (Fig. 3): the upper member, also the third member of the

Longmaxi Fm, consisting of varicolored grapholiths inter-bedded with thin carbonate rocks, 120—400 m thick and low in organic abundance (TOC < 1%), acts as the major cap rock; the middle member, also the second member of the Longmaxi Fm, consisting mainly of silty shales interbeded with thin very-fine sandstones and siltstones, 40—100 m thick and higher in organic abundance (TOC of 1%—2%), is the potential shale gas member; the lower member, also the first member of the WL Fm, consisting of black carbonaceous, siliceous, calcareous, and graptolite-rich shales, 80—120 m thick and very high in organic abundance (average TOC > 3%), is premium shale gas layer. The bottom of the first member of the Longmaxi Fm, composed mainly of siliceous organism-rich and calcareous shales, 30—50 m thick, with a porosity of 4.0%—6.0%, TOC of over 3%—6%, gas content of 4—7 m3/t, and pressure coefficient of 1.2—2.0, is comparative with the premium shale gas formations in North America in geologic characteristics, and the major shale gas pay in the WL Fm (Table 2) [2,11,13,20—23].

In 2010, the first shale gas well in Block Weiyuan, Wei 201, tapped a gas flow of (3—17) x 103 m3 a day in the WL Fm, which is a strategic breakthrough of shale gas exploration and development in China. In 2011, the first horizontal shale gas well in Block Changning, Ning 201-H1, and the horizontal well in Block Fushun-Yongchuan, Yang 201-H2, yielded a daily production of 150 x 103 m3 and 430 x 103 m3 respectively in the WL Fm, marking the commercial breakthrough of shale gas exploration and development in China. In 2012, four wells in Block Jiaoshiba, JY1HF to JY4HF, obtained high gas flow successively in the WL Fm, with gas production of (203—363) x 103 m3 per day and stable daily production of (55—300) x 103 m3 in production test, setting off a new craze

Fig. 3. Strata histogram of the WL Fm.

Table 2

Comparison of geologic characteristics of the shale gas layers in the WL Fm and North America.

Basins and blocks Formations Burial Area of Recoverable Depth/m Geochemical parameters Physical property Gas Brittleness parameters

depth/m favorable zone/km2 resource/108m3 parameters content (m3/t)

TOC/% RJ% Type of organic Porosity/% Permeability/nD Mineral componentsa Poisson Young's

matter ratio modulus/ 104 MPa

Weiyuan Upper Ordovician Wufeng-Lower Silurian Longmaxi 1300-3700 2800 2500 45 2.40 2.70 Sapropel, Sapropel-mixed 5.3 42 2.92 66.4%/33.6% 0.18-0.21 1.33-2.1

Fushun-Yongchuan Upper Ordovician Wufeng-Lower Silurian Longmaxi 3200-4500 3500 5100 80 3.80 3.00 Sapropel, Sapropel-mixed 4.2 233 3.5 61.3%/38.7% 0.23-0.28 2.3-3.1

Changning Upper Ordovician Wufeng-Lower Silurian Longmaxi 2000-4500 4300 5500 60 3.45 2.75 Sapropel, Sapropel-mixed 5.4 290 4.1 69.5%/30.5% 0.18-0.25 2.07-2.5

Zhaotong Upper Ordovician Wufeng-Lower Silurian Longmaxi 900-2200 1500 1100 38 3.20 2.95 Sapropel, Sapropel-mixed 5.0 1900 2.3 68.0%/32.0% 0.19-0.22 1.07-2.69

Jiaoshiba Upper Ordovician Wufeng-Lower Silurian Longmaxi 2100-3500 545 890 40 3.50 2.60 Sapropel, Sapropel-mixed 6.2 348 6.1 67.0%/31.4% 0.20-0.30 2.5-4.0

Coastal Basin of the Cretaceous Eagle 1220-4270 3000 5900 61 2.76 1.20 Sapropel-mixed 9.0 1000 2.8-5.7 45%-65%/35%-55% 0.2-0.3 1.3-3.5

Gulf of Mexico Ford

Fort Worth Basin Carboniferous Barnett 1980-2590 13000 12461 90 3.74 1.60 Sapropel-mixed 5.0 50 8.5-9.9 40%-60%/40%-60% 0.12-0.22 1.37-2.12

Salt Basin Jurassic Haynesville 3200-4200 23000 71083 80 3.01 1.50 Sapropel, sapropel-mixed 8.3 350 2.8-9.3 35%-65%/35%-65% 0.24 1.4-3.5

Arkoma Basin Carboniferous Fayetteville 305-2134 23000 11781 40 3.77 2.50 Sapropel, sapropel-mixed 6.0 50 1.7-2.6 40%-70%/30%-60% 0.23 1.4-3.2

Arkoma Basin Devonian Woodford 1829-3353 29000 3228 48 5.34 1.50 Sapropel, sapropel-mixed 5.0 50 5.6-8.5 50%-75%/25%-50% 0.1-0.25 1.2-2.4

West Canada Basin Triassic Montney 900-2740 142000 13875 105 2.79 1.50 Sapropel, sapropel-mixed 5.0 30 1.1-3.2 45%-70%/30%-55% 0.1-0.23 2.4-3.8

The number before "/" is the clay mineral content; the number after "/" is the brittle mineral content.

