AUTHORS

Frank Bilotti � Unocal E&E Technology; present address: Chevron ETC, 1500 Louisi- ana St., Houston, Texas 77002; frank.bilotti@chevron.com

Frank Bilotti is the structural geology team leader for Chevron ETC. Frank received a Ph.D. in structural geology from Princeton University and a B.S. degree in geology and mathematics from the University of Miami. He worked as a structural consultant in Texaco Exploration Technology and was most recently a structural geology specialist at Unocal E&E Technology. Frank’s current technical interests are in Gulf of Mexico salt tectonics and three-dimensional restoration technology.

John H. Shaw � Department of Earth and Planetary Sciences, Harvard University, Cam- bridge, Massachusetts

John H. Shaw is the Harry C. Dudley Professor of Structural and Economic Geology at Harvard University and leads an active research program in structural geology and geophysics, with emphasis on petroleum exploration and production methods. He received a Ph.D. from Princeton University in structural geology and applied geophysics and was employed as a senior research geoscientist at Texaco’s Explo- ration and Production Technology Department in Houston, Texas. Shaw’s research interests in- clude complex trap and reservoir characteriza- tion in fold and thrust belts and deep-water passive margins. He heads the Structural Ge- ology and Earth Resources Program at Harvard, an industry-academic consortium that supports student research in petroleum systems.

ACKNOWLEDGEMENTS

This manuscript benefited from very helpful reviews by Mark Rowan, Bradford Prather, Tom Elliott, and Carlos Rivero. The concepts for this work were originally developed as part of Texaco’s regional exploration effort in the Niger Delta. Support for refinement of these ideas and this article was provided by Unocal E & E Technology. John Suppe, Freddy Corredor, and Chris Guzofski provided valuable assis- tance in understanding and constraining the parameters used in the model. We are grateful to Veritas DGC, Ltd., for granting permission to publish the seismic data presented here.

Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles Frank Bilotti and John H. Shaw

ABSTRACT

We use critical-taper wedge mechanics theory to show that the

Niger Delta toe-thrust system deforms above a very weak basal de-

tachment induced by high pore-fluid pressure. The Niger Delta ex-

hibits similar rock properties but an anomalously low taper (sum of

the bathymetric slope and dip of the basal detachment) compared

with most orogenic fold belts. This low taper implies that the Niger

Delta has a very weak basal detachment, which we interpret to

reflect elevated pore-fluid pressure (l � 0.90) within the Akata Formation, a prodelta marine shale that contains the basal detach-

ment horizon. The weak basal detachment zone has a significant

influence on the structural styles in the deep-water Niger Delta fold

belts. The overpressured and, thereby, weak Akata shales ductilely

deform within the cores of anticlines and in the hanging walls of toe-

thrust structures, leading to the development of shear fault-bend

folds and detachment anticlines that form the main structural trap

types in the deep-water fold belts. Moreover, the low taper shape

leads to the widespread development of backthrust zones, as well as

the presence of large, relatively undeformed regions that separate

the deep-water fold and thrust belts. This study expands the use of

critical-taper wedge mechanics concepts to passive-margin settings,

while documenting the influence of elevated basal fluid pressures

on the structure and tectonics of the deep-water Niger Delta.

INTRODUCTION

The Niger Delta, located in the Gulf of Guinea, is one of the most

prolific petroleum basins in the world (Figure 1). The Delta con-

sists of Tertiary marine and fluvial deposits that overlie oceanic

AAPG Bulletin, v. 89, no. 11 (November 2005), pp. 1475– 1491 1475

Copyright #2005. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 4, 2005; provisional acceptance March 8, 2005; revised manuscript received June 9, 2005; final acceptance June 13, 2005.

DOI:10.1306/06130505002

crust and fragments of the extended African continen-

tal crust. Over the last decade, advances in drilling tech-

nology have opened the deep-water Niger Delta to ex-

ploration. At the deep-water toe of the delta, a series

of large fold and thrust belts (Figure 1) is composed

of thrust faults and fault-related folds (e.g., Damuth,

1994; Morley and Guerin, 1996; Wu and Bally, 2000;

Corredor et al., 2005). Recent discoveries in this fold

and thrust belt include the Agbami, Bonga, Chota, Ngolo,

and Nnwa fields, all of which have structural traps

formed by contractional folds.

