Reservoir Evaluation and Sweet Spot Optimization for Coal Rock Gas in Benxi Formation, Eastern Ordos Basin

doi:10.7657/XJPG20250302

Abstract

Abstract:

To determine the factors influencing coal rock gas productivity in the Carboniferous Benxi formation in the eastern Ordos Basin and identify favorable target areas for production, based on the fundamental geological characteristics and test data of the study area, the No. 8 coal seam was taken as an example for detailed reservoir characterization and analysis of factors controlling gas accumulation. A high-precision 3D geological model was constructed, and sweet spot areas were identified. The study area is a gently west-dipping monocline as a whole. The No. 8 coal seam is well developed and stable, with a thickness ranging from 6.0 to 12.0 m. The reservoir-caprock assemblage primarily consists of coal and mudstone, and the coal structure is mainly classified as Type Ⅰ and Type Ⅱ. In plane, coal rocks are distributed in a banded pattern, with a high gas content averaging 23.17 m3/t. The key factors controlling gas content include burial depth, thermal maturity, positive structure, fracture development, and reservoir-caprock assemblage. Based on the analyses of lithofacies, gas content, rock mechanics, in-situ stress, and fracture characteristics, and considering resources, structural features, coal seam properties, and stress regimes at the roof/bottom, a scheme for sweet spot optimization was proposed. As a result, approximately 777 km2 Class Ⅰ sweet spots and 560 km2 Class Ⅱ sweet spots were delineated.

Key words: Ordos Basin, Carboniferous, Benxi formation, coal rock gas, reservoir evaluation, sweet spot

CLC Number:

TE122.1

ZHANG Zhengtao, FEI Shixiang, LUO Wenqin, ZHONG Guanghao, LAN Tianjun, WANG Ye, CUI Yuehua, WANG Shujie, ZHANG Fang. Reservoir Evaluation and Sweet Spot Optimization for Coal Rock Gas in Benxi Formation, Eastern Ordos Basin[J]. Xinjiang Petroleum Geology, 2025, 46(3): 263-272.

Figures/Tables 12

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指标分类	储集层资源条件							构造保存条件					工程改造条件
指标分类	煤岩相	煤岩类型	煤层结构	煤层净厚度/m	镜质体反射率/%	平均含气量/（m³·t^-1）	资源丰度/ （10⁸ m³· km^-2）	地层倾角/ （°）	埋藏深度/ m	储盖组合	顶板石灰岩+泥岩厚度/m	底板泥岩厚度/ m	裂缝发育强度	煤层与顶底板应力差/MPa	水平主应力差/MPa
A类	高位覆水森林沼泽	光亮煤	Ⅰ分型为主	≥7	>1.6	>22	>2.0	≤1.0	2 000~3 000	煤-石灰岩、煤- 泥岩	>10	>8	较发育	>8	<4
B类	高位覆水森林沼泽、潮湿森林沼泽	半亮煤	I分型和 Ⅱ分型为主	5~7	1.2~1.6	19~22	1.5~2.0	≤1.5		煤-石灰岩、煤- 泥岩	>10	5~8	较发育	>8	<4
C类	低位覆水森林沼泽、植被繁盛低位沼泽	半亮煤、半暗煤	Ⅱ分型和 Ⅲ分型为主	3~5	<1.2	16~19	1.0~1.5	≤3.0		煤-泥岩、煤-砂岩	<10	<5	相对发育	<8	>4
D类	低位覆水森林沼泽、植被繁盛低位沼泽	暗煤、炭泥	薄煤线	<3	<1.2	<16	<1.0	≤3.0		煤-泥岩、煤-砂岩	<10	<5	不发育	<8	>4

指标分类	资源丰度（权重0.40）	地层倾角（权重0.06）	顶板石灰岩+泥岩厚度（权重0.16）	底板泥岩厚度（权重0.06）	裂缝发育强度（权重0.14）	煤层与顶底板应力差（权重0.08）	水平主应力差（权重0.10）
A类	10	10	9	10	9	9	10
B类	7	7	9	7	9	9	7
C类	4	4	3	3	4	3	4
D类	0	0	3	3	0	3	0