Fig. 4. Two accumulation modes of shale gas in the WL Fm.

of shale gas exploration and development in the Sichuan Basin. Several shale gas-producing areas including Changn-ing-Weiyuan, Zhaotong, Fushun-Yongchuan, and Jiaoshiba have been discovered successively since Paleozoic marine shale gas was targeted in 2009. By July 2014, 10515 km of 2D seismic survey and 1345.5 km2 of 3D seismic survey had been finished in the Sichuan Basin; two enrichment models of shale gas in the WL Fm had been discovered: structural slope-fracture model and anticline-fracture model (Fig. 4); favorable shale gas exploration and development play of (20-25) x 103 km2 and a recoverable resource volume of 3.38 tcm had been confirmed, as a result, the productivity construction goal of 7.5 bcm per year was set up. 142 wells had been drilled (125 horizontal wells), in which 105 had been fractured and tapped gas flow, and 60 had been brought on stream. Initial production of individual horizontal wells was (10-547) x 103 m3/d in formation testing, and the daily shale gas production of all horizontal wells was 4350 x 103 m3 in production test. The three classes of shale gas reserves in total were estimated at over 500 bcm, in which proved geologic reserves were 106.75 bcm. The shale-gas productivity of 2.5 bcm per year had been constructed primarily, and 0.7 bcm shale gas had been produced cumulatively, realizing the preliminary commercial scale production of shale gas.

3.2. Major discoveries in the Lower Paleozoic Qiongzhusi Fm

The Qz Fm, similar to the WL Fm, is another layer of black shale deposited mainly in half-deep to deep water shelf environment, with a thickness from 200 m to 600 m, in which shale with TOC of over 2% is 60-150 m thick. The bottom comprises siliceous shale and carbonaceous shale interlayers, with phosphorus minerals commonly seen; the middle is composed mainly of carbonaceous shale interbeded with dolomite and sandy dolomite; the top consists of dark gray shale, gray green shale, silty shale interbedded with siltstone strips. Wide in distribution, big in thickness of organic-rich concentrated section, and high in brittleness, the Qz Fm is an important layer series with higher potential of marine shale gas [12-14,17,24].

In 1966, the Qz Fm shale gas was first discovered in Well Wei 5, which produced (23.5-24.6) x 103 m3 of shale gas per day without any stimulation. According to statistics, there had

been 107 conventional hydrocarbon wells encountering the Qz Fm shale in the Sichuan Basin before the exploration of shale gas in 2009, and 41 of them got good gas shows. Since the beginning of shale gas exploration, four shale gas wells targeting the Qz Fm have been finished (three straight wells, one horizontal well) in the Sichuan Basin, and three of them have got commercial gas flow, signaling major shale gas discoveries. Wells Wei 201, Jinshi 1 and Wei 201-H3, produced shale gas of 10.8 x 103 m3, 25.0 x 103 m3, and 28.3 x 103 m3 a day respectively, whereas Well Ning 206 produced no gas but only water. Approximately 20 wells targeting the Qz Fm have been drilled outside the Sichuan Basin, and only less than 40% of them have got gas show or low rate gas flow, and the rest wells yield no gas or only water. The study shows that the formation and enrichment conditions of shale gas in the Qz Fm are quite different from those of the WL Fm. The Qz Fm shales are characterized by low content of biogenic siliceous and calcareous materials, old age (about 0.57 billion years B.P), high thermal evolution degree (Ro of 2.5-5.0%), strong tectonic reformation, strong carbonization of organic matters, poor physical properties of reservoir (4 of 1.5-4.2%, averaging at 2.4%), and low gas content (0.5-4.0 m3/t, averaging at 1.29 m3/t) [24,25].

