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Differentially Extended Rifted Margins: Implications for Hydrocarbon Charge in the Pre-Salt of the South Atlantic

Fraser, Scot 1; Sumner, Scott 1; Davies, Richard J.2
1 Cobalt International Energy L.P., Houston, TX.
2 Department of Earth Sciences, CeREES Durham University, Durham, United Kingdom.

Lithospheric extension of the South Atlantic margin during the Neocomian has generated rift-related fault geometries that describe a complex evolution of strain in the brittle crust. Planar and low-angle sub-basin scale extensional faults structurally partition the "hyper-extended" basin margin along strike and down-dip toward the continental -oceanic crust boundary. Whether these observations reflect inherited crustal heterogeneity or represent differential extension of the upper crust, the implications for syn-rift subsidence and rift related heat flow history are profound.

We have created 2D basin models of representative seismic dip profiles from the Brazil and West Africa conjugate margins based on new structural interpretations to better understand the temporal and spatial heat flow anomalies that would impact the maturation history of prolific pre-salt syn-extensional and syn-rift sag-related lacustrine source rock-bearing sequences. The models incorporate a tri-partite differential stretching profile and related heat flow history based upon the interpreted structural styles within the rifted basement.

The 2D basin modeling results allowed us to conclude that the "sag" sequence contains the effective source rock(s) for the pre-salt play regionally. Potential lacustrine source rock(s) in the deeper rift sequences were generally already in the gas window or thermally cracked prior to formation of sag sequence intraformational and salt top seals.

In terms of practical play application, we believe that in addition to the recognition of the importance of the sag sequence source rocks, the 2D basin modeling emphasizes the importance of identifying base salt closures for ultimate top seal and consequently oil charge focus. This work has allowed us to reduce exploration risk within the pre-salt play fairways of the South Atlantic.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009