Diagenetic heterogeneities in the Jurassic Samana Suk Formation, Foreland Sedimentary Fold and Thrust Belt, Pakistan: an integrated approach for reservoir behavior

The diagenetic heterogeneities seem to be the controlling factor for the reservoir quality of the oolitic carbonate rocks of the Samana Suk Formation (Jurassic) exposed in the Foreland Sedimentary Fold and Thrust Belt (FSFTB) of Pakistan. Limestones of Samana Suk Formation are constituted of four depositional textures: (i) grainstone (ooidal and peloidal grainstone), (ii) packstone (ooidal and bioclastic packstone), (iii) ooidal wackestone and (iv) fossiliferous mudstone, deposited in lagoonal and inner ramp coastal environment. The diagenetic features within this formation have been studied by integration of outcrop and core samples analyses with petrographic characterization and Sr isotope ratios. Iron-rich/iron-poor dolomitization and calcitization phases have been distinguished through the EDS equipped Scanning Electron Microscopy (SEM) and Electron Probe Micro Analyzer (EPMA) whereas cathodoluminescence (CL) studies differentiated micritization, calcite cementation, dolomitization, replacive calcite, fracture fillings by calcite through distinct luminescence colors of each diagenetic phase. By using these techniques, two phases of dolostone geobodies are observed in the Samana Suk Formation: (i) replacive dolomite (RD) and (ii) saddle dolomite (SD). And also four generations of calcite are identified. RD occurs as planar-s (RD-I) and planar-e (RD-II) dolomites. The calcite phases identified are: (i) pore filling (PFC), (ii) replacive (RC, post dolomitization), (iii) fracture filling FFC-I and (iv) a later fracture filling calcite FFC-II (post SD). ⁸⁷Sr/⁸⁶Sr ratios of fossils shells, ooids and matrix measured in the range of 0.707718 to 0.707856, agree with the seawater global curve of ⁸⁷Sr/⁸⁶Sr in Jurassic time, whereas saddle dolomite shows much higher ⁸⁷Sr/⁸⁶Sr ratios (0.711800) that denotes the contribution of different fluids, probably hydrothermal. PFC appears to have occluded primary porosity while secondary porosity is developed by dolomitization in an early stage of diagenesis. However, rock dissolution and precipitation of RC in a later stage clogged the porosity as indicated by calcitized remnants of dolomite. SD cross-cut by FFC in fractures developed in late stage diagenesis resulted in filling of the fracture porosity. No signs of porosity in core samples and open fractures in outcrop samples indicate that uplifting of middle Jurassic carbonates possibly is the major event that produced porosity in carbonates of Samana Suk Formation. These fractures have enhanced the overall reservoir quality of formation.