Diagenesis of microbial dolomite reservoirs in the second Member of Dengying Formation of Ediacaran in the Penglai area, Sichuan Basin: Insights into the formation and evolution of high-quality reservoirs

Abstract

Objective The microbial dolomite of the second Member of the Dengying Formation（Deng 2 Member） in the Penglai area of the Sichuan Basin is widely distributed and considered a potential target for deep carbonate oil and gas exploration. Unlike conventional high-quality reservoirs characterized by porosity and karst fractures, the carbonate rocks of the Deng 2 Member mainly consist of microbial dolomite. The genesis and diagenetic evolution of these high-quality reservoirs remain unclear. Methods This study employed petrographic thin sections, scanning electron microscopy (SEM), cathodoluminescence, and computed tomography （CT）scanning to conduct a detailed analysis of the microbial dolomite reservoirs in the Dengying Formation, using field outcrop and core samples. The aim was to gain a deeper understanding of the effects of diagenesis on pore formation and the development of high-quality reservoirs. Results and Conclusion The microbial carbonate reservoirs in the study area were characterized by low porosity and very low permeability, predominantly consisting of fracture-porosity (cavity) type microbial dolomite. The reservoir space was primarily composed of intergranular dissolution pores, residual framework dissolution pores, intragranular dissolution pores, and small to medium-sized dissolution cavities, with minor occurrences of intercrystalline pores and intercrystalline dissolution pores. The Deng 2 Member has undergone multiple diagenetic processes; penecontemporaneous dissolution and early epigenetic dissolution were key factors in enhancing porosity. Significance These findings enhance the understanding of the genesis of high-quality microbial carbonate reservoirs in the Ediacaran System of the Sichuan Basin and provide valuable information for deep oil and gas exploration and development in the region.

Keywords

• reservoir genesis
• microbial dolomite
• dengying formation
• penglai area
• sichuan basin