Symbiotic Characteristics of Multi-type Unconventional Natural Gas in Thin Interbedded Rock Series

Abstract

Thin interbedded rock series are usually composed of mudstone, coal, sandstone, carbonate rock, and other lithologies. These strata are widely distributed in the Upper Paleozoic-Cenozoic in China, and have favourable conditions for the symbiosis of shale gas, coal-bed gas, tight sandstone gas and conventional gas. Thin interbedded rock series usually contain gas as a whole: however, due to the low thickness of some single strata, it is difficult to achieve economic benefit when developing single gas resources. Comprehensive exploration and development of multi-type natural gas is important, but the related research remains sparse at present. The thin interbedded rock series of the Badaowan Jurassic Formation in the Turpan-Hami Basin were investigated. Based on outcrop, core, well-logging, and test data, the spatial superposition relationship of coal, shale, and compact sandstone in the target stratum is depicted vertically and horizontally. Based on the sedimentary background, three reservoir association modes were proposed: sand-shale-coal positive rhythm association in a riverine environment, shale-sand-coal reverse rhythm association in a deltaic environment, and a sand-shale inter-bed positive rhythm assemblage. The centres of coal-rock gas generation and shale gas generation have been determined and it is found that they do not coincide in plane. The intensity of coal gas generation is about twice that of shale. The vertical coupling relationship between physical properties of coal, shale and tight sandstone was further discussed, and the gas-bearing characteristics of different unconventional reservoirs were analysed. According to the evolution of the gas content of coal, shale, and tight sandstone reservoirs with depth, it is considered that from shallow to deep, the thin interbedded rock series is transformed and replaced by coal-bed methane dominance-tight sandstone gas dominance-shale gas dominance. These results can provide a reference for comprehensive evaluation and exploration of such natural gas resources.