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Tight Sandstone Gas Reservoir Characterization of Kuqa Depression, Tarim Basin, China

Abstract

Recent exploration in Kuqa depression of Tarim Basin,western China has discovered numerous tight sandstone gas reservoirs with reservoir porosity of less than 5% at depth of over 5000 m. However, lacking of better understanding on the complicated structural geology conditions and hydrocarbon accumulation mechanism of tight sandstone in Kuqa area restricted the target optimization in exploration and evaluation for hydrocarbon resources. Kuqa Depression in the northern Tarim Basin, north of the southern Tianshan orogenic belt south Tabei uplift, Tarim plate is developed in a northern edge of the Paleozoic platform, Cenozoic sedimentary depression (Yang Geng et al, 1995). Over 20 years' petroleum exploration and development practice shows that the rich oil and gas resources in Kuqa Depression, especially natural gas resource potential (Zhao MJ and Zhang BM, 2002). Currently, the Kuqa Depression has become one of the major oil and gas producing areas of the Tarim Basin to increase reserves and production. Crassus - Yiqikelike tectonic zone in northern Kuqa Depression is an important oil and gas structures belt. In recent years, Neogene and Quaternary strata found in shallow Dawanqi Oilfield, deep oil and gas exploration also received a major breakthrough, have discovered the Kela-2, Kela3, Dabei 1, Dabei 3, Keshen 2, Yinan2 et al oil and gas reservoirs, which showing the vast oil and gas exploration prospects. In order to getting a better understanding on tight sandstone reservoirs and tight gas accumulation, this study focused on the pore structure of tight sandstone and gas migration in hydrocarbon-bearing reservoir in Kuqa depression of Tarim basin. Detailed analyses are carried out using microscopic observation, capillary pressure curve test, poro- permeability measurement and gas charging physical simulation experiments. The results indicate that the main pore types of tight sandstone reservoir of Kuqa area are dominated by disslolved pores and slot pores, and the size of majority pore-throat radius is about 0.1μm. Besides, there is no obvious correlation between permeability and porosity, because the permeability is mainly controlled by pore structure and fracture. Finally, gas migration in tight sandstones requires certain starting pressure gradient, the higher the permeability is, the lower the starting pressure gradient is.