The mechanism of overpressure generation and its impacts for grand gas field formation in Kuqa depression, Tarim basin, China

摘要

According to modeling of coupling energy flow and mass flow and analyzing fluid potential process, four main mechanisms for overpressure generation in Kuqa depression can be identified quantitatively as: (1)Overpressure widely distributes in Baicheng-Yangxia sag in age of Triassic, Jurassic and Cretaceous. It is of typical characteristics of abnormal fluid pressure compartment, but the nonhomogeneous feature is very obvious within the inner compartment. (2) Extremely well sealed cap rocks consisting of salt and mudstone in Tertiary system (Ji-Di-ke Formation) is the prerequisite and key factor for forming and keeping overpressure in Kela-2 high pressure gas pools in Kuqa depression. (3)The high deposit ratio and well sealing character of the Tertiary cap rocks is the most important mechanism for generating overpressure condition in the Kela-2 area, but the compression stress. The contribution ratio of the stress for generation of the overpressure, however, is less than 20%, which is only as a factor to strengthen the abnormal pressure. (4)The stress of tectonic compression may enhance intensity of overpressure but it is not main mechanism. (5)The continuous natural gas charge is considered as a dynamic source for enhancing and maintaining balance of overpressure in Kela 2 gas field. Gas generation is considered as a mechanism for overpressure generation in Triassic and Jurassic source rocks. The tectonic thrust and uplifting in Himalayan episode could cause the hydrocarbons expulsion and charge into the gas pools.