Exploration Prospects of Shale Gas Resources in the Upper Permian

Jan 2, 2017 - Studies indicate that the Upper Permian Linxi Formation has thick black .... As the major rock stratigraphic units of the Late Permian, ...
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Exploration Prospects of Shale Gas Resources in the Upper Permian Linxi Formation in the Suolun-Linxi Area, NE China Renxing Lou,† Qingshui Dong,*,†,‡ and Hui Nie§ †

College of Earth Sciences, Jilin University, Changchun 130061, People’s Republic of China Key Laboratory of Oil Shale and Symbiosis Energy Minerals of Jilin Provence, Changchun 130061, People’s Republic of China § BGP Dagang Branch of Geophysical Research Institute, Tianjin 300280, People’s Republic of China ‡

ABSTRACT: The commercial exploitation of shale gas in the United States has changed the world’s energy structure. At present, many countries are increasing their commercial investments in this field, including China. On the basis of systematic analyses of laboratory test results, and drilling and outcrops investigation data in the Suolun-Linxi area, northeast of China, shale gas forming conditions and exploration prospects of the Upper Permian Linxi Formation are studied from such properties as the source rock sedimentary characteristics, the gas concentration of organic-rich shales, and the organic geochemical and reservoir characteristics. Studies indicate that the Upper Permian Linxi Formation has thick black shale (ranging from 50 to 485), excessive total organic carbon (TOC) content (ranging from 0.15% to 3.02%), a high thermal maturity with an average Ro value of 2.6% (ranging from 2.03% to 4.33%), and a high brittle mineral content with an average value of 71.2% (ranging from 58.5% to 97.5%). Moreover, abundant micro- and nanometer-sized pores and microcracks are developed in the mud shale, and the Upper Permian Linxi Formation has a formation with good potential shale gas resources. Using a comprehensive superposition method of geological information in the research area, we optimized two potential shale gas exploration areas: Bayinhushuo- Holingol and Jarud Banner.

1. INTRODUCTION Inspired by successful commercial exploitation in the field of shale gas in North America, the world has seen a shale gas revolution. According to 2015 U.S. Energy Information Administration (EIA) statistics, global technically recoverable shale gas resources are approximately 7,576.6 trillion cubic feet (Tcf), which is mainly distributed in China, Argentina, Algeria, U.S, Canada, Mexico, Australia, South Africa, Russia, and Brazil.1 In recent years, China has increased the exploration of shale gas. By May 2015, China had issued 54 exploration shale gas areas up to 170,000 km2. At the same time, it had accumulated a total of 840 shale gas drilling stations, 38 of which had a daily output of more than 0.035 billion cubic meters (Bcf). With further increases in the degree of exploration, shale gas production continues to increase. According to the latest report of EIA, in August 2016, in the United States, shale gas production accounted for more than half of U.S. natural gas production in 2015 and is projected to more than double from 37 Bcf/d in 2015 to 79 Bcf/d by 2040. Compared with the United States, China has only produced 0.5 Bcf/d of shale gas as of 2015.1 However, shale gas is projected to account for more than 40% of the country’s total natural gas production by 2040, which would make China the second-largest shale gas producer in the world after the United States. The northeast of China accounts for more than 20% of the country’s total geological reserves of shale gas. The study of shale gas in northeastern China is of great significance to improve the current situation and ease shortages of oil and gas. The Suolun-Linxi area is located in northeastern China, in which the Upper Paleozoic strata are widely distributed and have experienced a number of tectonic activities. These strata have been strongly deformed in local areas, but regional meta© XXXX American Chemical Society

morphism has not yet occurred. As one of the important raw hydrocarbon layer systems, the Upper Permian Linxi Formation developed a large number of high maturity dark mud shales, which are in the dry gas generation stage. Through a comprehensive analysis of source rock thickness, abundance of organic matter, maturity distribution, shale mineralogy, and the pore and fracture characteristics of the Linxi Formation, this paper studies this exploration prospect as a potential shale gas resource.

