Diagenetic and mineralogical impacts on clastic reservoir; A case study from the lower Indus Basin, Pakistan

Abstract

The Pab Sandstone, a significant and heterogeneous hydrocarbon development target in Pakistan’s Lower Indus Basin, offers a unique opportunity to investigate diagenetic and depositional controls on reservoir quality. A substantial uncertainty in reservoir quality assessment pertains to diagenetic effects. An integrated approach, including detailed petrographic analysis, scanning electron microscopy, X-ray diffraction, helium porosity, and permeability measurements, was used to analyze the impact of identified diagenetic elements on reservoir quality. Visual core investigations show deposition within a semi-arid, highly sand-prone coastal–alluvial braided plain under the periodic weak marine influence. The presence of trough-shaped crossbedding, large bedform structures, bioturbation, and some fossils in selective horizons indicate near-shore channel environments. The channel sandstones range from very fine to very coarse-grained quartz arenites. Analysis of some core samples shows an abundance of quartz arenites and kaolinite as the major authigenic clay minerals. Core-derived porosity and permeability were found to be 6.34 % and 69.90 mD, respectively. Main diagenetic processes include compaction, quartz overgrowth, calcite cementation, and authigenic kaolinite. Reservoir properties are strongly facies-dependent and are related to depositional texture, detrital mineralogy, together with their diagenetic overprint. Sandstones are typically very clean, with secondary porosity, which is associated with partial leaching of mudclasts, feldspars, and dissolution of volcanic rock fragments. Permeability exhibits a positive correlation with increasing grain size in the quartzose sandstones. Porosity decreases with poorer sorting. The research findings can be utilized to understand the complications of sandstone reservoirs and develop better exploration strategies minimizing the economic losses associated with non-productive wells.