Clay Mineral Studies of Barail and Tipam Reservoir Sands in Assam, India

Studies by X-ray diffraction and scanning electron microscopy of the matrix material from these sands indicated that both allogenic and authigenic clays are present in the reservoir. Matrix of Barail sands are characterized by illite, chlorite and kaolinite which have undergone pronounced diagenetic changes. Montmorillonite and expandable mixed-layers are important clay minerals associated with reservoir sands belonging to the Tipam series. Grains, pores and channels are all heavily coated with expandable clays in Tipam sands; the grains, pores and pore throats are partially coated with matrix clays in Barail sands with rare development of bridges in permeability channels.

Barail sands were deposited in a transitional environment (deltaic/estuarine) while the environment of deposition was fluviatile for the Tipams. Clay minerals in the Tipam sands are very sensitive to formation damage while those in the Barails could result in permeability barriers if the particles are dislodged in the sands. Accordingly, precise control of salinity of formation water in steps during production and development is suggested.