Structural characteristics and giant gas field in ultra-deep layer of salt-bearing basin,Kuqa Foreland basin, NW china

ABSTRACT Kuqa foreland basin islocated along the southern flank of the Tian Shan orogenic belt in westernChina. After year's exploration, serious gas fields were found in Cretaceoussandstone reservoir which have a low porosity (3%-5%) and very deep depth(6500m∼7800m) in ultra-deep subsalt layer. The total resource is over 1000billion cubic meters and the daily production per well is 300~1000 thousandcubic meters. The structure plays important roles for the gas accumulation. Ourmain findings are as follows: (1)Four layers dominate the stratigraphy:post-salt overburden, salt, subsalt strata, and basement. There are twodominant deformation zones, the Kelasu structural belt in the north and the Qiulitagestructural belt in the south. A series of subsalt thrusts occur across theKelasu belt, and some paleo-uplifts formed below salt along the Qiulitage belt.There are three related structural styles across the region: contractionalfeatures (thrusts, folds), salt structures, and strike-slip faults. (2)Thestructural evolution of the Kuqa foreland basin can be divided into four stageswhich we describe via the settings of ongoing deposition: late Permian—Triassicpaleo-foreland basin, Jurassic—Cretaceous depressional basin, Paleogene—Mioceneweak contractional flexure basin, and Pliocene—Quaternary intra-continentalforeland basin. (3)Rich oil and gas resources occur in Cretaceous sandstonereservoirs below salt layers across the Kuqa fold-thrust belt. Widelydistributed Triassic and Jurassic coal layers comprise the region's highquality source rocks. Thrusting of subsalt layers formed a series of anticlineswhich are the main traps for gas exploration, with the thick salt providingvery good seals for hydrocarbon preservation. Hydrocarbon migration primarilyoccurred during the Himalayan period, i.e., the main shortening period acrossthe Kuqa fold-thrust belt.