关键词: 四川盆地, 深层白云岩, 白云石胶结物, 孔隙, 龙王庙组

Abstract:

Deep dolomite has been a key focus of deep exploration in recent years, with dolomite cementation commonly found in its internal pores. However, there is relatively less research on the combination of dolomite cement grain occurrence and its effect on pores. Therefore, taking the Longwangmiao Formation of the Lower Cambrian in the Gaomo area of the Sichuan basin as an example, comprehensive data such as core samples, thin sections, cathodoluminescence, carbon-oxygen isotopes, and inclusions were used to study the types, characteristics, structures, volume fraction, and their effects on pores of dolomite cement. The crystals of dolomite cement mainly have shapes such as semi rhombus, rhombus, ring shaped, saddle shaped, etc., with particle sizes ranging from powder crystals to giant crystals. Cement crystals and pores form three types of structures: lining, bridging, and embedding. Using the frequency of development (the number of samples that develop a certain structure/the total number of samples of granular dolomite or grain dolomite) to characterize the degree of development of a certain structure, the embedding structure is the most developed in dolomite (48.3%-89.5%), followed by the bridging structure (22.4%-73.7%), and the lining structure is the weakest (8.6%-55.2%). The inner lining structure and bridging structure are mainly developed in granular dolomite, with volume fraction of 0 to 15% and 25% to 50%, respectively, and their development degree increases with the volume fraction of fine-grained dolomite cement. Embedding structures are commonly developed in dolomite, with a volume fraction mainly ranging from 50% to 100%, and their development is positively correlated with the volume fraction of medium crystalline to giant crystalline dolomite. The inner lining structure and bridging structure are mainly formed during the syndiagenetic stage to the middle diagenetic stage, while the embedding structure is mainly formed during the late diagenetic stage. The formation of inner lining and bridging structures earlier can reduce the damage to pores caused by solution compaction during diagenesis, while the embedding structure can damage residual pores. Based on the porosity of the layers with mainly developed embedding structures, the porosity of the layers with mainly developed inner lining and bridging structures can be increased by 50%.

Key words: Sichuan basin')">

Sichuan basin, deep dolomite, dolomite cement, pore, Longwangmiao Formation