Structural Styles of Hydrocarbon Traps in Angola

Inkollu, Murthy, Carlos Figueiredo, Leonor Binga, Bessa Teixeira, Sonangol, Luanda, Angola

Numerous hydrocarbon traps exist in the pre- and post-salt reservoirs of Angola. The traps span from early Cretaceous to Tertiary. These are grouped under structural and strati­graphic traps. The paper will outline the illustrations of various trap geometries with their seismic expression.

Presalt traps exhibit closures in fault-bound structures or subtle four-way or stratigraph­ic pinchout within a structure. Major traps are seen in the confluence areas where the base­ment related synrift faulting and, the associated transform fault zones are prevalent. Stratigraphic traps are noted in zones where an abrupt lateral pinchout of the reservoir to impermeable non-reservoir section.

Postsalt traps occur in Cretaceous and Tertiary with distinct mechanisms. Such traps have predominant influence of salt halokenisis.

Cretaceous traps in postsalt are essentially fault-bound four-way closures. Timing in listric fault development and salt diapirism are the key factors for the trap formation. Other notable component is stratigraphic trap within a structural closure.

Tertiary age traps are attributed primarily to salt diapirism and its timing. Notable traps are formed in channels and lobes associated with salt-induced four-way closures, fault­bound traps, salt flanks, stratigraphic pinchout and, in subsalt areas such as salt canopy zones. Further, traps in reverse fault structures are identifiable in salt compressive zones.

Understanding the reservoir trap geometry and its extent is the key to estimate reserves in prospect evaluation and, in particular for channel geometries.