CHINA PETROLEUM EXPLORATION Volume 21, Issue 3, May 2016 Discovery and exploration of Fuling shale gas field Guo Xusheng, Hu Dongfeng, Wei Zhihong, Li Yuping, Wei Xiangfeng Sinopec Exploration Company Abstract: The Fuling shale gas field is the first shale gas field put into commercial development, with the largest proven reserves and the highest average tested production in China. It was discovered on November 28, 2012, by Sinopec Exploration Company using the theory of binary enrichments for highly-evolved marine shale gas in complex structural areas in southern China. By the end of 2015, its total proven gas reserves were 3805.98 10 8 m 3, and the designed production capacity of 50 10 8 m 3 in Phase I was achieved. This gas field is characterized by its integration of source and reservoir, intensive layered distribution, and its well-endowed supply of gas. It is a typical self-generation and self-storage shale gas field. As a mid-deep, high-pressure, and high-quality natural gas reservoir, it contains high-quality shales in deep-water continental shelf, with fine roof and floor conditions and weaker structural transformation in later stage, which are the major factors controlling shale gas enrichment and high productivity in this area. Based on the exploration practices in the Sichuan Basin and its periphery used to discover the Fuling shale gas field, the main controlling factors for shale gas enrichment in complex structural areas in southern China were analyzed. The analysis highlighted the preservation condition as the key factor. Conclusions are drawn in three aspects. Firstly, extra-basin tectonic uplifting time, syncline width, burial depth, and fracture are the main parameters to evaluate the preservation condition in shale gas layers. Secondly, for areas at the basin margins, the evaluation should focus on the effects of basin-controlling faults on shale gas layers. Thirdly, the nature of and the scale of the faults and development features of high-angle fractures in the basin are critical for evaluating the shale gas preservation condition. From the discovery of Fuling shale gas field, the following points can be used for further shale gas exploration in complex structures in southern China. Firstly, shale gas exploration should be conducted depending on the basic laws of oil and gas generation. Secondly, several shale gas exploration targets exist in southern China, especially in the regions that are deeper than 4500 m and that have extra-basin normal pressure regions and continental shale gas areas. Thirdly, basic information, innovative ideas and critical technologies are essential for making breakthroughs in shale gas exploration. Key words: exploration and discovery history, gas pool features, preservation condition, enlightenment, Wufeng Fm-Longmaxi Fm, Fuling shale gas field The Fuling shale gas field is the first large shale gas field put into commercial development in China, and it is also the largest shale gas field around the world, except for those in North America. Its discovery pioneered the large-scale commercial development of shale gas in China. This historic breakthrough was not accidental, but rather was a result of continuous investigation, study and summarization. Internationally, some countries, including the United States and Canada, were the first to achieve large-scale development of shale gas, and they have developed a series of advanced and effective exploration and development models and technologies for shale gas in North America [1 5], which can be used as reference for exploration and development of shale gas in China [6 8]. However, the successful experience of the United States in shale gas exploration and development cannot be exactly copied in China because the marine shales in complex structural areas in southern China are characterized by old age, high thermal evolution degrees, and complexity as a result of superposition under multiphase tectonic movements, which was revealed by further research and exploration practices [9 22]. On the basis of these systematic studies on theories and technologies for marine shale gas in southern China, and after four years of exploration, high test productivity (20.3 10 4 m 3 /day) of shale gas flow was obtained in shale gas layers in the Wufeng Fm-Longmaxi Fm in Well Jiaoye 1HF on November 28, 2012, heralding the discovery of Fuling shale gas field. At the fifth global shale oil and gas summit held in the United States on November 5, 2014, Sinopec was awarded the Shale Oil and Gas International Pioneer Award in recognition of its first major commercial discovery in shale gas outside the North America. The discovery of the Fuling shale gas field is another significant achievement in oil and gas exploration in China, and it is also represents the progress of theory and practice of unconventional shale gas exploration in China. It shows the feasibility and hopeful Received date: 02 Mar. 2016; Revised date: 19 Mar. 2016. Corresponding author. E-mail: guoxs.ktnf@sinopec.com Foundation item: Basic Research Project of Sinopec Science and Technology Department Major controlling factors and prediction technologies for marine high-quality shale gas layers in Sichuan and Chongqing (P15074). Copyright 2016, Petroleum Industry Press, PetroChina. All rights reserved.
