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 E1 + 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.
Highlights
It is generally recognized that sedimentary basement that mainly consists of dense and hard Precambrian metamorphosed rocks have poor characteristics for hydrocarbon accumulation
Most basement reservoirs have been recognized on platforms or in intermontane basins, and they are unconformably overlain by sedimentary sequences
Most of the basement rock reservoirs that have been discovered around the world underlie thick mudstone with abundant organic matter, which acts as both a source rock and a good caprock
Summary
It is generally recognized that sedimentary basement that mainly consists of dense and hard Precambrian metamorphosed rocks have poor characteristics for hydrocarbon accumulation. Most basement reservoirs have been recognized on platforms or in intermontane basins, and they are unconformably overlain by sedimentary sequences. They usually occur on highs or are associated with mountain uplifts (Harris et al, 2002; Nelson et al, 2000; Landes et al, 1960). Fractures are ubiquitous in crystalline rocks and are usually clearly observed on the surface of drilling cores They control the rock strength and fluid transport, and two or more fracture systems can contact multiple oil-water or gaswater zones (Koning, 2003). The development of fractures can give rise to secondary porosity around them because flow mainly occurs in fractures, which creates more effective reservoir space in the basement rocks and allows the diffusion and flux of hydrocarbons in the basement rock (Ismar de Souza et al, 2013)
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