2. GEOLOGICAL SETTING The study area is mainly located in the western margin of the Songliao Basin and the central-southern areas of the Daxinganling Mountains. The area’s tectonic position is roughly equivalent to the south of the Xingan block that exists between the North China plate and the Siberia paleo plate (Figure 1). Moreover, this is the place that the Paleo-Asian Ocean residual basin final collision disappeared along the XarMoron River suture belt at the end of the late Paleozoic.2 Under the influence of the whole North China plate tectonic setting, the study area has suffered from multiphase tectonic events during the late Paleozoic. The Caledonian mountain-building movement results in the angular unconformity between the Devonian and the underlying Silurian strata. Since then, this area came into the regional extension phase. Until the late Early Carboniferous, the Ergun-Xingan block collisionally spliced with the eastern Songnen Massif, causing the study area to evolve from the terrestrial edge phase into open platform and neritic shelf phases. Received: August 3, 2016 Revised: October 31, 2016 Published: January 2, 2017 A

DOI: 10.1021/acs.energyfuels.6b01882 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Tectonic units in the northeast of China (modified with permission from ref 3). I−Okhotsk suture zone; IIDeerbugan tectonic zone; III− Hegenshan suture zone; IVXar Moron River suture belt; V−Nenjiang-Balihan strike-slip fault; VIMudanjiang fault belt; VII−Yitong-Yilan strikeslip fault; VIIIDunhua-Mishan strike-slip fault belt; IXXihot-Alin tectonic zone. reflectance, which was dominated on whole-rock particulate mounts by a MVP-3 microscope photomultiplier. Porosity and permeability were measured by means of the lowpressure N2 and He isothermal experiments, which were performed at the Ministry of Land Mineral Resources Supervision and Testing Center for mineral resources in Chongqing of China. The samples prepared for adsorption analysis were first outgassed at 120 °C under high vacuum in the apparatus to remove air, free water, and other gases. Previous studies show that the DFT molecular model can be used for PSD determination in the micropore scale as well as the mesopore scale.7−9 In this study, the micropores and mesopores play an important role in the pore structure of the shales; therefore, the N2 and He data collected on the samples were interpreted by applying DFT model analyses for PSD. In this work, bulk mineralogy was measured by X-ray diffraction (XRD) using 13 powdered samples with the device of D8 Advance at the Ministry of Land Mineral Resources Supervision and Testing Center for mineral resources in Chongqing of China, and the intensity data were collected in the 2θ range of 3−45° at steps of 0.02° (Cu Kα). Using FE-SEM (JSM 6610) equipped with an energy-dispersive spectrometer (EDS), a total of 13 mud shale samples (10 × 10 mm) that were subjected to ion milling using an Ar beam source were observed and analyzed at the Ministry of Land Mineral Resources Supervision and Testing Center for mineral resources in Chongqing, China. These

In a relatively stable extensional tectonic environment during the period between the Late Carboniferous and the Middle Permian, the study area has developed a large set of marine deposits.3,4 The Paleo-Asian Ocean was closed during the late Middle Permian− Early Triassic period, and the seawater retreated from the study area and a terrestrial setting occurred.5,6 The Linxi Formation (P3l) in the study area was deposited in the late Middle Permian resressice environment, which mainly develops a thick semideep lacustrine dark mud shale source rocks. Numerous fossils can be found in the research area, such as bivalves, conchostracans, plants, and pollen.

3. MATERIALS AND METHODS 3.1. Sample Collection. Only a few shale gas wells have been drilled in this area; therefore, it was difficult to obtain cores to better evaluate the shale gas resources in the study area. In this study, then, outcrop samples were mainly relied upon instead of cores. Through field outcrops and a few well drilling core samplings, a total of 22 samples (5 from wells) were collected from the Linxi shale in the study area. 3.2. Methods. The total organic carbon (TOC) content of 30 samples was measured using a Leco carbon−sulfur analyzer. The thermal maturity of 10 samples was determined according to the vitrinite B

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Energy & Fuels analyses were performed at a temperature of 25 °C and a humidity level of 35%. The isothermal adsorption experiments were done by using a magnetic levitation balance high-pressure isothermal adsorption instrument of ISOSORP-HP-type produced by the Rubotherm company of Germany. The field mud shale samples were analyzed at a temperature of 30 °C using CH4 as the adsorption gas, and the test results have been converted to the standard state.

