--> Abstract: The Pre-salt Geology of Block 4, Angola and Implications for the Post-salt Play, by J. Anderson, D. Craik, S. Drysdall, and N. Vivian; #90933 (1998).

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Abstract: The Pre-salt Geology of Block 4, Angola and Implications for the Post-salt Play

Anderson, John - Ranger Oil Limited; David Craik - Atlaslocal Limited; Steven Drysdall and Nigel Vivian - Ranger Oil Limited

In Block 4, pre-salt oils have been discovered in post-salt Pinda Group and labe Formation reservoirs. An understanding and full evaluation of the pre-salt hydrocarbon system is therefore of value in exploring for post-salt hydrocarbon traps charged by mature pre-salt source rocks.

The pre-salt sediments of Block 4 consist of syn-rift and early post-rift sequences of Valanginian to Aptian age (Fig. 1). Syn-rift Erva Formation ?red beds? are unconformably overlain by Bucomazi and Chela Formation sediments. The Bucomazi sediments in the central and eastern areas of Block 4 consist of a fluvially dominated, marginal lacustrine sequence containing thin lacustrine shales dispersed within a coarser clastic matrix. It is postulated on the basis of seismic evidence and regional models that the Bucomazi thickens and becomes more lacustrine and source-prone to the west.

From east to west the syn-rift basin is subdivided into:

- The "Inner Shelf" comprising faulted syn-rift alluvial and fluvial sediments of the Erva and Bucomazi Formations. The entire pre-salt section is lost to erosion towards the Eastern Hinge.

- The "Outer Terrace" where the Bucomazi Formation consists of fluvio-deltaics interbedded with lacustrine shales. Good to excellent source intervals are present, but thin. This zone is structurally complex consisting of numerous tilted fault blocks truncated by the base Aptian unconformity.

- The "Bucomazi Basin" lying to the west of the Atlantic Hinge. On seismic data the Bucomazi Basin is characterized by Chela and Bucomazi reflectors sub-parallel to the distinctive top pre-salt seismic event. The observed facies trend in this area to more distal lacustrine deposition from east to west infers the presence of a shale-dominated section with excellent source potential in the Bucomazi Basin.

Maturity modeling indicates that the Bucomazi shales of the Inner Shelf and Outer Terrace have reached sufficient levels of maturity to have locally charged the post-salt reservoirs with low to mid mature oils. Although mid mature oils are common, condensate and gas have also been tested. Regional maturity modeling indicates that only the Bucomazi shales to the west of the Atlantic Hinge are sufficiently mature to generate these high maturity products. This implies that long distance secondary migration through the Chela Formation and into the overlying post-salt reservoirs has occurred, possibly as a result of Tertiary-aged tilting of the basin margin. A viable post-salt play is therefore possible where the post-salt reservoirs can be charged from large volumes of mature Bucomazi to the west. Focusing of hydrocarbons towards the post-salt reservoirs can occur around pre-salt faults and flexures. For the play to work however, the Aptian salt must be absent or tectonically thinned beneath the post-salt reservoirs. Demonstrating that all of these elements are present is critical for successful post-salt exploration.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil