Distribution patterns and source-reservoir characteristics of Cambrian Qiongzhusi Formation shale in northern Sichuan Basin

The exploration degree of the Lower Cambrian Qiongzhusi Formation shale in the piedmont belt of the northern Sichuan Basin is relatively low. Clarifying its source-reservoir geological characteristics and the distribution patterns of high-quality shale is crucial for Cambrian shale gas exploration and sweet spot selection in this region. Through the observation of typical profiles and analysis of well data, using multiple analytical methods such as organic geochemical analysis, X-ray diffraction, mercury intrusion-nitrogen adsorption combined testing, and argon-ion polishing scanning electron microscopy, the spatiotemporal distribution, organic matter development, and reservoir characteristics of high-quality shale in the Qiongzhusi Formation of northern Sichuan Basin were systematically studied. The results showed that the average total organic carbon (TOC) content of shale in Qiongzhusi Formation in the northern Sichuan Basin exceeded 2.0%. Vertically, the TOC content was highest in the second member of the Qiongzhusi Formation. Horizontally, the organic matter abundance was greatest in the Deyang-Anyue aulacogen and the northern Sichuan foreland depression. The organic matter type was primarily sapropelic, predominantly in high to over-mature stages. The shale in Qiongzhusi Formation in the northern Sichuan Basin was characterized by a mineral composition of high siliceous content, medium clay content, and low carbonate content. The reservoir space type was dominated by inorganic pores, with pore size controlled by the degree of laminae development. The overall physical properties of the reservoir were relatively poor, but the brittleness was good. Temporally, high-quality shale in the aulacogen was concentrated at the bottom of the first to third members, while in the depression, high-quality shale was continuously deposited in the first and second members, with shale in the third member being poorly developed. Spatially, the sedimentary thickness of high-quality shale was greatest within the Deyang-Anyue aulacogen and the foreland depression in the northern Sichuan Basin. Based on a comprehensive analysis of key parameters including shale thickness, organic matter abundance, thermal maturity, and brittle mineral content, the favorable shale facies zones of the Qiongzhusi Formation in the northern Sichuan Basin are classified into three types. Among these, the western and eastern parts of Nanjiang are identified as the development areas for dominant favorable facies zones.