ABSTRACT

The Upper Palaeocene - Lower Eocene rocks in Moran oilfield of Upper Assam consist of sandstones with variable colour, compactness, particle size, porosity and permeability alternating with shales. Diagenesis has played a major role in determining reservoir quality of these sandstones. The thick- ness of these sandstones is very less (2-3metres in certain cases), but their productivity is very high. These sandstones contain large secondary pores and intragranular fractures which have developed mainly due to the activ- ity of interstitial solutions and tectonic effects. Authigenic growth of sec- ondary minerals in the intergranular spaces and in certain cases the devel- opment of overgrowth causes reduction in permeability, which finally leads to the significant losses in production. Permeability is also lost due to the presence of clay minerals such as kaolinite, illite and chlorite which tends to swell quite significantly in the presence of water. All of these clays are present in varying proportions within the Upper Palaeocene-- Lower Eocene reservoir sandstones.

Key words: Petrography, diagenesis, Upper Palaeocene -Lower Eocene, Moran oilfield.