Lithofacies diversity and geological sweet spots of shales deposited in different sedimentary systems of the Sichuan Basin, SW China

Abstract

The topic of shale petroleum systems holds significant prominence within the international petroleum industry. Lithofacies analysis provides a fundamental basis for understanding shale heterogeneity and identifying favorable exploration targets. However, the coexistence of multiple lithofacies classification schemes has hindered the comparability of results and constrained the development of a unified understanding of lithofacies diversity across different shale systems. Herein, we select shales from three distinct sedimentary systems (marine, marine-continental transitional, and lacustrine) in the two boreholes in the Sichuan Basin as our focal subjects. The lithofacies diversity of the shales has been captured based on mineralogy, TOC content, and sedimentary texture. The type of favorable exploration lithofacies (lithofacies assemblages) are summarized. Our results underscore that 12 lithofacies are identified within the three shale systems. The pore systems of different sets of shales are diverse, organic matter associated pores dominate the pore system of the shales of the Longmaxi formation. By contrast, clay mineral-associated pores dominate the shales of the Longtan formation and the Da’anzhai member. For the physical properties of the Longmaxi formation, the TOC content emerges as the major driver for pore development. Although TOC content does influence the Longtan formation and the Da’anzhai member shales, the clay mineral content remains the main influence on pore development. When integrating storage capacity, hydrocarbon content, and mineral composition, the laminated organic-rich siliceous shale within the Longmaxi formation emerges as the most promising exploration lithofacies. In the Longtan formation and the Da’anzhai member, the lithofacies assemblages constituted by organic-rich argillaceous shale interspersed with adjacent sandstones or carbonate interlayers emerge as favorable exploration targets. This study represents an attempt to describe lithofacies diversity across various shale systems using a unified lithofacies classification framework. The results enhance our understanding of lithofacies heterogeneity within the shale systems of the Sichuan Basin and provide theoretical support for future hydrocarbon exploration. Furthermore, this work offers a framework for systematically characterizing shale system heterogeneity, serving as a valuable reference for shale exploration in petroliferous basins worldwide.