Formation and accumulation of lower Jurassic tight gas sands field in Kekeya area of Tuha Basin, northwestern China

The lower Jurassic in Kekeya area of Tuha Basin contains great tight gas sand resources. This study reveals the geologic controls on the formation and accumulation of tight gas sand reservoirs in Kekeya area comprehensively and systematically, based on tectonic evolution, depositional setting, source rock characteristics, reservoir properties, charging-accumulation history and mechanisms. Our study reveals that tectonics and depositional systems play important roles in the tight gas sand petroleum system. Located at the thrust fold belt of Tuha Foreland Basin with continuous stratigraphic succession of braided fluvial - deltaic - lacustrine depositional systems, the lower Jurassic in Kekeya area contains favorable petroleum geologic conditions for the tight gas sands field. The source rocks contain abundant organic matters dominated by gas-prone type III kerogens and started generating primarily gas in early burial stage. The tight sand reservoirs are mainly composed of medium to coarse grained feldspathic litharenites with primary reservoir space as remnant interparticle pores, solution enlarged intraparticle pores as well as micro-fractures. The more active burial compactions and chemical diagenesis in coal-bearing stratigraphy along with the fast subsidence rate in Tuha Basin are responsible for the tightness of the sand reservoir. The favorable source - reservoir - seal arrangement along with the right timing of hydrocarbon generation, the developing of overpressure in source rocks as well as faulting and fracturing contribute to the widespread accumulation of tight gas sands in lower Jurassic in Kekeya area. The findings in this study would provide reference and guide for future exploration and development of tight gas sand fields in this area and in other basins.