Fault-controlled, bedding-parallel dolomite in the middle Jurassic Samana Suk Formation in Margalla Hill Ranges, Khanpur area (North Pakistan): petrography, geochemistry, and petrophysical characteristics

This study deals with superb fault-controlled, bedding-parallel dolomite in the shallow marine platform carbonates of the Samana Suk Formation (middle Jurassic), southern Hazara basin, NW Pakistan. Field observations, petrographic studies, mineralogical and isotopic analyses, and porosity-permeability analyses allow to unravel the diagenetic evolution of documented diagenetic phases and their impact on the reservoir behavior. Various diagenetic phases are recognized, which include (i) two episodes of replacive dolomites (Dol. I and Dol. II), (ii) initial phase of white calcite cement (WC-I), (iii) saddle dolomite (SD), (iv) late phase of white calcite cement (WC-II), and (v) transparent calcite cement (TC). In addition, other diagenetic features are bedding-parallel stylolites (PBSs), cataclastic deformation, and dissolution. Stable C- and O-isotope signatures confirmt that elevated temperature fluids resulted in the formation of the various dolomite and calcite phases. Porosity-permeability studies revealed enhanced porosity in the replacive dolomite (i.e., Dol. I and Dol. II), but late stage cementation in relation to calcite and dolomite cementation showed adverse effect on the reservoir quality. Furthermore, diagenetic alterations (i.e., brecciation and dissolution) along faults contributed in the enhancement of reservoir properties. In conclusion, dolomitization occurred due to the percolation of Mg-rich fluids along the Haro Fault and its splays. Dewatering of basinal fluids from the underlying siliciclastic rocks or deep and hot hydrothermal fluids may be the possible source of dolomitization.