2 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 prospects of unconventional oil and gas exploration in southern China. In this regard, a series of theories and supporting technologies with strategic significance in promoting shale gas exploration in China have been formed. pressure coefficient of 1.55. This gas field was found on November 28, 2012. By the end of 2015, its total proven gas reserves were 3805.98 108 m3. Basic parameters of this gas field are shown in Table 1. 1. 2. Overview of the Fuling shale gas field The Fuling shale gas field is situated in the eastern barrier-type folded belt of the Sichuan Basin, and to the west of the Qiyueshan fault at the basin boundary. It is administratively subordinate to the Fuling District of Chongqing City[9 11]. Currently, it is mainly producing in the Jiaoshiba structure, with the Triassic Jialingjiang Fm outcrops on surface (Fig.1). The Jiaoshiba structure is a rhombic faulted anticline controlled by two sets of faults (NE and near SN), with NE-striking axis. Its major part has weaker deformation, similar to a box with a faulted anticline. Specifically, its top is broad, with small stratigraphic dip and absent faults, and its two flanks are steep with faults[9 11]. The gas-bearing shale interval (TOC 1%) in the Wufeng Fm-Longmaxi Fm in this area is 50-100 m thick, and the fine gas-bearing shale interval (TOC 2%) is 35 45 m thick. The gas-bearing shale interval has average TOC of about 2.66%, Ro of 2.58%, an average porosity of about 4.53%, an average gas content of about 4.21 m3/t, and a formation Fig. 1 History of exploration and discoveries Surface petroleum mapping was carried out in the Fuling shale gas field during the 1950s. So far, the oil and gas exploration in this area can be divided into 4 milestone stages: (1) conventional gas exploration (1950-2009); (2) prospect evaluation and optimized targets drilling (2009-2012); (3) exploration breakthrough and evaluation (2012-2015); and (4) integrated exploration and development (2013-now). 2.1. Stage of conventional gas exploration (1950-2009) Geological mapping and petroleum exploration in the Fuling area has a long history. From the 1950s to the 1990s, the former Ministry of Geology and Mineral Resources of China carried out petroleum reconnaissance surveys and detailed geology surveys, with 14 2D lines (417.51 km), 4 MT lines (152.7 km), and 14 CEMP lines (470.7 km) deployed, which eventually contributed several anticlines (e.g. Jiaoshiba, Daershan, and Jiaozishan). Since 2001, Sinopec has evaluated hydrocarbons in the lower assemblage and Regional location of the Fuling shale gas field
Guo Xusheng et al., Discovery and exploration of Fuling shale gas field 3 Table 1 Basic parameters of the Fuling shale gas field Name Fuling shale gas field Discovery well (year) Jiaoye 1 (2012) Geographical location Fuling District, Chongqing City Production of discovery well 20.3 10 4 m 3 /day by testing Jiaoshiba structure in Baoluan-Jiaoshiba structural Regional tectonic location belt in Chuandong high-steep fold belt The major structure is relatively gentle, Tectonic setting without many faults Guided by the theory of binary enrichments and using the evaluation system and standard (including 3 major Discovery basis types and 18 evaluation parameters) for marine shale gas in southern China, several favorable exploration targets (Jiaoshiba etc.) were selected Features of shale gas layer First gas producing year 2012 Proven gas reserves (year) 3805.98 10 8 m 3 (2015) Recoverable gas reserves (year) 951.50 10 8 m 3 (2015) Reserves abundance 9.92 10 8 m 3 /km 2 Seismic data 2D: 19 lines (670 km); 3D: 1144.5 km 2 Gas reservoir features within proven gas-bearing scope in Phase I and Phase II Shale gas layer Wufeng Fm-the 1 st member of Longmaxi Fm Gas reservoir type Shale gas reservoir Sedimentary setting Deep-water shelf and shallow-water shelf Gas-bearing area (year) 383.54 km 2 (2015) Lithology Grey black carbonaceous graptolite shale with radiolarian, carbonaceous silty mudstone, carbonaceous graptolite shale, silty mudstone Distribution feature Continuous in vertical direction, no interlay in the middle, intensive layered distribution on plane Shale gas layer thickness 55.4 89.1 m Gas-bearing height 1430 m TOC 0.29% 6.79%, average 2.66% Depth in middle gas reservoir 2885 m Kerogen type Mainly type I Pressure in middle gas reservoir 43.87 MPa R o 2.22% 2.89%, average 2.58% Pressure coefficient in gas reservoir 1.55 Gas generation intensity 60.05 10 8 m 3 /km 2 Geothermal gradient 2.83 C/100 m Porosity 0.26% 8.61%, average 4.53% Gas composition Mainly CH 4 (96.10% 98.81%), low CO 2 (0 0.56%), no H 2 S Permeability 0.1307 1.2674 md, average 0.4908 md Gas type Over-mature dry gas Pore type Mineral composition Mainly nano-scale organic pores and inter clay micro pores, containing intercrystal pores and secondary corroded pores, etc. Siliceous mineral, K-feldspar, plagioclase, calcite, dolomite, pyrite, clay mineral (Illite-samoite mixed layer, illite, chlorite) Clay mineral content 10.7% 61.6% (average 32.7%) Silicoide content Average 42.6% Total gas content 0.35 9.63 m 3 /t, average 4.21 m 3 /t Gas saturation 66.8% 74.4%, average 67.7% Gas source Oil-gas-water relation Roof and floor feature Wufeng Fm-the 1 st member of Longmaxi Fm No gas-water contact The roof and floor are continuous deposits with shale gas bearing layers in conformable contact, with tight lithology and high breakthrough pressures others in several blocks (e.g. Fuling, Qijiang, and Qijiang South) in the southeastern Sichuan Basin to inspect their geological conditions. According to the evaluation results, the Baoluan-Jiaoshiba anticline belt-shimenkan anticline belt is regarded as a favorable exploration prospect in the lower marine assemblages in this area. However, as the exploration potential is not clear, there was no action taken in these blocks during this stage. 