3.3. Good signs of Triassic-Jurassic continental shale gas

Besides the major breakthrough in marine shale gas in the Sichuan Basin, good signs have also been found in continental shales. According to a preliminary evaluation, the Upper Triassic Xujiahe Fm and the Lower Jurassic Ziliujing Fm have high shale gas potential. Xujiahe Formation, shown as a complete marine-transitional- continental depositional cycle from bottom to top [26,27], is a set of transitional-limnetic coal-measure shales in wide distribution. The shales occur in the first, third, and fifth members of the Xujiahe Fm, with the 5th member being the major member [28], and the shales are composed of laminated black shale and siltstone, thin coal seam or coal vein. The Xujiahe Fm shales are 100-800 m thick in total, in which the shales in the first member, occurring mainly in the middle-south of the Western Sichuan Depression, are 10-300 m thick, with the maximum single layer thickness of 60 m; the shales in the third member of the Xujiahe Fm, occurring mainly in the middle of the Western

Table 3

Statistics of well-testing initial production of continental shale gas in the Sichuan Basin.

Play Serial number Well name Gas reservoir Test production gas (x104 m3/d)

Yuanba 1 Yuanba 101 Da'anzhai Member 13.97

2 Yuanba 102e1 Da'anzhai Member 23.78

3 Yuanba 11 Da'anzhai Member 14.44

4 Yuanba 21 Da'anzhai Member 50.7

5 Yuanba 5e1 Da'anzhai Member 4.227

6 Yuanba 9 Da'anzhai Member 1.15

7 Yuanye HF-1 Qianfoya Member Gas:0.715/oil:14 m3

8 Shiping 2-H Da'anzhai Member Oil:33.79t

Xinchang 9 Xinye HF-1 Xujiahe 0.3e0.5

10 Xinye HF-2 Xujiahe 1.5 e 4.0

Xinglongchang 11 Xinglong 101 Da'anzhai Member 11.01

Fuling 12 Fuye HF-1 Da'anzhai Member 1.8

13 Fuye 6-2HF Da'anzhai Member 2.6315

14 Fushi 1 Da'anzhai Member Gas:12.66/oil:68 t

Jiannan 15 Jian 111 Dongyuemiao Member 0.4

16 Jainye HF-1 Dongyuemiao Member 1.23

17 Jianye HF-2 Dongyuemiao Member 0.266

Sichuan Depression, are 10—200 m thick, with a maximum single-layer thickness of 50 m; the shales in the fifth member of the Xujiahe Fm, occurring mainly in the middle-south of the Western Sichuan Depression-central Sichuan, are 20e200 m thick, with a maximum single-layer thickness of 50 m. The Xujiahe Fm shales have a TOC of 1.2%—5.0% (the fifth member of the Xujiahe Fm has the highest TOC, with an average of 2.35% and a maximum of 16.33%), mainly type III kerogen, maturity to high maturity (Ro of 1.06%—2.40%, averaging at 1.43%), brittle mineral content of 41.3%—80.2%, averaging at 51.67%, and tested gas content of 1.18—3.77 m3/ t, averaging at 2.55 m3/t.

The Lower Jurassic Ziliujing Fm, experiencing three stages of sedimentary evolution: early lacustrine transgression, middle maximum lacustrine transgression, and late lacustrine regression, is a set of inland lake basin sedimentary rocks mainly composed of lacustrine shales interbedded with silt-stones and plentiful limestones and coquinoid limestones. The Ziliujing Fm can be divided into four lithologic sections from bottom to top: Zhenzhuchong, Dongyuemiao, Maanshan, and Da'anzhai. The shales were deposited in half-deep to deep lacustrine environment, in Daanzhai, Dongyuemiao, and Zhenzhuchong members [27]. With a total thickness of 20e240 m, the shales are thicker in Langzhong-Xuanhan-Wanzhou area (generally more 100 m), and thinner in the southwestern and western Sichuan Basin (20—50 m), and 80—100 m thick in Changshou of the southeastern Sichuan Basin. The Ziliujing Fm shales are 0.2%—2.4% in TOC value (0.58%e3.81% in Daanzhai member, 1.44% on average; 0.94—2.8% in Dongyuemiao, 1.87% on average), dominated by type III kerogen and in mature to high-mature stage (Ro of 0.9%—1.6%, 1.3% on average), 30%—54% in brittle mineral content, 41.53% on average, and 0.27e5.9 m3/t in tested gas content, 2.23 m3/t on average.

From 2010 to 2012, approximately 20 wells were drilled in Jiannan, Fuling, Xinchang, and Yuanba blocks by Sinopec, aiming at the continental shale of the Xujiahe and Ziliujing

Fms (Table 3). The initial test production reached (2.6—507) x 103 m3/d after large volumetric fracturing, thus confirming the existence of continental shale gas and the effectiveness of fracturing stimulation in getting productive gas-flow. Unluckily, the production test in Jiannan and Xinchang blocks witnessed short stable production period, fast production decline of shale gas, and water production, to date, the continental shale gas hasn't reached commercial productivity.

