Geochemistry Character of Longmaxi Formation Shale Gas in Western Hunan and Hubei Areas

doi:10.19657/j.geoscience.1000-8527.2019.04.19

Abstract

Abstract:

The Longmaxi Formation organic-rich shale, featured by high total organic carbon (TOC) and high thermal maturity, is located in western Hunan and Hubei region of the middle Yangtze plate. Organic matter in the region are type Ⅱ₁ to type Ⅱ₂ kerogen and are dominated by the former. In the region, brittle minerals (dominant) and clay minerals are abundant in the organic-rich shale. The CH₄ content of the gas samples is high (90.34% to 99.64%), and the gas also contains minor non-hydrocarbon components, as revealed by the experiment under 20℃, 65℃, and 90℃. CH₄ carbon isotopes of the hydrocarbon gases are of -41.9% to -30.8‰, whilst those of C₂H₆ and δ ²H_CH4 are of -42.3% to -36.2‰ and -193.4% to -156.0‰, respectively. With increasing maturity, carbon isotopes of CH₄ and C₂H₆, and hydrogen isotope of CH₄ all become heavier. Carbon isotopes of the hydrocarbon gases show clear reversals, i.e.δ¹³C₁>δ¹³C₂. Accurate calculation of the free gas and adsorbed gas in the shale gas is of substantial guiding significance for the future exploration and development. Therefore, our work establish the calculation formula of the proportion of free gas and adsorbed gas according to the gaseous isotope characteristics of the three temperature stages of the experiment and the gas bearing characteristics measured at each stage. The free gas volume and the amount of adsorbed gas contained in the total gas content could thus be determined.

Key words: Western Hunan and Hubei region, shale gas, carbon isotopes, δ²H_CH4, free gas and adsorbed gas

CLC Number:

TE122

ZHANG Peng, CHEN Shijing, LIU Ziyi, HUANG Yuqi, YANG Junwei, LIU Hongyang. Geochemistry Character of Longmaxi Formation Shale Gas in Western Hunan and Hubei Areas[J]. Geoscience, 2019, 33(04): 883-889.

Figures/Tables 8

References 28

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样品	层位	气体组分/%				碳同位素/‰		氢同位素/‰
样品	层位	CH₄	C₂H₆	CO₂	N₂	CH₄	C₂H₆	CH₄
YY2-1-1	S₁l	94.18	1.27	0.19	4.35	-41.9	-41.3	-193.4
YY2-1-2	S₁l	93.76	3.21	0.18	2.86	-38.1	-41.6	-173.0
YY2-1-3	S₁l	91.97	4.99	0.47	2.47	-32.2	-41.0	-160.4
YY2-2-1	S₁l	94.47	1.39	0.25	3.89	-41.8	-42.3	-187.8
YY2-2-2	S₁l	94.32	3.34	0.32	2.01	-38.2	-42.0	-168.6
YY2-2-3	S₁l	90.34	4.21	0.52	4.82	-36.7	-42.1	-160.2
LD1-1	S₁l	99.64	0.36	0.00	0.00	-30.8	-36.2	-171.0
LD1-2	S₁l	99.56	0.44	0.00	0.00	-32.9	-38.4	-176.0
LD1-3	S₁l	99.48	0.52	0.00	0.00	-32.7	-38.1	-156.0
LD1-4	S₁l	95.10	0.13	0.00	4.76	-36.3	-41.5	-162.0

样品	层位	气体组分/%				碳同位素/‰		氢同位素/‰
样品	层位	CH₄	C₂H₆	CO₂	N₂	CH₄	C₂H₆	CH₄
YY2-1-1	S₁l	94.18	1.27	0.19	4.35	-41.9	-41.3	-193.4
YY2-1-2	S₁l	93.76	3.21	0.18	2.86	-38.1	-41.6	-173.0
YY2-1-3	S₁l	91.97	4.99	0.47	2.47	-32.2	-41.0	-160.4
YY2-2-1	S₁l	94.47	1.39	0.25	3.89	-41.8	-42.3	-187.8
YY2-2-2	S₁l	94.32	3.34	0.32	2.01	-38.2	-42.0	-168.6
YY2-2-3	S₁l	90.34	4.21	0.52	4.82	-36.7	-42.1	-160.2
LD1-1	S₁l	99.64	0.36	0.00	0.00	-30.8	-36.2	-171.0
LD1-2	S₁l	99.56	0.44	0.00	0.00	-32.9	-38.4	-176.0
LD1-3	S₁l	99.48	0.52	0.00	0.00	-32.7	-38.1	-156.0
LD1-4	S₁l	95.10	0.13	0.00	4.76	-36.3	-41.5	-162.0

样品	地层	解析实验和同位素方法
样品	地层	总气量/(m³/t)	损失气量/(m³/t)	解吸气量/(m³/t)	残余气量/(m³/t)	游离气量/(m³/t)	吸附气量/(m³/t)
YY2-1	S₁l	2.302	0.738	1.448	0.116	1.619	0.683
YY2-2	S₁l	1.078	0.312	0.684	0.082	0.513	0.565

样品	地层	解析实验和同位素方法
样品	地层	总气量/(m³/t)	损失气量/(m³/t)	解吸气量/(m³/t)	残余气量/(m³/t)	游离气量/(m³/t)	吸附气量/(m³/t)
YY2-1	S₁l	2.302	0.738	1.448	0.116	1.619	0.683
YY2-2	S₁l	1.078	0.312	0.684	0.082	0.513	0.565