Reservoir Heterogeneity in Tide-Dominated Deltaic Systems: An Example from the Miocene Surma Group in the Sylhet Trough, Bangladesh
A study of core materials and well log data from the Miocene Bhuban and Bokabil Formations of the Surma Group in the Sylhet Trough of Bangladesh shows strong tidal influence in an outer estuarine to open marine environment. Cores indicate a gradual vertical transition from a more transgressive, estuarine paleoenvironment of the Bhuban (and lower Bokabil) Formation to a more progradational paleoenvironment of the upper Bokabil, culminating in the progradation of distributary mouthbars and delta front deposits towards the south during the late Miocene. A total of seven facies associations have been identified in the cores. Of those, delta front/mouthbar sandstones and sandy tidal bars/ ridges are the major reservoir facies in Bibiyana. Muddy tidal bars and tidal channels are the secondary reservoirs. Prodelta deposits, subtidal mudflats and shelf mud are identified as the non-reservoir units. The presence of gravity flow units in some of the interpreted delta front deposits appears somewhat problematic with respect to a shallow marine setting. However, recent studies indicate that high-density gravity flows can be generated from flood-dominated river deltas during humid climate periods that dramatically increase the sediment flux. We interpret the massive sandstones of Bibiyana to have developed by similar “hyperpycnites” during episodic flooding by the proto-Brahmaputra that fed the Surma basin deltas.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California