3.4. Prelimilary formation of shale gas E&D technologies and management systems

After nearly a decade of exploration and development practices, a set of matching technologies and management systems suitable for the geologic characteristics of the shale gas in the Sichuan Basin and shale gas reservoirs with a depth of less than 3500 m has taken shape (Table 4). The matching exploration and development technologies include evaluation methods for premium shale plays (gas pay), optimum & fast drilling technologies of horizontal wells, large volumetric fracturing technology, and "factory" operation model. In Jiaoshiba and Changning blocks, one-trip drilling of 1500e2000 m horizontal section, and 10e15 stages of volumetric fracturing (26 stages at maximum) have been realized, the "factory-like" staggering operation model of drilling, completion, fracturing, and production on a well pad with 4—8 horizontal wells has been established, resulting in the reduction of single well drilling time from 156 days to about 50 days, and the minimum drilling time of 36 days.

In terms of effective management system, an effective organization and integrated organization & management model have been established. A thorough and orderly running schedule and operating manual centering on "exploration, production, cost, safety, environmental protection" of shale gas have been formulated, thus realizing the availability of specific rules and normalized running of production process. Operation organization and internal competition are

Table 4

Summary of matching technologies and management systems for shale gas E&D.

Technologies/management systems

Main technologies/management connotations

Matching Technologies for E&D

Evaluation methods for favorable shales (gas pay)

Optimum & fast drilling technologies for horizontal wells

Large volume-fracturing stimulation technologies

"Factory-like" operation model

Effective organization & management system

Geologic evaluation method and standard; Resource evaluation method and standard; Zone evaluation method and standard; Key laboratory for experimental test. Compound drilling of air drilling, foam drilling and wash boring; Well trajectory control of horizontal wells; Oil-base mud + PDC horizontal one-trip drilling. Optimization design for large volume-fracturing stimulation; Large fluid volume, low proppant concentration, mixed-fracturing model; Long horizontal multistage volume-fracturing with drillable bridge plug; Clustering perforation devices, fracturing fluid system of retrievable slick water; Micro-seismic monitoring including earth surface, shallow wells and deep wells.

Standardized design for platform drilling of 4-8 horizontal wells; Cross-operation model of "drilling, fracturing, production".

Establishment of "exploration, production, working-site, cost, safety, environmental protection, disciplines" regulations and systems, specific rules and standardized running; Win-win relationship of enterprise-local government: local government providing great support in the use of land and water, and enterprise offering job opportunities and promoting local economic development.

standardized to improve services and ensure the implementation of technical measures etc; scientific & research and designing institutes are directly engaged in the engineering design and working-site problem-resolving. A win-win enterprise-local government relationship has been established, on one hand, local governments provide great support in the use of land and water, on the other hand, enterprises offer job opportunities and promote local economic development.

4. Understanding of enrichment conditions of high-mature marine shale gas

The study suggests that four major factors affect the formation and enrichment of the shale gas in the WL Fm: depositional environment, lithofacies combination, thermal evolution degree, and structural preservation.

4.1. Stable distribution of organic-rich shale of marine half-deep-deep water continental shelf, a favorable facies belt for the formation of shale gas in the WL Fm

Large-scale rich organic shale (large in continuous thickness and distribution area) is the important material basis for the formation and enrichment of shale gas. The formation of organic-rich shale needs two conditions: (1) abundant living organisms in water to provide sufficient organic matters for shale; and (2) quiet and anoxic water with sufficient sediments to provide good environment for the effective preservation of organic matter. Marine half-deep to deep water continental shelf with great water depth, poor water circulation, and likely dysaerobic or anoxic conditions in the bottom water, is the place favorable for the deposition of organic-rich shale.

From the end of Ordovician to the beginning of Silurian, in the background of worldwide continuous sea-level rise, the area where Yangtze Plate situated witnessed universal transgression. Affected by three palaeohighs - Middle Sichuan Uplift, Middle Guizhou Uplift, and Xuefeng Uplift in the

Upper Yangtze Cratonic platform, there formed widespread, low-energy, undercompensational, and anoxic marine half-deep to deep water shelf environment in and around the Sichuan Basin, to be specific, in southern Sichuan-northern Guizhou and eastern Sichuan-western Hubei. A large set of monolithologic, fine-grained, thick, organic-rich, and silicon/ calcite-rich black shale deposited in this environment (Fig. 5). As was mentioned above, the organic-rich shale of the WL Fm concentrate in its bottom, with a TOC of over 2%, large continuous thickness (generally 20-100 m), and stable lateral distribution. According to the drilling statistics, the shales in the concentration section of Fushun-Yongchuan area are 40-100 m thick, Weiyuan area 30-40 m thick, Changning area 30-60 m thick, and Fuling area 38-45 m thick [9,21-29].

