Deep-Water Niger Delta Fold and Thrust Belt Modeled

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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 mechanics states that once a wedge reaches some critical 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 defines the critical taper of the wedge. Conversely, we use measurements of the sea floor and the basal detachment 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 overpressure 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 function of the internal strength of the wedge of deforming 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 system 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 developed 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 ie et rtclTprWedge Taper Critical Delta Niger

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. Figure 4. Schematic depiction of a critical-taper wedge and fundamental equation relating taper shape to the internal and basal strength of the wedge (modified from Dahlen et al., 1984). a = surface slope of the wedge; b = basal detachment slope; mb = coefficient of friction at the basal detachment; f = angle of internal friction; l = ratio of pore-fluid pressure to lithostatic pressure in the wedge; lb = ratio of pore-fluid pressure to lithostatic pressure at the base of the wedge; r = density of wedge material; rw = density of seawater; C = cohesive strength of the wedge; S b = cohesive strength of the basal detachment; g is the gravitational acceleration constant; H is the local wedge thickness (see text for discussion and Table 1 for range of parameter values).

these tectonic systems, sediments are typically scraped wedge both at the deformation front, as the basal de- off the subducting slab and incorporated into an in- tachment propagates into the basin, and through active ternally deforming accretionary wedge; these wedges delivery of sediments to the back of the wedge from the have a shape generally governed by their internal and Niger River. This additional sedimentary input both basal strength (Chapple, 1978; Davis et al., 1983; Stock- modifies the taper of the wedge and provides more mal, 1983). Over the past two decades, critical-taper drive to the overall gravitational system. This modifi- wedge theory has been successfully applied to study cation of the wedge’s shape is more similar to subaerial many accretionary wedges and orogenic fold and thrust wedges, which are subject to erosion that constantly belts throughout the world (e.g., Zhao et al., 1986; modifies the shape of the wedge. Breen, 1987; Behrmann et al., 1988; Barr and Dahlen, 1989; Dahlen and Barr, 1989; Dahlen, 1990; DeCelles Formulation and Mitra, 1995; Braathen et al., 1999; Plesch and Oncken, 1999; Carena et al., 2002). The theory, how- The surface slope (a) and the dip of the basal detach- ever, is a general description of any wedge of material ment (b) are the first-order parameters modeled in the deforming by brittle frictional processes. The theory critical-taper wedge theory. The sum of a and b, the can be used to explain the first-order geometry of fold wedge taper, is a function of the relative strength of and thrust belts regardless of their driving mecha- the wedge and the strength of the basal detachment; nisms. Here, we demonstrate that the critical-taper the weaker the basal detachment and the stronger the wedge theory can successfully model the gravitation- wedge material, the narrower the wedge taper. The ally driven fold belts of the deep-water Niger Delta, wedge strength is defined by frictional coefficients and extending the application of this theory beyond oro- fluid-pressure values for both the internally deforming genic systems to passive-margin settings as suggested wedge and its basal detachment. Davis et al. (1983) by Bilotti and Shaw (2001) and Rowan et al. (2004). derived the equation for the wedge taper (a + b) from Although we will demonstrate that the general a force balance on a wedge at critical taper. An en- wedge theory is applicable to gravitationally driven hancement of the critical-taper equation, with cohe- wedges like the toe of the Niger Delta, it is important sion incorporated, was derived by Dahlen (1990) and to consider differences between these systems and their is given as tectonic counterparts in developing and interpreting ð1 À r =rÞb þ m ð1 À l ÞÀS =rgH wedge models. Foremost of these differences is the a þ b w b b b manner in which material is added to the wedge. At ð1 À rw=rÞþ2ð1 À lÞðsin f=1 À sin fÞþ C=rgH submarine convergent margins, the primary source of ð1Þ material input to the wedge is from sediments scraped from the subducting sea floor; otherwise, little modi- where r is the bulk density of the wedge material; rw is fication of the overall shape of the wedge is present. In the density of seawater (in the submarine wedge case); the case of the Niger Delta, material is added to the l and lb are the Hubbert-Rubey fluid-pressure ratio

Bilotti and Shaw 1479 Figure 5. Regional transects of bathymetric slope and the top of basement surface, which approxi- mates the basal detachment slope through the Niger Delta, based on seismic sections. These sections are the source of the taper measurements. See Figure 1 for transect locations. The top transect is from the shoreline across the extensional province to the distal edge of the contractional toe. The measured transects are re- stricted to the outer fold and thrust belt where we expect the critical- taper model to apply.