2.2. Stage of prospect evaluation and optimized targets drilling (2009-2012) Inspired by the rapid and successful development of shale gas in the United States, Sinopec officially launched shale gas exploration and evaluation while accelerating shale oil and gas exploration progress using strategic guidance relating to unconventional oil and gas resources [23]. In 2009, Sinopec Exploration Company carried out shale gas prospect evaluation in the Sichuan Basin and its periphery, successfully finished re-examination of more than 40 wells (e.g. Dingshan 1) and studies on 25 outcrop profiles of Xishui Qilongcun and others, and made a large number of analysis and tests. The basic geological conditions for forming marine shale gas in this area have been preliminarily identified. It is recognized that, compared with commercial shale gas fields in the North America, the marine shale gas in southern China is characterized by superposed transformation due to multiphase tectonic movements, high thermal evolution degrees, complex preservation conditions, and big differences in gas-bearing potential. Accordingly to these differences in defining characteristics, the theory and exploration technologies in the North America cannot be imitated completely. It is necessary to enhance shale gas preservation condition evaluation in the complex structures within southern China. This paper proposed the theory of binary enrichments for highly-evolved marine shale gas in complex structural areas in southern China [9 11], i.e. it proposed the idea that high-quality shale in deep-water continental shelf facies is the basis for marine shale gas enrichment, and fine preservation condition is the key for marine shale gas enrichment and high productivity. Moreover, this
4 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 paper presented the evaluation system and standard (including 3 types and 18 evaluation parameters) for marine shale gas in southern China. On this basis, a group of favorable exploration targets, such as Jiaoshiba, Dingshan and Pingbian, were selected. In order to identify the basic geological conditions of shale gas formation in the Fuling area and thereby promote commercial extraction of shale gas, Sinopec Exploration Company deployed the first marine shale gas parametric well - Jiaoye 1HF- in the Jiaoshiba block in September 2011, and spudded this well on February 14, 2012. Since then, unconventional shale gas exploration in the Fuling shale gas field has kicked off. 2.3. Stage of exploration breakthrough and evaluation (2012-2015) 2.3.1. Stage of exploration breakthrough Well Jiaoye 1 is the pilot hole of Well Jiaoye 1HF. Its drilling was finished on May 18, 2012, with a total depth of 2450 m in the Middle Ordovician Shizipu Fm. This well drilled through 89 m of shale gas layers in the Wufeng Fm-Longmaxi Fm, including 38 m of high-quality shale gas layers with TOC 2.0%. After the drilling of Well Jiaoye 1 was finished, it was decided not to do the vertical well fracturing test, but to rather drill a horizontal well to evaluate its productivity. The high-quality shale gas interval of 2395 2415 m in Well Jiaoye 1 was chosen as the target window of the horizontal section for lateral drilling to drill the horizontal well - Jiaoye 1HF. This well was finally drilled on September 16, 2012, with a total depth of 3653.99 m and a 1007.90 m long the horizontal section. In November 2012, massive hydraulic fracturing was conducted in 15 stages in the horizontal section (2646.09 3653.99 m) in Well Jiaoye 1HF, and a 20.3 10 4 m 3 /day commercial gas flow was obtained in testing on November 28, 2012, which was the same day as the announcement of the discovery of Fuling shale gas field. 2.3.2. Stage of evaluation After the commercial discovery in Well Jiaoye 1HF, three appraisal wells (Jiaoye 2, Jiaoye 3 and Jiaoye 4) were deployed to the south of Well Jiaoye 1HF, and they obtained moderate-high productivity commercial flows (33.69 10 4 m 3 /day, 11.55 10 4 m 3 /day and 25.83 10 4 m 3 /day, respectively) via fracturing tests. Thus, control on the major part of the Jiaoshiba structure was achieved. Meanwhile, 594.50 km 2 of 3D seismic surveys were deployed in the favorable prospects (less than 3500 m of burial depth) of the Jiaoshiba structure, providing a solid data foundation for Phase I production of the Fuling shale gas field. After the major part of the Jiaoshiba structure was controlled, five exploratory wells (Jiaoye 5, Jiaoye 6, Jiaoye 7, Jiaoye 8, and Jiaoye 9) were deployed and drilled in the peripheral regions in order to tap the potential of deep shale gas, which depende don the tectonic styles. In particular, Wells Jiaoye 5, Jiaoye 6, Jiaoye 7, and Jiaoye 8 obtained shale gas test flows of 4.5 10 4 m 3, 6.68 10 4 m 3, 3.68 10 4 m 3, and 20.8 10 4 m 3 respectively, which enlarged the exploration and development scope of the Fuling shale gas field. 2.4. Stage of integrated exploration and development (2013-now) After the commercial discovery in Well Jiaoye 1HF, Wells Jiaoye 2, Jiaoye 3, and Jiaoye 4 were drilled at the beginning of 2013 for the purpose of increasing reserves and production. Also, to find the appropriate gas field development mode and to evaluate the technical index of gas reservoir development, a 28.7 km 2 region in the Well Jiaoye 1 area was chosen as the location for the deployment of a pilot test well group for productivity evaluation, including 10 drilling platforms with 26 wells providing 5 10 8 m 3 /a of new productivity. On September 3, 2013, the National Energy Administration of China approved the establishment of the Fuling Chongqing National Shale Gas Demonstration Zone. On November 28, 2013, Sinopec approved the scheme to build the production capacity of 50 10 8 m 3 in Phase I in the Fuling shale gas field. On April 21, 2014, the Ministry of Land and Resources of China approved the establishment of the Fuling Chongqing Shale Gas Exploration and Development Demonstration Base. As of December 31, 2015, the Fuling shale gas field has had 290 wells spudded in, of which 256 wells were completed, and 180 wells put into production. This resulted in a total shale gas production of 43.91 10 8 m 3, and sales of 42.13 10 8 m 3. This also means that the production capacity of 50 10 8 m 3 /year for the Fuling shale gas field has been successfully completed. 3. Major geologic features of the Fuling shale gas field 3.1. High-quality shales developed in deep-water continental shelf During the late Ordovician-early Silurian period, the southeastern Sichuan Basin was covered in water ranging in depth from shallow to deep, and this continental shelf setting resulted in the production of thick dark organic-rich shales [9 11]. The drilling results of five wells, including Jiaoye 1, indicated that the organic-rich shales in the lower Wufeng Fm-Longmaxi Fm were stably distributed, with a thickness of 50 100 m. The high-quality shales (TOC 2.0%) in the deep-water continental shelf rest at the bottom of the
