关键词: 大安地区, 低阻页岩, 成因机理, 多因素耦合, 低阻模式

Abstract: Low-resistivity shale gas wells are widely distributed in Sichuan Basin, with relatively concentrated low-resistivity shale gas reservoirs forming in localized areas of southern Sichuan and western Chongqing. Traditionally, low-resistivity shale reservoirs have been viewed as low productivity of gas. However, drilling data have shown that some low-resistivity shale gas wells still exhibit high productivity, suggesting that low-resistivity shales possess significant exploration and development potential. This study focuses on the high-quality shale reservoirs of Longmaxi Formation in Da’an area of western Chongqing, and conducts a comprehensive analysis using transmission electron microscopy, scanning electron microscopy, wettability experiments, rock-electrical experiments, as well as core, logging, and seismic data. The research results indicate that the influence of formation water on shale resistivity predominates over other low-resistivity controlling factors. Specifically, the formation of low-resistivity shale is collectively governed by the endogenous water expulsion driven by hydrocarbon generation and drainage effects, combined with the direct exogenous water intrusion. Endogenous water plays a dominant role in this process. Although organic matter graphitization occurs in certain well sections of the Da’an area, its overall influence is weaker than that of formation water. The inherent conductivity of pyrite and the hydrophilicity of clay minerals contribute minimally to resistivity reduction while exerting negligible effects on gas content. A "water-dominant, exogenous fine-tuning" low-resistivity model is proposed for the low-resistivity shales in Da’an area. Its core characteristic is the synergistic decline of both shale resistivity and gas content, primarily driven by formation water. When the water saturation is below 25 %, the reservoir resistivity generally exceeds 15 Ω ·m, exhibiting favorable gas-bearing properties. When the water saturation ranges from 25 % to 45 %, the reservoir resistivity typically falls below 15 Ω ·m, indicating moderate gas saturation. When the water saturation exceeds 45 %, the reservoir resistivity is mostly below 5 Ω ·m, demonstrating low gas saturation. This study provides theoretical support for elucidating the genetic mechanisms and evaluating the productivity potential of low-resistivity shale gas reservoirs in Sichuan Basin.

Key words: Da’an area, low-resistivity shale, cause and mechanism, multi-factor coupling, low-resistivity model