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Deep-Water Niger Delta Fold and Thrust Belt Modeled

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Deep-Water Niger Delta Fold and Thrust Belt Modeled Deep-water Niger Delta fold AUTHORS Frank Bilotti Unocal E&E Technology; and thrust belt modeled as present address: Chevron ETC, 1500 Louisi- ana St., Houston, Texas 77002; a critical-taper wedge: The [email protected] Frank Bilotti is the structural geology team influence of elevated basal fluid leader for Chevron ETC. Frank received a Ph.D. in structural geology from Princeton University and a B.S. degree in geology and mathematics pressure on structural styles from the University of Miami. He worked as a structural consultant in Texaco Exploration Frank Bilotti and John H. Shaw 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 ABSTRACT restoration technology. We use critical-taper wedge mechanics theory to show that the John H. Shaw Department of Earth and Niger Delta toe-thrust system deforms above a very weak basal de- Planetary Sciences, Harvard University, Cam- tachment induced by high pore-fluid pressure. The Niger Delta ex- bridge, Massachusetts hibits similar rock properties but an anomalously low taper (sum of John H. Shaw is the Harry C. Dudley Professor the bathymetric slope and dip of the basal detachment) compared of Structural and Economic Geology at Harvard with most orogenic fold belts. This low taper implies that the Niger University and leads an active research program Delta has a very weak basal detachment, which we interpret to in structural geology and geophysics, reflect elevated pore-fluid pressure (l 0.90) within the Akata with emphasis on petroleum exploration and Formation, a prodelta marine shale that contains the basal detach- production methods. He received a Ph.D. from ment horizon. The weak basal detachment zone has a significant Princeton University in structural geology and influence on the structural styles in the deep-water Niger Delta fold applied geophysics and was employed as a senior research geoscientist at Texaco’s Explo- belts. The overpressured and, thereby, weak Akata shales ductilely ration and Production Technology Department in deform within the cores of anticlines and in the hanging walls of toe- Houston, Texas. Shaw’s research interests in- thrust structures, leading to the development of shear fault-bend clude complex trap and reservoir characteriza- folds and detachment anticlines that form the main structural trap tion in fold and thrust belts and deep-water types in the deep-water fold belts. Moreover, the low taper shape passive margins. He heads the Structural Ge- leads to the widespread development of backthrust zones, as well as ology and Earth Resources Program at Harvard, the presence of large, relatively undeformed regions that separate an industry-academic consortium that supports the deep-water fold and thrust belts. This study expands the use of student research in petroleum systems. critical-taper wedge mechanics concepts to passive-margin settings, while documenting the influence of elevated basal fluid pressures ACKNOWLEDGEMENTS on the structure and tectonics of the deep-water Niger Delta. This manuscript benefited from very helpful reviews by Mark Rowan, Bradford Prather, INTRODUCTION Tom Elliott, and Carlos Rivero. The concepts for this work were originally developed as part of Texaco’s regional exploration effort in the The Niger Delta, located in the Gulf of Guinea, is one of the most Niger Delta. Support for refinement of these prolific petroleum basins in the world (Figure 1). The Delta con- ideas and this article was provided by Unocal sists of Tertiary marine and fluvial deposits that overlie oceanic E & E Technology. John Suppe, Freddy Corredor, and Chris Guzofski provided valuable assis- tance in understanding and constraining the Copyright #2005. The American Association of Petroleum Geologists. All rights reserved. parameters used in the model. We are grateful Manuscript received January 4, 2005; provisional acceptance March 8, 2005; revised manuscript to Veritas DGC, Ltd., for granting permission to received June 9, 2005; final acceptance June 13, 2005. publish the seismic data presented here. DOI:10.1306/06130505002 AAPG Bulletin, v. 89, no. 11 (November 2005), pp. 1475–1491 1475 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). crust and fragments of the extended African continen- meters (Corredor et al., 2005). The detachment fold tal crust. Over the last decade, advances in drilling tech- belt is a transitional zone between the inner and outer nology have opened the deep-water Niger Delta to ex- fold and thrust belts that is characterized by regions of ploration. At the deep-water toe of the delta, a series little or no deformation interspersed with broad de- of large fold and thrust belts (Figure 1) is composed tachment anticlines that accommodate relatively small of thrust faults and fault-related folds (e.g., Damuth, amounts of shortening (Bilotti et al., 2005). 