四川盆地天宫堂地区龙一1亚段页岩白云石特征及其对储层孔隙发育的影响OA

[Objective and Methods]This study investigates the types,origins,distribution,and impact on reservoir properties of dolomite in organic-rich marine shale of the first sub-member of the First Member of Longmaxi Formation in Tiangongtang area of the Sichuan Basin.[Results]Utilizing thin-section observation,X-ray diffraction(XRD),scanning electron microscopy,electron probe analysis,and other experiments,we have identified three dolomite types:Dispersed,layered,and aggregated dolomite.Dolomite crystals are rhombic,featuring a distinct core-ring structure.The core is poorly crystalline and compositionally characterized by high magnesium,high calcium,and almost no iron.In contrast,the ring is well-crystalline with high magnesium,high calcium,and high iron content.The distinct differential compaction boundaries and rhombic crystals at the edges of dolomite grains suggest that the ankerite ring may have formed in the methane anaerobic oxidation zone during the early shallow burial diagenesis.The rounding characteristics of the dolomite core suggest a detrital origin.Additionally,ankerite spots in the dolomite core may be due to the pore-filling cementation.The dolomite content in Tiangongtang area is highest in the first sub-member of the First Member of Longmaxi Formation shale,and gradually decreases upwards.The dolomite content shows a gradual increase from the deep-water area in the southeast to the shallow-water area in the northwest of the study area.Analysis of the dolomite content and visible porosity indicates that the development of inorganic pores in Longmaxi Formation shale in Tiangongtang area is closely related to the presence of dolomite.When the dolomite content is below 15%,it promotes the development of inorganic pores.When the dolomite content reaches about 15%～20%,its contribution to pore development peaks,and further increases in dolomite content no longer significantly affect reservoir pore development.Dolomite grains exhibit strong anti-compaction properties,retaining intergranular pores and effectively improving the reservoir properties of shale.In particular,the large number of intergranular pores between dolomite grains with layered and aggregated distributions makes a significant contribution to the reservoir space,whereas dispersed dolomite has little impact on pore development.[Conclusion]The research results offer valuable insights for shale gas exploration and development.