4.2. The organic matter with high organic abundance and good kerogen-type generated gas by thermal cracking, providing important source gas for the Wufeng-Longmaxi gas reservoir

The major shale gas pays in the WL Fm are high in organic carbon content. Increasing from top to bottom, the TOC of the overall WL Fm is more than 2%, generally from 2.5% to 4.0%, and up to 8.6%. Weiyuan tectonic area has a TOC of 2.7-3.0%, Changning 3.1-4.0%, and Jiaoshiba 3.2-3.8%. All areas have good sapropel-mixed type of kerogen. The organic matter is moderate in thermal evolution degree, with Ro ranging from 2.1% to 3.6%, generally less than 3%, representing the stage of effective gas generation from thermal cracking of high-mature crude. Well drilling has proved that the WL Fm across the whole area is gas-bearing, and gas accumulation forms in large areas. Compared with the WL Fm shales, the Qz Fm shales are low in gas content despite high TOC, generally less than 2.0 m3/t (Fig. 6), and its initial test production is only (1.0-2.8) x 104 m3/d. A large quantity of statistics suggest that the thermal evolution (Ro > 3.0%) degree of the Qz Fm shale is too high, resulting in the carbonization

Fig. 5. The early stage (SQ2) sedimentary facies of the WL Fm.

of shale reservoir, decrease of organic porosity [24,25], low gas content, and low productivity.

4.3. Rich silicic and calcific shales with well-developed matrix pores and fractures act as favorable reservoirs in the WL Fm

Siliceous and calcareous shales are the most favorable petrologic facies for shale gas reservoirs. The major gas pays of the WL Fm are composed dominantly of siliceous and calcareous shales abundant in microfossils such as radiolarian and calthrop, etc. The siliceous and calcareous shale sections are biologic and biochemical origins [30], and high silicon and high calcium content are conducive to the formation of matrix pores and fractures. The storage space in the WL Fm shales is made up of matrix pores and fractures. Matrix pores include

Fig. 6. Comparison of gas-bearing characteristics of the WL Fm and the Qz Fm.

intercrystalline pores and organic nanopores of clay minerals, intergranular pores and intragranular dissolution pores of clastic particles, with the diameter ranging from 5 nm to 200 nm generally. Intercrystalline pores and organic nanopores of clay minerals are the main types of storage space for shale gas.

Under various structural settings, there developed abundant fractured storage space in the WL shales, including lamella-tional, structural and joint fissures, which are likely to form network fracture systems in structural folding regions. The existence of a large quantity of natural fractures not only provides sufficient storage space for shale gas, but also decreases the fracture initiation pressure in shale reservoir stimulation, making it easier to form artificial fracture network and increasing the artificial fracture bulk volume. The WL Fm shale reservoirs feature good physical properties, with a porosity ranging from 2.78% to 7.08%, averaging at 4.65%, and permeability ranging from 42 nD to 1900 nD, averaging at 412.79 nD, meeting the porosity and permeability standard of premium shale reservoirs.

4.4. Stable structures, good preservation conditions, and formation overpressure contribute to the enrichment and high production of the Wufeng-Longmaxi shale gas

Compared with North America, the Sichuan Basin underwent multiple phases of complicated tectonic movements, so

Table 5

Relationship between formation pressure and shale gas production.

Structural location Pressure Test production

coefficient (x104 m3/d)

Areas with normal pressure inside and 0.85-1.2 <2.50

outside the basin

Overpressure areas inside the basin 1.2-1.5 2.50-7.0

Areas with abnormal high pressure >1.5 7.0-55.0

in the basin

the shale formation suffered various degrees of damage [31,32]. Therefore, the shale gas targets should be anti-clinorium (synclinorium) with relatively stable structures, where the shale formation is in a large area, and free of damages by faults and foldings. The principal shale gas producing area in Changning of southern Sichuan is located in the southwestern wing of Changning Anticline in the southern Sichuan low-steep structural belt, and is actually a gentle syncline under the anticlinal structural background, far away from faults, especially Tongtian Fault. The WL Fm, characterized by gentle occurrence, absence of large-scale fault belts, and relatively good preservation conditions, is conducive to the formation of shale gas core area. At present, the area is the key exploration and development area of marine shale gas in the southern Sichuan Basin, where the average daily production of single horizontal well reaches 100 x 103 m3/d. In contrast, the gas-bearing property of shales outside the basin is commonly poor. The gas content of Well Zhao 101 in Zhao-tong, Yunnan, is only 0.17-0.51 m3/t, with an average of 0.33 m3/t, and no commercial hydrocarbon-flow obtained.

