Geological characteristics and natural gas exploration targets of the Jurassic coal-measure whole petroleum system in Northwest China

Abstract

Coal-measure whole petroleum system is generally featured by dual-source multi-reservoir coupling, coexistence of three dynamic fields controlling hydrocarbon accumulation, and sequential accumulation of deep coal-rock gas (shale gas), proximal tight sandstone gas (fractured tight gas), distal tight gas/conventional natural gas, and shallow coalbed methane. To reveal the common geological characteristics of coal-measure WPS in the Jurassic coal-bearing basins in Northwest China, this paper analyzes the geological characteristics of coal-measure WPSs in Kuqa Depression of Tarim Basin and Thrust Belt of Southern Junggar Basin, such as structure and hydrocarbon accumulation. It is pointed out that the Jurassic whole petroleum system of coal measures in northwestern China is significantly different from that of the Carboniferous–Permian in North China. Four types of source-reservoir coupling accumulation models are mainly developed in the Jurassic of Northwest China, including structural-type shallow to deep conventional gas outside the source, structural-type deep to ultra-deep tight gas outside the source, near-source/in-source tight gas, and self-generating and self-preserving coal-rock gas. It is indicated that the Jurassic strata in Northwest China belong to the giant continental sedimentary region on the passive continental margin of Neo-Tethys Ocean, including six subsidence zones, and containing coal measures and coal rocks that are consistent in the region but distinct from basin to basin. Thus, this region stands as the largest Mesozoic coal-measure area and a giant natural gas accumulation area in China. Resource assessment demonstrates that the Jurassic coal-measure WPS in Northwest China holds natural gas resources up to 30×10¹² m³, in which merely 11% has been proved, and coal-rock gas accounts for 17×10¹² m³, possessing a tremendous exploration potential. Deep and ultra-deep tight gas and coal-rock gas in Kuqa Depression and Southwestern Depression of Tarim Basin, southern margin of Junggar Basin, northern margin of Qaidam Basin, and Taipei Sag of Turpan-Hami Basin will serve as key natural gas exploration targets in the future.