indicators of shale gas source rocks are mainly organic matter abundance, type, and maturity. Through organic geochemistry tests and analyses of 113 dark mudstone and mud shale samples that were collected from field outcrops and drill cores, the characteristics of shale gas source rocks of the Linxi Formation are evaluated. Because of the high level thermal evolution of hydrocarbon source rocks in the Linxi Formation, the hydrogen index (HI) and the potential hydrocarbon generation amount (S1 + S2) are low (Table 2), and they cannot accurately reflect the abundance of organic matter. Therefore, this paper uses a combination analysis of residual organic carbon content (TOC) and saturated hydrocarbon gas chromatographic analyses to evaluate the abundance and type of organic matter in the Linxi Formation. Through organic carbon content (TOC) tests of black mudstone and mud shale samples in the Linxi Formation collected from the Suolun-Linxi area, results show that the organic carbon content (TOC) of the samples ranges between 0.15% and 3.02% (Table 2), with an average of 0.75%. Additionally, the TOC value of several samples is more than 2%, while in most it is 0.6% ∼ 1.6%, which reveals that almost all test samples basically reached the standard of hydrocarbon source rocks. The sterane triangle kerogen type diagram of samples in the Linxi Formation shows the kerogen of the organic matter in the study area is predominantly type II1−II2 (Figure 3). From the relationship between TOC and the hydrocarbon-generating potential of dark mudstone in the Suolun-Linxi area (Figure 4), it can be observed that the hydrocarbon-generating potential of the outcrop samples in the Linxi Formation on the whole is relatively lower than that of drilling core samples, and more than 75% of them reached the medium−good standard of hydrocarbon source rocks, which illustrates the hydrocarbon source rocks of the Linxi Formation have good hydrocarbon generation potential. The influence of organic maturity on the formation of shale gas is complex. When the shale reaches maturity, the shale gas potential gradually decreases with increasing maturity, but the shale micropores, adsorption capacity, and shale gas content correspondingly increase. The US shale gas development practice has shown that the vitrinite reflectance (Ro) of shale in recent, commercial exploitation is 1.1%−3.0%.13,14 Study of predecessors on the thermal evolution of source rocks in the Linxi Formation in the Suolun-Linxi area in northeastern China demonstrates that the vitrinite reflectance (Ro) values of the mud oil shale are mainly in the range 2.0% ∼ 4.0%, and the organic maturity indicators OEP and CPI were 1.00−1.24 and 1.07−1.35, respectively, indicating that the degree of hydrocarbon source rocks’ evolution and maturity are high.15 Similarly, the maturity test of six Linxi Formation shale outcrop samples collected from different parts of the study area shows that the vitrinite reflectance (Ro) of samples in the Jarud Banner and Wulagai area is 0.79% ∼ 2.27%, averaging 1.62%, while it is 2.90% ∼ 4.33% in Suolun and Guangdi town, with an average of 3.57% (Table 2, Figure 2). The rock pyrolysis of 22 dark mud shales shows that the Linxi Formation has a high thermal temperature, and the maximum pyrolysis temperature (Tmax) is mainly distributed in the range 455−588 °C, with an average of 523 °C. According to the geochemical evaluation criteria of continental hydrocarbon source rocks of China (SY/T57351995), comprehensive analysis of vitrinite reflectance (Ro) and the maximum pyrolysis temperature (Tmax) demonstrate that the source rocks of the Linxi Formation in the study area in general are in the high and overmatured stage (Figure 5).

4. RESULTS AND DISCUSSION 4.1. Sedimentary Characteristics. As the major rock stratigraphic units of the Late Permian, the Linxi Formation is widely distributed in the Suolun-Linxi area, eastern Inner Mongolia of North China. Through six field section measurements of the Linxi Formation in Guandi and Taohaiyingzi of the eastern Inner Mongolia, combined with the analysis of drilling core facies with HZK1 in the study area (Table 1), the results Table 1. Black Mud Shale Parameters of the Upper Permian Linxi Formationa Sec No/Bh No

FT (m)

DSL

TTS (m)

MTS (m)

