Abstract:
The Ghazij Formation in the Southern Indus Basin, Pakistan, is a shaledominated Eocene sequence containing carbonate-rich limestone intervals whose source rock quality and unconventional reservoir potential remain poorly characterized. This study integrates geochemical and petrographic analysis of the Ghazij Formation in Zamzama-5 and Mehar-1 to evaluate the quantity, quality, and thermal maturity of organic matter and to assess its unconventional hydrocarbon reservoir potential. Total organic carbon (TOC) analysis, Rock-Eval pyrolysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were applied to drill-cutting samples from Zamzama-5. Mehar-1 was evaluated by TOC and visual cutting examination only, as all samples fell below the 0.5 wt.% analytical threshold. XRF and XRD indicate carbonate-dominated limestone at Zamzama-5, ranging from calcareous quartzose limestone at shallow depths to a purer limestone at depth. Clinochlore as the only clay phase (4–5%) and a diagenetic dolomite-to-ankerite transition at the deepest interval. TOC values of 2.71– 5.05 wt.% indicate very good to excellent source rock quality, exceeding the 2% unconventional hydrocarbon threshold throughout. Rock-Eval pyrolysis identifies Type III indigenous gas-prone kerogen (HI: 87–183 mg HC/g TOC), with the production index indicating immature to early mature conditions in the sampled intervals. SEM-EDS identifies five pore types defining a multi-scale pore system, with interparticle pores at shallower depths and more conspicuous natural microfractures at the deepest sample. Mineralogy-based brittleness indices of 29.5–39.6% and 90.3– 96.8% indicate a favourable mineralogical brittleness framework for hydraulic fracture stimulation. However, the sampled Ghazij intervals are currently immature to early mature, representing the principal limitation to their present thermogenic hydrocarbon potential. More thermally mature Ghazij intervals may occur in deeper parts of the Southern Indus Basin and require investigation using samples from more deeply buried wells. At Mehar-1, TOC values of 0.32–0.45 wt.% and mixed shale-limestone cuttings confirm a lean organic-poor facies equivalent. The Ghazij limestone at Zamzama-5 exhibits the principal organic, mineralogical, and pore-system characteristics favourable for an unconventional tight carbonate gas play, but its present immature to early mature thermal state remains the principal limitation. It is therefore proposed as a target for further investigation, particularly in deeper parts of the Southern Indus Basin where more mature intervals may occur.