(Hubbert and Rubey, 1959) for the wedge and the basal toe of the Niger Delta in 10 profiles (Figure 5; locations detachment; Sb and C are the cohesive strengths of the shown in Figure 1). The sea floor represents the upper basal detachment and the wedge material, respectively; free surface of the wedge, and we use a linear approx- g is the gravitational acceleration constant; H is the imation of the seafloor slope to measure the angle a, wedge thickness; and mb is the coefficient of basal slid- the angle below horizontal of the upper surface of the ing friction. The cohesive strength of the wedge (C)is wedge. For the angle b, we use the observation that more important in the thin toe of the wedge; therefore, the basal detachment of the fold and thrust belt is gen- the taper also depends on the distance from the toe of erally parallel to the underlying basement reflector, as the wedge. This is because the ratio of cohesive strength shown in Figure 3. Within the representative 10 pro- to frictional strength is larger at shallow depths, hence, files, we use a linear fit to the basement reflector to mea- the C/rgH term in the wedge equation. sure the dip of the basal detachment of the wedge, b. In the next section, we use this formulation to es- The measured wedge taper (a + b)oftheNigerDelta tablish that the Niger Delta fold belts are at critical toe is 2.5 ± 0.4j (Figure 6). This taper (Figure 7) is lower taper. Subsequently, we examine the strength of the than all of the accretionary wedges at active margins basal detachment in the Niger Delta using internal reported in Davis et al. (1983). This anomalously low wedge parameters derived from well data and fault pat- taper suggests that either the toe of the Niger Delta is terns observed in seismic reflection data. not at critical taper, or it has substantially different mechanical properties compared to most active margins. In the subsequent discussion, we first establish that the MODELING THE TOE OF THE NIGER DELTA deep-water fold belts are at critical taper and then explore the possibility of anomalous mechanical proper- Measuring the Taper ties for the Niger Delta through critical-taper wedge modeling. The overall shape of the distal toe of the Niger Delta fits the form of a tapered wedge, but to investigate Is the Toe of the Niger Delta at Critical Taper? whether it acts mechanically like a critical-taper wedge, we make quantitative measurements of the wedge Critical-taper wedge theory predicts a linear relation- shape as required by the Dahlen (1990) formulation. ship with negative slope between the dip of the basal We measure the taper of the fold and thrust belt at the detachment and the seafloor slope of a contractional

1480 Niger Delta Critical Taper Wedge wedge once a wedge has reached its critical taper. An active wedge that has not yet reached critical taper should not exhibit this relationship, nor should it propagate forward (i.e., grow wider). Instead, a subcriti- cal wedge would internally deform until it thickened to its critical taper before it grew wider. Given these considerations, we propose that the toe of the Niger Delta has indeed reached critical taper based on two main observations:

1. The wedge has grown appreciably wider by means of the basal detachment propagating into the basin. The fold and thrust belt shown in Figure 3 is more than 40 km (25 mi) wide, and the entire contractional toe is on the order of 100 km (62 mi) wide (Figure 5). A noncritical wedge first builds taper by internal deformation and then propagates basinward after reaching critical taper. A pronounced widening of a contractional wedge is an indication Figure 6. Plot of wedge shape measured in 10 transects across that it has reached critical taper and has continued the deep-water Niger Delta fold and thrust belts. The transect to grow. number is labeled on each point. The overall negative slope 2. The regionally consistent taper of the wedge as of the points is consistent with a critically tapered wedge with shown by the negative slope of a graph of a versus b taper (a + b) between 2.3 and 2.9j. Transect locations are j shown in Figure 1. indicates a consistent taper of about 2.5 ± 0.4 over a range of values for a and b (Figure 6). Critical-taper