MR

P-GD-01 P-GD-02 P-GD-03 P-GD-04 P-GD-05 P-TH-01 HZK1

105.1 61.31 58.5 27.09 43.93 239 844.5

8 6 5 3 13 16 27

61.84 56.13 43.93 9.55 38.77 93.3 483.65

16.72 20.99 18.84 4.17 18.84 21.02 32.95

0.59 0.92 0.75 0.35 0.88 0.39 0.57

a

Abbreviations: Sec No/Bh No: section number/borehole number; FT: formation thickness; DSL: dark mud shale layers; TTS: the total thickness of dark mud shale; MTS: the maximum single layer thickness of dark mud shale; MR: Mud ratio.

show that the Upper Permian Linxi Formation mainly developed continental lakes and fan delta deposits, whose lithology is mainly dark gray-black mudstone, mud shale, silty shale, siltstone, and fine sandstone. On the whole, the Linxi Formation has a great single layer thickness of dark mudstone and a high mud ratio. Furthermore, its mud shale is interbedded with sandstone with different thicknesses. The thickness planimetric distribution of the Upper Permian black mud shale in the study area shows that the semideep lacustrine black mud shale is mainly developed in TuquanHolingol-Xi Ujimqin Banner in the middle of the research area, and its average thickness is greater than 500 m (Figure 2). However, as is the same as the Jarud Banner region in the southern area, the Suolun region in the northern area is dominated by fan Delta facies, the black mud shale of which is relatively undeveloped. As a whole, it is a large continental lacustrine depositional system in the study area during the deposition of Linxi Formation,10 which developed a large number of black mud shales with great thickness up to 483.65 m, while the single layer can be up to 32.95 m (Table 1). Compared to the shale thickness with Barnett shale of the Fort Worth Basin (60−300 m) and Lewis shale of San Juan Basin (152−579 m),11,12 which is present in the shale gas exploitation stage in North America, the study area also has a thick dark mud shale of Linxi Formation (44−844 m). These thick dark mud shales contain large amounts of organic matters, which are able to provide adequate material basis for shale gas formation. 4.2. Organic Geochemical Characteristics. As with conventional hydrocarbon source rocks, the organic geochemical C

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Figure 2. Comprehensive evalution of Linxi Formation dark mud shale and shale-gas favorable area forcast in Suolun-Linxi area, NE China.

4.3. Shale Gas Reservoir Characteristics. Brittle mineral content has a major influence on the physical porosities of mud shale and is associated with microfracture development, gasbearing, and subsequent fracturing.16,17 Through X-ray diffraction (XRD) quantitative analysis of 13 field samples in the Linxi Formation in the study area (Table 3), results show that those samples have a high brittle mineral content, ranging from 58.5% to 97.5% (average 71.2%), of which the quartz content is highest, accounting for 41.7% ∼ 53.6% (average 47.0%) of the total. The rest of the brittle minerals such as feldspar content is 0.3% ∼ 7.2% (average 2.3%), plagioclase content is 5.9% ∼ 45.4% (average 16.5%), calcite content is 0.3% ∼ 2.2% (average 1.3%), dolomite content is 0.4% ∼ 1.8% (average 1.4%), siderite content is 0.6% ∼ 2.9% (average 1.8%), and anatase content is 1.1% ∼ 3% (average 1.8%). Compared with brittle minerals, the clay mineral contents of those mud shales of the Linxi Formation are relatively low, ranging from 2.5% to 41.5% (average 29.1%). Its clay mineral composition is mainly dominated by Illite of 24% to 89% (average 63.5%), chlorite 9%, Illite smectite mixed layer 11% to 76% (average 34.2%), and chlorite - smectite layer 5%, not

containing expansion clay minerals such as montmorillonite, kaolinite, etc. (Figure 6). Previous studies have shown that the content of clay minerals in shale gas reservoirs has a certain relevance with adsorbed gas content. Illite and chlorite concentrations on adsorption of shale gas are the most influential, but swelling clay minerals such as montmorillonite and kaolinite are detrimental to the late-made reservoir fracturing,18 which is not conducive to late shale gas absorption. X-ray diffraction quantitative data of 13 field samples show (Table 3) that the silica content of black mud shale in the Linxi Formation is high, and the clay minerals are dominated by Illite and Illite smectite mixed layers, excluding swelling clay minerals, such as montmorillonite and kaolinite, etc. These properties are all conducive to the late reformation of the reservoir and shale gas adsorption. Compared to the mineral compositions with the Ohio shale in the Appalachian basin, the Barnett siliceous shale of the Fort Worth basin, and the Bossier shale of the East-Texas basin in North America,19−21 the Linxi Formation mud shales in the study area are characterized by low carbonate content and high quartz-feldspar content (Figure 7), the overall mineral D

