High Resolution Sequence Stratigraphy and its Implication in Mixed Siliciclastic Carbonate Sequence: A Case Study From Early to Middle Eocene Sylhet Formation, Assam and Assam-Arakan Basin, India

Abstract

The Early to Middle Eocene 170m thick Sylhet Formation of Assam and Assam-Arakan Basin belongs to an overall fining-up 2nd order transgressive systems tract (TST). It consists of a mixed siliciclastic-carbonate sequence and the overall facies architecture implies deposition in a tide affected marginal marine (deltaic) to inner shelf carbonate ramp with episodic clastic supply from western hinterland by isolated proto-rivers during regressive phases. High resolution sequence stratigraphic analysis based on core and electrolog data allows to subdivide the 2nd order TST into 3 systems tracts of 3rd order, (i) basal TST, represented by shale and limestone followed by (ii) a highstand systems tract (HST) incorporating sandstone, limestone and shale, and (iii) a TST at the top. The HST can be further subdivided into 4th order parasequence-sets separated by marine flooding surfaces. The isopach and isolith maps of 4th order units as well as the conceptualised depositional model reveal a minimal sand supply at beginning of 3rd order HST and a significant increase in fluvial sand thereafter, resulting basinward progradation of facies and paleo shoreline due to a punctuated slow rise of relative sea-level that leads to constructive delta building till end of 3rd order HST. The topmost 3rd order TST marks a landward retreat of facies due to paucity of sand supply and rise in relative sea-level. As basin margin areas are more prone to periodic exposures related with lower order sea-level fluctuations, the secondary porosity development within sandstone and limestone has resulted due to percolation of meteoric water leading to significant dissolution of early formed calcite cement. Such phenomena are well evident by petrographic and SEM data and are helpful in finding the locales of better reservoir facies. Sylhet Formation is characterised by abundant glauconitic horizons at selected stratigraphic levels that represent intervals of marine flooding events (MFE) of 3rd and 4th order sea-level change. The glauconites associated with MFE, result in condensed section with minimum sediment supply. The glauconitic horizons provide a key parameter to build the sequence stratigraphic architecture of Eocene sequence. The depositional model and diagenetic events of such mixed siliciclastic-carbonate system are correlated that helps to explain the extent of short-term transgressive-regressive cycles, improving elucidation of hydrocarbon-bearing sand units for future exploration.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018