Figure 7. Comparison in range of bathymetric slope and de- collement (basal detachment) dip for the Niger Delta versus active submarine fold and thrust belts at convergent margins (modified from Davis et al., 1983). The Niger Delta has much lower taper (a + b) than most other measured fold and thrust belts. Those with similar tapers (Makran and the toe of the Barbados accretionary wedge) are known to have high basal fluid pressures (Davis et al., 1983), implying the same con- dition for the Niger Delta. l = lb line assumes mb =0.85and m = 1.03.

Bilotti and Shaw 1481 Figure 8. (a) Measured basal step-up angles for thrust ramps in the Niger Delta. We interpret the left peak of the graph to represent the basal step-up angle at which the thrusts initially propagate, which defines db. The second peak represents imbricated thrust ramps or thrusts that initially propagated as kink bands. (b) Relationship between the basal step-up angle (db) and the angle of internal friction (f) for the wedge (modified from Dahlen et al., 1984).

wedge theory predicts that the properties of the measurements of the angle that thrusts step up from

wedge and basal detachment prescribe a wedge ge- the basal detachment, db, to estimate the angle of in- ometry that is as consistent as the regional material ternal friction, f. We use the relationship from Dahlen strength properties. Conversely, if the wedge was et al. (1984, equation 27): dominated by sedimentary processes, such as the angle of repose, we would not expect a consistent relationship between the basal detachment geome- db ¼ p=4 À f=2 À yb ð2Þ try and the seafloor slope.

where m = tan f and yb is the angle between the maxi- Based on these considerations, we conclude that mum principal compression (s1) and the basal detach- the toe of the Niger Delta is a contractional wedge at ment (Figure 8). critical taper, and we now explore which properties of From 49 measurements of the dips of thrust faults the wedge give it its uniquely low taper. To model the from the fold and thrust belts of the Niger Delta (Cor- Niger Delta as a critical-taper wedge, we need to pro- redor et al., 2005), we derive the histogram of values vide constraints on the main parameters of the formu- for db presented in Figure 8. The bimodal graph has lation of Dahlen (1990). The following sections dis- peaks at about 22 and 32j. The corresponding values cuss the constraints on these parameters for the toe for m = tan f (for yb =2j) are m = 0.90 and m = 0.40, of the Niger Delta. respectively. The value for m (0.40) corresponding to

db =32j falls outside of the range of empirically derived rock strength values 0.6 m 1.0, known as Wedge Strength Parameters Byerlee’s Law (Byerlee, 1978). Therefore, we suggest

that db =32j does not represent a fundamental step-up Internal Coefficient of Friction angle in the area but instead reflects a typical value for We can constrain the internal coefficient of friction imbricated faults (Suppe, 1983; Shaw et al., 1999) or (m) using the geometry of the wedge as well as basic perhaps faults that propagated as kink bands (Dahlen rock mechanics. Because a critical-taper wedge is on et al., 1984). In contrast, the value for m (0.90) corre- the verge of failure throughout the wedge, two planes sponding to db =22j is consistent with Byerlee’s Law. are oriented at angles ± (p/4 À f/2), measured with This suggests that db =22j represents the funda- respect to the maximum principal compression vector, mental ramp step-up angle, and we employ the corre- s1, on which the failure criterion is satisfied (Jaeger and sponding frictional value (m = 0.90) in our subsequent Cook, 1979) (Figure 8). We use this relationship and modeling.

