Three shale gas fields, Fuling, Weirong, and Wulong, in the Sichuan Basin exhibit significant differences in the production performance of the Wufeng-Longmaxi shales. This study investigates the total organic carbon (TOC) content, mineralogical properties, and organic matter (OM) pore characteristics of these fields to identify key factors governing sweet spots. The Fuling Gas Field shale reservoirs exhibit a large thickness, high TOC and quartz contents, high hydrocarbon potential, high surface porosity, and well-developed OM macropores under good preservation conditions, resulting in high initial production and high estimated ultimate recovery (EUR) per well. The Weirong Gas Field features thin carbonate-bearing siliceous shales with medium-high TOC content and well-developed OM macropores but low surface porosity. This leads to high initial production but low EUR per well under good preservation conditions. Wulong Gas Field's thick, high-TOC siliceous shales have low surface porosity and poorly-developed OM macropores, causing low initial production and EUR per well under poor preservation conditions.
For the formation of shale gas sweet spots, sedimentary conditions serve as an intrinsic decisive factor as they control the mineral composition and organic matter content of shales, while preservation conditions serve as an extrinsic decisive factor by controlling OM macropore development. Deep-water shales in transgressive systems tracts (TSTs) exhibit better-developed OM macropores and higher TOC content compared to highstand systems tracts (HSTs). OM macropores are more prevalent in siliceous shales, followed by carbonate-bearing siliceous shales and then argillaceous shales. Furthermore, good preservation conditions are conducive to OM macropore development. Therefore, the sweet spots of Wufeng-Longmaxi shales are largely determined by organic richness, lithofacies, and preservation conditions.
The shale gas revolution in the United States has spurred large-scale shale gas exploration and development activities in China since 2009 (Zou et al., 2010). The largest shale reservoirs in China, first discovered in the Ordovician Wufeng-Silurian Longmaxi Formation in the Sichuan Basin, have formed the shale gas fields such as Fuling, Wulong, and Changning with burial depths of less than 3500 m (11,483 ft). Breakthroughs have been made in deeply buried shale gas reservoirs at depths of 3500-4500 m (11,483-14,764 ft) at the Weirong, Dingshan-Dongxi, and Luzhou fields, among others. The proven shale gas in place in the Sichuan Basin exceeds 2.0 × 1012 m3 (70.63 tcf), with an annual production of about 22.8 × 109 m3 (805.17 bcf) (Zou et al., 2022). These shale gas fields vary greatly in lithofacies, preservation conditions, and estimated ultimate recovery (EUR) (Guo, 2014).