Guo Xusheng et al., Discovery and exploration of Fuling shale gas field 5 Wufeng Fm-Longmaxi Fm, with a thickness of 38 43.5 m (Table 2) and characteristics that include low silty content, high carbonaceous content, the presence of enriching graptolite, and well-developed lamellation fractures. Compared with the whole shale gas layer (TOC 1.0%), the high-quality shale gas layers in the deep-water continental shelf in the Wufeng Fm-Sub-member I of the 1 st member of Longmaxi Fm are characterized by four highs (i.e. high TOC, high porosity, high gas content, and high siliceous content) (Table 2 and Fig.2) in addition to their features of bigger thickness, good organic matter type (type I), and a moderate thermal evolution degree (R o =2.22% 2.89%, averagely 2.58%). Taking Well Jiaoye 1 as an example, its high-quality shale gas layers reveal a TOC of 1.04%-5.89% (average 3.77%), porosity of 2.78% 7.08% (average 4.65%), total gas contents of 3.52 8.85 m 3 /t (average 6.03 m 3 /t), and brittle mineral content (mainly siliceous mineral) of 31.0% 70.6% (average 44.8%). The electric properties of the high-quality shale gas layers show a well logging response feature of four highs and three lows, i.e. high GR, high uranium, high AC, high resistivity, low density, low neutron, and low KTh (Fig.2). Table 2 Analysis and comparison of main evaluation parameters of the Wufeng-Longmaxi shale gas layers in Well Jiaoye 1-Well Jiaoye 5, the Fuling shale gas field Well Gas layer Interval/m Thickness/m TOC/% Porosity/% Siliceous content/% Total gas content (m 3 /t) Jiaoye 1 Shale gas layer 2326 2415 89 0.55 5.89 (2.52) 1.17 7.22 (4.52) 18.4 70.6 (37.3) 1.52 8.85 (4.30) High-quality shale gas layer 2377 2415 38 1.04 5.89 (3.77) 2.78 7.08 (4.65) 31.0 70.6 (44.8) 3.52 8.85 (6.03) Jiaoye 2 Shale gas layer 2477 2575 98 0.82 5.25 (2.78) 1.85 8.61 (6.20) 19.6 67.3 (45.4) 1.94 8.90 (5.06) High-quality shale gas layer 2533 2575 42 2.17 5.25 (3.76) 1.85 8.61 (6.20) 35.4 67.3 (50.2) 4.41 8.90 (6.46) Jiaoye 3 Shale gas layer 2313 2414 101 0.46 4.53 (2.21) 1.22 5.39 (3.54) 0.63 9.63 (4.12) High-quality shale gas layer 2370.5 2414 43.5 0.19 4.53 (2.99) 3.16 4.33 (3.66) 4.08 9.63 (6.23) Jiaoye 4 Shale gas layer 2512 2595 83 0.58 6.79 (2.95) 4.41 7.80 (5.78) 22.9 80.5 (49.6) 1.19 8.83 (4.83) High-quality shale gas layer 2557 2595 38 0.58 6.79 (3.67) 5.02 7.8 (6.19) 22.9 80.5 (53.0) 4.12 8.83 (5.93) Jiaoye 5 Shale gas layer 2972 3086 114 0.26 3.94 (1.76) 0.86 5.16 (2.44) 31.0 76.6 (43.7) 0.34 5.98 (4.14) High-quality shale gas layer 3043 3086 43 1.53 3.94 (3.15) 2.02 5.16 (3.35) 31.1 76.6 (45.8) 3.21 5.98 (4.72) Note: The data in brackets are the average values Fig. 2 Composite columnar section of Wufeng Fm and the 1 st Member of Longmaxi Fm in Well Jiaoye 1, the Fuling shale gas field
6 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 3.2. Good preservation condition and gas-bearing potential Similar to conventional gas accumulation, shale gas accumulation is a balance between accumulation and effusion [24 25]. Shale gas exploration practices in the Sichuan Basin and its periphery indicate that the roof and floor conditions (and later, tectogenesis intensity and time) control the effusion pattern, degree, and abundance of shale gas. Studies show that the Fuling shale gas field has fine roof and floor conditions, and its later tectogenesis intensity was weaker, which led to good gas-bearing potential (Table 2) and high formation energy in shale gas layers, which is one of the critical factors for high and stable shale gas production. 3.2.1. Fine roof and floor conditions The shale gas layers in the Wufeng Fm-Longmaxi Fm in the Sichuan Basin and its periphery generally underwent two stages: early continuous deep burying and late continuous uplift. During the early continuous deep bury stage, shale gas layers were formed through dynamic evolution of hydrocarbon generation, expulsion, and detention in dark shales. At the max burial depth before continuous uplift, the formed shale gas layers usually underwent high pressure or extra high pressure. The major factors for influencing shale gas detention and preservation were the roof and floor conditions. Several factors, such as the contact relationship between the roof, floor and shale gas layers, lithology assemblage, thickness, and physical properties, control the room s ability to seal in the shale gas. The roof and floor of the shale gas layers in the Wufeng Fm-Longmaxi Fm have received continuous deposits of shale gas layers in conformable contact, and the roof and floor have features of thick, tight lithology and a high breakthrough pressure. The roof of the shale gas layers is a grey-dark grey moderate-thick siltstone interwoven with thick silty mudstone in the 2 nd member of the Longmaxi Fm, with a thickness of around 50 m, an average porosity of 2.4% in siltstone, an average permeability of 0.0016 md, and a breakthrough pressure of 69.8 71.2 MPa. The floor of the shale gas layers is dark grey argilliferous nodular limestone, and the limestone in the Linxiang Fm and the Baota Fm has a thickness of around 30 40 m, an average porosity of 1.58%, an average permeability of 0.0017 md, and a breakthrough pressure of 64.5 70.4 MPa. The features listed above imply that the roof and floor of the shale gas layers in the Wufeng Fm and the 1st Member of the Longmaxi Fm have better sealing capacity for the shale gas layers. 3.2.2. Weaker tectogenesis during later period During the stage of later continuous uplift, the tectogenesis was intense, which could have stopped hydrocarbon generation, and reduced or eroded the formation above the gas-bearing layers, and then led to decreasing overburden pressure, which would have resulted in the shale gas breaking the cap rocks and effusing upwards. Apatite fission track analysis showed that [26 28] the starting time for the continuous uplift of the shale gas layers in the Wufeng Fm-Longmaxi Fm in the Sichuan Basin and its periphery featured an orderly progression from outside of the basin to the inside of the basin from early to late periods. The late effusion time of the shale gas took up the whole stage of later continuous uplift of the shale gas layers. The effusion of the shale gas has occured for a longer time outside of the basin (from early Cretaceous to present), and for a shorter time inside the basin (from late Cretaceous to present). Moreover, from the outside to the inside of the Sichuan Basin and its periphery, there are different tectonic deformation styles and geologic structure features [29 30]. In the region to the west of the Qiyueshan faulted belt, it is chiefly a intrabasinal baffle deformation belt, with weak deformation in marine structural layers, good formation continuity in lateral and vertical directions, low-micro amplitude fold structures, and smaller denudation thickness (Fig.3). Specifically, the Jiaoshiba structure has eroded about 4500 m from the surface since the Late Cretaceous, while the Mesozoic and Paleozoic systems are well preserved, which is good for hydrocarbon preservation because it reveals a high pressure feature. However, the periphery of the Sichuan Basin is mainly in a trough-like folds-thrust belt with many big faults; the deformation in whole marine structural Fig. 3 Division of structural deformation zones in the Sichuan Basin and its southeast margin
