Seismic Stratigraphy and Seismic Geomorphology of a Slope Environment — Offshore Angola, West Africa

Henry W. Posamentier
ETC, Chevron Energy Technology Company, Houston, TX

The stratigraphy of the slope offshore Angola, West Africa, is characterized by multiple, though subtle, unconformities. These unconformities are defined by stratal discordance and most are associated with small (<150m wide) to large (>7km wide) slope valleys that “hang” from these surfaces. Multiple channels commonly hang from these surfaces. The focus of this study is the details of the evolving geomorphology and associated small channels.

The variety of features that characterize the section include small leveed channels, with sinuosities ranging from nearly 1.0 to 1.5, polygonal fracturing, some filled with injectites, and small gas-escape structures. The range of channel flow direction is narrow; flow is towards the west-northwest around a narrow range of 20-30 degrees. The width of levees, measured from their pinchouts on either side of the channels range from 30 times the width of the channel to less than 10. Straighter channels tend to have broader levees, compared with more sinuous channels. Polygonal fracturing tends to characterize entire sections, however, enhanced localized fractures are observed to overlie associated small channels (i.e., <150 m wide). Some of these fractures appear to be filled with injectite material. Small (<75m) circular gas-escape structures seem to be associated with specific horizons but seem randomly distributed along these surfaces.

Intense polygonal fracturing characterizes the upper part of the section. Within this section there is no evidence for turbidite deposits. The section characterized by turbidite channels is notable for the near absence of polygonal fracturing. However, where polygonal fracturing is observed, in the more turbidite-rich part of the section, it is far less intense and more localized. Moreover, the fractures in the lower part of the section are wider and appear to be filled possibly with sand.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery