Enhancement of Petroleum Systems by Raft Tectonics: A Type Example from Offshore Northern Angola

Abstract

Raft tectonics (tectonique en radeaux) is the most extreme form of thin-skinned extension. Gravity-induced stretching of the cover causes the overburden above a decollement of Aptian salt to separate into allochthonous fault blocks or [open quotes]rafts.[close quotes] Widening, deepening depocenters between rafts are filled with younger, mainly Tertiary sediments. Such a high degree of extension creates a special tectonic style, which is particularly visible on the continental shelf of northern Angola. More relevant, though, is the economic importance of raft tectonics, which substantially enhance the petroleum systems of the area. Where the cover is typically thin, the generative petroleum subsystem is [open quotes]subsalt,[close quotes] as source rocks are only mature in lacustrine environments of the rift basins or the cratonic lowstand wedge of earlier ages. If vertical migration of hydrocarbons is prevented by a seal of undeformed, laterally continuous salt, found in [open quotes]prerafts[close quotes] of moderately stretched overburden, low-impedance conditions prevail and most hydrocarbons migrate laterally and disperse updip, forming the well known outcropping asphalts. The migration-entrapment petroleum subsystem may work, however, under favorable conditions made possible by raft tectonics in several ways and illustrated in the paper using post-stack migrated seismic sections and case histories of specific fieldsmore » of the area (Lombo East, Sulele). (1) Vertical migration is made easier by salt seals breached during extensional salt welding. (2) Albian carbonate reservoir porosity of the Pinda formation is enhanced by shoaling over tilted fault blocks. (3) Extensional structural traps impede lateral migration of hydrocarbons from the Pinda. Therefore, an accurate picking of the evaporitic interval, combined with a thorough discrimination of the structural targets, are essential to the success of exploration.« less