Main controlling factors for variations in gas content and resource potential assessment of the Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation shale, Southeastern Chongqing

Objective The evaluation of shale gas−bearing properties plays a pivotal role in shale gas exploration and development. The associated parameters are fundamental for resource assessment, prospective zone prioritization, delineation of sweet spot targets, and formulation of development plans.

Methods This study focuses on the Wufeng Formation−First Member of Longmaxi Formation shale in Southeastern Chongqing. By integrating multiple methods including field tests, laboratory analyses, and well logging interpretation, the influencing factors of its gas−bearing capacity were analyzed.

Results The research findings indicate that the gas−bearing properties of the shale in this region exhibit pronounced variations both vertically and horizontally. Vertically, the total gas content of sub−layers ⑥～⑨ ranges from 0.33 to 4.30 m³/t, with an average of 1.47 m³/t. In contrast, that of sub−layers ①～⑤ spans from 0.77 to 10.88 m³/t, averaging 3.55 m³/t. Evidently, the gas−bearing properties enhance from the upper layers to the lower ones. Horizontally, the average total gas content of the high−quality shale segments outside the basin, along the basin margin, and within the basin amounts to 3.04 m³/t, 3.46 m³/t, and 4.27 m³/t respectively. Moving from the exterior to the interior of the basin, the gas−bearing properties progressively improve, accompanied by a successive increase in single−well production.

Conclusions The factors influencing the gas−bearing properties of shale can be categorized into static and dynamic types. The quality of the shale reservoir represents a foundational static factor. Variables such as the sedimentary environment, lithology, and total organic carbon (TOC) content determine the original hydrocarbon generation and supply capacities. Preservation conditions, a crucial static factor, encompass parameters like burial depth, distance from the erosional zone, and fault characteristics. These conditions exert a significant influence on the formation pressure coefficient and the retention of shale gas within the reservoir. Reservoir formation and evolution constitute decisive dynamic factors. Disparities in the timing of tectonic deformation and shale gas escape lead to alterations in the total gas content and the proportion of free gas, ultimately impacting single−well production. Based on key parameters including material basis (source rock conditions), preservation conditions, and gas−bearing properties, the total shale gas resources in Southeastern Chongqing are comprehensively evaluated to be 2.06 × 10¹² m³, of which the resources within the Class I area amount to 3458 × 10⁸ m³.