川南罗布向斜五峰组—龙马溪组页岩孔隙分形特征与主控因素

四川盆地南缘罗布向斜上奥陶统五峰组—下志留统龙马溪组页岩具有厚度大、有机碳含量高、分布范围广、有机质热演化程度适中等特点,有可能成为下一步页岩气勘探的重要领域。为更好地表征研究区页岩非均质性及其对页岩气富集的影响,根据岩矿、地化、测井资料以及CO₂和N₂吸附实验,对研究区五峰组—龙马溪组龙一₁亚段各小层开展了页岩分形特征研究。结果表明,研究区各小层的总有机碳含量、矿物成分和孔隙度均存在较大差异,储集层非均质性强,页岩孔隙分形维数为2.711 0~2.794 8,平均为2.747 0,其中五峰组和龙一31小层的分形维数较高,龙一41小层的分形维数最低;有机碳和生物成因石英对页岩微孔的发育具有促进作用,页岩平均孔径越小,孔隙数量越多,孔隙结构越复杂,分形维数越大。

Fractal Characteristics and Main Controlling Factors of Wufeng-Longmaxi Formation Shale in Luobu Syncline,Southern Sichuan Basin

In the Luobu syncline, southern Sichuan basin, the shale in the Upper Ordovician Wufeng formation (O₃w)-Lower Silurian Longmaxi formation (S₁l) is characterized by large thickness, high organic carbon content, wide distribution range and moderate thermal evolution of organic matter, making it an important target for subsequent shale gas exploration. In order to better characterize the heterogeneity of shale pore structure and clarify the influence of heterogeneity on shale gas enrichment in the study area, the fractal characteristics of the shale in layers of O₃w-S₁l11 in the study area were analyzed using the rock-mineral analysis, geochemical and well logging data, and CO₂ and N₂ adsorption analysis. The results show that the layers in the study area are very different in TOC content, mineral compositions and porosity, and the reservoir is highly heterogeneous. The fractal dimension of shale pores ranges from 2.711 0 to 2.794 8, with an average of 2.747 0; the value is relatively high for O₃w and Layer #1 in S₁l11, and the lowest for Layer #4 in S₁l11. The organic carbon and bio-quartz can promote the development of shale micropores. The smaller the average pore size of shale, the more the pores, and the more complex the pore structure, and the larger the fractal dimension.