Detailed Investigation of Bedding Parallel Dolostone in Samana Suk Formation (Middle Jurassic), Nizampur Area, NW Himalayas, Pakistan: Possible Mechanism of Dolomitization and Implication on Reservoir Characteristics

Abstract

The Jurassic Samana Suk Formation is exposed in various sections in Nizampur Basin. The formation is well exposed in the study area having a thickness of 104m. Upper contact of the formation is covered by alluvium and thrusted lower contact with the Cretaceous Lamshiwal Formation in the NE and Kawagarh Formation in the NW. The main focus of the present study is to investigate, diagenetic changes, bedding parallel dolomitization in the Samana Suk Formation Kahi Section and possible mechanism of dolomitization and its effect on reservoir characterization. Field observations show both the signatures of host Oolitic, Micritic limestone and bedding parallel dolomitization in the Samana Suk Formation. Dolomitization is multiphase and various matrix replacive dolomites bodies were recognized on the basis of colour contrast i.e., dark grey colour replacive dolomite, light grey dolomite, brownish dolomite and yellowish dolomite. Beside replacive phase voids and fracture filling cementing saddle dolomite (SD), and cementing calcites (CC) are also recognized in the field. The conformations of field observations were carried on detailed conventional and Cathodoluminescence (CL) petrographic examinations of various thin sections of selected samples taken from field. Petrographic study shows the complex diagenetic history of the Samana Suk Formation from near surface diagenesis micritization to burial diagenesis stylolization. From petrographic investigations, it is observed that RD1 is fine to very fine grained, RD2 is medium to coarse grained and anhedral crystal shape, RD3 is medium to coarse grained and subhedral crystal shape, RD4 is coarse to very coarse grained and planner Euhedral Zoned dolomite. Cementing saddle dolomite has large crystal, curved faces and shows undulose extinction in crossed polarized light, cementing calcite CC3 are twin and CC4 are ferroan in nature. Petrographic investigations reveal an extra replacive dolomite RD5 coarse grained subhedral to Euhedral and ferroan in nature, very fine grained cementing dolomite CD1, coarse grained void filling cementing calcite CC1, and pink calcite CC4. Geochemistry of various dolomites and dolomite phases shows a varied range of values. Extensive depleted δ18O values ( -4.79‰ to -10.73‰ V-PDB) of dolomites and calcites supports the diagenetic alteration of the Samana Suk Formation by dolomitization and calcitization at numerous temperatures. Kahi thrust-1 along which the Jurassic Samana Suk Formation thrusted over the late Cretaceous Kawagarh Formation has provide pathway for Mg rich fluids from the underlying siliciclastic Formations to form fault related dolomites. Fault related & burial compaction are the main contributor in dolomitization of Samana Suk Formation. Reservoir characterization observations were carried out by image analysis of photomicrographs. Vuggy, fenestral, moldic, and fracture porosity are observed in petrographic examination. Porosity caused by dissolution is much greater than limestone part of the formation.