Gas Content Evaluation and Analysis of the Main Controlling Factors of Qiongzhusi Shale Gas, Sichuan Basin

The substantial resource capacity and commercial extraction prospects of the Cambrian Qiongzhusi shale have been increasingly highlighted by recent exploration milestones in the Ziyang region of the Sichuan Basin. To elucidate the in situ gas-content characteristics and their controlling factors across different sublayers of the Qiongzhusi Formation shale, this study conducted field desorption experiments, physical property measurements, and organic petrographic analyses on 211 shale samples from the Ziyang area. A three-dimensional (3D) continuous flow model was constructed based on Fick’s diffusion law and Langmuir isothermal adsorption theory. Furthermore, an evaluation methodology for in situ gas content and the adsorbed/free gas ratio was established and validated against measured data from pressurized coring. Our findings suggest the following: (1) The field-desorbed gas content from samples of wells Z1 and Z2 is mostly below 0.4 m 3 /t, showing a distinct positively skewed distribution, and is positively correlated with total organic carbon (TOC) content and porosity. (2) The developed 3D continuous flow model for porous media incorporates both the actual core geometry and gas flow dimensionality. This overcomes the limitations of traditional gas content restoration methods in their theoretical basis (which considers only diffusion) and assumptions (a one-dimensional (1D) approximation of 3D flow). Calculations indicate that significant errors arise in the 1D continuous flow model when the core height-to-diameter ratio ( H /2 r ) ≤ 5 (for shorter cores). (3) The USBM method exhibits substantial errors in evaluating in situ gas content, yielding an average value only one-third of that calculated by the 3D continuous flow model. A comprehensive evaluation of wells Z1 and Z2 shows that this error ratio is directly controlled by the gas loss proportion and indirectly influenced by parameters such as rock adsorption capacity and the gas diffusion coefficient. (4) The evaluated in situ gas content in wells Z1 and Z2 ranges from 1.84 to 9.13 m 3 /t (averaging 3.88 m 3 /t), with the adsorbed gas ratio (AGR) ranging from 2.3% to 94.7% (averaging 52.0%). The in situ gas content shows positive correlations with TOC content, effective porosity, and Langmuir volume ( V L ), while the AGR exhibits a negative correlation with effective porosity and a positive correlation with V L, which are identified as the main controlling factors governing the gas occurrence state. (5) An integrated analysis of vertical physical properties and in situ gas characteristics in wells Z1 and Z2 identifies sublayer 5 as the optimal “gas-bearing sweet spot”. In well Z1, sublayer 5 is characterized by high TOC content (averaging 1.85%), large effective porosity (averaging 1.87%), and high in situ free gas content (averaging 1.47 m 3 /t). Similarly, well Z2 exhibits this “triple-high” characteristic (TOC averaging 1.85%, effective porosity averaging 2.70%, in situ free gas content averaging 3.87 m 3 /t). This study presents significant theoretical and technical insights into the enrichment mechanisms of deep shale gas and optimizing exploration and development strategies.