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Conoco Inc., Houston, Texas
A new depositional model relates the formation of Niger Delta toe thrusts and growth faults to source rock maturation. Specifically it is proposed that an oil prone source rock at early maturity generates enough fluid and pressure to initiate major fault movement by gravity sliding.
Recently acquired, high quality deepwater seismic data show a well-defined decollement surface that links toe thrusts, growth faults, and counter-regional faults in the progradational wedge. It is believed the decollement occurs in an Aptian source rock known to be present in West Africa and Brazil.
Regional cross-sections were constructed through the delta, using the published positions of shorelines through time and assuming a maximum post-Aptian sediment wedge of eight kilometers. Maturation modeling was performed and the results at various times from Late Cretaceous to Recent were plotted and compared to paleopositions of the shelf edge and toe of slope. The modeling illustrates how the maturation of an Aptian source rock may have significantly influenced structural development and facies distribution in the delta.
The present-day time slice shows a strong correlation between the location of the early mature window of the proposed source rock and the position of the leading toe thrust in the delta. A similar correlation is found between the location of the post gas-mature window and the position of significant fluvial facies accumulation. The fluvial facies occurs as the result of shoreline advance caused by the loss of source-related lubrication of the decollement surface.
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