Geological characteristics and exploration orientations of the Upper Carboniferous and Middle Jurassic coal rock gas in Qaidam Basin

doi:10.3969/j.issn.1672-7703.2025.03.006

Abstract

Abstract: Field outcrops and drilling data reveal that the Paleozoic–Mesozoic coal rocks are widely distributed in Qaidam B asin, with certain hydrocarbon generation potential and good reservoir performance, which is a new exploration field in the basin. However, the level of research and understanding is relatively low. A systematic study on coal rocks in the Middle Jurassic Dameigou Formation and the Upper Carboniferous Keluke Formation in Qaidam Basin is conducted, including sedimentary environment, distribution, coal properties, reservoir characteristics, resource amount, and gas enrichment rules. In addition, gas resource potential in coal rocks is evaluated, and exploration deployment orientations are proposed in Qaidam Basin. The study results show that: (1) The limnic facies coal rocks in the Middle Jurassic have a single layer thickness of 2 – 30 m and a distribution area of 11100 km²; The transitional facies coal rocks in the Upper Carboniferous have a single layer thickness of 1 – 6 m and a distribution area of 5689 km². (2) The coal rocks in the Middle Jurassic and the Upper Carboniferous are semi-bright to bright coals in a middle coal rank stage, all possessing gas generation capacity; TOC of coal rocks in the Middle Jurassic Dameigou Formation is 32.22%–79.50%, with an average of 62.85%, and R_o is 0.77%–1.38%, with an average of 0.9%; TOC of coal rocks in the Upper Carboniferous Keluke Formation is 35.67%–98.34%, with an average of 72.40%, and R_o ranges in 0.92%–1.82%, with an average of 1.57%. (3) The coal rock reservoirs are characterized by good porosity and permeability properties, and the coal cleats have a high density, showing a reticular distribution pattern and good connectivity; The matrix pores such as stomata, mold pores, dissolution pores, intercrystal pores and plant tissue pores are observed; The measured porosity of coal rocks ranges from 5.26% to 34.01%, with an average of 15.65%; The permeability is 5.11–12.60 mD, with an average of 8.91 mD. (4) Five types of gas accumulation and dispersion combinations are classified vertically in coal rocks, among which the widely distributed coalrock–mudstone and coal rock–limestone gas accumulation combinations have good sealing conditions and high peak values of total hydrocarbon gas logging shows, indicating the most favorable combinations for coal rock gas enrichment. (5) The favorable areas of the Middle Jurassic in Yuandingshan–Jiulongshan area in the northern margin of Qaidam Basin and the Upper Carboniferous in Dulan–Wulan in Delingha area are optimally selected for further strategic exploration deployment. The above understanding is expected to guide the strategic deployment of coal rock gas in Qaidam Basin, open up a new field for coal rock gas exploration, and unveil a new chapter in natural gas exploration in the basin.

Key words: Qaidam Basin, coal rock gas, Upper Carboniferous, Middle Jurassic, coal rock reservoir, gas enrichment condition

CLC Number:

TE122.2

Liu Guoyong, Zhang Yongshu, Xue Jianqin, Long Guohui, Ma Feng, Wang Bo, Wang Yongsheng, Zhang Changhao, Zhou Fei, Tian Jixian, Sun Xiujian, Wu Zhixiong. Geological characteristics and exploration orientations of the Upper Carboniferous and Middle Jurassic coal rock gas in Qaidam Basin[J]. China Petroleum Exploration, 2025, 30(3): 75-88.

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