University of Perugia Department of Physics and Geology

From “Structural styles in the deep-water fold and thrust belts of the Niger Delta”

(By Freddy Corredor, John H. Shaw, and Frank Bilotti)

(Petrophysics and Reservoir Geology)

Andrea Fabbrizzi

a. y. 2015-2016

REGIONAL GEOLOGIC SETTING OF THE DEEP-WATER NIGER DELTA

• Boundaries

• On the continental shelf: differential loading cause gravitational collapse forming large growth normal faults soled onto the basal detachment which emerges in deep water as thrust faults.

Stratigraphy and Structural setting -Two Tertiary main formations on prograding depositional environment are: • Akata Fm: shale sequence containing Source Rock with turbidite sands intercalated (potential Reservoir) • Agbada Fm: siliciclastic sequence of delta-front, delta-topset and fluviodeltaic environments. -Five major Structural Provinces: 1) Extentional (basinward-dipping growth rollover normal faults); 2) Mud-diapir zone (diapir depocenter on the slope); 3) Inner fold-and-thrust belt (basinward-verging imbricated thrusts); 4) No-deformation zone (detached fold zone); 5) Outer fold-and-thrust belt (basin and hinterland-verging faults)

IMBRICATE-SHEAR Fault-Bend Folding

A: -More accomodation space for growth strata and downlap stratigraphic unconformities.

-Changing of the ramp dip angle: the inflection point between the 2 dips is the top of decollement.

B: -Shear transmitted toward the hinterland, with or without further slip of the older thrusts. -Constant dip angle for the backlimb ramp.

ROLE OF BASAL DETACHMENT in Style of Imbrication

(Type I)

(Type II)

Single detachment. Thin-skinned.

Two basal detachments. Anticline on the master ramp of the thickened Akata Fm.

SEQUENTIAL RESTORATION of the Thrust Belt

Type I: 25 km of total shortening. Single detachment

(Present day)

(Past) Hinterland oldest thrust Foreland youngest thrust

Coeval deformational activity in the inner and outer belt

Break-backward, based on thickness of growth strata onlap of backlimb.

Type II: 17.5 km of total shortening. Multiple detachment.

(Present day)

(Past)

Origin of the system first occurs along shallow detachment as in Type I

Last evolution along deep detachment through the master ramp connecting the two decollement

The oldest thrust is not located toward the hinterland, but it sit above the backlimb anticline

Thank you for your attention!