Influence of Fold and Salt-wall Growth Rates on Deepwater Sedimentary Systems in an Active Salt Mini-Basin, Offshore Angola

Jones, Gemma L.; Lonergan, Lidia; Mayall, Michael; Dee, Stephen

The infill history of a salt withdrawal mini-basin in the contractional domain of the gravity-driven salt system on the Angolan passive margin has been reconstructed using a high resolution three-dimensional seismic dataset. Well-constrained biostratigraphy has allowed calculation of the rate of shortening and the rate of crestal growth across the basin-bounding structures. These measurements are used as a proxy for the growth rate and indicate that the depositional style of sediments preserved in the basin varies in response to changes in the rate of growth of these coeval, adjacent salt structures.

We subdivide the basin history into intervals according to the amount of structural movement occurring during that time and the corresponding type of sediment deposited. These intervals can broadly be grouped into three: low shortening rates (<10 m/Ma) where ponding dominates, moderate shortening rates (10-30 m/Ma) which influence the flow pathways of channel complex systems and high shortening rates (>30 m/Ma) generating and affecting the distribution of local and regional-scale mass transport complexes.

In all intervals, development of sea floor topography influenced the deposition of sediments in the basin. In the channelised intervals, moderate to high growth rates across the structures caused channels to react to pre-existing or contemporaneously developing sea floor topography through deflection, diversion, off-set stacking and incision. Subsequent salt movement, and a concomitant increase in the growth rate of basin-bounding anticlines, led to more elevated topography and extensive slumping of sediment into the basin center forming large mass transport deposits. Locally sourced mass transport complexes can be traced back to structures that had high growth rates at the time of initiation. Ponded fans formed during periods of low structural growth, and infill topographic lows generated during periods of earlier high growth.

Understanding the impact growing structures can have on sediment distribution is invaluable in the exploration and production of oil and gas. In this area, the association of channels with structures has formed reservoir-trap combinations for four major oil fields, so these interactions need to be taken into account in the development plans, risks and uncertainties required for each field.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013