Abstract

Kela-2, a large gas field developed in a sub-thrust anticline within the Kuqa depression of the Tarim Basin and with reserves of 7.55×1012 SCF, represents the largest unsegmented gas pool in China. Detailed petrographic, geochemical, structural and fluid inclusion studies show that the field is a result of a favourable interrelationship of geological events, involving several periods of compressive structuring, multiple periods of oil and gas migration and a complex diagenetic history. The Kela-2 discovery well encountered abundant oil and gas shows over a 448 m gas column within Cretaceous to Lower Tertiary Bashijiqike and Baxigai Formation sands. Gas is trapped in a large, heavily faulted, northeast-east striking fold with a décollement horizon developed along a gypsiferous horizon. The structure, with a closure of 55 km2, was initiated in the Late Tertiary compressive phase and was accentuated during Quaternary Xiyu compression.

The source of the gas is Triassic and Jurassic coals, which communicate with the reservoir vertically through faults. It is possible through fluid inclusion studies to identify two periods of secondary oil migration and two periods of re-migration within the reservoir. The reservoirs were deposited in a continental environment, within which secondary porosity has been developed by leaching of carbonate cement that infilled original primary pore spaces. The gypsiferous mudstones of the Lower Tertiary provide a common seal to both reservoirs, which are abnormally pressured.

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