Shale gas wells of high production in both North America and China have abnormal high pressure. Formation overpressure is an important indication of good preservation conditions of shale reservoirs. Shale gas production of single well is positively correlated with pressure coefficient obviously, because high formation pressure is mostly the result of hydrocarbon generation of organic matters. Formation overpressure can lead to the continuous hydrocarbon expulsion, good gas-bearing property, and high production (Table 5).

Exploration practices prove that the formation pressure coefficient of the WL Fm in the Sichuan Basin exceeds 1.2,

suggesting common overpressure. The gas content of shale members is higher than 4 m3/t generally, 4.1 m3/t in Changning area, and 6.1 m3/t in Fuling area. The gas content of the Longmaxi Fm is commonly higher than that of the Qz Fm. Detailed analysis shows the shale gas pay of the Longmaxi Fm is overlain by very thick grapholiths with good plasticity, and underlain by the Baota limestone with high mud content and good stability. Fractures are not developed in both the overlying and underlying formations, resulting in strong self-sealing ability and the formation of overpressure shale gas layers. By contrast, the Qz Fm is overlain by fractured area-ceous shales and limestone, and underlain by weathered dolomitic aquifer with active hydrodynamics, resulting in serious dissipation of gas, and low gas content (Fig. 7).

5. Main challenges

Though major breakthroughs have been made in geologic understanding, evaluation methods, E&D technologies of shale gas, there is still a long way to go to achieve effective scale development of shale gas in the Sichuan Basin. There are a series of challenges ahead, including deepening geologic understanding, improving technical level, strengthening environmental protection and reducing development cost.

5.1. Unclear sweet spots of shale gas resources

Among the factors contributing to the success of shale gas in North America, "geologic" and "resource" factors are the paramount. Only drilling in the "core area" with comprehensive advantages of high resource quantity, good reservoir quality, and good drilling and completion conditions, can give high single well production, high EUR, and high economic benefit. The exploration of shale gas is not necessarily successful because its anisotropy is stronger than any other oil and gas resources. There is high uncertainty in predicting the results of an area or a well with that of another area or well. The "sweet spots" of shale gas resources can only be found out after geologic prediction and drilling evaluation. The geologic setting of China as a whole is quite complicated, especially in the south. The marine shales in the south are characterized by

Fig. 7. Shale gas preservation conditions of the WL Fm and the Qz Fm.

wide distribution, early formation, high thermal evolution, multiple tectonic changes and wide variation of burial depth, leading to complex formation and enrichment patterns of shale gas. The enrichment conditions of the Paleozoic Marine shale gas revealed now are mainly based on the results of the WL Fm in the Changning-Weiyuan and Jiaoshiba pilot areas in the Sichuan Basin, their universality needs to be verified by further exploration practice. The formation and enrichment conditions of shale gas and distribution patterns of "sweet spots" in other areas and systems remain unclear and further exploration and development practices are needed to provide more information and evidence.

5.1.1. Uncertain enrichment conditions and high resource risk in the Qz Fm

Although commercial gas flow has been obtained in the Qz Fm in several wells, productivity has not been constructed yet, resulting in an increasing risk in its exploration and development. The comparative analysis suggests that the organic pores of the Qz Fm are not as developed as those of the WL Fm, which is likely the results of excessive evolution degree of organisms or differences of organic types [32—34]. More importantly, the preservation condition of the Qz Fm may be the fatal weakness for its enrichment of shale gas. Despite the lower requirement on preservation conditions for shale gas enrichment than for conventional hydrocarbon enrichment, it's still impossible for shale gas to accumulate in poor or even no preservation condition [23] (Figs. 4 and 7, Table 6). The shale gas resource of the Qz Fm as a whole is high in risk, and "sweet spots" can only exist in places where the preservation conditions are fairly good.

5.1.2. Unclear resource potential and prospects of the transitional-continental shale gas

Shale gas has been discovered in the Xujiahe and Ziliujing Fms, but no productivity breakthrough has been made yet. Formation and enrichment conditions, distribution pattern of "sweet spots", and resource potential of this kind of shale gas still remain unclear [35]. The study suggests that despite wide

distribution, the continental organic-rich shales are thin in concentrated section and poor in lateral continuity, containing mainly terrigenous organisms, lower in thermal maturity; so oil and gas are paragenetic in most areas, while gas mainly occurs in deeper-burial areas or the center of lake basin. In addition, the resource potential of continental shale gas needs to be further examined because of its high content of clay minerals, lower diagenetic degree and undeveloped matrix pores and fractures.

