The evolution of Jurassic carbonates is globally significant for understanding the depositional framework, diagenetic phases and sedimentary characteristics of shallow marine shelf deposits. For this purpose, two outcrop sections of the Jurassic carbonates with a road distance of 121 km in the Trans Indus Ranges, NW Himalayas, were included in this study. Geological fieldwork was conducted for sedimentological data, and representative samples were collected for microfacies analysis and diagenetic evolution complemented by carbon and oxygen isotope analysis. Results show that eight microfacies were identified in both sections where mudstone microfacies was only present in the Chichali section, whereas wackestone and packstone facies widely existed in both sections. The diagenetic evolution interpreted that dolomitization and stylolization were pronounced in the Paniala section, while micritization and calcite cementation were prevalent in the Chichali section. The interpreted depositional setting implies the wide range from supratidal to outer ramp shallow marine for the Chichali section, suggesting a wide range and relatively deeper environment, alongside merely intertidal to middle ramp settings for Paniala section. Diagenetic evolution suggests marine to meteoric influence in the Chichali section, while burial and uplift phases were dominant in the Paniala section. The diagenetic events were also validated by the isotopic analysis, where most of the samples with values up to −4‰ VPDB δ¹⁸O, corresponding to a carbon isotope range of up to +4‰, were interpreted as the burial phase of diagenesis; meanwhile, a few samples with −2 δ¹³C and −7‰ VPDB δ¹⁸O isotope signatures were marked as meteoric influx in the Paniala section. This study indicates the diversity of the depositional environment and diagenetic heterogeneity by integration of thin sections using isotope data, which are quite applicable to shallow marine carbonates.

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.