Guo Xusheng et al., Discovery and exploration of Fuling shale gas field layers is stronger, with poorer formation continuity in the lateral and vertical directions and a bigger denudation thickness (Triassic-Jurassic systems only remain in fewer synclines). Thus, the marine shale gas layers usually show normal pressures. The above features indicate that, compared with the regions outside of the Sichuan Basin, the Fuling area is characterized by later tectonic reformation time and a weaker tectonic reformation degree because it is located inside the basin. 3.2.3. Fine structural styles and moderate burial depths are favorable for later preservation The major part of the Jiaoshiba structure is a box-like stable faulted anticline. Its top is broad and gentle, and its two flanks are steep dips. There are fewer faults in the major part, and faults are mainly developed in the eastern and southwestern margins of the Jiaoshiba area. These two sets of faults are apparent reverse faults, with good sealing capacity (Fig.4). Moreover, the major part of the Jiaoshiba structure is at moderate burial depths (generally >2000 m), with chief outcropping formations of Jurassic-Triassic systems. There is no outcropping in the Wufeng Fm-Longmaxi Fig. 4 4. 7 Fm in the Jiaoshiba area because of the apparent lack of pressure release zones in the lateral direction. 3.3. Typical self-generation and self-storage continuous shale gas reservoir The gas reservoir in the Wufeng Fm, the 1st member of the Longmaxi Fm, is a typical self-generation and self-storage continuous shale gas reservoir in the middle-deep layers, with a low geothermal gradient and a high pressure. Analysis of the gas source indicates that the shale gas in the Wufeng Fm-Longmaxi Fm in the Jiaoshiba area came from the mud source rocks, which gives reason to believe that source and reservoir are integrated. The middle of this gas reservoir is 2885 m deep, with an average geothermal gradient of 2.83 C/100 m and a formation pressure coefficient of 1.55. The dominant gas composition is CH4 (96.10% 98.81%), with low CO2 content (0 0.56%) and no H2S, making this a high-quality dry gas reservoir (Table 1). Based off testing, there is no water in the gas reservoir. The flowback rate of the fracturing liquid is very low (average 2.9%) in the fracturing wells. Seismic section and well distribution map of the Jiaoshiba structure in the Fuling shale gas field Discussion 4.1. Whether the Fuling gas reservoir is a typical shale gas reservoir? In the initial stage, the Fuling shale gas field realized an average single-well production up to 32.72 104 m3/d via testing, with a success ratio of 100%. Thus, Chinese scholars have argued over several controversies relating to the Fuling shale gas field. For examples, is the Fuling shale gas field a typical shale gas field? Does the fractured gas reservoir or do other lithologic interlayers in the shales provide more productivity? In this paper, the authors will prove that the Fuling shale gas field is a typical self-generation and self-storage continuous shale gas reservoir with respect to the shale gas source, reservoir bed, and production law of gas wells, etc. 4.1.1. Shale gas accumulated in integrated source and reservoir by on-site retention The reservoir in the Fuling shale gas field is composed of
8 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 organic-rich shales deposited continuously, with a shale ratio near 100%, continuity in the vertical direction, and no interlayer (Fig.2). The average TOC value is 2.66%, the major organic matter is type I, and the average R o is 2.58%. This data shows that, during geologic history, the shale in the Wufeng Fm-Longmaxi Fm had potential to generate a great quantity of oil and gas. Through calculations, the hydrocarbon generation intensity of the source rock (TOC 1.0%) in the Wufeng Fm-Longmaxi Fm in the Fuling shale gas field was estimated to be up to about 60.05 10 8 m 3 /km 2. Natural gas in the Wufeng Fm, the 1 st member of Longmaxi Fm in the Fuling shale gas field, came from the source rock in these formations. According to geochemical comparative analysis of the natural gas, the Fuling natural gas is characterized by high CH 4 content (96.10% 98.81%), a high dry coefficient, and no H 2 S (Table 1), which is apparently different from the natural gas in the Lower Jurassic system, which has a higher moisture content and some H 2 S in its natural gas in the Upper Permian and Lower Triassic systems. Additionally, according to the δ 13 C change law in gas source rocks, the high mature natural gas should come from the source rocks with heavier δ 13 C (1 3 ). Based on correlations between the carbon isotopes, the δ 13 C feature of the Fuling natural gas is most similar to the kerogen carbon isotope values in the Wufeng Fm-Longmaxi Fm and Middle-Lower Permian series. However, the Middle-Lower Permian series had no overpressure, and their generated natural gas was unable to migrate to the Longmaxi Fm (it was tighter and generated hydrocarbons) and form overpressure; δ 13 C values of kerogen in the Longmaxi source rock ranged from 30.81 to 29.21, corresponding to (about 5% higher than) the δ 13 C values of the natural gas in its reservoir bed (Fig.5). This means that the most probable source of the natural gas in the Wufeng Fm, the 1 st member of Longmaxi Fm in the Fuling shale gas field, is the source rock within them. In addition, the carbon isotope values of CH 4, C 2 H 6 and C 3 H 8 in natural gas in the Fuling shale gas field have an apparent inverse feature (i.e., CH 4 δ 13 C> C 2 H 6 δ 13 C>C 3 H 8 δ 13 C)(Fig.6), which also serves as evidence that the shale gas came from the source rocks in the Wufeng Fm-Longmaxi Fm. Shale gas in the Longmaxi Fm is chiefly pyrolysis gas, and came from