The contractional part of the deep-water Niger

Delta is divided into three major zones (Connors et al.,

1998; Corredor et al., 2005): the inner fold and thrust

belt, the outer fold and thrust belt, and the detachment-

fold province (Figure 1). The inner fold and thrust belt

is a highly shortened and imbricate fold and thrust belt,

whereas the outer fold and thrust belt is a more classic

toe-thrust zone with thrust-cored anticlines that are

typically separated from one another by several kilo-

meters (Corredor et al., 2005). The detachment fold

belt is a transitional zone between the inner and outer

fold and thrust belts that is characterized by regions of

little or no deformation interspersed with broad de-

tachment anticlines that accommodate relatively small

amounts of shortening (Bilotti et al., 2005).

The deformation in the contractional toe of the

Niger Delta is driven by updip, gravitational collapse of

shelf sediments. Basinward motion of these shelf sedi-

ments is accommodated by normal faults that sole to

detachments within the prodelta marine strata that lie

above the basement (Figure 2). Slip on the detachments

is transmitted to the deep water, where it is diverted

onto thrust ramps and consumed by contractional folds

in deep-water fold and thrust belts (Figures 2, 3). This

style of gravitationally driven, linked extensional and con-

tractional fault systems is common in passive-margin

deltas (Rowan et al., 2004), including the Gulf of Mexi-

co basin (e.g., Peel et al., 1995). The Niger Delta fold

and thrust belts occupy the outboard toe of the delta in

Figure 1. Map of the offshore Niger Delta showing the location of regional transects (1–10) used in this study, bathymetry, and major offshore structural provinces (modified from Connors et al., 1998; Corredor et al., 2005).

1476 Niger Delta Critical Taper Wedge

water depths ranging from 1 to 4 km (0.6 to 2.5 mi)

below sea level (Figure 1) and create a very gentle (

Figure 3. Regional two-dimensional seismic line through the contractional toe of the Niger Delta, showing the tapered, wedge shape of the deep-water fold and thrust belts modeled in this article. Note that the bathymetric slope is caused by the underlying deformation, and that the basal detachment is subparallel to the top of underlying oceanic basement. Data provided courtesy of Veritas DGC, Ltd.

1 4

7 8

N iger

D elta

Critical Taper

W edge

these tectonic systems, sediments are typically scraped

off the subducting slab and incorporated into an in-

ternally deforming accretionary wedge; these wedges

have a shape generally governed by their internal and

basal strength (Chapple, 1978; Davis et al., 1983; Stock-

mal, 1983). Over the past two decades, critical-taper

wedge theory has been successfully applied to study

many accretionary wedges and orogenic fold and thrust

belts throughout the world (e.g., Zhao et al., 1986;

Breen, 1987; Behrmann et al., 1988; Barr and Dahlen,

1989; Dahlen and Barr, 1989; Dahlen, 1990; DeCelles

and Mitra, 1995; Braathen et al., 1999; Plesch and

Oncken, 1999; Carena et al., 2002). The theory, how-

ever, is a general description of any wedge of material

deforming by brittle frictional processes. The theory

can be used to explain the first-order geometry of fold

and thrust belts regardless of their driving mecha-

nisms. Here, we demonstrate that the critical-taper

wedge theory can successfully model the gravitation-

ally driven fold belts of the deep-water Niger Delta,

extending the application of this theory beyond oro-

genic systems to passive-margin settings as suggested

by Bilotti and Shaw (2001) and Rowan et al. (2004).

Although we will demonstrate that the general

wedge theory is applicable to gravitationally driven

wedges like the toe of the Niger Delta, it is important

to consider differences between these systems and their

tectonic counterparts in developing and interpreting

wedge models. Foremost of these differences is the

manner in which material is added to the wedge. At

submarine convergent margins, the primary source of

material inpu