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Energy & Fuels Table 2. Rock Pyrolysis Data of the Upper Permian Linxi Formation Sample

HI

Tmax (°C)

S1 (mg/g)

S1+S2 (mg/g)

TOC

Ro

B004-1 GD14-2 SL-2 LX-8 TH-10 HE-2 B14002 B14007 B14010 B14014 B14017 B14018 B14019(1) B14021 B14027 B14028 B14031(3) B14043 TJ14101 HZK1-72 HZK1-53 HZK1-81

0.66 1.32 0.66 0.71 5.00 1.03 1.50 1.47 1.62 1.34 1.31 2.34 2.22 4.33 10.13 4.53 1.84 1.04 1.80 5.00 19.00 12.00

537.00 527.00 527.00 522.00 455.00 528.00 541.00 529.00 540.00 518.00 505.00 490.00 532.00 549.00 503.00 521.00 549.00 331.00 543.00 580.00 586.00 588.00

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.05 0.02

0.03 0.02 0.02 0.02 0.05 0.02 0.03 0.03 0.04 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.02 0.03 0.06 0.22 0.10

3.02 0.76 1.51 1.40 0.80 0.97 1.15 1.59 1.64 1.13 1.42 0.85 0.98 0.45 0.15 0.32 1.28 1.30 1.28 0.88 0.88 0.65

2.03 3.47 4.33 2.05 0.79 2.90

Figure 4. Relationship between TOC and the hydrocarbon-generating potential of dark mudstone in the Suolun-Linxi area.

Figure 5. Tmax distribution of Linxi Formation in the Suolun-Linxi area, NE China.

nanopores, intragranular nanopores, dissolved nanopores, organic matter nanopores, clay minerals interlayer nanopores, microcracks, etc. Studies of shale gas reservoirs at home and abroad indicate that the organic matter nanopores and the clay mineral interlayer nanopores are the major contributors to the matrix pore of mud shale reservoirs.22 Together with the fissures in the mud shale, they form the major shale reservoir space. Scanning electron microscopy of 13 mud shale samples in the Linxi Formation confirmed that the organic matter has appeared 32 times, of which the diameter is generally 10−700 nm, has a mainly dispersed distribution, and is local banded. With the honeycomb pores developed internally (Figure 8a), the organic matter is in the adsorbed natural gas main reservoir spaces. In the high and overmature stage, thermal degradation and hydrocarbon generation of kerogen will produce a large number of secondary pores, microfissures, and nanopores between organic matter and minerals (Figure 8b). These nanopores and fissures easily form an interconnected oil network without capillary resistance with microlayers of mud shale, which greatly improves the adsorption capacity.23,24 In addition to the nanopores of organic matter, the interlayer nanopores of clay minerals also play an important role in the transformation of mud shale reservoirs. The dehydration

Figure 3. Kerogen type ternary diagram of the Linxi Formation based on the relative content of sterane.

composition of which is exactly similar to that of the Barnett siliceous shale. With high brittle mineral content and hard-brittle rock, the mud shale of the Linxi Formation is conducive to the formation of natural fractures and late artificial fracturing, which has great prospects for shale gas accumulation. As unconventional reservoirs, mud shales are characterized by ultralow porosity and permeability, with diverse types of reservoir space. The pore and fracture characteristics of mud shale are observed simultaneously in two places: Chongqing mineral resources supervision and inspection center of Ministry of Land and Resources and the Test center of Jilin University. Using high-powered scanning electron microscopes, the reservoir microscopic features of 13 field mud shale samples of the Linxi Formation in the study area are studied. The results show that the mud shales developed micropores and microcracks (Figure 8), of which the types mainly are intergranular E

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Table 3. Mineralogical Composition of the Dark Mud Shale Based on XRD Analysis in the Suolun-Linxi Area, NE China Sample

quartz (%)

feldspar (%)

