The Jurassic shale in the northeastern Sichuan Basin is one of the main target intervals for continental shale gas exploitation. Research on the pore structure and gas-bearing properties of shales is the key issue in target interval optimization. Through core observation, geochemistry, bulk minerals, scanning electron microscopy, nitrogen adsorption, and isothermal adsorption experiments, various lithofacies with different pore structure characteristics were clarified. In addition, the factors that control gas-bearing properties were discussed, and a continental shale gas enrichment model was finally established. The results show that the Jurassic continental shale in the northeastern Sichuan Basin can be classified into six lithofacies. Organic pores, intergranular pores, interlayer pores in clay minerals, intercrystalline pores in pyrite framboids, and dissolution pores can be observed in shale samples. Pore structures varied in different shale lithofacies. The contact angle of shales is commonly less than 45°, leading to complex wettability of pores in the shales. Free gas content is mainly controlled by the organic matter (OM) content and the brittleness in the Jurassic shale. The adsorbed gas content is mainly controlled by the OM content, clay mineral type, and water saturation of the shales. The enrichment mode of the Lower Jurassic continental shale gas in the northeastern Sichuan Basin is established. Paleoenvironments control the formation of organic-rich shales in the center part of lakes. The “baffle” layer helps the confinement and high pressure, and the complex syncline controls the preservation, forming the enrichment pattern of the complex syncline-central baffle layer.

针对川东北地区下侏罗统自流井组大安寨段陆相页岩储层非均质性强,岩性变化快,岩相类型复杂,页岩黏土矿物含量高等问题,结合川东北地区已有成果资料、岩心观察描述、扫描电镜、CO2吸附、N2吸附和高压压汞孔径联合实验分析结果,对研究区自流井组大安寨段进行储层定量表征综合研究.结果表明,研究区主要发育岩相为富有机质纹层状混合质页岩、富有机质块状黏土质页岩、含有机质纹层状混合质页岩、含有机质层状黏土质页岩、含有机质纹层状硅质页岩和贫有机质纹层状混合质页岩,总计6种岩相;主要发育孔隙类型为:有机质孔隙、粒间孔隙和粒内孔隙,不同岩相类型中,富有机质纹层状混合质页岩孔隙发育类型最多、孔数量发育最多且孔隙连通性良好;微孔体积为0.0014～0.0026 cm3/g,平均为0.002 cm3/g;中孔体积为0.0075～0.0166 cm3/g,平均为0.0123 cm3/g;宏孔体积为0.0578～0.2105 cm3/g,平均0.1221 cm3/g;微孔比表面积为4.52～9.82 cm2/g,平均为6.74 cm2/g;中孔比表面积为3.41～13.87 cm2/g,平均为8.73 cm2/g;宏孔比表面积为0.04～0.48 cm2/g,平均为0.13 cm2/g;储集能力最优质的岩相是富有机质纹层状混合质页岩.

川南地区龙马溪组深层页岩展现了极大的勘探潜力,但目前对此类储层的孔隙连通性发育特征仍然缺乏详细深入的认识.文中选取泸州和长宁西地区6 口深层井龙马溪组的6块样品为研究对象,用扫描电镜直接观察样品孔隙形态,运用低温气体吸附和高压压汞等实验手段研究孔隙结构特征,采用去离子水/正癸烷自吸斜率评估亲水/亲油孔隙网络的连通性特征.基于实验结果,讨论了页岩优势矿物组成、镜质组反射率R.、总有机碳质量分数TOC和孔隙结构参数对孔隙连通性的影响.有机质孔、有机质-黏土矿物复合孔、有机质-黄铁矿复合孔为连通性较好的孔隙类型,粒间孔及溶蚀孔的连通性相对较差;亲油孔隙网络较亲水孔隙网络更为发育,也更有利于页岩气的运聚;高含量的石英、适量的黏土矿物、适宜的热演化程度、高TOC以及高孔隙体积易于形成优势连通通道,进而提高储层孔隙连通性.整体上,泸州地区储层孔隙连通性要优于长宁西地区.

