Geological constraints of giant and medium-sized gas fields in Kuqa Depression

Abstract

There is a gas-rich and well-charged petroleum system in the Kuqa Depression where Triassic and Jurassic source rocks play important roles. Distributed in an area of more than 10000 km and with a thickness of up to 1000 m, they are composed of dark mudstones, carbonaceous mudstones and coal seams containing 6%, 40% and 90% of TOC, respectively, and are mainly the humic organic matter. As high-quality regional cap rocks, the Neogene and Eogene gypsum rocks and gypseous mudstones matched well with the underlying Neogene and Cretaceous-Eogene sandstones. They have formed the most favorable reservoir-seal assemblages in the Kuqa Depression. Also the Jurassic sandstones and mudstones formed another favorable reservoir-seal assemblage. The traps are shaped late in the fold-thrust belt, mainly fixed in the Tertiary-Quaternary, where ten structure styles have been distinguished. These traps spread as a zone in N-S, are scattered like a segmental line in W-E and show tier-styled vertically. The best traps are gypsum-salt covered fault-bend anticlines related to the passive roof duplex. This petroleum system is characterized by late accumulation. In the early Himalayan Movement, mainly gas condensate and oil accumulated and were distributed in the outer circular region of the kitchen; whereas in the middle and late Himalayan the gas accumulations mainly formed and were distributed in the inner circular region near the kitchen. The overpressure of gas pools is common and is formed by seal capacity of thick gypsum layers, extensive tectonic compression and large uplift. The well-preserved anticline traps underlying the high-quality regional cap rocks of the Tertiary gypsum rocks and gypseous mudstones are the main targets for the discovery of giant and medium-sized gas fields. Above conclusions are important for the petroleum geology theory and the exploration of the fold-thrust belt in foreland basins in central and western China.