two sources: one source is the gas from crude oil cracking, and the other source is the gas from kerogen or colloidal bitumen cracking. When these two types of methane are mixed to a certain ratio, inverse features occur. Many researchers in China and other countries think that both the mud and shale sealing system with a high evolution degree and the mixture of natural gas that came from the same source but was generated in different periods may be the major reasons for these inversed carbon isotope values in paraffin gas in the Longmaxi Fm [15, 31 32]. Fig. 5 Distribution of kerogen δ13c of source rocks in various strata in the Sichuan Basin 4.1.2. The reservoir beds have features of intensive layered distribution and general gas endowment The distribution of the shale gas layers in the Fuling shale gas field is dominantly controlled by the sedimentary facies belt. The sedimentary facies in the Wufeng Fm, the lower 1 st member of Longmaxi Fm, are both deep-water and shallow-water continental shelves that containin shale gas layers in both deep-water and shallow-water continental
Guo Xusheng et al., Discovery and exploration of Fuling shale gas field 9 Fig. 6 Distribution of carbon isotopes of natural gas in the Fuling shale gas field shelf sub-facies. According to the data of 9 exploratory wells, more than 200 horizontal wells, and 3D seismic data acoustic impedance inversion in the Fuling shale gas field, the lithology and electric property in the shale gas layers display a strong correlation, with features including continuous vertical distribution, no apparent interlayers, and gradually improving parameters (TOC, gas content, fragile mineral content, etc.). The thicknesses of the shale gas reservoir beds with TOC 1% are laterally stable (55.4 89.1 m thick), showing intensive layered distribution. The shale reservoir beds develop a large number of nanoscale pores, with few fractures. The major storage spaces are 1.5 50 nm pores, with more organic pores. Here, the average shale porosity is 4.53%, and the average specific surface area is 18.9 m 2 /g, which is favorable for shale gas adsorption and storage as well as general gas endowment in the shale gas layers. There was no water produced in test wells, and no water in the producing test; additionally, there was no water layer in the well logging interpretation, and there was no apparent gas-bearing boundary and gas-water contact. 4.1.3. Single well production capacity is controlled by artificial fracture network scale Generally, shale gas wells only produce micro gas or have no natural production capacity in testing. Commercial gas flows with high productivity can be formed only through large-scale staged fracturing tests in horizontal wells. In the Fuling shale gas field, some findings were obtained via experiments of production capacity and volume fracturing in shale gas wells. Firstly, the vertical wells have low production capacity in perforation and testing. By APR testing combination technology, the 2359.5 2361.5 m (2 m/1 layer) shale gas interval in the Wufeng Fm in Well JY11-4 was perforated and tested. The working system of the testing included two openings and two closings, lasting 341 hours. The calculated absolute open flows were 40 93 m 3 /d by single point method. Secondly, there is an apparent relationship between production capacity and fracturing scale in horizontal wells. According to the statistics of several horizontal wells with producing test in the Fuling shale gas field, it was concluded that the following working system is favorable for high productivity and higher stable productivity in single wells: 1300 1700 m horizontal section length, 16 22 sections, 70 80 m single section length, 50 60 total clusters, 1700 1900 m 3 fracturing liquid volume in single section, and 55 60 m 3 fracturing sand volume in a single section. For the horizontal wells with apparently smaller fracturing scales, their testing and producing test production and pressures were lower. Taking Well Jiaoye 9-2HF as an instance, there were only two sections of fracturing in 180 m horizontal interval in this well. From this well, a testing production of only 5.9 10 4 m 3 /d was obtained along with a lower producing test production and lower pressures than other wells. As of December 31, 2015, the surface casing pressure of this well was 5.66 MPa, with a daily gas production of 1.95 10 4 m 3 and a total gas production of 2092 10 4 m 3. 4.1.4. It is not a fractured gas reservoir Typical fractured gas reservoirs usually have features such as low reservoir matrix porosities, mainly fractured storage spaces, strong reservoir heterogeneity, big testing production differences between adjacent wells, high initial testing production in single wells, and quick pressure decline, etc. The study results indicate that the Fuling shale gas layers have differences from fractured gas reservoirs in terms of features of reservoir beds and well production. Firstly, according to core observation, FMI well logging interpretation, scanning electron microscope observation of argon ion polishing, and other analysis, since the major part of the Jiaoshiba structure has gentle formation occurrence and undeveloped macro fracture system (especially in that there are fewer high-angle fractures), the shale reservoir beds mainly develop nanoscale organic matter pores and nanoscale micro fractures. Secondly, in the major part of the Jiaoshiba structure, there was almost no drilling fluid leakage or well kick and well blowout while the exploratory wells and horizontal shale gas wells were being drilled in the shale gas layers. Thirdly, just as mentioned earlier, shale gas layers are controlled by the deposit settings of the deep-water continental shelf, causing features like intensive layered distribution and gas bearing throughout the whole structure. Shale gas evaluation parameters had smaller lateral differences, and the shale gas reservoir beds have weaker heterogeneity in lateral direction. Fourthly, within 383.54 km 2 of the proven gas-bearing area, all of the 142 testing wells penetrated the high-quality shale gas layers and obtained moderate-high yield shale gas flows, with an av-
