Characterization of the shale gas formation process based on fluid inclusion evidence: An example of the Lower Cambrian Niutitang shale formation, Xiushan section, southeastern Chongqing

doi:10.13745/j.esf.sf.2022.5.42

Abstract

Abstract:

Direct geological evidences for the shale gas formation process are scarce. However, a clear understanding of the shale gas formation process in terms of oil/gas generation stages and the relevant temperatures and thermal maturity indicators is essential for shale gas exploration. Fluid inclusions can record the geological information on fluid activities during geological time therefore can provide evidence to reconstructing the fluid evolution history. In this study, shale samples are collected from the Lower Cambrian Liutitang Formation, Xiushan section, southeastern Chongqing to investigate the fluid inclusions entrapped in veins, using integrated fluid inclusion analysis method. Three types of hydrocarbon inclusions are detected, which are bitumen + gas, bitumen and gas inclusions indicating different stages of the shale gas forming process. The shale gas forming process can be divided into four stages: primary oil generation, oil cracking into wet gas, wet gas cracking into dry gas, and shale gas loss, where abnormal high temperatures associated with acid fluids activity play a key role in the process of transforming oil into gas. These findings can be applied to advancing shale gas exploration in areas that have a history of abnormal high temperature associated with thermal fluid activity and experienced relatively weak late-stage tectonic deformation.

Key words: shale gas, gas forming process, veins, fluid evolution, fluid inclusion, Niutitang Formation, southeastern Chongqing area

CLC Number:

P618.13

LIU Xiuyan, CHEN Honghan, XIAO Xuewei, LI Peijun, WANG Baozhong. Characterization of the shale gas formation process based on fluid inclusion evidence: An example of the Lower Cambrian Niutitang shale formation, Xiushan section, southeastern Chongqing[J]. Earth Science Frontiers, 2023, 30(3): 165-180.

Figures/Tables 11

References 32

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脉体编号	层位	岩性	宿主矿物产状	成因	盐水包裹体均一温度/℃		盐度(w(NaCl))/%		流体幕次
脉体编号	层位	岩性	宿主矿物产状	成因	范围	平均值	范围	平均值	流体幕次
DQ15-1	牛蹄塘组	暗色泥岩	顺层含黄铁矿方解石脉	原生	95.8 ~ 104.9	100.3 (n=6)	0.2 ~ 4.0	1.9 (n=4)	C1
DQ12-1	牛蹄塘组	暗色泥岩	顺层方解石脉	原生	115.5 ~ 128.9	123.0 (n=22)	1.7 ~ 6.2	2.5 (n=8)	C2
				原生	135.6 ~ 153.5	144.6 (n=23)	0 ~ 5.3	2.7 (n=18)	C3
				原生	161.6	161.6 (n=1)	5.3	5.3 (n=1)	C3
				次生	97.1 ~ 103.5	100.4 (n=4)	2.9 ~ 3.2	3.1 (n=2)	C6
			顺层石英脉	原生	163.7 ~ 169.8	166.9 (n=6)	5.1 ~ 7.2	6.1 (n=5)	Q1
				次生	189.5 ~ 199.9	195.1 (n=5)	11.6	11.6 (n=1)	Q2
				次生	230.7 ~ 236.1	232.5 (n=3)	—	—	Q3
				次生	120.6	120.6 (n=1)	—	—	Q6
				次生	143.4	143.4 (n=1)	17.5	17.5 (n=1)	Q5
DQ9-1	牛蹄塘组	暗色泥岩	垂直脉中方解石	原生	124.6 ~ 129.0	126.7 (n=3)	2.7 ~ 4.8	3.8 (n=2)	C5
			垂直脉中方解石	原生	139.6 ~ 150.9	143.9 (n=6)	2.4 ~ 7.0	4.4 (n=4)	C4
			垂直脉中方解石解理面	原生	104.1 ~ 111.6	107.9 (n=4)	2.1 ~ 4.8	3.1 (n=4)	C6
			垂直脉体壁上石英	原生	160.2	160.2 (n=1)	4.6	4.6 (n=1)	Q1
				次生	163.1 ~ 172.3	167.7 (n=2)	—	—	Q4
				次生	194.5	194.5 (n=1)	6.2	6.2 (n=1)	Q2
DQ14-1	牛蹄塘组	暗色泥岩	垂直脉中方解石	原生	150.1	150.1 (n=1)	2.4	2.4 (n=1)	C4
			垂直脉中方解石解理面	原生	107.9	107.9 (n=1)	1.7	1.7 (n=1)	C6
			垂直脉体壁上石英	原生	162.1 ~ 166.2	165.0 (n=7)	6.0 ~ 6.7	6.3 (n=3)	Q1
				次生	215.8 ~ 225.6	220.7 (n=2)	—	—	Q3
				次生	161.1 ~ 171.5	164.8 (n=6)	3.9 ~ 7.2	5.5 (n=2)	Q4
				次生	142.0 ~ 158.7	151.9 (n=6)	0.4 ~ 3.5	2.3 (n=6)	Q5
				次生	114.5 ~ 126.0	118.3 (n=4)	3.9	3.9 (n=1)	Q6
DQ14-2	牛蹄塘组	暗色泥岩	垂直脉外侧纤维状方解石	原生	135.6 ~ 145.4	141.5 (n=7)	0.9 ~ 8.0	4.4 (n=5)	C4
				原生	164.3	164.3 (n=1)	11.3	11.3 (n=1)	C4
				原生	203.8	203.8 (n=1)	—	—	C4
				原生	124.1 ~ 134.2	129.4 (n=3)	1.4 ~ 7.3	4.4 (n=2)	C5
			外侧纤维状方解石解理面	原生	103.3 ~ 103.4	103.4 (n=2)	0.9 ~ 1.9	1.4 (n=2)	C6