1482 Niger Delta Critical Taper Wedge Figure 9. Histogram of values for the Hubbert-Rubey fluid-pressure ratio (l) derived from pore-pressure measurements from 13 wells in the deep-water Niger Delta. The effect of the water column (D) is removed from the dis- played Hubbert-Rubey fluid-pressure values, as shown by the equation (inset) and as required by the wedge mechanics formulation (Dahlen et al., 1984).

Internal Fluid Pressure of l values for 13 wells in the deep-water Niger Delta. The Hubbert and Rubey (1959) fluid-pressure ratio is The average value is l = 0.54, and we use this value in the ratio of fluid pressure to the lithostatic pressure. subsequent models. For comparison, lambda for hy-

This value is from Hubbert and Rubey’s formulation drostatic fluid pressure, lh, is approximately 0.43, in- of effective stress that sought to explain how large dicating that the Niger Delta section is, in general, thrust sheets could be translated long distances with slightly overpressured. little internal deformation from friction on the underlying fault. Through critical-taper wedge theory, Davis Cohesive Strength et al. (1983) showed that high fluid pressures were The cohesive strength of the wedge material can be an not necessary to explain the large-scale geometry of important property for thin wedges and near the tip of fold and thrust belts; critical-taper wedge theory ex- high-taper wedges. In the toe of the Niger Delta, the plains the first-order geometry of fold and thrust belts relative significance of the cohesive strength of the without calling on significant overpressure. However, rocks in the wedge, C, is small because the wedge tip the fluid pressure is still an important component of is at the distal end of the basal detachment, which is the effective strength of the wedge material. typically about 3 km (1.8 mi) below the sea floor For the Niger Delta, we obtain regional values for (Figure 5). At depth, the effect of the increasing fric- the Hubbert-Rubey fluid-pressure ratio, l, generalized tional strength of the material overwhelms the effect for the submarine case from formation pressures mea- of the relatively small cohesive strength. Because the sured in deep-water wells: result is relatively insensitive to the cohesive strength of the material, we can safely estimate the cohesive

l ¼ðPf À rwgD=rghÞð3Þ strength of the wedge from rock-mechanics experiments (Hoshino et al., 1972) to be 10 ± 5 MPa for our where D is the water depth, h is the depth of the deep-water sediments. measurement below the sea floor (Dahlen et al., 1984), and r is an estimate of the bulk rock density Basal Strength Parameters derived from many well-density logs. This formulation removes the effect of the overlying water because the The basal detachment of the toe of the Niger Delta hydrostatic and lithostatic pressure curves are identical lies within the Akata Formation, a thick marine shale in the water column. Figure 9 shows the distribution that is thought to contain the source section for some

