Analysis of continental shale gas accumulation conditions in a rifted basin: A case study of Lower Cretaceous shale in the southern Songliao Basin, northeastern China

Abstract The analysis of shale gas accumulation conditions is the basis of shale gas exploration. Previous research has mainly focused on the Upper Cretaceous shale in the Songliao Basin of northeastern China, and few studies have analyzed the Lower Cretaceous shale in this area. Based on an integrated analysis of the sedimentary facies, shale samples and seismic interpretation from the Lower Cretaceous Shahezi, Yingcheng and Denglouku Formations of the Fulongquan Depression in the Songliao Basin, the distribution of the shale and the generation, accumulation and preservation conditions of the shale gas are evaluated and discussed. Core observations and sedimentary facies analysis show that the sedimentary facies types in the Shahezi, Yingcheng and Denglouku Formations are mainly fan delta, braided river delta, sublacustrine fan and lacustrine facies. The semideep lake, shallow lake and delta front sedimentary subfacies are favorable for the deposition of shale. The study of the shale distribution indicates that the shale was mainly deposited near the maximum flooding surface of the third-order sequences, and the shales in the Shahezi and Yingcheng Formations are darker and thicker than those in the Denglouku Formation. In addition, the shale in this study area is laterally distributed in the depression belt and the steep slope belt near the boundary faults of the basin. According to the geochemistry tests, rock properties and mineral constituents, the shales in the Shahezi and Yingcheng Formations have favorable gas generation potential. Compared with the properties of the major gas-producing shales in China and the United States, the shales in the Shahezi and the Yingcheng Formations are thick and have a favorable organic matter type, high average organic matter abundance, high organic matter thermal maturity that reached a high maturation stage, high porosity and high gas contents; these properties meet the standards of industrial exploitation, and these shales can therefore be considered favorable targets for unconventional energy exploration.