Sequence Stratigraphic Exploration for Deep-Water Cretaceous Reservoirs, Offshore South Africa

L.F. Brown
Bureau of Economiv Geology Jackson School of Earth Sciences, The University of Texas at Austin, Austin, TX

Exploration by former Soekor Ltd. (Pty) from 1987-1993 focused on the sequence stratigraphy of offshore Pletmos, Bredasdorp and Orange Basins. Robust data (wireline logs, microfossils, source rock analyses, 2-D and some 3-D seismic data provided a basis for lithogenetic, sequence and seismic stratigraphic analysis of post-rift, divergent and passive-margin Cretaceous strata (126 to 65 Ma).

A variety of microfossil types (benthonic, planktonic, calcareous nannos, radiolaria, and dinoflagellate cysts provided chronostratigraphic control for correlating South African depositional sequences with internationally accepted sea-level cycles.
Second-, third- and fourth-order sequences exist between lower Valanginian (126 Ma) and Danian (~67 Ma), including 37 type-one unconformities and 36 third-order sequences (average ~1.5 my’s). South African sequences correlated with 36 sequences recognized elsewhere in the world. Four of six second-order sequences agree in age with Exxon’s second-order sequences. All major Atlantic anoxic events are preserved in the Cretaceous succession.

A unique aspect of South African sequences is presence of seismically recognizable fourth-order sequences, best developed in Pletmos Basin where sixty two ~100,000-year sequences were recorded because of high sedimentation rates and low subsidence rates.

Drilling showed that all twelve commercial basin-floor fan reservoirs were deposited on second-order type1 unconformities. Sequence stratigraphic exploration resulted in unusually high success rates for predicting density-flow basin-floor reservoirs. The relatively rapid discovery by Soekor of commercial reservoirs in offshore South Africa clearly documents the utility of sequence approaches in oil exploration.

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