Bilotti and Shaw 1483 of the major oil fields of the deep-water Niger Delta. that for reasonable values of wedge strength and basal A few wells have approached this section, but we strength, the wedge geometry is very sensitive to the know of no penetrations of the deeper parts of the basal fluid pressure. Based on the observed wedge shape, formation where the basal detachment of the fold and we are readily able to distinguish the effects of basal thrust belt resides. The mechanical properties of the fluid pressure and conclude that the low taper of the basal detachment cannot be measured directly and contractional toe of the Niger Delta is the result of must be inferred from rock-mechanics experiments or strongly elevated fluid pressure at the basal detachment. derived from the critical-taper modeling. Elevated basal fluid pressures have also been in- Three parameters in the generalized wedge equa- voked to explain the anomalously low taper of some tion of Dahlen (1990) address the strength of the basal orogenic fold and thrust belts, including the Barbados detachment of the wedge: the basal friction, the basal accretionary wedge (Figure 7) (e.g., Behrmann et al., cohesive strength, and the basal fluid-pressure ratio. In 1988). We suggest that the elevated basal fluid pressure this analysis, we assume that the basal coefficient of is the dominant cause of low-taper fold and thrust belts, friction is similar to that derived for the wedge mate- regardless of whether they are orogenic or gravitation- rial, mb = 0.91, which is consistent with Byerlee’s Law ally driven. In the case of the Niger Delta, the elevated (Byerlee, 1978). In addition, because the basal detach- basal fluid pressure is also consistent with the anoma- ment is a regionally continuous surface that is actively lously low compressional wave speeds (2500 m/sec; sliding, we assert that it is working as a frictional fault 8202 ft/sec) in the Akata Formation, as well as the with low or no cohesive strength. We assume in our general structural styles manifest in this shale tectonic modeling that the basal cohesion is equal to zero. province (Wu and Bally, 2000; Rowan et al., 2004; Cor- With the values as described above (summarized redor et al., 2005). In the next section, we examine in Table 1), we can directly compute a value for a range how the elevated basal fluid pressure and the corre- of basal fluid pressures from the critical-taper wedge sponding weak basal detachment in the Niger Delta equation. Figure 10 shows the predicted wedge taper influence the regional structural architecture of the for values of lb compared to the measured taper values deep-water fold belts, as well as the structural styles from the toe of the Niger Delta. Critical-taper wedge expressed by individual toe-thrust structures. theory predicts Hubbert-Rubey fluid-pressure ratios of 0.89 < lb < 0.92 for the range of measured transects and our preferred values for each parameter. The total PROPERTIES OF LOW-TAPER WEDGES WITH range of predicted fluid pressures using the low- and ELEVATED BASAL FLUID PRESSURES high-pressure end members of each parameter is 0.82 < lb < 1.01. Figure 11 shows the sensitivity of the wedge Elevated basal fluid pressure in the Niger Delta has had model to more typical values of lb using the wedge a substantial influence on the architecture and deforma- formulation of Dahlen et al. (1984). These models show tional history of its deep-water fold belts. The elevated

Table 1. Critical-Taper Wedge Model Parameters

Parameter Value Method of Determination

Surface slope a 0.7–1.3j Measured from 10 regional transects of seismic-derived seafloor map (see Figure 5) Detachment dip b 1.2–2.2j Measured from 10 regional transects of seismic-derived basement map (see Figure 5 and text for justification) Density r 2400 kg/m3 Deep-water well-density logs (approximate) Fluid-pressure ratio l 0.54 ± 0.15 Pressure data from 13 deep-water wells (see Figure 9)

Basal step-up angle db 22j Measured from seismic data (lower peak of graph in Figure 8) Internal coefficient of friction m 0.91 ± 0.06 Calculated from basal step-up angle

Basal coefficient of friction mb 0.91 ± 0.3 Similar to wedge material strength and Byerlee’s Law (Byerlee, 1978) Wedge cohesion C 5–15 MPa Hoshino et al. (1972); see discussion

Basal cohesion S b 0–5 MPa Hoshino et al. (1972); see discussion

1484 Niger Delta Critical Taper Wedge Figure 10. Plots of seafloor slope (a) versus detachment dip (b) for 10 transects across the toe of the Niger Delta, with curves of constant basal fluid pressure (lb). The plots iot n Shaw and Bilotti display model results for the high (A), middle (B), and low (C) fluid-pressure end members of the model parameters. The observed wedge taper values are consistent with Hubbert- Rubey basal fluid-pressure ratios (lb) between 0.82 and 1.01, which are greatly elevated above the observed fluid pressures in the wedge (see Figure 9). 1485 Figure 11. Model bathymetric profiles showing the effect of varying each parameter over its range of model values (see Table 1). These graphs use Dahlen et al. (1984, equation 40), which includes the distance from the wedge tip but not separate cohesion strength parameters for the wedge and the basal detachment. Within our control of the parameters, lb and S 0 show the largest range of predicted bathymetric slope. fluid pressure presumably localized the basal detach- are common in the deep-water Niger Delta, reflecting ment in the Akata Formation and controlled the over- distributed deformation in the weak Akata Formation all shape of the thrust belts as expressed by its narrow as it slides above a basal detachment (Bilotti et al., 2005). taper. Furthermore, the low strength of the Akata For- The deep-water Niger Delta also exhibits other, mation and the weakness of the basal detachment have perhaps more enigmatic, structural characteristics that dictated the structural styles expressed in the fold belt. we also attribute to its fluid overpressure. Specifically, Individual toe-thrust structures involve large compo- the widespread occurrence of backthrusts and the physi- nents of shear within their hanging walls, forming shear cal separation of the inner and outer fold belts can be fault-bend folds (Suppe et al., 2004) as documented attributed to the weak basal detachment and corre- by Corredor et al. (2005). Moreover, detachment folds spondingly low wedge taper based on the critical-taper formed by ductile thickening of the Akata Formation concept, as discussed below.

