AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins – Genesis, Evolution and
June 4-7, 2006 – Port of Spain, Trinidad & Tobago
Structural Styles of Shale-Dominated
Gravity-Driven Thrusting, Southern Atlantic
Margins of Africa and Brazil
thrusts developed in shale-dominated deposystems form proven and potential
hydrocarbon traps along lengthy segments of the southern Atlantic continental
margins; exploration in Nigeria,
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.