Structural Styles
of Shale-Dominated Gravity-Driven Thrusting, Southern Atlantic Margins of
Africa and Brazil
Gilbert, Ed1,
Kirk Geno2, Doug Ware2, Clay Fernandez2,
Morris Hall2, Mike Hankins2, Gerald Montgomery2,
William Wimberg2, Mike Hiner2, Jack StJohn3 (1)
Devon Energy, Houston, TX (2) Devon Energy, International Division (3)
Consulting Geologist, 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 parts of 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 simple folding, 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. This structural model impacts the depositional
systems of shallow reservoirs, hydrocarbon maturation and expulsion histories,
and migration pathways within structures.
The Rio Muni segment of Equatorial
Guinea and the northern margin of Brazil provide examples of true downslope gravity sliding, with the sedimentary cover
detached along discrete stratigraphic horizons. In
the northern Brazilian Barreirinhas Basin, Santonian age thrusts are mechanically problematical, as
the combined headwall detachment zone and thrust belt is broad (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 thrusting continues for prolonged
periods.
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