Strata-bound Dolomitization in the Eocene Laki Formation, Matyaro Jabal Area Lakhi Range, Sindh, Pakistan

Abstract

The limestone of Eocene age Laki Formation of Matyaro Jabal area, Lakhi Range SindhPakistan has been studied to see different sedimentary features and diagenetic overprinting. The most diagnostic diagenetic feature of the Laki Formation is the formation of strata bound dolostone over extensive area. The dolostone beds which are separated by non-dolomitic limestone have developed at three different stratigraphic levels whose thickness vary from few centimeters to about 5 meters. Interbedded non dolomitic limestone is characterized by highly fossiliferous to less fossiliferous white chalky limestone with significant secondary porosity. The dolostone beds make lower erosional contact with chalky limestone while upper contact is sharp as well as transitional. The dolostone beds are very hard to soft with well developed dissolution cavities and karstification horizons. As a result of dolostone formation, the primary sedimentary features of rock fabrics and bioclasts are poorly preserved. However, few bioclastic grains show partial preservation with enhanced dissolution and biomoldic porosity. Dolomitization and different porosity types such as; intragranular, vuggy, molidic, intercrystalline, fracture and fenestral have made the limestone of Laki Formation as potential hydrocarbon reservoir rock. The mechanism of stratabound dolostone formation within Laki Formation is due to the mixing of seawater and fresh water with optimum Mg:Ca ratio. The Mg rich sea-water circulated through highly porous and permeable strata which was responsible for stratabound dolostone formation in the Laki Formation. The extrinsic factors such as sea level fluctuations and tectonics also played a vital role for dissolution along with porosity and permeability enhancement followed by dolomitization