Key geological factors governing sweet spots in the Wufeng-Longmaxi shales of the Sichuan Basin, China

Abstract

Production performance of the Wufeng-Longmaxi shales varies significantly among Fuling, Weirong, and Wulong fields in the Sichuan Basin. Total organic carbon (TOC) content, mineralogy, and organic matter (OM) pore characteristics are investigated to identify key factors governing sweet spots. Siliceous shales with good preservation conditions in the Fuling Field exhibit large thickness, high TOC content and thin-section porosity (TSP), and well-developed OM macropores, thus high initial production and estimated ultimate recovery (EUR). Thin carbonate-containing siliceous shales with good preservation conditions in the Weirong Field feature medium-to-high TOC and well-developed OM macropores but low TSP, leading to high initial production but low EUR. Siliceous shales with poor preservation conditions in the Wulong Field are characterized by large thickness, high TOC, low TSP and poorly-developed OM macropores, causing low initial production and EUR.Both sedimentary and preservation conditions are intrinsic decisive factors of sweet spots, as they control the mineral composition, TOC, and OM macropore development. Deep-water shales in transgressive systems tracts (TSTs) exhibit better-developed OM macropores and greater TOC compared to highstand systems tracts (HSTs). OM macropores are most prevalent in siliceous shales, followed by carbonate-containing siliceous shales and then argillaceous shales. Furthermore, good preservation conditions are conducive to retain OM macropores with low pore aspect ratio (PAR). Comparison among the three fields shows that high-TOC silicious shales with good preservation conditions are the highest in TSP and EUR. Therefore, organic richness, lithofacies, and preservation conditions are the major factors which determine OM pore development, governing the sweet spots of the Wufeng-Longmaxi shales.