Anatomy of a gas-bearing submarine channel-lobe system on a topographically complex slope (offshore Nile Delta, Egypt)

Abstract

The characterization and predictability of submarine channel-lobe systems on topographically complex slopes have proven challenging, due to the complex responses of such systems to interacting flows and seafloor topography, in terms of their temporal evolution and spatial changes in morphology and architecture. Detailed subsurface studies can reveal important depositional and morphological elements in such settings and help develop predictive models for hydrocarbon exploration. Here, we document a gas-bearing submarine channel-lobe system on a topographically complex slope, from the Pliocene succession offshore the west Nile Delta where the Giza gas field has been discovered. Based on an integrated analysis of 3D seismic reflection and borehole data, several distinct deep-water depositional elements, including amalgamated channel-fills, aggradational channel-fills, mud-prone channel-fills, lobes, levees and mass-transport deposits are recognized. They are developed on a tectonically influenced continental slope, forming a channel-lobe system that features four main evolutionary phases over a period of ca. 0.26 Myr: 1) Channel belt incision, 2) infill of channel belt by laterally amalgamated channel deposits, 3) development of leveed aggradational channels, with lobes developed within and outside the channel belt, 4) channel avulsion leading to the abandonment of the channel belt, resulting in the formation of a new ponded lobe adjacent to the original channel belt.Spatially, the channel-lobe system shows marked changes in architecture and planform morphology in the areas affected by folds and faults. As the channels cut across a faulted anticlinal structure, they show straightening with some degree of diversion on the upstream limb, while on the downstream limb, channels exhibit increased lateral migration and sinuosity, widespread high-amplitude reflections (interpreted as sand-rich channel-fills), and channel to lobe transitions. In contrast, the last-stage avulsion channel appears to divert along several normal faults at a high angle to the regional slope, and forms a ponded lobe external to the main channel belt. This study shows how the channels and lobes throughout these evolutionary stages respond differently to the presence of folds and faults. It provides a unique analog for submarine channel-lobe systems developed during a third-order sea-level lowstand to transgressive phase with the influence of structurally induced topography and mass transport. The results of this study provide insights into the spatio-temporal development of various types of slope channels and lobes, and may greatly enhance the prediction of associated reservoirs in deep-water systems in tectonically active areas.