为了建立合理准确的川南五峰组—龙马溪组页岩TOC含量预测方法,以长宁、泸州等地区的测井曲线及17口井实测TOC含量数据为基础,利用主成分分析法对这些资料进行预处理,基于BP神经网络和梯度提升决策树(GBDT)方法建立2种TOC含量预测模型,并将之与传统TOC含量预测方法进行对比.结果表明:①2种新模型的准确度均高于传统方法,预测结果与实际值吻合度均满足要求.②与BP神经网络模型相比,GBDT预测精度更高,均方根误差仅为0.0387.利用GBDT方法所建立的TOC含量预测模型具有低成本、高效、连续等特点,能够快速准确地预测目的层TOC含量.该成果可为提高页岩油气勘探开发效率提供有效技术支撑.

A breakthrough in shale gas exploration was made in the lacustrine shale of Qianfoya Formation located in the Sichuan Basin; a gas analysis has shown that the reservoir therein is a condensate gas reservoir. To enhance the production of shale gas in the Sichuan Basin, reduce the exploration risk, and facilitate development, the marine dry gas shale of the Longmaxi Formation was selected as a comparison to analyze the pore structure characteristics of condensate and dry gas reservoirs. The results showed that the dry gas shale reservoir is mainly developed as organic matter (OM) pores, followed by clay mineral (CM) pores. In condensate gas shale reservoirs, the OM pores, CM pores, intragranular dissolution (ID) pores, and microfractures are common. In the dry gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 10%–65%, 0%–23%, 0%–22%, and 0%–13%, respectively. Micropores were mainly provided by OM pores; mesopores were mainly provided by OM and CM pores; macropores were mainly provided by OM, CM, and ID pores. In the condensate gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 0%–52%, 6%–34%, 0%–18%, and 0%–21%, respectively. Micro-mesopores were mainly provided by OM and CM pores, while macropores were mainly provided by CM pores and microfractures. Because of the influences of material composition, diagenetic evolution, and hydrocarbon generation at the burial depth, there were significant differences in the surface porosities of the matrix components and the distribution and proportions of the different pores between the dry and condensate gas shale reservoirs. This essentially led to a difference in the reservoir space.

A number of wells in the Sichuan Basin of China have tested industrial gas flow pressure arising from the shale of the Da’anzhai section of the Ziliujing Formation, revealing good exploration potential. Microfractures in shales affect the enrichment and preservation of shale gas and are important storage spaces and seepage channels for gas. In order to increase productivity and to reduce the risks associated with shale gas exploration, the types, connectivity, and proportion of microfractures in the Da’anzhai Member have been studied in this work by core and thin section observations, micro-CT, scanning electron microscopy, nitrogen adsorption, and high-pressure mercury intrusion. The results show that four types of fractures have developed in the shale of the Da’anzhai section: microfractures caused by tectonic stress, diagenetic shrinkage fractures of clay minerals, marginal shrinkage fractures of organic matter, and microfractures inside mineral particles. Among these, structural fractures and organic matter contraction fractures are the main types and are significant for shale reservoirs and seepage. The structural microfractures are mainly opened and are well-developed in the shale, with a straight shape, mainly between bedding, with the fracture surface being curved, fully opened, and mainly tensile. Organic matter fractures often develop on the edge of the contact between organic matter and minerals, presenting a slit-like appearance. The fractures related to bedding in the shale are particularly developed, with larger openings, wider extensions, intersecting and expanding, and forming a three-dimensional interconnected pore-fracture system. Based on image recognition, generally speaking, microfractures account for about 20% of the total pore volume. However, the degree of the microfractures’ development varies greatly, depending upon the structural environment, with the proportion of microfractures in fault-wrinkle belts and high-steep zones reaching 40% to 90% of the total pore space. On the other hand, micro-fractures in areas with underdeveloped structures account for about 10% of the total pore space.