--> Facies Control on Selective Dolomitization in the Samana Suk Formation (Middle Jurassic), Southern Hazara Basin (NW Himalaya, Pakistan): Implications on Reservoir Characterization

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Facies Control on Selective Dolomitization in the Samana Suk Formation (Middle Jurassic), Southern Hazara Basin (NW Himalaya, Pakistan): Implications on Reservoir Characterization

Abstract

The studied rocks form part of the southern Hazara basin in the NW Himalayan Fold and Thrust belt, Pakistan. Thick inner to mid ramp carbonate buildups of Samana Suk Formation (Callovian-Bathonian) mostly composed of medium to thick bedded limestone with occasional dolostone beds are exposed in various localities. In the present studies, attempt has been made to understand fluid-rock interaction phenomenon of dolomitizing fluids with various limestone facies and its impact on the reservoir behavior of altered rocks. Field investigations, petrographic studies, geochemical analyses and porosity/permeability studies were performed to achieve above mentioned objective. Field observations revealed various limestone types (Oolitic, burrowed, dolomitic, micritic and sandy), whereas dolomite existed as completely replacive, partially replacive and burrow-filling types. Petrographic studies showed various limestone and dolomite facies, which include: (i) Grainstone facies (bioclastic grainstone, peloidal grainstone, ooidal grainstone & pel-bioclastic grainstone), (ii) Packstone facies (bioclastic packstone & peloidal packstone), (iii) Wackstone facies (bioclastic wackstone), (iv) Mudstone facies (lime mudstone, partially-dolomitized mudstone, patchy-dolomitized limestone) and (v) Dolomite facies respectively. Early stage calcitization resulted in the pore-filling of grainstone-packstone facies, whereas mudstone-wackstone facies remained unaltered due to low porosity and permeability. Late stage dolomitization caused complete to partial alteration of mudstone-wackstone facies, in contrast grainstone-packstone facies are not affected by dolomitizing fluids due to the fact that the pore network of these coarser facies had already been occluded by the preceding calcitization event. Porosity and permeability analyses revealed high porosity values (4–8%) in the dolomitized facies, whereas unaltered limestone facies showed considerably low porosity/permeability values (>1%). In conclusion, less porous/permeable fine-grained facies evolved into more porous and permeable units due to the interaction of dolomitizing fluids, which confirms that the original sedimentary texture of rock has implications on the selectivity of any diagenetic alteration.