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Factors Affecting Shale Gas Accumulation in the Overmature Shales-Case Study from the Lower Cambrian Shale in the Western Sichuan Basin, South China Yuying Zhang, Zhiliang He, Shu Jiang, Shuangfang Lu, Dianshi Xiao, Guohui Chen, and Jianhua Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03544 • Publication Date (Web): 11 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018
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Factors Affecting Shale Gas Accumulation in the Overmature Shales-Case Study from the Lower Cambrian Shale in the Western Sichuan Basin, South China †
Yuying Zhang , Zhiliang He
‡,§
‖
†
†
†
, Shu Jiang* , Shuangfang Lu* , Dianshi Xiao , Guohui Chen ,
⊥
Jianhua Zhao †
Research Institute of Unconventional Oil & Gas and Renewable Energy, China University of Petroleum
(East China), 266580 Qingdao, China ‡
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development,
100083 Beijing, China §
Petroleum Exploration and Production Research Institute, SINOPEC, 100083 Beijing, China
‖
Energy & Geoscience Institute, The University of Utah, 84102 Salt Lake City, UT, USA
⊥
School of Geosciences, China University of Petroleum (East China), 266580 Qingdao, China
Abstract: A series of comprehensive and systematic experimental data and log data of lower Cambrian Qiongzhusi Formation from the first successful Well Jin-1 in western Sichuan Basin were analyzed to investigate the properties of lower Cambrian organic-rich shale and to illustrate its shale gas potential, so as to investigate the factors affecting shale gas potential in overmature and ultratight shale. Trace element ratios (V/Cr and Ni/Co) and the concentrations of Cu+Zn imply the lower Cambrian shale deposited under suboxic-anoxic bottom water with high primary productivity. The organic matter enriches in Qiongzhusi Formation as sapropelic (TypeⅠ) mainly, and Ro (2.65 % 2.97 %) values indicate the organic matter in the overmature stage. Pores are dominated by organic pores in lower organic-rich shale and dominated by inorganic intragranular and intergranular pores in upper organic-rich shale, respectively. Generally, organic-rich shale was deposited in anoxic shelf (high ratios of V/Cr and Ni/Co) with high productivity (high concentration of Cu+Zn), and shale gas has accumulated and been preserved in the region with stable tectonic setting. Based on the comprehensive analysis of the properties in overmature shales (Marcellus, Longmaxi and Qiongzhusi) and their effect on shale gas, it shows that (a) the organic matter formed material basis for shale gas generation, (b) the porosity provided the capacity for the shale gas accumulation, (c) the moderate high thermal maturity (Ro up to 3 %) improved the storage space through elevating porosity. This indicates the TOC, porosity, and thermal maturity in overmature shales are the main factors affecting shale gas accumulation. Key word: lower Cambrian, organic-rich shale, shale property, shale gas, Sichuan Basin
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1. INTRODUCTION Sichuan Basin and its adjacent areas in South China have been considered as promising shale gas play areas,1 where shales in Lower Silurian Longmaxi Formation and Lower Cambrian Qiongzhusi/Niutitang Formation with high to over maturity are regarded as the most potential shale gas plays. 2 Previous researches suggested the accumulation and preservation of shale gas were determined by sedimentary environment, tectonic setting and structural activities. Since shale gas generated and accumulated in organic-rich shale, thus organic-rich shale is both source rock and reservoir rock, as well as seal rock.3-5 Properties of organic-rich shale (e.g. geochemistry, mineral composition, thermal maturity, porosity) are important parameters in evaluating promising shale gas play, which determines shale gas content largely.6-11 By comparison, Marcellus shale in Appalachia Basin in North America has similar properties with Cambrian Qiongzhusi equivalent shale and Longmaxi shale, such as thickness, depth, organic matter, thermal maturity, mineral components, and Marcellus shale and Silurian Longmaxi shale have been achieved success in commercial production. 12-14 However, it has not achieved significant breakthrough for shale gas exploitation in overmature and ultra-tight shale like lower Cambrian shale in South China, in which the majority drilled wells failed to find commercial shale gas accumulation due to very high maturity, poor reservoir quality (ultratight), and poor preservation condition comparing to Longmaxi and Marcellus shale.15-21 So far, properties of lower Cambrian shale and their effect on shale gas have not been well understood since very few type wells with limited cores inside Sichuan Basin are available to study the Cambrian shale reservoir properties and shale gas content, while most wells with no or low production were drilled in the tectonically disrupted areas outside the basin. In this study, Well Jin-1, a successful shale gas well targeting the very old lower Cambrian Qiongzhusi Formation shale in western Sichuan Basin has been selected as research object since it has a suite of log data, abundant tested data, and core samples covering the whole shale interval. Furthermore, the series of data from Marcellus and Longmaxi shale have been collected for analyzing the pattern and controlling factors of shale gas accumulation in other similar shales with high thermal maturity. The analysis of these data, additional massive systematic experimental data and comparison with other similar shales will help reveal the reservoir quality and factors affecting shale gas accumulation of lower Cambrian shale in South China and similar overmature shales, which will provide fundamental insights for
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predicting shale sweet spot of overmature shales with multiple complex tectonic activities and high thermal maturity. It would provide help to realize the commercial development of shale gas in deep zone (e.g. Lower Paleozoic) and basin with long geologic history and very old organic-rich shale (e.g. Sichuan Basin).
