Impact of Diagenesis on Microbial Carbonate Reservoirs in the Upper Indus Basin, NW Himalayas

Microbial carbonates are globally known petroleum reservoirs. However, the complex interplay between deposition and diagenesis significantly influences the pore network distribution in these microbial carbonate reservoirs. The present study aims to discuss diagenetic alterations in the Jurassic microbial carbonate successions from foreland basins in the NW Himalayas. Geological field observations, petrographic analysis, scanning electron microscopy, and isotopic analysis were applied to highlight the role of diagenesis in reservoir characterization of shallow marine carbonates. The results indicate that dolomitization, dissolution, and fracturing during the early to late phase of diagenesis enhanced the reservoir pore network. However, cementation, micritization, and mechanical compaction considerably reduced the reservoir pore distribution. Furthermore, fractures and stylolites that developed perpendicular to bedding planes indicate the role of convergent tectonics in developing the fracture network that allowed fluid migration and improved the pore spaces in microbial carbonate reservoirs. Isotopic data revealed shallow-burial diagenesis with marine and meteoric influx that provides avenues for the movement of fluids. These fluids are associated with microbial activity in carbonate rocks along the faults and fractures that were developed because of compressional tectonics, evident from the perpendicular fracture network. This study recommends the integration of deposition and diagenesis to refine the pore network distribution and characterization of carbonate reservoirs around the globe.