Controlling effect of three major faults on gas accumulation in the Xujiaweizi faulted depression, Songliao Basin

Understanding the major controlling factors of gas accumulation in the Xujiaweizi faulted depression of the Songliao Basin is of great significance to future gas exploration. However, they were seldom studied in the context of major faults including Xuxi, Xuzhong and Xudong. Based on fine interpretation of 3D seismic data acquired in this area, the controlling effect of these three faults on gas accumulation and distribution was identified through a composite analysis of the three faults and the gas accumulation conditions considering the characteristics of development and distribution of the three major faults. The following conclusions were obtained. (1) These faults jointly control the development of coal measure source rocks in the Lower Cretaceous Shahezi Fm. Such rocks show three high-value areas of thickness (with the maximum thickness of over 1000 m) and have a large potential of hydrocarbon generation. These faults provide passages for gas to migrate upwards to the overlying reservoirs. (2) These faults act as the pathways for magma to protrude upwards into the shallow crust during the deposition of Early Cretaceous Yingcheng Fm. Volcanic rocks are well-developed around the faults, forming favorable fractured volcanic reservoirs. (3) These faults provide effective sealing conditions for gas accumulation in the volcanic reservoirs. If the fault cutting Ying 1 Member is juxtaposed by volcanic reservoirs and sandstones or conglomerates at both walls, the fault is sealed laterally, which is favorable for gas accumulation, and gas layer is proved through drilling; otherwise, if juxtaposed by volcanic rocks, the fault is not sealed laterally, which is unfavorable for gas accumulation, and water layer is confirmed through drilling.