2. GEOLOGICAL SETTING Sichuan Basin is located in Yangtze Platform, including central-eastern Sichuan Province and western Chongqing Province. Yangtze Platform evolved from a rift basin to a passive continental margin basin at ca.750 Ma -690 Ma when separated from Rodinia supercontinent during its final breaking down.22, 23 Central-eastern Sichuan Basin was a carbonate platform during early Cambrian, while western Sichuan Basin was an intracratonic sag depositing shale and chert from Mianyang to Yibin (Figure 1a), formed by Xingkai taphrogenesis during late Proterozoic-early Cambrian.24, 25 A rapid large scale transgression happened in Early Cambrian, which resulted in global ocean transforming to anoxic muddy deep shelf from oxygenated carbonate platform in late Ediacaran.2628
The lower Cambrian strata (Qiongzhusi Formation) disconformably overlie upon the top of Ediacaran dolomite (Dengying Formation) with the thickness of 466 m in the study area (Figure 2), which is conformably overlain by sandstone in Yuxiansi Formation. The lower Cambrian organicrich shale (LCOS) in Qiongzhusi Formation can be divided into two intervals: (1) lower organicrich shale in Qiongzhusi Formation (LOSQF) from 3560 m - 3640 m, composed of phosphoric siliceous dolomite, dolomitic mudstone, black shale, marlstone; (2) upper organic-rich shale Qiongzhusi Formation (UOSQF) from 3276 m – 3310 m, composed of black shale and limestone (Figure 2). Over 200 m of argillaceous siltstone interbeded with mudstone was deposited between LOSQF and UOSQF. Since several intense tectonic activities occurred in Yangtze Platform since Indosinian period (Triassic), especially the intense compression in Yanshan movement, which resulted in a series of faulted fold belts in eastern Sichuan Basin and its surrounding regions, the preservation condition of shale gas has been severely disrupted. Fortunately, western Sichuan Basin is located in a relatively stable tectonic unit without major faults or folds, which is favorable for shale gas preservation compared with other areas in Yangtze Platform in South China (Figure 1b).24, 25, 29
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Figure 1a. Paleogeography and lithofacies during early Cambrian in Sichuan Basin in southwest China and location of Well Jin-1, modified from Ye et al. (2017);10 1b. The tectonic section in a nearly east-west direction of Sichuan Basin, modified from Deng (2013).28
3. MATERIALS AND METHODS A total of 35 samples were collected from LOSQF and UOSQF of Well Jin-1, including black shale, phosphoric siliceous dolomite, dolomitic mudstone, black shale, marlstone, siltstone and silty mudstone. TOC and shale gas content (total gas, free gas, absorbed gas) were generated from log data calibrated by experimental data due to limited quantities of samples. Shale gas content
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(absorbed gas and free gas) was calculated by TOC and porosity, and all the data of shale gas are provided by China Petroleum & Chemical Corporation (SINOPEC). Combined experimental data with log suite data, we characterize the geochemistry, depositional environment, thermal maturity, mineral components, reservoir, and shale gas content. Trace element concentrations (V, Cr, Ni, Co, Cu, Zn) were determined with inductively coupled plasma-atomic emission spectrometry (ICP-AES), using a modified multi-acid solvent (HNO3-HF) to dissolve samples. This is a high-resolution measurement since the error range is within 4%. They were accomplished at the laboratory of Beijing Research Institute of Uranium Geology, China National Nuclear Corporation. Thermal maturity data were determined by vitrinite reflectance (Ro), calculated from bitumen reflectance (Rb) according to the formulation suggested by Jacob (1989): Ro=0.618 Rb +0.4,30 and Rb was determined by MPV-Ⅲ microscope photometer in the State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, SINOPEC, the relative error is less than 5 %. Mineral components were analyzed via D8 ADVANCE XRD Diffractionmeter at Key Laboratory of Hydrocarbon Accumulation, SINOPEC, the quantity of them was measured by the individual spectra of the anticipated mineral components. Porosity was measured with the instrument of QK-98 porosimeter, and permeability was measured with the instrument of GDS-90Fpermeameter, both of which were accomplished by the State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, SINOPEC, the relative errors of both are less than 1 %.
