Depositional Environment and Sand Distribution Patterns in Rudrasagar and Demulgaon Formations of Barail Group, Lakwa-Lakhmani Area, North Assam Shelf

Sumit K. Chakrabarti¹, Jayanta Bhattacharya¹, Mir Alam Shah Bora¹, and Gautam K. Ray^2
1Basin Research Division, KDM Institute of Petroleum Exploration, Dehradun, India
²MBA Basin, ONGC, Kolkata, India

Occurrence of hydrocarbons has been established in North Assam Shelf long back. In addition to the established reserves within Barails in Lakwa-Lakhmani, Sonari etc., recent hydrocarbon finds in Laiplingaon and Tiphuk has generated interest in further exploration for hydrocarbons north of Lakwa structure.

Sand distribution patterns of the Barail Group is highly variable as a response to the fluvio-deltaic environment of deposition. An intensive study was carried out to generate sand distribution models of various units within Rudrasasgar and Demulgaon formations (BCS and BMS) in Lakwa-Lakhmani and surrounding areas.

The sands within Barail Group have been classified into six informal units covering Disangmukh, Demulgaon and Rudrasagar formations. Sand distribution pattern combined with seismic attributes and log motifs suggests a fluvial dominated constructive deltaic depositional environment prevailing during the deposition of the Barails. The study suggests a delta front environment for the Demulgaon Formation, which continued upto LBS-1. A major change in depositional environment above LBS-1 has been indicated by the first appearance of coal and change from upward coarsening profiles to a broad fining upward pattern within BCS. The Rudrasagar Formation was deposited in a dominantly fluvial delta plain setting characterized by aggradation dominating over lateral progradation.

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