Diagenetic Events, Reservoir Compartmentalization and its Relationship With Lower Order Relative Sea-Level Fluctuation in Early-Middle Eocene Sylhet Formation, South Assam Shelf (SAS), Assam and Assam-Arakan Basin

Abstract

The present paper is focused on diagenetic events, reservoir quality and its relationship with sea-level fluctuation for the reservoir sands restricted in upper part of Early-Middle Eocene Sylhet Formation within South Assam Shelf. Synergy between sequence stratigraphy and diagenesis, however, enables prediction of spatial and temporal distribution of diagenetic alterations and post-depositional evolution of reservoir quality. It also provides information on diagenetic baffles and barriers for fluid flow, thus potential diagenetic reservoir compartments and seals. Sylhet Formation consists of mixed siliciclastic-carbonate sequence deposited in tide affected marginal marine to inner shelf carbonate ramp environment with episodic siliciclastic supply during regressive pulses within an overall fining-up transgressive unit. 21 cores along with electrologs have been investigated to infer lithofacies, mineralogy and diagenetic facies controlling reservoir quality using petrography, XRD and SEM analysis. The dominant microfacies are calcareous quartz arenite/quartz wacke and arenaceous foraminiferal wackestone with minor dolostone. 12 diagenetic facies have been identified within Sylhet Formation. The salient diagenetic features responsible for reservoir deterioration are intense early calcite cementation, authigenic pore filling kaolinite and chlorite. However, during lower order sea-level fluctuations, diagenetic alterations owing to percolation of meteoric water below subaerially exposed sequence boundaries can cause extensive dissolution of calcite cement enhancing secondary porosity (~30%). Based on textural relationship among different diagenetic facies, a paragenetic sequence depicting two major diagenetic events, has been identified and mapped to bring out the diagenetic history. Paragenetic sequence includes early calcite cementation and its immediate dissolution in eodiagenetic stage, subsequent dissolution of feldspar and precipitation of kaolinite and quartz cements, followed by patchy calcite cementation in mesodiagenetic stage. Porosity distribution map depicts good porosity in NE-SW corridor with isolated poor porosity pods. However, in some areas, primary porosity is preserved where localized acidic environment inhibits precipitation of early calcite cement. Integration of diagenesis and sequence stratigraphy will constitute a powerful tool for prediction of the evolution of reservoir quality and of diagenetic baffles for fluid flow and seals.

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