Shale Gas Accumulation Conditions and Gas-bearing Properties of the Lower Carboniferous Jiusi Formation in Western Guizhou

doi:10.19657/j.geoscience.1000-8527.2020.099

Abstract

Abstract:

In order to study the shale gas accumulation conditions and gas-bearing characteristics of the Jiusi Formation, we conducted thin-section microscopic identification, X-ray diffraction (XRD) analysis, overburden porosity and permeability, scanning electron microscopy (SEM), low-temperature nitrogen adsorption, organic geochemistry, in-situ gas content, and isothermal adsorption analyses. The results show that the organic-rich Jiusi Formation (Fm.) shale is widely developed (mainly 30 to 100m thick) and has high TOC. It is mainly of type II and in the stage of over-mature dry gas generation. The shale contains mainly quartz and clay mineral, and with high brittleness index (>50%), ultra-low porosity and permeability. The rocks have also various pore types, medium pore size, large specific surface area and total pore volume, and good shale gas accumulation conditions. The fractal dimension of micro-pores is large, with average D₁ and D₂ of 2.804,1 and 2.753,8 respectively, reflecting complex pore structure and strong heterogeneity. The gas and adsorbed gas contents are of 1.5 to 3.0 m³/t and 1.58 to 4.52m³/t, respectively. The results indicate good gas-bearing performance, suggesting that the Jiusi Formation has good shale gas exploration potential.

Key words: shale gas, Jiusi Formation, accumulation conditions, pore structure, gas-bearing properties, western Guizhou

CLC Number:

TE121.1

JIANG Bingren, YANG Tongbao, SHI Fulun, HAN Minghui, FU Wei. Shale Gas Accumulation Conditions and Gas-bearing Properties of the Lower Carboniferous Jiusi Formation in Western Guizhou[J]. Geoscience, 2021, 35(02): 338-348.

Figures/Tables 13

Fig.1 Geological background and sampling location in western Guizhou

Fig.2 Stratigraphic column of Carboniferous sedimentary system in western Guizhou

Fig.3 Core photos and thin-section microscopic characteristics of the Jiusi Formation in western Guizhou

Fig.4 Distribution frequency of TOC with depth in the Jiusi Formation shale of LY1 well

Table 1 Comparison of major development parameters of shale gas series between western Guizhou, North America and Sichuan basins [1,23-24]

地区	页岩层系	地层年代	厚度/m	有机质类型	TOC/%	R_o/%	石英 /%	孔隙度/%	含气量/ (m³·t^-1)	地层压力系数
福特沃斯	Barnett	石炭纪	61~300	Ⅱ	1.0~13.0	1.0~2.1	35.0~50.0	1.0~6.0	8.49~9.91	0.99~1.02
阿巴拉契亚	Ohio	泥盆纪	91~610	Ⅱ	0.5~23.0	0.4~4.0	45.0~60.0	2.0~11.0	1.70~2.83	0.35~0.92
密歇根	Antrim	泥盆纪	49	Ⅰ	0.3~24.0	0.4~0.6	20.0~41.0	2.0~10.0	1.13~3.50	0.81
涪陵	龙马溪组	志留纪	200~300	Ⅰ、Ⅱ	0.5~7.1	2.4~2.8	18.8~70.6	1.2~8.6	0.63~9.63	1.00~1.55
昭通	龙马溪组	志留纪	200~300	Ⅰ、Ⅱ	2.1~6.7	2.5~3.2	35.0~50.0	1.6~4.8	1.00~5.50	1.00~2.05
黔西地区	旧司组	石炭纪	30~100	Ⅱ	0.6~4.5	1.7~3.2	36.9~67.0	0.7~4.0	0.90~3.00	0.84~0.90

Fig.5 Discrimination of organic-matter types in the Jiusi Fm. Shale [23,24]

Fig.6 Mineral composition of the Jiusi Fm. shale in western Guizhou and comparison with shale in shale-gas development zones[23,26]

Fig.7 Pore types of organic-rich shale in the Jiusi Formation in western Guizhou

Fig.8 Adsorption desorption isotherm curve of typical shale samples in the Jiusi Formation

Fig.9 Pore-size distribution of organic-rich shale in the Jiusi Formation

Table 2 Pore structure parameters and fractal dimension of the Jiusi Fm. shale by nitrogen adsorption

样品编号	比表面积 /(m²·g^-1)	孔体积 /(m³·g^-1)	平均孔径 /nm	D₁	D₂
JS-1	10.853	0.021 4	7.68	2.684 1	2.717 2
JS-5	19.016	0.019 1	4.86	2.735 3	2.813 5
JS-7	9.366	0.007 8	4.42	2.767 4	2.835 6
JS-12	16.841	0.019 6	5.59	2.752 1	2.774 0
JS-13	15.245	0.015 5	5.20	2.735 6	2.784 5
JS-15	30.287	0.011 3	3.15	2.848 3	2.899 6

Fig.10 Fractal characteristics of micro-pores of organic-rich shale in the Jiusi Formation

Fig.11 Isotherm adsorption curve of organic-rich shale samples from the Jiusi Formation

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