Structural Deformation Styles in Yanqi Basin and Their Control on Oil and Gas

Yanqi Basin is a Mesozoic－Cenozoic overlapping basin Located between Kuluketarg fold belt and South Tianshan Mountain fold belt． The Mesozoic prototype basin was a strike－slip foreland basin in front of Kuluketarg Mountain where developed a wedgelike Triassic－Jurassic coal－bearing sequence which was thick in the south while thin in the north． The Cenozoic prototype basin was an intermountain compressive rift－subsidence basin of Tianshan Mountain where developed a set of wedgelike red clastic rock sequence of flood and fluvial facies which is thick in the north while thin in the south． Two fault structure belts with a near EW－NWW trend and a NNW trend respectively developed in the basin．The former parallels to the south and north mountain system with compresso－shear thrusting characteristics while the latter is a companion result of compresso－shear action of the former．The structure deformation which was strong in the south while weak in the north resulted in the remarkable difference between Jurassic oil and gas system in the south depression and that in the north depression．In the south depressionreservoirs formed earlierreservoir damage was more serious and oil and gas distribution was poorer than that in the north depression．The NW trend secondary structural zone in the north depression is the most favorable area for oil and gas accumulation and the fill factor is large in anticline traps related to faults．The NW trend secondary structure zone with relatively weak deformation in the south depression is the most favorable area for the development of prototype oil and gas reservoirs of Yanshan Period．The Zhongmachang thrusted rift between the north and south depressions has poor oil and gas preservation conditions．