高演化页岩纳米孔隙在过熟阶段的形成演化特征及主控因素:中扬子地区寒武系水井沱组页岩含水热模拟实验

我国中-上扬子地区海相寒武系页岩现今普遍处于过熟阶段，该套页岩储层内地质流体活动相对频繁，然而其对页岩储层孔隙发育影响程度及作用机制尚不清楚.选取中扬子宜昌黄陵隆起区演化程度相对较低的寒武系水井沱组页岩样品进行了封闭体系含水热模拟实验，获得了热成熟度介于R_o=2.26%~4.01%之间的页岩储层样品，对这些页岩样品进行了碳-硫和矿物组成、氮气吸附和扫描电镜观测等分析.实验结果显示:随有机质演化程度增加，页岩TOC变化不明显，硫、无机碳和粘土矿物含量持续减少，长石含量持续增加，在R_o≥2.7%时，石英含量显著降低，透辉石含量显著增加.这表明实验条件下，高演化页岩生排烃能力相对较弱，黄铁矿、碳酸盐岩、粘土矿物和石英均发生了不同程度的溶蚀，与此同时，形成了长石和透辉石等矿物.地质流体作用下，高演化页岩内纳米孔隙发育主要受黄铁矿、碳酸盐岩、粘土矿物和石英等矿物溶蚀控制，矿物溶蚀有利于页岩内介孔，尤其宏孔发育，宏观上表现为矿物含量与总孔和宏孔体积之间具有显著负相关关系；矿物生成对页岩内微孔发育不利，对介孔和宏孔发育较有利，这是矿物溶蚀占据主导地位的进一步体现；烃类生成和排出对高演化页岩纳米孔隙发育影响较小，这与该阶段页岩生排烃能力较弱相吻合.随矿物溶蚀或有机质演化程度增加，微孔丰度、体积和比表面积逐渐降低，并逐步向介孔和宏孔转化，表现为微孔体积和比表面积与介孔和宏孔体积和比表面积呈负相关.该研究成果对于进一步深入认识地质流体作用下高演化页岩储层内纳米孔隙发育机理及主控因素具有重要意义. 

Formation and Evolution of Nanopores in Highly Matured Shales at Over-Mature Stage: Insights from the Hydrous Pyrolysis Experiments on Cambrain Shuijintuo Shale from the Middle Yangtze Region

The Cambrian marine shales are currently over-matured in the Middle-Upper Yangtze region. Several phases of geofluids are detected within these shale reservoirs. However, the impacts and relevant mechanisms of geofluids on the occurrence of nanopores within these shale reservoirs are still unclear. A series of pyrolysis experiments were conducted on the highly matured Cambrian shale from the Shuijingtuo Fm. in the Huangling anticline within a closed hydrous system. The measurements of C-S, XRD, N₂ adsorption and FE-SEM were done on these pyrolyzed shale samples with thermal maturity of R_o=2.26%-4.01%. The results demonstrate no obvious change of TOC, continuous decrease of sulfur, inorganic carbon and clays and increase of feldspar throughout the experiments as well as the significant decrease of quartz and increase of diopside at R_o≥2.7% with increasing thermal maturity, which indicates the slight hydrocarbon generation and expulsion, the dissolution of pyrite, carbonate, clays and quartz to varying levels and the formation of feldspar and diopside under the experimental conditions. The nanopore formation within highly matured shale reservoirs is regulated mainly by the mineral dissolution with the addition of geofluids, it is feasible for the occurrence of mesopores and macropores as indicated by the strong negative correlations between the mineral contents versus the total pore and macropore volumes; the formation of feldspar and diopside restricts the occurrence of micropores and promotes the development of mesopores and macropores, further implying the predominant effects of mineral dissolution on nanoporosity under the experimental circumstances; hydrocarbon generation and expulsion have the minor impacts on nanopore development due to the limited capability of petroleum generation and expulsion at the over-mature stage. With increasing mineral dissolution or thermal maturity, the abundance, volumes and specific surface areas of micropores gradually decrease and evolve into mesopores and macropores subsequently as suggested by the negative correlations of pore volumes and specific surface areas of micropores versus those of mesopores and macropores. This study should be helpful in the better understanding of the occurrence mechanisms and relevant controlling factors of nanopores within highly matured shale reservoirs with the presence of geofluids in nature.