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AAPG/GSTT HEDBERG CONFERENCE

Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

 

 

Structural Styles of Shale-Dominated Gravity-Driven Thrusting, Southern Atlantic Margins of Africa and Brazil

 

Ed Gilbert

International Division, Devon Energy, Houston, TX

 

Gravity-driven thrusts developed in shale-dominated deposystems form proven and potential hydrocarbon traps along lengthy segments of the southern Atlantic continental margins; exploration in Nigeria, Equatorial Guinea, and Brazil reveals the considerable variation in structural style.

 

In the Niger Delta fold belt (including northern Equatorial Guinea) the structural style is quite variable. High quality seismic reveals primarily shear-fault-bend folds, with localized and less common fault-bend and fault-propagation folds. “Plastic shales” are commonly invoked as a mechanism for folding in numerous structures, but improved seismic imaging and balanced cross-sections of fold complexes indicate that deep structures are in part duplexes of a mechanically competent deep (“Akata”) section. Geometric differences implicit in the formation of true plastic shale masses versus coherent duplex fault blocks impact the hydrocarbon maturation and expulsion history, and migration pathways within structures. It is therefore important to discriminate between fault-cored structures and those produced by true plastic shale deformation.

 

The Rio Muni segment of Equatorial Guinea and the northern margin of Brazil provide examples of true downslope gravity sliding above massive shales. In the northern Brazilian Barreirinhas Basin, Santonian age thrusts are detached along discrete stratigraphic horizons, but are mechanically problematical, as the combined headwall detachment zone and thrust belt is broad (up to 70km) but thrusting occurs above a very shallow detachment (<1km below water bottom). Young (Late Miocene to present) thrusting on the Barreirinhas margin occurs along curved detachment surfaces that do not follow stratigraphic horizons, and more closely approximates a simple slump model.

 

In all cases major sea level falls appear to control the initiation of thrusting, though once initiated the thrusting continues for prolonged periods.

 

AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago