Formation and distribution of “continuous” gas reservoirs nd their giant gas province: A case from the Upper riassic Xujiahe Formation giant gas province, Sichuan Basin

Abstract

The comprehensive analysis of geological setting, accumulation condition, trapping mechanism and distribution regularity of oil-gas of the Upper Triassic in Sichuan Basin reveals that the gas accumulation in Xujiahe Formation is a layered lithologic giant gas province. Coal source rocks in layers expelled hydrocarbon evaporatively. Under the setting of the episodic fold-thrusting and compression in the boundary orogen, multi-sources charged quickly and formed a coarse deltaic environment with a large-scale delta plain and a small-scale delta front. In gentle structures, Darcy and non-Darcy flow mechanisms coexist because of the limited buoyancy of low permeability sandstone, thus forming the coexisting oil-gas-water system. This paper classifies the diagenetic facies quantitatively using compaction percentage, cementation percentage and corrosion percentage, and defines eight diagenetic subfacies and twenty diagenetic microfacies. Besides, the origin of diagenetic facies is analyzed by using Ro and formation water parameter. This kind of giant gas province originated mainly from stable large-scale continental depressions, gentle foreland slopes and paralic facies in central China. It is characterized by large-area layered distribution, no obvious trap and cap formation, large-scale resource, low-abundance reserves, and difficult to exploit. Key words: coal source rock, layered giant gas province, layered evaporative hydrocarbon expulsion, differential entrapment, unconventional trap, “continuous” gas reservoirs