脉体编号	层位	岩性	宿主矿物产状	成因	盐水包裹体均一温度/℃		盐度(w(NaCl))/%		流体幕次
脉体编号	层位	岩性	宿主矿物产状	成因	范围	平均值	范围	平均值	流体幕次
DQ15-1	牛蹄塘组	暗色泥岩	顺层含黄铁矿方解石脉	原生	95.8 ~ 104.9	100.3 (n=6)	0.2 ~ 4.0	1.9 (n=4)	C1
DQ12-1	牛蹄塘组	暗色泥岩	顺层方解石脉	原生	115.5 ~ 128.9	123.0 (n=22)	1.7 ~ 6.2	2.5 (n=8)	C2
				原生	135.6 ~ 153.5	144.6 (n=23)	0 ~ 5.3	2.7 (n=18)	C3
				原生	161.6	161.6 (n=1)	5.3	5.3 (n=1)	C3
				次生	97.1 ~ 103.5	100.4 (n=4)	2.9 ~ 3.2	3.1 (n=2)	C6
			顺层石英脉	原生	163.7 ~ 169.8	166.9 (n=6)	5.1 ~ 7.2	6.1 (n=5)	Q1
				次生	189.5 ~ 199.9	195.1 (n=5)	11.6	11.6 (n=1)	Q2
				次生	230.7 ~ 236.1	232.5 (n=3)	—	—	Q3
				次生	120.6	120.6 (n=1)	—	—	Q6
				次生	143.4	143.4 (n=1)	17.5	17.5 (n=1)	Q5
DQ9-1	牛蹄塘组	暗色泥岩	垂直脉中方解石	原生	124.6 ~ 129.0	126.7 (n=3)	2.7 ~ 4.8	3.8 (n=2)	C5
			垂直脉中方解石	原生	139.6 ~ 150.9	143.9 (n=6)	2.4 ~ 7.0	4.4 (n=4)	C4
			垂直脉中方解石解理面	原生	104.1 ~ 111.6	107.9 (n=4)	2.1 ~ 4.8	3.1 (n=4)	C6
			垂直脉体壁上石英	原生	160.2	160.2 (n=1)	4.6	4.6 (n=1)	Q1
				次生	163.1 ~ 172.3	167.7 (n=2)	—	—	Q4
				次生	194.5	194.5 (n=1)	6.2	6.2 (n=1)	Q2
DQ14-1	牛蹄塘组	暗色泥岩	垂直脉中方解石	原生	150.1	150.1 (n=1)	2.4	2.4 (n=1)	C4
			垂直脉中方解石解理面	原生	107.9	107.9 (n=1)	1.7	1.7 (n=1)	C6
			垂直脉体壁上石英	原生	162.1 ~ 166.2	165.0 (n=7)	6.0 ~ 6.7	6.3 (n=3)	Q1
				次生	215.8 ~ 225.6	220.7 (n=2)	—	—	Q3
				次生	161.1 ~ 171.5	164.8 (n=6)	3.9 ~ 7.2	5.5 (n=2)	Q4
				次生	142.0 ~ 158.7	151.9 (n=6)	0.4 ~ 3.5	2.3 (n=6)	Q5
				次生	114.5 ~ 126.0	118.3 (n=4)	3.9	3.9 (n=1)	Q6
DQ14-2	牛蹄塘组	暗色泥岩	垂直脉外侧纤维状方解石	原生	135.6 ~ 145.4	141.5 (n=7)	0.9 ~ 8.0	4.4 (n=5)	C4
				原生	164.3	164.3 (n=1)	11.3	11.3 (n=1)	C4
				原生	203.8	203.8 (n=1)	—	—	C4
				原生	124.1 ~ 134.2	129.4 (n=3)	1.4 ~ 7.3	4.4 (n=2)	C5
			外侧纤维状方解石解理面	原生	103.3 ~ 103.4	103.4 (n=2)	0.9 ~ 1.9	1.4 (n=2)	C6

样品编号	δ¹³C_V-PDB/ ‰	δ¹³C/ ‰ (1σ)	δ¹⁸O_V-PDB/ ‰	δ¹⁸O/ ‰(1σ)
DQ-9-1	-3.94	0.01	-12.94	0.03
DQ-12-1	-6.67	0.02	-12.42	0.03
DQ-14-1	-2.31	0.01	-12.36	0.02
DQ-15-1-1	-2.23	0.02	-7.09	0.03
DQ-15-1-2	0.27	0.02	-7.49	0.05

样品编号	δ¹³C_V-PDB/ ‰	δ¹³C/ ‰ (1σ)	δ¹⁸O_V-PDB/ ‰	δ¹⁸O/ ‰(1σ)
DQ-9-1	-3.94	0.01	-12.94	0.03
DQ-12-1	-6.67	0.02	-12.42	0.03
DQ-14-1	-2.31	0.01	-12.36	0.02
DQ-15-1-1	-2.23	0.02	-7.09	0.03
DQ-15-1-2	0.27	0.02	-7.49	0.05