Differential sedimentary mechanisms of Upper Ordovician-Lower Silurian shale in southern Sichuan Basin, China

Abstract

The Wufeng-Longmaxi Formations are the focus of shale gas exploration in China. Their sedimentation is affected by global and regional geological events, and the resulting heterogeneity hinders the expansion of exploration results. This study combines logging, mineralogy, and geochemistry to reconstruct the sedimentary mechanisms of shale in the southern Sichuan Basin and discusses its implications for exploration. The shale can be divided into third-order sequences, in which the Wufeng Formation and lower Long-1 Member are transgressive systems tracts (TST1 and TST2), and the Guanyinqiao Member and the middle-upper Long-1 Member are highstand systems tracts (HST1 and HST2). Volcanic eruption and glaciation evolution are the driving factors of the sedimentary environment during the TST. The area is in a cold-warm climate, characterized by anoxic conditions, slow sedimentation, booming productivity, and low terrigenous influx, thereby depositing four types of siliceous shale. The siliceous shale is rich in biogenic microcrystalline quartz and high TOC, which are the keys to becoming the “sweet spot” of shale gas. Microcrystalline quartz fills the intergranular pores to support the entire pore network framework. The organic matter undergoes thermal evolution to produce abundant organic matter pores, which constitute the main place for adsorbed gas and free gas. For the highstand systems tract, HST1 is the product of the peak of glaciation, mainly composed of calcareous shale and carbonate rocks. The sedimentation of HST2 is affected by turbidity currents and violent orogeny. The area is in a warm climate, characterized by high terrigenous influx, dysoxic-oxic conditions, medium productivity and rapid sedimentation rates. The lithofacies gradually changed from siliceous shale to mixed shale and argillaceous shale. Excessive clay minerals and low TOC make it difficult for HST2 to realize the commercial development of shale gas.