Characterization of the reservoir-caprock of the large basement reservoir in the Dongping field, Qaidam Basin, China

Abstract

The basement gas reservoir in the Dongping field in the Qaidam Basin is a large reservoir that is different from other basement reservoirs around the world. The basement reservoir does not contain thick mudstone with abundant organic matter that acts as both a source rock and a caprock. The natural gas came from lateral Jurassic source rocks. The basement lithologies in wellblocks Dp3, Dp1, and Dp17 are granite, granitic gneiss, and limestone with slate, respectively, but they all provide effective reservoir space for gas accumulation. The average porosities are 3.3%, 5.2%, and 3.6%, respectively, and the average permeabilities are 0.66 mD, 0.60 mD, and 0.57 mD, respectively. Tectonic fractures are the main factor for improving the physical properties of the reservoir, and secondary solution space is the key factor for the high and stable gas production in the study area. The E 1 + 2 Formation, which contains abundant anhydrite, unconformably overlies the basement rock. Some of the anhydrite was deposited as cement and filled the fractures and pores, which led to decreased porosity and to the formation of a tight caprock with a high breaking pressure for hydrocarbon accumulation. The caprock becomes thinner from the lowland to the uplift, and it is missing in wellblock Dp3, which led to the heterogeneous distribution of gas. Anhydrite-bearing caprock is the dominant factor that controls the gas accumulation in the basement rock reservoir in the Dongping field. Studying the spatial distribution of the anhydrite-bearing caprock is important to the exploration and development of basement gas reservoirs in the Qaidam Basin. This unique gas accumulation mechanism in a basement rock reservoir may inspire new ideas for exploring basement oil and gas reservoirs around the world.