1994; Morley and Guerin, 1996; Wu and Bally, 2000; The deformation in the contractional toe of the Corredor et al., 2005). Recent discoveries in this fold Niger Delta is driven by updip, gravitational collapse of and thrust belt include the Agbami, Bonga, Chota, Ngolo, shelf sediments. Basinward motion of these shelf sedi- and Nnwa fields, all of which have structural traps ments is accommodated by normal faults that sole to formed by contractional folds. detachments within the prodelta marine strata that lie The contractional part of the deep-water Niger above the basement (Figure 2). Slip on the detachments Delta is divided into three major zones (Connors et al., is transmitted to the deep water, where it is diverted 1998; Corredor et al., 2005): the inner fold and thrust onto thrust ramps and consumed by contractional folds belt, the outer fold and thrust belt, and the detachment- in deep-water fold and thrust belts (Figures 2, 3). This fold province (Figure 1). The inner fold and thrust belt style of gravitationally driven, linked extensional and con- is a highly shortened and imbricate fold and thrust belt, tractional fault systems is common in passive-margin whereas the outer fold and thrust belt is a more classic deltas (Rowan et al., 2004), including the Gulf of Mexi- toe-thrust zone with thrust-cored anticlines that are co basin (e.g., Peel et al., 1995). The Niger Delta fold typically separated from one another by several kilo- and thrust belts occupy the outboard toe of the delta in 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 (<2j), inked to regional seafloor slope away from the coast. In this article, we show that the Niger Delta toe- thrust system, composed of a sloping sea floor, a basal detachment system, and an internally deforming wedge ceanic crust. This section of sediments, can be successfully modeled as a critical- taper wedge like those found in accretionary wedges at active margins (Davis et al., 1983). The taper of a wedge is the angle between its free surface and basal detachment. The theory of critical-taper wedge me- chanics states that once a wedge reaches some criti- cal taper, it grows self-similarly as material is added to or removed from the wedge. Given the strength of the wedge and its basal detachment, the theory de- fines the critical taper of the wedge. Conversely, we use measurements of the sea floor and the basal de- tachment to define the wedge shape and subsequently employ measured values of density, fluid pressure, and basal thrust step-up angles to examine the strength of the deforming wedge and its basal detachment. Specifically, we predict the magnitude of fluid over- pressure in the basal detachment zone that would be required to produce the observed wedge shape. Fi- nally, we consider the implications of this model for the structural styles and evolution of the Niger Delta fold and thrust belts. CRITICAL-TAPER WEDGE MECHANICS Theory Critical-taper wedge mechanics theory explains the first-order geometry of fold and thrust belts as a func- tion of the internal strength of the wedge of deform- ing material and the strength of the basal detachment on which the wedge slides (Davis et al., 1983; Dahlen et al., 1984). For the deformation front to propagate, the basal detachment of the wedge must be weaker than the wedge material. The analogy of a bulldozer pushing a pile of snow or sand is commonly used to illustrate the first-order mechanics of this system (Figure 4). Once the critical taper is reached, the wedge grows self-similarly, and internal deformation of the wedge maintains the taper. The taper of a specific wedge sys- tem is determined by the strength of the basal detach- Regional geologic cross section through the central Niger Delta modified from Shaw et al. (2004). The extensional collapse ofment the shelf sediments is l relative to the internal strength of the wedge. Critical-taper wedge mechanics theory was devel- oped initially to explain accretionary wedges that form contractional deformation in the deep-waterapproximately fold follows and transect thrust 5 belts from by a Figure regional 1. detachment in the Akata Formation, a prodelta marine shale above the o Figure 2. at convergent plate boundaries (Davis et al., 1983). In Bilotti and Shaw 1477 1478 Niger Delta Critical Taper Wedge 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.
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