Petrography and diagenetic evolution of the Barail sandstones of Naga Schuppen belt, North East India: implication towards reservoir quality

Rock-thin section, scanning electron microscopy and X-ray diffraction analyses have been employed to describe in detail the mineralogical constituents, diagenetic alterations and their impact on reservoir quality of Oligocene Barail sandstones of Naga Schuppen belt, North East India. The Barail Group comprises of alternate beds of hard and compact sandstones with siltstone, shale, carbonaceous shale, and a few thin intermittent coal seams in the upper part of the rock sequence. Petrographic analysis indicates that quartz (42.02- 55.02%) is the most dominant mineral constituent followed by rock fragments (6.85-15.67%) and feldspars (0.00-1.97%) with different types of cement in the studied sandstones. Quartz overgrowth, formation of pseudo matrix, authigenic growth of secondary minerals and precipitation of clay within the pore spaces tend to reduce the primary and secondary porosities of the rocks. However, in certain samples, the grain coating restricts or hinders cementation and preserves porosity during deep burial, but decreases permeability at pore throats. Partial dissolution and intragranular fracturing of the framework minerals provide sites for pore growth. Pyrite framboids and iron oxides inhibit quartz cementation, but infill pore spaces. The present study shows that original pore morphologies, as well as secondary porosities within the sandstones tend to be destroyed to a large extent by the diagenetic processes.