Microfacies and stable isotope analysis of Kuldhar Member Limestone (Callovian-Oxfordian), Jaisalmer Basin, western Rajasthan: implications for depositional environment and diagenetic evolution

Microfacies, diagenesis, and stable isotope analysis of Kuldhar Member Limestone (KML) has been carried out to reconstruct the depositional environment and palaeogeography as well as to evaluate the influence of diagenesis on its reservoir quality. The KML is characterized by thin bedding, a high abundance of fossils, and fine to medium grain size. The grains range from angular to well rounded and vary in their sphericity. The KML is composed of diverse array of skeletal grains such as brachiopods, echinoderms, bivalves, gastropods, corals, foraminifera, and bryozoans, as well as non-skeletal grains such as intraclasts, ooids, and peloids, with poor to well-sorted nature. Twelve microfacies were identified in the KML based on detailed petrographic studies, which are echinoderm grainstone (MF1), coated bioclastic grainstone (MF2), oncoid grainstone (MF3), oolitic wackestone (MF4), peloidal wackestone (MF5), ooidal grainstone (MF6), algal packstone (MF7), aggregate-grain grainstone (MF8), bioclastic packstone (MF9), bioclastic lithoclastic packstone (MF10), whole fossil wackestone (MF11), and bioclastic rudstone (MF12). The KML was deposited in shoal, lagoon, and open marine environments and inferred to be a product of fluctuating sea level and varying energy conditions which include shallowing as well as deepening of the sea throughout the deposition. Micritization, cementation, physical and chemical compaction, dissolution, fracturing, neomorphism, and precipitation of calcite veins in some fractures are the main diagenetic processes affecting the KML. The investigations of diagenetic aspects and stable isotopes (delta 13C and delta 18O) show that the KML was influenced by early marine, meteoric, and burial diagenesis. Micritization, cementation, neomorphism, physical compaction, and precipitated calcite veins had a detrimental effect on the reservoir quality. On the other hand, dissolution, fracturing, and chemical compaction had a positive impact and enhanced the reservoir quality, making the rocks conducive to hydrocarbon accumulation. This study could point to a potential carbonate resource and aid in future exploitation of the under-explored Jurassic carbonates in the Jaisalmer Basin.

Automated summary

Microfacies, diagenesis, and stable isotope analysis of the Kuldhar Member Limestone (KML) were conducted to reconstruct its depositional environment and evaluate the impact of diagenesis on reservoir quality. The study revealed that diagenetic processes such as micritization, cementation, and compaction affected the reservoir quality negatively, while dissolution and fracturing had a positive impact.