Characterising Depositional-Diagenetic Aspects and Geochemistry for Assessing Source Rock Potential of the Thanetian Lockhart Limestone, Lesser Himalayas, Pakistan (Eastern Tethys)

Abstract

Depositional and diagenetic processes constrain organic matter characteristics in siliciclastic rocks; however, such controls are elusive in carbonates. To increase oil and gas production and expand exploration opportunities, it is essential to assess the potential of non-traditional source rocks, for example, carbonates. This study investigates the depositional facies and diagenetic constraints on the organic matter (OM) accumulation, preservation and source rock potential of the Palaeocene Lockhart Limestone exposed at the Kala-Chitta Range, Lesser Himalayas, Pakistan. An integrated approach combining conventional methods for source rock evaluation (i.e., sedimentology, petrography, cathodoluminescence, TOC, Rock-Eval pyrolysis, and vitrinite reflectance) with stable isotopes (δ¹³C and δ¹⁵N) analysis is employed. Three microfacies are identified, that is, mudstone (MFT-1), foraminiferal wackestone (MFT-2), and foraminiferal packstone (MFT-3). Carbonate diagenesis revealed the presence of eo-, meso- and telo-diagenetic transformations that affected both microfacies and source rock potential. The δ¹³C_org and δ¹⁵N_org proxies suggest suboxic to anoxic conditions, facilitating OM preservation. Microfacies, OM enrichment, stable isotopes and C/N ratios infer that the Lockhart Limestone was deposited within a shallow marine carbonate-dominated inner (lagoon) to middle ramp setting. The total organic carbon (TOC) content exceeding 0.3 wt.% and S₂ yield > 0.2 mgHC/g rock represent OM enrichment and source rock potential of the formation. Based on OM geochemical analyses, the Lockhart Limestone is dominated by Type-ΙΙΙ kerogen exhibiting over-thermal maturity (T_max > 470°C) and is prone to dry-gas generation. The organic geochemical characteristics elucidate indigenous OM, implying the generation of hydrocarbon during meso-diagenesis. The present data suggest that the studied bituminous limestones are poor to moderate source rocks for potential hydrocarbon generation.