湘鄂西龙马溪组页岩解析气同位素组成特征及应用

doi:10.19657/j.geoscience.1000-8527.2019.04.19

摘要/Abstract

摘要：

湘鄂西地区位于中扬子板块,研究区龙马溪组富有机质页岩的有机质类型以Ⅱ₁型为主,Ⅱ₂型次之,TOC含量高,有机质热成熟度高,页岩矿物以脆性矿物为主,黏土矿物次之。通过分析三个实验阶段(20 ℃、65 ℃、90 ℃)解析气样发现,气体中CH₄含量较高(90.34%~99.64%),含有少量非烃气体。龙马溪组页岩气CH₄碳同位素值为-41.9‰~-30.8‰,C₂H₆碳同位素值为-42.3‰~-36.2‰,δ²H_CH4为-193.4‰~-156.0‰,随着成熟度的增加,CH₄碳同位素、C₂H₆碳同位素以及CH₄氢同位素都有升高的趋势。烃类气体碳同位素具有明显的倒转现象,即δ¹³C₁>δ¹³C₂。根据解析实验三个阶段的气体同位素特征和测得的各阶段含气特征建立游离气与吸附气所占比例的计算公式,进一步推算出解析过程总含气量中所含有的游离气量以及吸附气量。

关键词: 湘鄂西地区, 页岩气, 碳同位素, CH₄氢同位素, 游离气与吸附气

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

中图分类号:

TE122

张鹏, 陈世敬, 刘子驿, 黄宇琪, 杨军伟, 刘洪洋. 湘鄂西龙马溪组页岩解析气同位素组成特征及应用[J]. 现代地质, 2019, 33(04): 883-889.

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.

图/表 8

图1 湘鄂西地区龙马溪组页岩显微组分

Fig. 1 Maceral of the Longmaxi Formation shale in western Hunan and Hubei region

图2 湘鄂西地区龙马溪组页岩矿物特征

Fig.2 Mineral feature of the Longmaxi shale in western Hunan and Hubei region

表1 湘鄂西地区龙马溪组解析实验气体的地球化学特征

Table 1 Geochemical characteristics of shale gases released in the experiment from the Longmaxi Formation in well YY2 and well LD1

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

图3 甲烷、乙烷稳定碳同位素图版(修改自Dai et al.,2014)

Fig.3 Plot of the δ13C1 and δ13C2 values (modified after Dai et al., 2014)

图4 甲烷碳、氢同位素图版(修改自Dai et al.,2014)

Fig.4 Plot of the δ13C1 and δ2HCH4 values (modified after Dai et al.,2014)

图5 湘鄂西地区龙马溪组YY2井解析实验

Fig.5 Experiment on the Longmaxi Formation(in well YY2)in western Hunan and Hubei region

图6 解析实验不同阶段甲烷碳、氢同位素

Fig.6 Plot of δ13C1 and δ2HCH4 of the different temperature stages in the experiment

表2 解析实验和同位素分析方法计算游离、吸附气量

Table 2 Free gas and adsorbed gas contents calculated from the method of the experiment and isotopic analysis

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

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