4. RESULTS 4.1 Trace element concentrations Table 1 lists the data of V/Cr and Ni/Co ratios and Cu+Zn concentrations from the inorganic geochemistry tests for Lower Cambrian Qiongzhusi Formation shale. V/Cr ratios and Ni/Co ratios in LOSQF (lower interval) range from 1.45 to 4.99 (with an average of 2.36) and from 2.41 to 7.65 (with an average of 3.45), respectively (Table 1, Figure 2). V/Cr ratios and Ni/Co ratios in UOSQF (upper interval) range from 1.05 to 7.85 (with an average of 3.21) and from 2.48 to 16.22 (with an average of 8.03), respectively (Table 1, Figure 2). Cu+Zn concentrations are between 65.7 ppm-
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179.9 ppm (with an average of 112.1 ppm) in LOSQF and between 57.5 ppm-248 ppm (with an average of 120 ppm) in UOSQF (Table 1, Figure 2).
Figure 2 Lower Cambrian stratigraphic column with a series of parameters of geochemistry, mineralogy, petrophysics, and gas content in Well Jin-1. Curves of TOC, brittle minerals, quartz, clay minerals, porosity, total gas, free gas and absorbed gas are from log data; Edi.: Ediacaran, LOSQF: lower organic-rich shale in Qiongzhusi Formation, UOSQF: upper organic-rich shale in Qiongzhusi Formation
4.2 Organic Geochemistry LCOS have high organic matter abundance and high thermal maturity. TOC values vary from 0.2 % to 4.82 % (averaging 1.43 %) in LOSQF and from 0.64 % to 3.77 % (averaging 1.75 %) in UOSQF,
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on the contrary, TOC values in argillaceous siltstone or mudstone between them are relatively low (less than 0.5 % overwhelmingly). Ro values are calculated from bitumen reflectance (Rb) due to lack of vitrinite in Cambrian (see section 3. Materials and Methods for calculating formulation). On the basis of calculating results, Ro values are in 2.65 % - 2.97 % (Table 1).
Table 1. Geochemical proxies of Qiongzhusi Formation in Well Jin-1ᵅ Samples
Depth
V/Cr
Ni/Co
Cu+Zn
Ro
ppm
%
m Jin-01
3285.42
1.52
2.57
176.3
JY-019
3285.68
6.45
16.22
81
JY-018
3290.50
7.85
7.72
96.2
2.69
JY-017
3291.68
2.45
13.64
248
2.73*
JY-016
3294.60
3.17
13.99
151.2
2.70*
Jin-04
3295.00
1.81
3.18
149.6
Jin-05
3295.65
2.24
3.42
144.2
JY-013
3296.17
4.36
10.70
57.5
2.65*
JY-015
3299.95
3.05
7.74
64.6
2.73*
JY-014
3302.32
3.08
11.98
94.1
Jin-07
3308.30
1.05
2.48
90.7
2.67
JY-012
3309.02
1.58
2.67
86.6
2.66*
JY-011
3408.62
1.77
2.00
69.5
JY-010
3528.59
1.66
3.15
96.6
JY-009
3544.55
1.45
2.41
65.7
JY-007
3575.13
1.45
2.46
72
JY-008
3582.29
1.46
2.60
90.6
JY-004
3583.94
2.82
3.39
131.9
Jin-09
3585.88
--
--
--
JY-006
3588.90
1.46
2.57
80.4
JY-005
3594.32
1.75
7.65
91.2
JY-002
3604.29
3.79
3.43
146.3
JY-003
3608.11
4.99
4.17
179.9
JY-001
3616.09
2.81
2.68
166.3
2.66*
2.97
ᵅData with * are from Xie et al. (2015).42
4.3 Mineral Components Based on experimental data and log data (Table 2, Figure 2), quartz and clay minerals are the major mineral components in LOSQF and UOSQF (more than 70 % in general, Figure 3, Figure 4), except for only few individuals dominated by carbonate, e.g. Jin1-03, Jin1-10, and Jin1-13. In Qiongzhusi
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Formation, brittle minerals are composed of quartz (28 % on average), feldspar (K-feldspar and plagioclase, 6.2 % on average), carbonate and pyrite (8.8 % on average). The content of clay minerals is between 9 % and 55 % (averaging 34.4 %).
