Geologic Controls from Sequence Stacking and Architecture of Eocene Siliciclastic Deposits in Upper Assam Foreland Basin, India

Mayadhar Sahoo¹ and Kshetradhar Gogoi^2
1ONGC, Nazira, India
²Department of Petroleum Technology, Dibrugarh University, Dibrugarh, India

The Eocene sediments of Upper Assam Foreland Basin were deposited in a passive margin set up, when Indian plate was completely detached from Antarctica plate in Late Cretaceous and further deformed in a foreland phase during Neogene period. The exploration success in Eocene succession of NW part of the basin has warranted a detailed sequence stratigraphic analysis. Therefore the present sequence stratigraphic study of fine siliciclastic deposits offered in this paper is in contrast to earlier works mainly of coarse clastic deposits.

The present study is an integrated analysis of available lithological, core, well log data and seismic inputs to produce detailed sequence stacking pattern and stratigraphic architecture to understand the basin evolution process during Eocene period. The study thus subdivided the Eocene section of Paleocene - Oligocene 2nd order sequence into three 3rd order sequences and nine parasequences (S1a, S1b, S1c, S2a, S2b, S3a, S3b, S3c and S3d). The base of the Eocene succession is characterized by 100m to 200m thick prograding fluvial to marine sandstone, mudstones forming Lowstand clastic wedge, overlain by 100m to 250m thick aggradational succession of carbonate deposits and 500m to 700m transgressive marine shales with alternation of fine sandstone and mudstone deposited in estuarine environment. The stratigraphic architecture and thickening trend of parasequence sets indicate, the main source of sedimentation from NW part of the craton. The recognition of sequence boundaries suggest that siliciclastic rocks were subaerially exposed during Eocene, hence it is concluded that the stratigraphic architecture and stacking pattern was controlled dominantly by eustasy not extensional tectonics.

The study is expected to offer valuable inputs to predict the reservoir heterogeneities for field development and to locate potential stratigraphic traps.