B004-2 SL-2 LX-8 XW-5 TH-10 HE-2 B14002 B14014 B14018 B14021 B14027 B14031 B14101

49.3 41.7 43.2 45.3 43.5 53.6 47.8 45.5 48.9 51.9 43.5 50.7 45.6

10.6 22.9 8.0 10.1 13.9 7.3 20.2 9.9 11.8 20.2 46.5 28.0 34.7

calcite (%)

dolomite (%) 0.4

1.8 1.8 2.7

2.2

1.8 0.3 1.1 1.0

siderite (%)

0.9 0.6 1.7 2.9

1.7 1.8 1.7 1.3

anhydrite (%) 1.0 5.5 2.9 1.7

anatase (%)

Total clay (%)

1.3 3.0

38.8 29.2 41.5 38.4 40.9 34.9 26.0 40.3 30.5 21.7 2.5 18.1 15.5

1.1 5.4 2.6 2.4 4.1 6.4 1.5 1.9

content of clay minerals in the Linxi Formation is up to 29.1%, of which a large proportion is Illite with an average content of more than 60%. The high Illite ratio provides a good condition for the formation of interlayer nanopores. The porosity and permeability test results of the Linxi Formation mud shale show that the porosity is generally 0.21% ∼ 4.42% in this area with an average of 1.64%, and the permeability is generally 0.0013 × 10−3 ∼ 0.2758 × 10−3 μm2 with an average of 0.0492 × 10−3 μm2, which indicates that the mud shale belongs to the ultralow porosity and ultralow permeability reservoir. However, a large number of cracks can be observed in field outcrops and drilling cores of the Linxi Formation, and even many local net crack structures are formed (Figure 9). Scanning electron microscopy also showed that the tectonic microfissures of the Linxi Formation mud shale in the study area are well developed, and its fissure width is mainly 67− 240 nm with local fracture width up μm(Figure 10a, Figure 10b). These bedding fissures and microstructural fractures, which began in the late Permian-Triassic Indosinian, greatly improved the shale reservoir porosity and permeability and also provided the venue for the post shale gas accumulation. 4.4. Shale Gas-Bearing Characteristics. The shale gasbearing characteristic is an important indicator of its resource potential evaluation and favorable zone optimization. By analyzing field mud shale samples at a temperature of 30 °C, the test results show that the adsorption content of samples in the Linxi Formation in this region is 1.22−1.91 cm3/g with an average of 1.56 cm3/g (Figure 11). Compared with the Barnett Shale gas content (8.5−9.9 cm3/g) of samples in the Fort Worth Basin in the USA,11 this test result is low. It is related to the longterm exposure to the surface of the outcrop samples, causing the shale gas to escape, which requires further study.

Figure 6. Dark mud shale clay minerals relative content of the Linxi Formation in the Suolun-Linxi area, NE China.

Figure 7. Ternary diagrams showing the mineralogy comparison between typical shales in North America (data from the studies by Han et al.20 and Jarvie et al.21) and Linxi Formation dark mud shale in the Suolun-Linxi area, NE China.

transformation of clay minerals occurs in a diagenesis process and releases many interlayer waters, resulting in a large number of microcracks. The clay mineral transformation forms in the study area and includes smectite conversion into Illite and Illitesmectite mixed-layers, Illite-smectite mixed-layer conversion into Illite, and kaolinite conversion into chlorite. Using scanning electron microscopy and energy spectrum observation and analysis of 13 mud shale samples, a large number of interlayer microcracks can be observed among the curling-flake-like and silk-thread-like Illite. The crack widths are generally 150−750 nm (Figure 8c, Figure 8d), the largest of more than 1 μm, which have good connectivity. The relative content of the clay mineral distribution in the study area showed (Figure 6) the average

5. DISCUSSION According to the shale gas accumulation characteristics and the favorable geological factors of the reservoir, combined with the geological characteristics in the study area and existing test data, the comprehensive information superposition method is used to analyze the exploration prospects of the shale gas resource of the Linxi Formation in the Suolun-Linxi area in northeastern China. Using a combination of stratigraphic thickness, organic matter abundance, thermal evolution of organic matter, porosity, and other reference indexes, preferably in the study area, two shale gas exploration prospect favorable zones are optimized (Figure 2). There are Bayinhushuo-Holingol exploration prospects in the F

