Although the Palaeocene-Eocene reservoirs of the Assam shelf have been prolific producers of oil and gas, the production profiles of various fields have caused concerns because of their departure from the geological model originally envisaged. An attempt is made to understand the temporal and spatial reservoir heterogeneity within a broader depositional model framework using micro- resistivity image and open hole log data from four fields in the assam Shelf; Baghjan, Katahloni, Bhogpara and Nahorkatia. Using image data, whole cores, dip patterns, openhole logs, image derived neural network facies and image derived textural facies, the key reservoir facies associations are identified. Facies associations in different wells are made use of to reconstruct depositional sequences, which enable to conceive a broader depositional model for the sedimentation of major Plaeocene to Eocene formations, namely Langpar, Lakadong, Narpuh, Prang and Kopili.

This study infers marginal marine to coastal plain channelized sedimentation for the Langpar formation with reservoirs likely to be oriented in a NW-SE to E-W direction. These basal clastics are overlain by lagoon–barrier island to coastal plain channel environment of the Lakadong formation with reservoir orientation along E-W to NW-SE direction. Beach-strand plain to chenier plain depositional environment is encountered in the Narpuh formation with the reservoir appearing to have an extension along N- S to NW-SE direction. Overlying Prang and Kopili formations are inferred as restricted marginal marine deposits. The Prang shows occurrences of carbonate facies whereas the Kopili shows feeder channels facies of thick argillaceous sequence. Overall, the Eocene-Oligocene petroleum system seems to have been deposited in a marginal marine depositional setting. Analysis of depositional units in a sequence stratigraphic context suggests that the overall sequence has evolved in a transgressive system. Using a sequence stratigraphic framework allowed us to better define reservoir distribution patterns of the Assam Shelf, and these facies associations may prove useful in future reservoir modeling studies.

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