Delineation of thin and discrete sand reservoir facies from shale-dominated Kopili Formation (Middle to Late Eocene) using the post-stack seismic inversion and neural network algorithm: A case study from Assam Basin, India

In hydrocarbon exploration, delineating deep-seated thin and discrete sand reservoir facies in shale-dominated sedimentary succession (Kopili Formation) is one of the major challenging tasks. Although the marine shale of the Kopili has been a proven potential source rock in the Assam Basin, the success percentage in commercial hydrocarbon production is minimal in the Lakwa field. In the present study, an integrated analysis using post-stack seismic inversion, total porosity and litho-facies predictions has been performed to delineate the thin and discrete sand reservoir facies from Kopili Formation. The features of moderate impedance (10,000–11,300 gm/cc*m/s) and moderate total porosity (21–27%) are interpreted as the sand reservoir facies. While dominant low-impedance (<10,000 gm/cc*m/s) and high-total porosity (>27%) features are shale facies, high impedance (>11,300 gm/cc*m/s) and low total porosity (<10%) are interpreted as carbonate facies. Predicted litho-facies illustrate the presence of thin and discrete sand reservoir facies between the borehole locations with improved resolution and accuracy. Cumulatively, electro-log-based litho-facies interpretation and seismic-derived attributes analyses confirm the presence of relatively thick sand facies in the Upper-Kopili and relatively thin interbedded sand facies from the Lower-Kopili. The sand facies are found to be widespread but discrete elongate bodies that thicken towards the southeast direction. The lateral and vertical heterogeneities in reservoir facies distribution estimated from reservoir characterization directly impact oil/gas reserve estimation. The integrated attribute analysis minimizes uncertainty in delineating depth/thickness and spatial distribution of reservoir facies. The present study may give a fresh impetus to exploration opportunities from Kopili Formation in the Lakwa field, Assam Basin.