Table 2. Experimental data for relative mineral abundances of lower Cambrian organic-rich shale in Well Jin-1 Samples
Depth
Quartz
K-feldspar
Plagioclase
Carbonate
Pyrite
Clay minerals
Others
m
%
%
%
%
%
%
%
Jin1-01
3285.42
26
1
5
8
5
52
3
Jin1-02
3290.8
29
2
6
7
5
50
1
Jin1-03
3294.8
4
--
1
79
3
13
--
Jin1-04
3295
25
2
6
7
4
55
1
Jin1-06
3300.8
37
3
11
7
3
35
4
Jin1-07
3308.3
43
3
12
6
4
30
2
Jin1-09
3556.3
31
3
11
13
5
36
1
Jin1-10
3579.5
2
--
--
40
30
28
--
Jin1-11
3583.6
21
1
--
18
7
53
--
Jin1-13
3606.3
19
1
--
28
25
26
1
Jin1-14
3612.12
70
--
--
14
6
9
1
Figure 3. Percentage of mineral components for lower Cambrian organic-rich shale in Well Jin-1
4.4 Porosity and Permeability The porosity ranges from 0.51 % to 6.21 % (averaging 1.62 %) in LOSQF, while it ranges from 2.03 % to 5.84 % (averaging 3.82 %) in UOSQF (Table 3). In the meantime, permeability is from 0.003 mD to 0.09 mD without obvious difference between LOSQF and UOSQF (Table 3). Compared to typical producing North America shales and Wufeng-Longmaxi shale in South
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China,31, 32 LCOS in Sichuan Basin show relatively low porosity and low permeability.
Figure 4a., 4b. Microphotograph of biogenic quartz in lower Cambrian organic-rich shale under optical microscope, showing foraminifera shape (a. plane-polarized light, b. cross-polarized light), 3603 m; 4c., 4d. Microphotograph of mineral components in lower Cambrian organic-rich shale under scanning electron microscopy (SEM), 3296.17 m (Q: quartz, C: clay minerals, Py: pyrite).
Table 3. Porosity and permeability of lower Cambrian organic-rich shale in Well Jin-1 Sample
Depth
Porosity
Permeability
m
%
md
Jin1-01
3285.42
3.99
0.00543
Jin1-15
3287.68
2.03
0.031
Jin1-02
3290.8
4.17
Jin1-16
3291.28
Jin1-03
Sample
Depth
Porosity
Permeability
m
%
md
Jin1-19
3549.17
0.61
0.049
Jin1-20
3552.74
0.78
0.094
0.00564
Jin1-09
3556.3
0.51
0.00738
3.74
--
Jin1-21
3579.24
0.78
0.013
3294.8
3.59
--
Jin1-22
3582.89
0.75
0.016
Jin1-04
3295
2.56
0.00305
Jin1-11
3583.6
6.21
--
Jin1-17
3297.31
4.74
0.097
Jin1-23
3590.36
1.05
--
Jin1-06
3300.8
4.15
--
Jin1-24
3612.09
1.19
--
Jin1-18
3301.44
3.39
--
Jin1-14
3612.12
3.45
--
Jin1-07
3308.3
5.84
0.0134
Jin1-25
3615.9
0.94
0.028
Jin1-08
3523.4
1.57
0.00955
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5. DISCUSSION 5.1 Sedimentary Paleoenvironment from Inorganic Geochemical Proxies Although the formation of marine organic-rich shale was a comprehensive result from multi-factors in sedimentary environment, main factors controlling it are: (1) high primary productivity;33-35 (2) anoxic preservation condition under bottom water.36-38 Massive previous studies proved that trace element concentrations and ratios could be used as indicators representing redox conditions and primary productivity. In this study, V/Cr and Ni/Co have been used to evaluate paleoredox conditions, and the total concentration of Ni and Cu has been used to evaluate primary productivity. As suggested by Jones and Manning (1994)39, V/Cr
