How Well Do Asymmetrical Rift Models for the Conjugate Margins of Brazil and Angola Compare to Deep-Penetration Seismic Reflection Data?

Abstract

The conjugate margins of Brazil and Angola have been the sites of some of the largest, and most productive oil fields in the world the past decade. While three decades of previous studies have been conducted with several different models proposed, the opening and general understanding of the South Atlantic remains controversial. Previous models can be subdivided into two groups: 1) Low-angle detachment models that include Lister et al. (1986), Wernicke and Tilke (1989), and Reston (2009); and 2) Higher angle fault dip models involving more mantle control on the crustal structure. Comparison of these model predictions with five published deep-penetration lines reveals the following conclusions: 1) Low-angle faults are not apparent on any of these lines giving credibility for more steeply-dipping and less asymmetrical opening mechanisms; 2) despite the lack of low-angle detachments, the Brazil-Angola margin shows pronounced asymmetry with the wider, rifted footwall alternating with more narrow, less rifted hanging wall blocks, along strike of the rifted margins; 3) asymmetry includes a wider zone of hydrocarbon-rich sag basins associated with the wider footwall margin. 2D gravity modeling of the conjugate margins confirms the broad extent of the hyperextended footwalls underlying productive sag basins. The Santos basin (Brazil), Kwanza basin (Angola), and Benguela basin (Angola) all fall into the hyperextended footwall domain, while their conjugates all exhibit narrow/ non-hyperextended crustal structures associated with hanging wall blocks. Margin widths range from 100-150 km (Kwanza, Santos, respectively) across their footwall blocks as opposed to 30-50 km (Namibe, Campos, respectively) across their conjugate hangingwall blocks.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016