Abstract

The Middle to Late Eocene Kopili formation is an important source and reservoir for hydrocarbons in the Dhansiri Valley area of South Assam Shelf. Isopach and sand shale ratio maps suggest the development of good reservoir facies sandstones in the lower Kopilis and fair to moderately well developed reservoir facies in the upper Kopilis in an estuarine setting in the study area between Borholla and Khoraghat structures. Correlation of base level fluctuations has revealed the presence of at least two complete sequence stratigraphic cycles, of which the first (Cycle-I or Sequence-I) extends from the onset of forced regression above the Pre-Cambrian basement to the onset of the next forced regression at the beginning of the hiatus towards the end of the deposition of the Barail Group (the Oligocene unconformity). This cycle of base level changes includes the Tura, Sylhet, Kopili formations and the Barail Group of sediments and is constituted of fluvial to shoreface LST, TST and HST deposits. The Kopili formation has been deposited in a dominantly transgressive set up and includes the Maximum Flooding Surface for the first sequence stratigraphic cycle, which is followed by a period of normal regression during which the delta front and delta plain sediments of the Barail Group were deposited. The Sylhet-Kopili and the KopiliBarail transitions transition appear to be within sequence surfaces whereas the BMS-BCS transition within the Barail Group appears to be a a Within Trend Normal Regressive Surface within the HST phase of Cycle-I.