Control effect of three large faults on the gas pools in the Xujiaweizi faulted depression, Songliao Basin

The study of 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 control effect of these three major faults on the gas pools was studied through a composite analysis of the three faults and the gas accumulation conditions based on the characteristics of development and distribution of the said major faults. The following conclusions were obtained. (1) These major faults jointly controlled the development of coal-seam source rocks in the Lower Cretaceous Shahezi Fm. Thickness of hydrocarbon source rocks exist three high-value areas (with the maximum thickness of over 1000 m) and have a large potential of hydrocarbon generation. The three major faults acted as carriers for the migration of gas upwards to the overlying reservoirs. (2) The three major faults also acted as the pathways for magma to protrude upwards into the shallow crust during the Early Cretaceous Yingcheng period. Volcanic rocks are well-developed around the faults and consist of fractured volcanic reservoirs of high quality. (3) These faults provided effective sealing conditions for the gas accumulation in the volcanic reservoirs. If the volcanic rocks in the one fault bock are juxtaposed with sandstone or conglomerate in the other fault bock, the lateral seal of the fault is favorable for gas accumulation, thus gas reservoirs can be formed. In contrast, if the volcanic rocks in the one fault block are juxtaposed with volcanic rocks in the other fault block, no lateral seal exists, thus being favorable for the formation aquifers instead of gas reservoirs.