1486 Niger Delta Critical Taper Wedge In thrust wedges with very low taper, the max- therefore, most of the thrust displacement in wedges imum principal compressive stress (s1) is necessarily with large tapers occurs on forethrusts. The lack of pref- subhorizontal and very nearly parallel to the basal de- erence for forethrusts over backthrusts in many parts of tachment (yb 0) as well as the sea floor. When this is the Niger Delta results from its low taper and weak the case, as it is in the Niger Delta, only a very small basal detachment (Figures 12, 13). Backthrusts are most component of s1 acts on the basal detachment. This common in the lowest taper zones in the outer fold and configuration yields no mechanical preference for devel- thrust belt, as expected based on the critical-taper oping fore- or backthrusts. This relationship is quan- wedge theory. tified by yb, the angle between s1 and the basal de- Another consequence of a low-taper wedge is that tachment (Figure 8), which is also a measure of the sedimentary deposits can readily enable the wedge to difference between the dips of fore- and backthrusts. reach critical taper, even in the absence of deformation.

Regionally, in the Niger Delta, we estimate yb to be The seismic line in Figure 14 shows such a case. The part 1–5j. As expected, the basal step-up angle of the back- of the outer fold and thrust belt shown in the seismic thrusts ranges from 15 to 25j (excluding thrusts that image is separated from the inner fold and thrust belt seem to be folded or imbricated), similar to the range by more than 35 km (21 mi) of essentially flat-lying of primary step-up angles for forethrusts shown in seismic reflectors. This relatively undeformed zone, Figure 8. Furthermore, once the faults are formed, the however, has the necessary critical taper and is sliding low bathymetric slope means little difference exists stably without requiring internal deformation. Close between the overburden shoreward or offshore from a inspection of the shallow strata reveals that the taper of given fault, making forethrusts and backthrusts equally the wedge in this area is generated by a young wedge of mechanically efficient at accommodating shortening. sediments coming from the continental shelf. This il- In contrast, wedges with large tapers and steep surface lustrates how normal sedimentary input to a part of the slopes have backthrusts that dip much more steeply wedge can achieve the necessary taper, which has the than forethrusts (Figure 12). This makes the forethrusts effect of retarding deformation in the underlying part much more efficient at accommodating shortening, and of the wedge. This phenomenon is presumably most

Figure 12. Schematic models illustrating the effects of taper angle on the tendencies for fore- and backthrusting in a deforming wedge. In high and moderate taper wedges (left), forethrusts typically dip at a lower angle than backthrusts and, thus, are more effective at accommodating shortening. In extremely low taper wedges (right), fore- and backthrusts should have essentially the same dip values and are equally effective at accommodating shortening. Areas of the Niger Delta where backthrusts are prevalent (Figure 14) are generally associated with regions of very low taper, consistent with this theory.

