Deep-water Niger Delta fold AUTHORS and thrust belt ... · PDF file on the structure and...

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Transcript of Deep-water Niger Delta fold AUTHORS and thrust belt ... · PDF file on the structure and...

  • AUTHORS

    Frank Bilotti � Unocal E&E Technology; present address: Chevron ETC, 1500 Louisi- ana St., Houston, Texas 77002; [email protected]

    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