Seismo-stratigraphic analysis of the Begumganj Structure in the Hatia Trough, Bengal Basin: implications for tectonostratigraphic evolution and trapping mechanisms

Abstract

 The Begumganj Structure of the Hatia Trough in the Bengal Basin is well-known for being a gas field. The trapping mechanism and tectonostratigraphic evolution of the trough are poorly understood due to lack of exposed rock units. In this drawback, this research aimed at seismo-stratigraphic analysis to understand the hydrocarbon potentiality, structural style, structure-stratigraphic interactions, and possible timing of the Begumganj Structure. Results show that the Begumganj Structure is elongated - oval shaped, asymmetrical gentle anticline with NNW-SSE-trending anticlinal axis. Most of the reflectors are strongly affected by channels, especially at the western flank. Wireline log interpretation of the well BG#1 shows a 16 m thick gas-bearing zone at depth of 2995 m (within the Miocene Bhuban Formation). Results from borehole data and interpretation of the seismic transects indicate the presence of mud or shale-filled erosional channel in the western flank of the Begumganj Anticline which results in a combinational trapping mechanism (i.e., structure-stratigraphic combinational trap). Results also show evidence of stratigraphic (pinch-out) trap at greater depths. Based on the reflection quality, lapping geometry, and reflection configuration from seismic transects and lithological information from the boreholes, three seismo-stratigraphic units have been identified. The oldest (Oligocene?) seismo-stratigraphic unit 3 shows south- or southeast-ward prograding clinoforms. The highly channelized seismo-stratigraphic unit 2 (equivalent to Miocene Surma Group) shows gradual south- or southeast-ward thickening in the lower part and relatively uniform thickness in the upper part. The topmost unit 1 (equivalent to Pliocene Tipam and Pleistocene Dupi Tila Groups) shows relatively high-frequency reflectors that are gradually onlapping to the anticlinal crest. This youngest (Pliocene to Recent) seismo-stratigraphic unit is also representing the syn-kinematic package, indicating a coeval time for the structural activation in response to the youngest episode of the Indo-Burmese subduction processes to the far east.