The Mechanism Of Gas Accumulation In The Overpressured Kuqa Fold-Thrust Belt

The Kuqa depression is a Mesozoic-Cenozoic foreland basin in the northern Tarim Basin, northwest China. The objective of this paper is to analysis the petroleum accumulation system in the Kuqa fold-thrust belt. Kela 2 gas field located at the Kelasu tectonic zone. There mainly are two sets of source rocks which are composed of Triassic-Jurassic lake facies mudstone or coal-bearing formation. Reservoirs consist of conglomerate at the bottom of the Eogene Kumugelimu formation, and sandstone in the Cretaceous Bashijiqike formation. The seal is very thick, composed of Eogene salt-gypsum. The gas field belongs to extraneous overpressure gas system that the coefficient of overpressure ranges from 1.8 to 2.0. Based on the analysis of tectonics, the traps of Kela 2 gas field formed at Kangcun period, Miocene Epoch. Tectonic compression and uplift is the main mechanism of overpressure formation. The burial and gas generation history simulation indicates that the source rocks possess strong ability to generate gas, and the filling of gas into the traps started at 5.2Ma, which offer enough gas source for accumulation. The most efficient structural assemblage zone for gas accumulation is the thrust fault and nappe thrust belt developed in a variety of salt-related due to tectonic compression and salt flexible fluid. Faults became the pathway for fluid migration, and the folds related to thrust faults help to form traps. The excellent pathway and overpressure promoted the rapid episodic accumulation of gas. Abundant gas source, efficient fluid-migration and tectonic movement in the late stage are the fundamental conditions for accumulation of the Kela 2 gas field.