10 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 erage testing production of 32.72 10 4 m 3 /d in single wells and small differences in production between adjacent wells. Fifthly, the wells with higher shale gas production capacity in the Fuling Jiaoshiba area are basically distributed in the major part of the Jiaoshiba structure, while the wells with lower shale gas production capacity are close to the Daershan faulted belt. According to the statistics, the average testing production in the tested wells near the fracture development zones is 17.1 10 4 m 3 /d, only 49.01% of those in the major part of the Jiaoshiba structure. Furthermore, the wells with lower production capacity have features composed of a large drilling leakage, slightly lower measured pressures, etc. This means that the macro fractures near faults have no positive contribution to shale gas production capacity, but instead lead to reduced gas-bearing abundance by shale gas effusion. Sixthly, the production test wells have stable production test pressures and production. As of February 2016, out of the 183 production test wells, 34 wells reached production levels of over 4000 10 4 m 3. During production tests, the production test pressures and production declined at a relatively slower rate. For Well Jiaoye 1HF, the well with the longest production time, it was put into a production test on November 28, 2012, and its production proration was made with 6 10 4 m 3 /d on January 9, 2013. During the initial stage, this well produced compressed natural gas, and this was transported by pipeline transportation since September 15, 2013. As of February 23, 2016, this well has continuously produced for 1182 days, with daily gas production of about 6 10 4 m 3, tubing head pressure at 8.36 MPa, casing pressure at 7.74 MPa, total gas production of 7475.3 10 4 m 3, and total water production of 397.5 m 3 (Fig.7). Fig. 7 Shale gas production test curves for the Wufeng Fm-Longmaxi Fm in Well Jiaoye 1HF 4.1.5. Producing test curves have similar features with producing wells in the North America Currently, the Fuling shale gas field has two production modes. (1): Constant production rate. For instance, the production proration of Well Jiaoye 1HF was 6.0 10 4 m 3 /day. (2) Production caused by a large pressure drop. For instance, a producing test in Well Jiaoye 6-2HF was carried out following the pattern of production by increased pressure drop at initial stage, then production by constant production rate at later stage. Its initial daily gas production was 39.0 10 4 m 3. At present, it produces with a constant wellhead pressure at 6 MPa with a daily gas production of 20.0 10 4 m 3. As of February 23, 2016, it has continuously produced with high yields for 850 days, creating the highest record of total shale gas production by one well in shale gas development in China (2.0 10 8 m 3 ). Actually, the above two production modes are conceptually similar to the production law of shale gas wells in the North America. During the early stage, Well Jiaoye 6-2HF and the Haynesvile shale gas wells (11.906 mm choke) used the same production system with big pressure drops, all of which featured a quick production decline. Well Jiaoye 1HF was characterized by a constant production rate and small choke, mimicking the change law of the Haynesvile shale gas wells (5.556 mm choke), i.e., low initial production, but stable production, slow decline, and high total production (Fig.8). Fig. 8 Production test curves of wells in the Jiaoshiba area and North America
Guo Xusheng et al., Discovery and exploration of Fuling shale gas field 11 4.2. Differences of evaluation factors for preservation conditions in different regions In the United States, where there are shales in the major shale gas basins, there is also shale oil and gas. But in southern China, the tectonic reformation is violent; thus, the hydrocarbon preservation condition is an important factor for shale gas accumulation [9 18, 20]. According to the exploration details, the roof and floor conditions of the Wufeng Fm-Longmaxi Fm shale gas layers in the Sichuan Basin and its periphery are superior, and the main factor for influencing preservation condition is later reformation. However, in various tectonic zones in extrabasinal, basin-marginal, and intrabasinal areas, different factors have different degrees of influence on shale gas preservation. 4.2.1. Extrabasinal areas: uplifting time, syncline width, burial depth and fault The extrabasinal folded areas have strong tectonic reformation and large deformation, with the major tectonic style being that of residual syncline, developing faulted anticlines, and micro uplifts in some local regions, with more faults. The Wufeng Fm-Longmaxi Fm extensively outcrops to the surface in the anticline regions, and no complete regional cap rocks are present. In syncline areas, the burial depths of shales are generally 500 3500 m, with a pressure coefficient of 0.8 1.2. Several oil companies and local enterprises have made positive explorations for shale gas in the Wufeng Fm-Longmaxi Fm deep-water continental shelf in extrabasinal areas of the Sichuan Basin. But up to now, commercial gas flows were obtained only in syncline WL and syncline SZP. The testing production of Well LY1HF (with a formation pressure coefficient of 1.1) in syncline WL was (5 6) 10 4 m 3 /d, which is higher than that in Well PY1 (with a testing production of 2.52 10 4 m 3 /d and a formation pressure coefficient of 0.96) in syncline SZP. By detailed comparison between the above two areas, it is found that the higher formation pressure coefficient of the shale gas layer in Well LY1HF is attributed to two aspects. Firstly, comprehensive analysis of apatite fission tracks and inclusions show that the initial uplifting time of Well LY1HF was around 93 Ma, while the initial uplifting time of Well PY1 was about 125 Ma, thus showing that Well LY1HF was uplifted later than Well PY1. Secondly, syncline WL has a bigger area and wider flanks. The distance between the shale gas layer outcropping areas of the two flanks of syncline WL is 25.1 km, while that of syncline SZP is 20.8 km. Thirdly, the shale gas layer in Well LY1HF is deeper. The bottom depth of the shale gas layer in Well LY1HF is 2872 m, while that of Well PY1 is only 2160 m. In addition, faults are well-developed in extrabasinal areas, which can lead to worse gas-bearing potential and a lower pressure coefficient of the shale reservoirs. Thus, it can be concluded that Well LY1 has more favorable conditions for shale gas production. Well LY1 is situated in composite syncline LC. Imaging logging and core analysis show that the Wufeng Fm-Longmaxi Fm contains a large number of high-angle fractures and highly conductive fractures, all of which are unfavorable preservation conditions for shale gas. The total hydrocarbon potential is lower (<1%), with an average gas content of only 0.26 m 3 /t. It can be concluded that uplifting time, faults, syncline width, and burial depth of shale gas layers are the major factors for evaluating preservation conditions of shale gas layers in extrabasinal areas. 