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.