Paleoenvironmental and Diagenetic Analysis of the Middle Palaeocene Ramp Carbonates in Northwestern Pakistan: Implications for Deciphering the Reservoir Potential

Abstract

Carbonate rocks are the most impressive petroleum reservoirs in the world, constrained by depositional and diagenetic processes. With Paleocene Lockhart Limestone (PLL) strata, the Hazara Basin in northwest Pakistan is a growing petroleum basin. It is crucial to evaluate how PLL depositional and diagenetic processes affect reservoir potential. A combination of field observations, petrographic analysis, and Xрray fluorescence is applied to achieve the desired goals. PLL consists mainly of densely fossiliferous nodular limestone that exhibits three main facie associations: low-energy lagoon facies represented by abundant green algae and micritic matrix, foraminifera-rich inner ramp with low-energy conditions, and shallow marine middle ramp with restricted water circulation and abundant non-skeletal allochems. PLL is interpreted to be formed in middle-inner ramp to lagoon depositional setting representing type 3 transgressive-regressive systems tract. The geochemical results show that CaCO₃ range (82.20–94.86 wt %) is dominant than other element oxides indicating that calcareous influx is more, which inhibits the clastic rocks influence in the deposition and the influence of magnesium enrichment process is minimal, which ultimately supports the formation of ramp setting. The diagenetic features include compaction, micritization, stylolites, dissolution, cementation, dolomitization, fractures, and recrystallization representing marine, marine-meteoric mixing, and burial diagenetic environments. Dissolution and fracturing can improve the net porosity of the formation, making it a better reservoir for petroleum exploration. PLL is correlated with stratigraphic equivalents involving the Jahrum Formation (Iran) and Zongpu Formation (China) for better understanding the reservoir properties of other carbonate petroleum fields.