AAPG Geoscience Technology Workshop

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Evidence of Fluid-Flow Phenomena and their Implications on Deep and Ultra-Deep Petroleum Systems on the Nigeria Transform Margin

Abstract

In the past decade, the tectonic evolution and deep and ultra-deep petroleum systems of Central Atlantic transform margins have attracted increased industry interest and exploration activities due to successes from world-class Jubilee Field, Venus and Odum Fields. The study area is located in the western part of Nigeria covering Oil Prospecting Licenses 312, 313 and 314, with over 2,845 km2 3D high-resolution seismic data acquired to confirm the presence of deeply buried lower and upper Cretaceous play fairways (> 5km) already discovered along the West African Transform and Brazilian margins. Study revealed wide range of overburden and seabed fluid-flow phenomena with large-scale mass-transport complexes (MTCs) and deep-water channel complexes. Fluid-flow features are formed from fluid movement within the basin; this could be water, gas, oil or a combination of all three. Series of pockmarks, vertical pipes, direct hydrocarbon indicators (DHIs), gas-hydrate Bottom Simulating Reflections (BSRs) and seabed mound have been mapped extensively in the study area. They are spatially distributed above fault planes, structural highs and active and deep-water channels. A thermogenic origin has been proposed as the fluid source as hydrocarbon generation in the basin started in Late Miocene and continued till the present-day, which coincides with the appearance of fluid-flow phenomena suggesting they could have formed during hydrocarbon generation/migration. These fluid flow features provide evidence of presence of an active deeply buried petroleum system, such that the fluids migrated through zones of weaknesses of faults and channel margins. The study area is intensely deformed by mass transport complexes (MTCs) characterized on seismic as low-amplitude, highly distorted, chaotic and dis-continuous reflections. They constitute up to 25% of the entire stratigraphy and attributed to intense tectonism during margin evolution and increased rate of sedimentation through time. No relationship has been observed with fluid flow features and MTCs; with the former mapped in the eastern part devoid of MTCs and the latter concentrated in the central and western part of the area devoid of fluid-flow features. Presence of these fluid-flow indicators on the Nigeria Transform Margin provides direct indications for hydrocarbons and bode well for hydrocarbon prospectivity. However, they also provide evidence of fluid leakage from the deep and ultra-deep petroleum systems and detailed risk assessment of the petroleum systems must be carried out in the area. Pockmarks and mounds on seabed may affect installations of drilling equipment and must be carefully mapped prior to installation. This study provides a very detailed spatial distribution of these fluid-flow features and their relationship with underlying structural and deep-water depositional systems useful in building geologic models and making predictive models that can reduce exploration risks on the margin and other basins in the world.