--> --> Dissolution Related Collapse Breccia within the Eocene Margalla Hill Limestone in the Southern Hazara and Kashmir Basins: Mechanism of Brecciation and its Impact on Reservoir Properties

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Dissolution Related Collapse Breccia within the Eocene Margalla Hill Limestone in the Southern Hazara and Kashmir Basins: Mechanism of Brecciation and its Impact on Reservoir Properties

Abstract

Brecciation due to dissolution in carbonate rocks resulted in collapse of large blocks (metre-scale), which is responsible for porosity enhancement. Source and composition of the fluids, timing and migration pathways are the important parameters to be determined when solution brecciation occurred, which helps in interpreting the post-depositional history of the studied rocks. Faults provide pathways to control fluid flux due to upwelling of heated, basinal brines, which resulted in hydrothermal karst. Besides, these provide channels for downward movement of meteoric water, which results in the formation of dissolution halos. In order to understand the genesis of breccias, this will help in reconstruction of the tectonostratigraphic evolution of a basin and in predicting porosity distribution in carbonate reservoirs. In this context, present studies were carried out to investigate excellent brecciated zones in the Margalla Hill limestone (Eocene) in southern Hazara (Texila-Khanpur road section) and Kashmir basins (Yadgar, Batlian and Seri Dara sections). These surface analogues were studied to establish field relationship, petrographic changes, and geochemical variations. Field observations revealed that brecciated limestone is restricted to the upper middle part of the formation, where the upper the lower part of the Margalla Hill limestone is undisturbed. Such evidences exclude the possibility of possible tectonic activity responsible for such brecciation. Petrographic studies revealed that non-luminescent, twinned calcite with pore-filling behavior mostly occur in between the brecciated limestone. Stable isotope signatures indicate depleted δ18O and δ13C values of the above mentioned pore-filling calcite. These observations indicate telogenetic nature of the calcite and hence meteoric water influx responsible for dissolution related brecciation. In conclusion, faults provided pathways for the downward movement of surficial waters to develop karstification. Such phenomenon may result in the collapse of the overlying blocks of limestone and hence resulted in collapse breccia. Such zones have high dissolution porosity as well as permeability. Hence it gives positive effect on the reservoir character of the studied rock.