Sandstone diagenesis and its impact on reservoir quality of the Arenaceous Unit of Barail Group of an oilfield of Upper Assam Shelf, India

The potential of a sandstone reservoir to produce hydrocarbons is intimately related to its diagenetic history. The Barail Group of rocks within the Upper Assam Basin is considered as one of the world's major deltaic deposits with known hydrocarbon potential. The Arenaceous Unit of the Barail Group constitutes a significant percentage of the known hydrocarbon reserves, but the heterogeneity of the distribution of the diagenetic properties in these reservoir sandstones makes the wide variation in crude oil production from well to well within the reservoirs. Precipitation of authigenic mica, chert, quartz overgrowth and crystallization of cementing material are some of the diagenetic changes responsible for porosity reduction. Moreover, the presence of clay minerals in the pore spaces and their bridging nature especially of illite makes certain reservoir horizons less productive. On the other hand, dissolution and replacement of framework grains, intragranular fracturing and floating type of texture enhance the porosity as well as permeability and make the reservoir sandstone highly productive.