DOI: 10.1021/acs.energyfuels.6b01882 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. Dark mud shale porosity characteristics of the Linxi Formation. OM: organic matter. (a) organic honeycomb mesopores (LX07); (b) mesopores between organic matter and mineral (GD-4-04); (c) curling sheet Illite interlayer mesopores (HR-16); (d) elemental energy spectrum analysis of M in panel C.

northwest region and the Jraud Banner exploration prospect area with a total area of 1.74 × 104 km2. These prospective shales have the following common features: (1) The sedimentary environment is mainly for the reduction− strong reduction semideep lacustrine facies. (2) The mud shale has a large total thickness (50−485 m), high mud ratio (average 0.63), and adequate material basis. (3) The average total organic carbon content is greater than 1.0%, has a maximum up to 3.02%, and reaches a medium−good standard of hydrocarbon source rocks. (4) The organic matter of mud shale is generally in high and overmature stages of evolution, which is in favor of shale gas generation. (5) The common development of mud shale fractures made up for a lack of porosity and low permeability. (6) The brittle content of minerals such as quartz is high, which is conducive to the formation of natural fractures and artificial

Figure 9. Dark mud shale net cracks of Linxi Formation in HZK1.

Figure 10. Microcracks’ characteristics of the Linxi Formation dark mud shale: (a) microcracks (HR-25); (b) microcracks (GD-4-04). G

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ACKNOWLEDGMENTS This work is financially supported by the Tracking and Evaluation of Oil and Gas Investigation Project In the Southeast of Songliao Basin (Grant 12120115001701-2), the Tectonic Evolution and Petroleum Resource Potential of The Late Paleozoic Basin In Suolun-Linxi Area, Inner Mongolia (Grant DDYZ201330-3), and the 1:5 Million Geology and Mineral Comprehensive Resources Survey of Democratic Community, Han Temple, Chijiapu, Xisha, Inner Mongolia (Grant 12120115031701). We also appreciate the experimental support from those institutes mentioned above.

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Figure 11. Adsorption isotherm curves of the Linxi Formation mud shale in the Suolun-Linxi area, NE China.

fracturing in the late stage. In conclusion, the mud shale of the Linxi Formation of the Suolun-Linxi area in northeastern China has the basic conditions of a shale gas reservoir and therefore contains huge shale-gas exploration prospects.

6. CONCLUSIONS (1) The geochemical analysis of the Linxi Formation black mud shale in the study area demonstrated that the organic carbon content of the sample is mainly distributed between 0.15% and 3.02%, the organic type is mainly II 1 and II2, the vitrinite reflectance (Ro) ranges between 0.79% and 4.33%, and the maximum pyrolysis temperature (Tmax) mainly ranges between 455 and 588 °C, with an average of 523 °C. The relative content of brittle minerals such as quartz is between 58.5% and 97.5%, while the clay mineral contents are between 2.5% and 41.5%. These test results indicate that the Linxi Formation black mud shale in the study area has great potential to form shale gas reservoirs. (2) Scanning electron microscopy of the study area showed that the mud shale of the Linxi Formation has a great number of developed nanopores and microcracks, of which the types mainly include intergranular nanopores, intragranular nanopores, dissolved nanopores, organic matter nanopores, clay minerals interlayer nanopores, and microcracks. The nanopore diameters are mainly from tens to hundreds of nanometers, and few can reach the micron level. The development of a pore-fissure network structure of the Linxi Formation black mud shale greatly improves the porosity and permeability of tight reservoirs and is conducive to the migration and storage of shale gas. (3) Using a comprehensive superposition method of geological information in the Suolun-Linxi Area in northeastern China, we optimized two potential shale gas exploration areas: Bayinhushuo-Holingol and Jarud Banner. The total exploration prospect area is 1.74 × 104 km2.

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AUTHOR INFORMATION

Corresponding Author

*(Qingshui Dong) Telephone:0431-88502603. E-mail: [email protected]. Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.energyfuels.6b01882 Energy Fuels XXXX, XXX, XXX−XXX