5.2. Immature exploration and development technologies for shale gas at a depth of more than 3500 m

The shale gas resources of the Sichuan Basin are abundant, accounting for 65% of the total in Southern China. Shale gas resources at a depth of more than 3500 m account for more than 50% [11,12]. Researches and practices suggest that the deep structures in the Sichuan Basin are more complicated, in particular, the complexity of the deep ground stress has a great influence on the success rate of drilling and completion of shale gas horizontal wells and the effect of reservoir volume fracturing, and the effect of ground stress increases with depth. Major breakthroughs need to be made in exploration and development technologies for shale gas at a depth of more than 3500 m. According to the statistics of available drilling data, shale gas wells of more than 3500 m deep can't form effective commercial gas flow because of their low production and short time of stable production (Table 7). There are four major technological difficulties in the development of deep shale gas resources at a depth of more than 3500 m: (1) big burial depth of target shales, complex structures leading to high accident rate of horizontal drilling, and difficulty in well path control; (2) high formation pressure, great difficulty in reservoir volume fracturing, and poorer effect of stimulation; (3) poorer reservoir quality, lower single well production, lower EUR, and high cost of exploration and development; (4) high requirements on the quality of matching devices and tools for high-temperature and high-pressure conditions.

Table 6

Comparison of geologic characteristics between the WL Fm and the Qz Fm shales.

Key parameters Qz Fm WL Fm

History (Ma) 541—529 445.2—438.5

TOC (%) 1.0—5.0 1.1e6.3

Ro (%) 2.5—5.0 2.0e3.0

Porosity (%) 1.0—3.0 3.0e8.0

Pore diameter (nm) 10—20 80e90

Surface area to volume ratio (m2/g) 2—8 10—35

Gas content (m3/t) 0.3—1.3 3.0e5.0

Brittle minerals including quartz, etc. Mainly detrital material of terrigenous origin Detrital material of terrigenous origin + biogenesis

Pay formation composition Overlying formation Pore-fractured shales Grapholith

Major pay Silty shales Siliceous, calcareous, graptolite-rich, and carbonaceous shales

Underlying formation Fractured shales Tight limestones

Table 7

Statistics on test production of wells of over 4000 m deep.

Well name Well depth/m Production (x104 m3/d)

L101 4700 0.99

DY2HF 5700 2.53

NY1HF 5820 /

G202-H1 5200 3.33

D201-H1 5003 1.7

D202-H1 5300 12

6. Prediction of shale gas development in the Sichuan Basin

Compared with the shale gas development history of about 200 years in North America, the shale gas development in the Sichuan Basin started much later and has only realized preliminary commercial production by far. However, the Sichuan Basin, with abundant shale gas resources and many organic-rich shale layer systems, is deemed to fulfill major development and be mainstay in shale gas development in China with the deepening of geologic understanding and advancing of exploration and development technologies.

6.1. Abundant shale gas resources and multiple organic-rich shale layer systems in the Sichuan Basin

As was mentioned before, there are six sets of well-developed organic-rich shales in the Sichuan Basin. According to the evaluation of national shale gas resources by Ministry of Land and Resources in 2012 [2], the national geologic resource volume of shale gas is 134.4 x 1012 m3, and that of the Sichuan Basin is 40.02 x 1012 m3, accounting for 30%; and national technologically recoverable resource volume is 25.08 x 1012 m3, and that of the Sichuan Basin is 6.45 x 1012 m3, accounting for 26% (Table 8). As we can see, the Sichuan Basin is really abundant in shale gas resources and has a bright development prospect.

6.2. Conditions for forming scale-production of shale gas in the WL Fm

The gas generation characteristics in the WL Fm are very similar to that of the three productive shale gas plays in North America (Table 9) [11]. The proved geologic reserves of shale gas in the WL Fm in Fuling and Jiaoshiba area are 106.75 x 109 m3, and the controlled probable geologic

Table 8

Distribution of the shale gas resource volume in the Sichuan Basin.

Formations Geologic resource volume (x1012 m3) Recoverable resource volume (x1012 m3)

Jurassic 10.82 2.14

Triassic 8.47 1.67

Longmaxi Fm 9.90 1.73

Qiongzhusi Fm 10.83 0.91

Total 40.02 6.45

reserves are approximately 200 x 109 m3. The confirmed three classes of geologic reserves in Changning-Weiyuan and Zhaotong areas are more than 200 x 109 m3. By far, the cumulative production of shale gas in the WL Fm has reached 0.7 x 109 m3. CNPC and Sinopec will build the productivity of 7.5 x 109 m3 per year in 2015, and will produce 5 x 109 m3 of shale gas per year then. In addition, many enterprises including Chongqing Energy Investment Group and China Huadian Corporation have been conducting exploration of shale gas in the Sichuan Basin and its surrounding areas and have finished large quantity of geologic research and exploration evaluation. The reserve and production of shale gas in the Sichuan Basin will be further improved if some breakthroughs are made during the 13th Five-Year Plan.

