Since the consecutive discovery of several gas fields from 2004 to present, the Rakhine Basin has been an active area for petroleum exploration in the Bay of Bengal. High-resolution 3D seismic data and well data from blocks AD1, AD6 and AD8 offshore northwest Myanmar are used to study the Miocene-Pleistocene depositional architecture and sedimentary evolution in the Rakhine Basin. Analysis of seismic facies and seismic attributes indicates that deep-water architectural elements include submarine canyons, confined slope channel complex systems, aggradational channel-levee complexes, isolated channels, frontal splays and mass-transport complexes, which have variable characters (shape, dimension, sedimentary architecture) within predominantly background deep-water slope-basin floor facies. Most of the sediments are interpreted to be sourced from the Ganges-Brahmaputra fluvio-deltaic system to the north with only minor lateral input from the Indo-Myanmar Ranges to the east. Investigation of the depositional evolution and architectural elements transformation during the filling history of the Rakhine Basin suggests the Rakhine Basin experienced rapid progradation during the Oligocene-Middle/Upper Miocene, gradual retrogradation during the Middle/Upper Miocene-Early Pliocene and gradual progradation during the Early Pliocene-Pleistocene. Published exploration results indicate that the main reservoirs of the discoveries in blocks A1 and A3 are Pliocene frontal splays and channel-levee fills, dominated by fine and very fine-grained sandstones, in structural and structural-stratigraphic traps. Analytic results from seismic characters and several exploration wells indicate that channel complexes and associated overbanks and frontal splays with fine-grained sandstones and siltstones trapped by the four-way closures are primary reservoir targets.

Supercritical flows are ubiquitous in natural environments; however, there is rare 3D anatomy of their deposits. This study uses high-quality 3D seismic datasets from the world’s largest submarine fan, Bengal Fan, to interpret 3D architectures and flow processes of Pliocene undulating bedforms that were related to supercritical flows. Bengal undulating bedforms as documented in this study were developed in unconfined settings, and are seismically imaged as strike-elongated, crescentic bedforms in plan view and as rhythmically undulating, upstream migrating, erosive, discontinuous reflections in section view. Their lee sides are overall 3 to 4 times steeper (0.28° to 1.19° in slope) and 3 to 4 times shorter (117 to 419 m in length) than their stoss flanks and were ascribed to faster (high flow velocities of 2.70 to 3.98 m/s) supercritical flows (Froude numbers of 1.53 to 2.27). Their stoss sides, in contrast, are overall 3 to 4 times gentler (0.12° to 0.27° in slope) and 3 to 4 times longer (410 to 1139 m in length) than their lee flanks and were related to slower (low velocities of 2.35 to 3.05 m/s) subcritical flows (Froude numbers of 0.58 to 0.97). Bengal wave-like features were, thus, created by supercritical-to-subcritical flow transformations through internal hydraulic jumps (i.e., cyclic steps). They have crests that are positive relative to the surrounding region of the seafloor, suggesting the predominant deposition of draping sediments associated with net-depositional cyclic steps. Turbidity currents forming Bengal wave-like features were, thus, dominated by deposition, resulting in net-depositional cyclic steps. Sandy deposits associated with Bengal net-depositional cyclic steps are imaged themselves as closely spaced, strike-elongated high RMS-attribute patches, thereby showing closely spaced, long and linear, strike-elongated distribution patterns.

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