Reservoir Heterogeneity in the Fluvio-Deltaic Deposits: A Case Study from the Surma Group Succession Exposed in the North-Eastern Surma Basin, Bangladesh

Well-exposed fluvio-deltaic Surma Group successions of the northeastern Surma Basin provide a direct analogue to its equivalent subsurface successions—the main hydrocarbon reservoirs in Bangladesh. The various levels of lateral and vertical heterogeneities lead to changes in the subsurface sandbody distributions and their reservoir qualities across fields. The present study aims to identify the heterogeneities in the fluvio-deltaic Surma Group successions by detailed sedimentological analysis of the Mio-Pliocene-aged Surma Group successions. Eleven lithofacies have been identified and based on the recurring pattern of these facies within the Surma Group, five facies associations have been identified. The facies associations indicate that these deposits originated from diverse depositional settings, ranging from the prodelta to the alluvial to delta plain. As a whole, they represent a progressive shallowing upward succession with two transgressive events in the upper and lower parts of the Surma Group. Five levels of heterogeneities such as prodelta mud (HL-I), tidal heterolithic deposits (HL-II & III), interbedded mud units in delta front (HL-IV), and intercalated mud laminae, drapes (HL-V) have been identified in the fluvio-deltaic Surma Group. The study suggests that the various levels of reservoir heterogeneities are controlled by the occurrence and distribution of various facies associations.