A Study on Logging Evaluation Methods for Organic-Rich Shale Reservoirs in the Qiongzhusi Formation of the Ziyang Area, Sichuan Basin

The deposition of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin was controlled by a trough configuration, with multiple sets of shale developed vertically. The formation is rich in shale gas resources and holds significant development potential. Fine logging evaluation of reservoirs is crucial for the optimization of favorable areas in the Qiongzhusi Formation shale. However, logging evaluation methods and reservoir sweet spot evaluation criteria for the Qiongzhusi Formation shale have not yet been established. This paper focuses on the Qiongzhusi Formation shale in the Ziyang area, integrating core experimental data and logging data to study the sensitivity of lithology, physical properties, and gas-bearing characterization parameters. Ultimately, a set of logging evaluation processes and methods for shale gas in the Qiongzhusi Formation was developed. The results indicate: (1) The mineral composition of the Qiongzhusi Formation shale is primarily quartz, feldspar, and clay, with an average brittle mineral index of 69.7%; (2) Uranium content and density show good correlation with shale organic carbon content. The established logging evaluation model for organic matter has a relative error of less than ±8% compared to core TOC content. It was found that organic-rich shale in the Ziyang area is mainly developed in the fifth sublayer of the second submember of the Qiongzhusi Formation's first member, with TOC generally exceeding 2%; (3) A multi-parameter porosity calculation model based on acoustic, density, and uranium content was developed, with a relative error of less than 8% compared to core experimental results. The study confirmed that organic-rich shale reservoirs have good physical properties, with an average porosity of 4.6%, which increases with higher TOC content and feldspar content; (4) A gas content calculation model was established based on pressure-preserved coring and isothermal adsorption experiments. The results show that organic-rich shale has good gas-bearing properties, dominated by free gas. Total gas content increases with porosity, TOC content, and burial depth, averaging 7.7 m3/t. The research findings provide strong technical support for the vertical target optimization and favorable facies division of the Qiongzhusi Formation shale in the Ziyang area.