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

摘要： 四川盆地广泛分布低电阻率(低阻)页岩气井,并在川南和渝西局部地区形成相对集中的低阻页岩气藏。传统上,页岩储层的低阻特征常被视为低产标志,但实际钻探证实,部分低阻页岩气井仍具有高产特征,表明低阻页岩同样具备可观的勘探开发潜力。以渝西大安地区志留系龙马溪组优质页岩储层为研究对象,综合运用透射电子显微镜、扫描电镜、润湿性实验、岩电实验以及岩心、测井和地震资料进行系统分析。研究结果表明,地层水对页岩储层电阻率有显著影响,超越了其他低阻控制因素:生烃排水效应所驱动的内源水的排出程度与外源水的直接侵入共同控制着页岩储层低阻现象的形成,其中,内源水发挥着主导作用。大安地区部分井段虽受有机质石墨化影响,但整体上其作用弱于地层水。黄铁矿固有的导电性与黏土矿物的亲水性对储层电阻率的降低作用有限,且对含气性影响较小。针对大安地区低阻页岩提出了"水主导-他源微调"低阻模式,其核心表现为页岩的电阻率与含气性受地层水主导,具有协同衰减特征:当含水饱和度低于25 % 时,储层电阻率普遍高于15 Ω ·m,含气性较好;当含水饱和度为25 % ~45 % 时,储层电阻率多降至15 Ω ·m以下,含气性中等;当含水饱和度超过45 % 时,储层电阻率大多低于5 Ω ·m,含气性较差。该研究为四川盆地低阻页岩气藏的成因机理阐释及产能潜力评价提供了理论支撑。

关键词: 大安地区, 低阻页岩, 成因机理, 多因素耦合, 低阻模式