川南筇竹寺组页岩储层天然裂缝发育特征及对页岩气富集高产的影响OA

The development scale of natural fractures has a significant impact on the enrichment and preservation of shale gas,and also plays a controlling role in reservoir stimulation volume.However,the characteristics of natural fractures in the Qiongzhusi Formation and their influence on the enrichment and production of shale gas are still unclear,which limit the fine evaluation of reservoirs and the selection of sweet spots.This study takes the organic-rich shale reservoirs of the Qiongzhusi Formation in southern Sichuan as an example.Based on drilling and imaging logging data,methods such as scanning electron microscopy(SEM),focused ion beam SEM,X-ray fluorescence spectrometry,and whole-rock mineral X-ray diffraction were used to clarify the types and characteristics of natural fractures,identify the controlling factors of natural fracture development in the area,and reveal the influence of natural fractures on the enrichment and production of shale gas.The results study show that in the tectonically deformed areas,the fractures in the Qiongzhusi Formation are mainly NEE-NWW trending shear fractures and S-N tensile-shear fractures,with a small number of NE trending tensile fractures.In non-deformed areas,bedding fractures and fluid overpressure fractures have mainly developed.The natural microfractures are mainly intergranular and intragranular.The laminated,long,fine-grained siliceous siltstone shale has relatively developed fractures,while the calcareous,pyrite,and tuff laminae affect the longitudinal extension of fractures.The fracture density increases with an increase of brittle minerals such as quartz and feldspar,while the total organic carbon and clay minerals reduce the fracture density.Overall,these characteristics are significantly different from those of the Longmaxi Formation.Tectonic deformation and faults have a relatively weak influence on the development of fractures in the study area.Bedding fractures,intra-layer microfractures,and other medium-and small-scale fractures are conducive to the enrichment of shale gas,while cross-layer shear fractures at the bottom of the layer allow shale gas to escape.Fractures with an inclination angle greater than 60°,NEE-NWW trending fractures,and shear and tensile-shear fractures with an angle less than 30° to the direction of in situ stress are more likely to be activated after stimulation to form complex fracture networks,have more connected gas-bearing layers,and have a significant contribution to the productivity of shale gas.