A morphometric analysis of the fluid flow features of the southern Orange Basin, South Africa

Abstract

Fluid flow features can be used to delineate the migration pathways of fluid from gas sources to the present-day seafloor. A quantitative and qualitative study of these features can reveal linkages between modern mass wasting processes, submarine geomorphology and fluid dynamics in the sub-strata. This study examines 2219 pockmarks from the seafloor of the western margin of South Africa, located in the gas-rich Orange Basin. High resolution bathymetry and 2D seismic reflection data reveal multiple pockmarks which are the manifestation of fluid migration from an Aptian gas reservoir in the basin's drift succession. Six units (Seismic Facies 1–6) and five unconformity surfaces (Surface A–E) are defined, and fluid flow features are studied within the context of this seismic stratigraphy. The fluid migration process formed blowout pipes and brightened reflectors and occured via four mechanisms (Case 1–4). Gas pipes may have cut through the stratigraphy in a near vertical fashion to terminate on the seafloor as pockmarks (Case 1). Peculiar vertical-lateral stepping of gas pipes along seismic units, dominated by lateral flow, comprised Case 2. Case 3 encompassed the termination of gas pipes along stratigraphic boundaries associated with changes in lithology. Lastly, Case 4 involved the migration of fluid along pipes and into palaeo-canyon channels. Using a hydrological extraction technique, the pockmarks are quantified for density, depth and distribution. There appears to be three distinct regions of pockmark occurrences (Zone A–C). Zones A and C are densely populated by pockmarks, whereas Zone B expresses fewer pockmark features. Pockmark morphologies include circular, elongate, crescentic, composite and stringed-types. The diversity in pockmark morphology reflects the local effects of bottom currents. This study show how fluid-flow features may control mass wasting and the development of distinctive seafloor morphologies such as submarine canyons.