DefinePK

Abstract

We describe the lithofacies, depositional sequences and diagenetic fabric of the Early Jurassic Datta Formation for the characterization of its hydrocarbon reservoir potential in the Salt Ranges, Northwest Pakistan. In the study area (Nammal Gorge Section) the Datta Formation is predominantly composed of fine to coarse grained sandstone interbedded with clays and limestone at places. We collected the outcrop data and used microfacies analysis techniques to categorize five different facies which are 1) Cross bedded sandstone (DF1), representing deposition in a braided fluvial environment 2) Thin to medium bedded sandstone and interbedded clays (DF2), representing deposition in a meandering fluvial environment 3) Coal beds/organic clays (DF3), representing deposition in a deltaic (swampy) environment 4) Laterites (DF4), representing prolonged exposure of the platform under oxidizing conditions and 5) Lime mudstone-wackestones (DF5), representing deposition in a lagoonal environment. Based on the facies synthesis a dynamic depositional model is presented, which suggests seven repeated exposure cycles of the platform (SB1-SB7 sequence boundaries), represented by the Laterite facies (DF3) and associated channel sandstone deposits of the braided river system that are interpreted to have been deposited in a regressive systems tract. The unconformable sedimentation pattern is punctuated by subsequent marine transgressive cycles. These cycles preserve coal beds/ carbonaceous matter along with the carbonate facies in marginal marine settings and also caused flood plain deposition in a meandering river system in the transgressive systems tract.
The diagenetic fabric of the potential hydrocarbon reservoir intervals (DF1 and DF2 and DF5 facies) is also documented. The fractured quartz grains and bending in muscovite manifests compaction of rock while the presence of siliceous, calcitic, ferruginous, glauconitic and clayey cements indicate different phases of cementation. The observed visual porosity within the DF1 and DF2 facies is effective and includes intergranular, intra-granular and fractured types. The experimental values of plug porosity and permeability along with sub-mature nature of sandstones confirm good hydrocarbon prospects of the Datta Formation in the region.