EVIDENCE OF A LARGE PALEO-POCKMARKED SURFACE IN THE ORANGE BASIN: IMPLICATIONS FOR AN EARLY EOCENE MASSIVE FLUID-ESCAPE EVENT OFFSHORE SOUTH AFRICA

Pockmarks, mud volcanoes, gas chimneys, pipe structures, and other seismic anomalies are widely accepted as evidence for fluid and gas migration and leakage processes. Some of these expressions have been shown to exist along the West African continental margin (Gay et al., 2007; Anka et al., 2009). Evidence of fluid leakage processes, such as mud diapirs and seismic chimneys, have previously been identified in some areas of the Orange Basin, South Africa (Kuhlmann et al.). Here we present the integration of these results with our ongoing investigations on 3D seismic reflection data, which provides new insights into the timing and dynamics of fluid migration and seepage on the Orange Basin.

The interpretation of 3D seismic data in the NW of the Orange Basin has led to the identification of a pockmark-covered horizon within Paleogene slope sediments. The pockmarks have average sizes from 200 to 350 m. Their density increases upslope where they form interconnected structures similar to those described by Pilcher & Argent (2007) in the Congo basin. This pockmarked surface extends for an area of approximately 2800 km² to the North of the basin and it occurs above a well constrained Cretaceous/Cenozoic unconformity and below a Miocene erosional surface. Possible driving factors for this massive release event are currently being investigated: (1) Cenozoic uplift, erosion, and/or Paleogene sea-level fluctuations, (2) Initiation and shifting of ocean currents during early Cenozoic, (3) Local increase in paleo-heatflow due to Paleocene/Eocene and Eocene/Oligocene volcanism, which could have triggered generation and subsequent seepage of thermogenic methane along existing migration pathways.

References:

Anka, Z., et al. 2009. AAPG Search and Discovery Article #90090.

Gay, A., et al. 2007. Marine Geology (244): 68-92.

Kuhlmann, G. et al. 2010, Marine and Petroleum Geology, 27(4): 973-992.

Pilcher, R. and Argent, J. 2007. Marine Geology 244, 15-32