Miocene seismic stratigraphy and geomorphology of Bibiyana gas field, Surma Basin, Bangladesh

Abstract

An integrated approach using a three-dimensional (3-D) seismic survey over the Chevron-operated Bibiyana field in Bangladesh along with petrophysical well data and cores provides new insights into the stratigraphic development of the Miocene–early Pliocene Ganges Delta. From this, a sequence stratigraphic model was constructed for the Bibiyana field that more accurately depicts the internal architecture of the reservoir units of the field. As of 2019, Bibiyana field was producing approximately 1.3 billion SCF/day of gas and accounted for more than 45% of Bangladesh’s daily gas supply. The 3-D seismic volumes covering the field area were stratigraphically flattened using multihorizon seismic flattening techniques. The well-log data were also flattened to enable the wells to be tied and visualized within the flattened domain. Core data from the field were examined, interpreted, and integrated into the stratigraphic model. Reservoirs at Bibiyana are divided into a series of sequence stratigraphic units, each recognized by a seismically defined sequence boundary at both the base and the top of the units. Two types of major reservoir sandstones were observed: (1) erosional remnants of shallow-marine, tidal sandstones below sequence boundaries and (2) channel sandstones at the base of incised canyons above sequence boundaries. The focus of this investigation was on the architecture of the main reservoir known as the BB60 series (Miocene Bokabil Formation), containing approximately 50% of the field reserves. The BB60 series reservoir is an erosional remnant below a major sequence boundary (SB60) sealed by the thick canyon-fill prodelta shale of the BB55 interval. Channelized sandstones and associated mass transport deposits are present locally at the base of the canyon and have reservoir properties that contrast with the underlying and adjacent shallow-water, tidal-dominated deltaic sandstones of the BB60 reservoir. Multihorizon flattening of the seismic and well data in concert with 3-D visualization reveals the complexly eroded canyon system, incised into the underlying tidal deltaic deposits of the BB60 series. Similar erosional patterns are present in the underlying BH series reservoirs (Miocene Bhuban Formation), and the conceptual model developed for the BB60 reservoir can be applied and aide the understanding of the deeper reservoirs that are less well imaged by the existing seismic data. The recognition that unconformities with overlying canyon-fill shales seal many of the reservoirs at Bibiyana is critical in understanding and modeling the reservoir architecture and performance over time.