Deepwater Niger Delta fold-and-thrust belt modeled as a critical-taper wedge: The influence of a weak detachment on styles of fault-related folds Frank Bilotti1, Chris Guzofski1, John H. Shaw2 1 Chevron 2Harvard University GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger delta “outer” fold-and-thrust belt very low taper Odd fault-related folds “Ductile” thickening Forethrusts and backthrusts in close proximity GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Outline • The nature of the toe of the Niger Delta • Basics of critical-taper wedge theory • The Niger Delta outer fold-and-thrust belt is at critical taper • Model parameters and results (high basal fluid pressure) • Applicability in 3D & subsequent work • Implications of high basal fluid pressure for contractional fault-related folds GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger Delta Bathymetry Slope fold-and-thrust belt deepwater fold-and-thrust belt GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Fold-and-thrust belts of the Niger Delta GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Regional Geologic Setting Inner Fold and Outer Fold and Thrust belt 1 i Thrust belt Detachment fold belt Extensional Growth Faults t 3250 Lobia-1 0 k m a 0 k m . l m p 4 m . numerouscr estal crestal growthfaults growth faults numerous growthfaults ? ( 3 ma ? mudd apir (?) ? 5 m P n i e i y R it u x c : e n d her s d 5 l 1 0 5 m l l i 2 5 mud diapr (?) 2 m N m s t i 5 k m velocty sag(?) 5 k m e 2 m a .5 ma s u i basal detachment a .0 i 5 a as e veoct y ag(?) velocty sag(?) . a 1 . a After Shaw et al., 2001 i 21 10 km 10 km 5 5 SW NE 0 10 Km SW NE Quaternary Pliocene Continental Alluvial Abada Fm. (Benin Fm.) Late OFTB deformation Middle Miocene k IFTB deformation Early Early l c . Oligocene t e ? Marine Shales Agbada Fm. (Akata Fm.) . Deltaic Facies i Eocene Paleocene K. After Lawrence et al., 2002 GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger Delta toe Inflection in bathymetry Seismic data courtesy of Veritas DGC Ltd. frontal thrust Basal detachment basement GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Critical taper wedge mechanics Convergent margins erosion buttress Sediment input subduction Internally deforming wedge Whose shape is dictated by its internal Robinson, 2003 Passive margins strength and basal detachment strength Sediment input Gravitationally driven Chapple (1978) – plastic wedge Davis, et al (1983) – Coulomb wedge Dahlen, et al. (1984) – Cohesive Coulomb wedge theory GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Critical taper wedge equation (Dahlen, 1990) λ and λb - Hubbert-Rubey (1959) pore fluid ratio ρ – bulk density of the wedge µ and µb– coefficients of friction S0 – Cohesive strength GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Critical taper wedge equation Basal strength Wedge taper Wedge strength (Dahlen, 1990) λ and λb - Hubbert-Rubey (1959) pore fluid ratio ρ – bulk density of the wedge µ and µb– coefficients of friction S0 – Cohesive strength GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Niger Delta Bathymetry/Basement Bathymetry (upper free surface) Basement (as shape proxy for basal detachment) GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Measured wedge taper (after Davis, et al., 1983) GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Low-taper wedges Nankai trough Barbados accretionary wedge (Fitts and Brown, 1999) GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Is the toe of the Niger Delta at Critical Taper? 1. Negative slope of α and β plot 2. Propagation of the fold-and-thrust belt GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Wedge model parameters GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Model basal fluid pressure GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Model bathymetry GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Pseudo 3d modeling 0 100 km Mechanical parameters: ρ from Velocity Model Boundary regional Vp model 10 km Viewing direction VE- 1:3 1.4 2.5 km/s GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Model basal detachment geometry: prediction Use the bathymetry (α) to solve for the detachment geometry (β) Model Prediction Observation GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Model based mechanical parameters: λb Using the surface bathymetry and basement dips, we can invert for mechanical parameters GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Predicted λb for interpreted transects GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Coupled Fluid-mechanical models GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Ings and Beaumont, 2010 Structural implications of low taper & high basal fluid pressures • Regional – Deformation continues very far offshore – Large zones of little compressive deformation – No preference between fore and back-thrusts • Prospect-scale – Weak Akata shales result in detachment folds and shear fault-bend folds GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Undeformed zone GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Thrust vergence and wedge taper GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop INSIGHTS FROM ANALOG MODELING [COSTA & VENDEVILLE, 2002] Brittle sand cover over weak, viscous décollement (silicone polymer) 10 cm Costa and Vendeville [2002] • Bivergent directed thrust and fold anticlines separated by broad synclines • Coeval to nearly coeval activation of contractional structures • General structural thickening of the décollement unit at deep thrusts locations Brittle sand cover over strong, frictional décollement (glass microbeads) 10 cm Costa and Vendeville [2002] • Deformation mainly accommodated by slip along break–forward propagation mode • Closely space thrust ramps and folded hanging–walls • Continuous individual thrust–fault planes (up to the depth of detachment) GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Detachment fold Growth by limb-rotation Weaker rocks between deltaic section and basal detachment GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Toe-thrust geometry GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Shear fault-bend folding simple-shear fault-bend fold Classic fault-bend folding Suppe et al., 2004 pure-shear fault-bend fold GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Possible sources of elevated basal fluid pressure • Undercompaction • Horizontal compaction • Hydrocarbon maturation (e.g. Frost 1996, Cobbold, ) GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Shale Diapirism? Corredor, et al., 2005 With better seismic data, we see fewer “diapirs” - steeply dipping anisotropic beds - top of overpressured zones tend to be transparent in seismic data - large dip contrasts (angular unconformities) are not imaged well GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop What about the inner fold-and-thrust belt? A A’ Inner fold-and-thrust belt • Much more complicated deformation • older, deeper, polyphase • Larger, more variable wedge taper • Much more robust petroleum system A’ A GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop Conclusions • Basal detachment at the toe of the Niger Delta is very weak • Probably due to elevated pore pressure λb ≈ 0.91 compared to λ=0.59 measured in deltaic section • Hypothesis is robust in 3D and in more sophisticated modeling • Low taper that results from weak detachment facilitates distal thrusting, zones with little or no deformation, and back-thrusting • Weakness of Akata formation results in detachment folds and shear fault-bend folds • Subregional variations in physical properties have strong implications for the petroleum system and prospectivity GeoPrisms Rift Initiation and Evolution Scientific Planning Workshop .