Laminae characteristics of shale and their influence on reservoir performance in Permian Wujiaping Formation of Hongxing area, eastern Sichuan Basin

Multiple shale laminae types exist vertically in the Permian Wujiaping Formation of the Hongxing area, eastern Sichuan Basin. Currently, research on the characteristics of shale laminae in the Wujiaping Formation and their influence on reservoir performance remains unclear. Based on core observation and description, thin-section identification, micro-area quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), argon ion-polished scanning electron microscopy, conventional physical property measurements, and nitrogen adsorption, the characteristics of different laminae types and laminaecombinations in the second member of the Wujiaping Formation and their influence on reservoir quality were studied. The results showed that the shale in the second member of the Wujiaping Formation developed five laminae types: tuffaceous laminae, siliceous laminae, radiolarian laminae, mixed bioclastic laminae, and mud crystal calcite laminae, as well as two laminae combination types: radiolarian + siliceous and mud crystal calcite + tuffaceous. Among them, the average total organic carbon (TOC) content of the radiolarian + siliceous combination was 10.87%, the average brittle mineral content was 86.85%, and the average reservoir properties and gas content were 4.80% and 6.67 m3/t, respectively, mainly distributed in the upper part of sub-layer ③ and the lower and middle parts of sub-layer ④. The average TOC content of the mixed bioclastic laminae was 7.37%, the average brittle mineral content was 86.15%, the average reservoir properties and gas content were 3.91% and 4.35 m3/t, respectively, mainly distributed in the middle part of sub-layer ③. The average TOC content of the mud crystal calcite + tuffaceous laminae combination was 4.21%, the average brittle mineral content was 60.69%, and the average reservoir properties and gas content were 2.19% and 2.57 m3/t, respectively, mainly distributed in the middle and lower parts of sub-layer ③. In addition, the second member of the Wujiaping Formation as a whole also developed blocky siliceous shale, calcareous shale, and mixed shale without laminae structures, mainly distributed in sub-layers ④, ⑤, and ③, respectively. Comparisons of different shale types in brittle mineral content, TOC content, reservoir properties, and gas content showed that the radiolarian + siliceous laminae combination shale has the best reservoir quality, followed by siliceous shale, mixed shale, and calcareous shale, while the mixed bioclastic laminae shale and the mud crystal calcite + tuffaceous laminae combination shale were the worst. In summary, the upper part of sub-layer ③ and the lower and middle parts of sub-layer ④ in the second member of the Wujiaping Formation are comprehensive engineering-geological sweet spot intervals for shale gas exploration and development.