6.3. Prediction of shale gas production

Based on comparison with Barnett shale gas play in the Fort Worth Basin [11], it is predicted that the shale gas production of the Sichuan Basin can reach (30—60) x 109 m3. The Fort Worth Basin has the favorable shale gas zone of 10400 km2, core zone of 4162 km2, and technologically recoverable resource volume of 1.25 x 1012 m3. The reservoirs there are 2000e2500 m deep, 30e300 m in effective thickness, 60—90 m on average, and 4.2—9.9 m3/t in gas content. Over 15000 producing wells have been drilled in this basin, with gas production of 47.26 x 109 m3 in 2013 (Table 10) and the highest annual production of 52 x 109 m3 (2011).

Compared with the Fort Worth Basin, the favorable shale gas zone in the Sichuan Basin has an area of 94300 km2, a core zone area of approximately 64400 km2 and an estimated technologically recoverable resource volume of 2.64 x 1012 m3. The shales in the Sichuan Basin are 1500—4500 m deep generally, 30—120 m thick, averaging at 60 m, and 2.0—4.0 m3/t in gas content. It is estimated that the annual production of shale gas in Sichuan Basin can reach (30—60) x 109 m3.

7. Conclusions

1) With three types of widely-developed formations of marine, transitional, and continental facies, and six sets of organic-rich shales, the Sichuan Basin has superior formation and enrichment conditions for shale gas, huge resource volume and bright development prospect. A major breakthrough of exploration and development has been made, and preliminary commercial production of shale gas has been realized in the WL Fm, proving it the most favorable interval of shale gas.

2) The Sichuan Basin is the most successful area of shale gas development in China, with an estimated production of 1.2 x 109 m3 in 2014 and a prospective production of (30—60) x 109 m3 in the future. The year of 2015 will be an important turning point of shale gas development in this basin, if further progress is made, the practice in the Sichuan Basin will provide significant support for the shale gas development in China.

Table 9

Comparison of producing pay characteristics between the WL Fm in the Sichuan Basin and three typical producing pays in North America.

Producing pays Thickness (m) TOC (%) Ro (%) Porosity (%) Gas content Free gas Pressure Average production of EUR (108 m3)

or producing areas (m3/t) ratio (%) coefficient single well (x104 m3/d)

Haynesville 60-90 0.5-4.0 1.2-3.0 4.0-14.0 2.8-9.3 80 1.6-2.0 14.2-70.8 1.6

Marcellus 15-60 3.0-12.0 1.2-3.5 4.0-12.0 1.7-4.2 55 1.1-1.4 7.1-76.5 1

Barnett 30-180 4.5 1.0-2.1 4.0-6.0 8.5-9.9 50 0.9-1.2 2.8-48.1 1.2

Jiaoshiba 38-80 2.1-6.3 2.2-3.0 2.5-7.1 4.7-7.2 60-75 1.35-1.55 11.6-54.7 1.13

Changning 30-60 1.9-5.3 2.5-3.0 2.9-6.5 2.4-5.5 65 1.25-2.0 1.9-20 0.42-1.11

Weiyuan 30-40 1.1-6.3 2.1-2.8 2.8-5.5 1.9-4.8 55-60 1.1-1.5 0.7-16 0.47-0.74

Table 10

Paleozoic marine shale gas production forecast simulation table.

Comparison parameters

Barnett in America

Sichuan Basin

Comparison conclusions

Technologically recoverable resource volume (108 m3)

Burial depth (m)

Area of favorable zone (km2)

Area of core zone (km )

Porosity

Thickness of organic-rich shales (m) Gas content (m3/t) Production in 2013 (108 m3) Maximum annual production (108 m3)

2000-2500

30-300/average: 90

4.2-9.9

520 (in 2011)

1500-4500

30- 120/average: 60

2.0-4.0

(Predicted) 300-600

Approximately 2 times Deeper

Approximately 9 times Approximately 10 times Approximately 2/3 Approximately 2/3 approximately 1/4—1/2

3) Compared with North America, the Sichuan Basin has a later start in the exploration and development of shale gas and is confronting a series of challenges. The only way to realize scale development of shale gas in the future is deepening the understanding of resources, developing key technologies, strengthening environmental protection and reducing development cost.

Fund project

Major National Petroleum Projects "Shale Gas Evaluation for Key Areas" (No. 2011ZX05018-001), "Study on Global Remaining Hydrocarbon Resources and Fast Evaluation Techniques of Hydrocarbon Assets (Phase II)" (No. 2011ZX05028), State Key Development Program for Basic Research of China (Program 973) (No. 2013CB228001).

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