4.2.2. Basin-marginal areas: basin-controlling faults Due to the continuous influence of the Jiangnan-Xuefeng uplift and the Central Guizhou uplift, the structures at the first row in the western Qiyueshan faulted belt in southeastern Sichuan Basin show various tectonic styles (faulted anticline and faulted nose, etc.). High-quality shales are developed in the deep-water continental shelf in the Wufeng Fm-Longmaxi Fm of these areas. In addition, the burial depths of the shale gas layers are moderate (generally 2000 4500 m). Thus, this area is one of the most realistic exploration areas currently in the Sichuan Basin and its periphery. However, drilling of several wells reveals that the basin-controlling fault (Qiyueshan fault) plays a critical role in gas-bearing potential of shale gas. Typically, after vertical dissipation of shale gas along faults and fractures occured in varying degrees near the Qiyueshan faulted belt, the shale gas far away from the fault belt migrated laterally to fault and fracture development zones driven by concentration differences before effusing away. One typical example is the DS area (Fig.9). Both Well DY1 and Well DY2 are situated in the Dingshan nose-like faulted anticline. The shale gas layers in the Wufeng Fm-Longmaxi Fm in Well DY1 are at a burial depth of 2054 m, with a high-quality shale thickness of 26 m, an average gas content of 3.07 m 3 /t, and a formation pressure coefficient of 1.08. Through testing, a 3.4 10 4 m 3 /day commercial gas flow was obtained from the horizontal well. Well DY2 is an exploratory well targeting deep shales, with a shale gas layer depth of 4367.5 m, a high-quality shale thickness of 35.5 m, a formation pressure coefficient of 1.55, and a testing commercial gas flow of 10.5 10 4 m 3 /day form the horizontal well. Lower gas-bearing potential and lower pressure coefficients in the shale gas layers of Well DY1 are caused by two factors. Firstly, there are more high-angle fractures in the shale gas layers, which leads to some dissipation in the vertical direction. Secondly, it is close to the Qiyueshan faulted belt (8.5 km), where a steep pressure drop causes the fluid to migrate a short distance, leading to considerable effusion in the lateral direction. Compared with Well DY1,
12 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Fig. 9 Section of the Wufeng-Longmaxi shale gas layers in the DS area Well DY2 is further away from the Qiyueshan faulted belt and outcrop area; thus, it has a better preservation condition. The factors listed above show that for the structures at the first row in the western Qiyueshan faulted belt at a moderate burial depth, their shale gas-bearing potentials are apparently related to their distance from the Qiyueshan fault belt. 4.2.3. Intrabasinal areas: faults and high-angle fractures Shale gas exploration breakthroughs have been achieved in several tectonic styles (e.g. anticline, uplift and slope) in relatively stable areas of the Sichuan Basin. The exploration success ratio is high, the testing production in single wells is great (generally >5.0 10 4 m 3 /d), and the pressure coefficients of shale gas layers are higher than normal (1.2 2.0). On one hand, high-quality shales are developed in deepwater continental shelves via exploration breakthroughs. On the other hand, the formations in these areas are fully preserved, and the tectonic deformation is weak; most of the Wufeng Fm-Longmaxi Fm is not exposed to the surface, and shale depths are generally deeper than 2000 m. Therefore, most areas (except for the Changning area) don t have shale gas outcrops and lateral effusion. However, with the gradual expansion of exploration and research, some complexities have been found relating to shale gas exploration in intrabasinal areas. Even for a shale gas field, different tectonic positions may yield different production capacities in shale gas wells, as they have different degrees of development for faults and high-angle fractures. Exploration and development of the Fuling shale gas field reveal that the stable areas of the Jiaoshiba structure have marked differences in productivity, drilling fluid leakages, and measured shale gas well pressures, particularly from the belts with well-developed faults and high-angle fractures. Specifically, drilling fluid leakage is more serious in the southwestern area near the Wujiang fault and the eastern areas of the Shimen fault and the Daershan fault. Analysis indicates that fault characters, fault scales, and differing densities and fillings of the derived high-angle fractures are the major underlying causes for these differences (Fig.10). 1 Fault characters. The character of the regional structures (thrusting during early stage, strike-slipping during late stage) in southeastern Chongqing city caused the differences in regards to fault characters. NE striking faults were mainly affected by early thrusting, and less importantly, strike-slipping; NW striking faults were mainly affected by late sinistral strike-slipping. For the reverse faults, the the strike-slipping character was the character with the larger effect on shale gas effusion,; the strike-slipping character is also one of the major reasons that drilling fluid leakage is at its most serious near the NW striking Wujiang fault in the Jiaoshiba area. 2 Fault scales. The small faults (Himalayan period) in the third-order tectonic units of the Jiaoshiba structure have less of an effect, as seen through the presence of fewer high-angle fractures and good gas-bearing potential in its shale gas layers;. In contrast, drilling fluid leakage can more easily occur in the regions near peripheral large faults, and gas-bearing potential in those shale gas layers becomes worse. Additionally, the big faults surrounding the major part of the Jiaoshiba structure also indicate that, with the increase of fault scales, drilling fluid leakage has become more serious. To further prove this point, the Diaoshuiyan fault and Tiantaichang fault in NW flank have the smallest displacements and lengths; thus, drilling fluid leakage decreases in these near wells. With the sequential increase of fault displacements and fault lengths of the Shimen, Daershan, and Wujiang