Enrichment mechanism of organic-rich shale under tectonic–depositional differentiation: A case study from the lower Cambrian Qiongzhusi Formation, northern Sichuan Basin, China

The Cambrian Qiongzhusi (QZS) Formation on the northern margin of the Sichuan Basin, in front of the Micang-Daba Mountains, features extensive marine fine-grained sedimentary rocks, particularly organic-rich shale. Understanding its spatial distribution, controlling factors, and enrichment mechanisms is crucial for advancing shale gas exploration in this region. However, the complex depositional and diagenetic history has led to significant heterogeneity in organic matter distribution, necessitating a comprehensive investigation. By integrating field geological surveys, drilling data, and geochemical analyses, including total organic carbon (TOC) content measurement and elemental analysis, this study systematically assesses the influence of various factors on the spatiotemporal distribution of organic-rich shale. The results indicate that the TOC content generally exceeds 2%, with organic matter abundance showing a cyclical vertical decrease. Laterally, the sedimentary zones of organic-rich shales can be classified into three types: favorable sedimentary zone, moderately sedimentary zone, and poor sedimentary zone. Moreover, the shale is thicker with higher organic matter abundance in the south and west, whereas it thins out and exhibits lower organic matter content toward the north and east. The spatial heterogeneity of organic enrichment is primarily controlled by the sedimentary paleogeographic framework, dominated by a "intracratonic sag–uplift–depression" structure, and regional tectonic evolution further accentuates these variations. Sea-level fluctuations and paleoclimate evolution govern the cyclical vertical distribution of the shale, where the abundance of plankton and bacteria contributed to organic matter accumulation, and a sufficient material supply facilitated clastic sedimentation of the QZS Formation. The organic-rich shale was influenced by warm and humid conditions, medium to high paleoproductivity, terrestrial input, and rapid deposition of organic matter occurring under deep-water reducing conditions with preservation enhanced by thick overlying cap rocks. • The spatial and temporal distribution of organic-rich shales exhibits significant regularity. • Key controlling factors influencing the formation and development of organic-rich shales are thoroughly discussed. • An enrichment model along with a classification scheme for favorable shale gas zones is proposed.