Bilotti and Shaw 1487 1488 ie et rtclTprWedge Taper Critical Delta Niger

Figure 13. Seismic reflection profile across the toe of the Niger Delta in a region dominated by backthrusts. Data provided courtesy of Veritas DGC, Ltd. iot n Shaw and Bilotti Figure 14. Seismic reflection profile through the northern part of the Niger Delta fold and thrust belt, showing a large (30 km [18 mi] wide) undeformed zone lying to the east of a series of toe-thrust structures. The critical taper of this undeformed zone is provided by a lobe of sediments that thin toward the deep water, making it unnecessary for this zone to internally deform to maintain taper. The inferred weakness of the basal detachment caused by elevated fluid overpressure also facilitates sliding of this undeformed zone without internal deformation. Data provided courtesy of Veritas DGC, Ltd. 1489 apparent in low-taper, passive-margin wedges such as on the magnitude of the maximum compressive stress, the Niger Delta because sedimentary deposits alone we are unable to quantify this effect. could not likely achieve critical taper in wedges with Finally, Frost (1996) proposed that the matura- higher surface slopes. tion of the source facies of the Akata Formation pro- duced a sufficient change in fluid pressure to start structural growth of the toe-thrust belt. The change in volume caused by the generation of oil from type II CAUSES OF ELEVATED BASAL kerogen could be as large as 25% (Meissner, 1978; FLUID PRESSURE Swarbrick et al., 2002). If hydrocarbon maturation is in fact linked to the elevated basal fluid pressures we Several possible causes exist for the elevated fluid model, there may be both a temporal and spatial re- pressure that controls the shape of the Niger Delta lationship between the shape of the Niger Delta thrust thrust wedge, including rapid burial, tectonic forces, wedge and the maturity state of the Akata source rocks. and increased fluid volume caused by hydrocarbon maturation. Disequilibrium compaction in shales is common in sedimentary basins (e.g., Osborne and Swar- brick, 1997) because permeability declines abruptly CONCLUSIONS with burial of shales, preventing fluid expulsion and mechanical compaction. However, once mechanical We have shown that the contractional toe of the Niger compaction stops, subsequent burial causes the pore- Delta acts as a critical-taper wedge similar to the fold fluid pressure to rise only as fast as the lithostatic gra- and thrust belts found at active margins. The toe of dient (Osborne and Swarbrick, 1997). To produce fluid the Niger Delta is unique in that it has a very low pressure as high as we predict in the critical-taper taper that results from a very weak basal detachment. wedge model (l = 0.90), fluid retention would need By using measured properties of the wedge material to begin at about 500 m (1640 ft) below mud line. and making reasonable assumptions about the prop- Permeability in mudstones at 500-m (1640-ft) depth erties of the basal detachment strength, we calculate is generally too great to cause fluid retention; to pro- that the basal detachment in the Akata Formation is duce fluid pressure in these relatively permeable rocks, strongly overpressured, with a Hubbert-Rubey fluid- sedimentation would have to strongly outpace fluid pressure ratio lb 0.90. That is, 90% of the weight of expulsion. Predicted sedimentation rates to produce the outer Niger Delta is supported by pore fluids in fluid retention at 500-m (1640-ft) depth are in excess the Akata Formation. This result explains the wide- of 3000 m/m.y. (10,000 ft/m.y.) (Mann and MacKenzie, spread occurrence of detachment folds, shear fault- 1990), certainly unreasonable rates to sustain at the bend folds, backthrusts, and the large, relatively un- toe of the Niger Delta. Although it is likely that some deformed regions that separate fold belts in parts of fraction of the total basal overpressure in the Akata the Niger Delta. The elevated fluid pressure that we Formation is caused by disequilibrium compaction, this model is likely caused by the combined effects of dis- process alone is probably not capable of elevating fluid equilibrium compaction, tectonic stresses, and per- pressure to the level we infer. haps, increased fluid volume caused by hydrocarbon Horizontal tectonic forces can also elevate fluid maturation. pressure (Osborne and Swarbrick, 1997). Because we see very young or active thrusts in the Niger Delta wedge (Corredor et al., 2005), the maximum principal compressive stress is subhorizontal today, and some REFERENCES CITED fraction of the overpressure we model may be caused by horizontal stress. In this case, the upper limit of Barr, T. D., and F. A. Dahlen, 1989, Brittle frictional mountain overpressure is the minimum principal stress (Swar- building: 2. 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