Abstract: Inversion structures are now recognized in most orogenic belts and in many othertectonic settings, however, this has not always been the case. For many years the thin-skinnedparadigm dominated the interpretation of thrust belts which resulted in many inversion structuresbeing erroneously interpreted as thin-skinned thrusts. In rift basins, intra-cratonic basins andon passive margins inversion structures were often interpreted as having alternative origins,for example, strike-slip deformation. The classic paper of Bally described the geometric character-istics of inversion structures, in which he identified the extensional fault geometry and its sub-sequent compressional reactivation as essential requirements of an inversion structure. Theincreased recognition of inversion structures in a wide variety of tectonic settings is illustratedby four hydrocarbon exploration case studies. The geometric characteristics of inversion structuresare remarkably consistent irrespective of the tectonic setting and by applying a few simple criteriathey can easily be identified. The impact of inversion on hydrocarbon prospectivity is a functionof the control on the petroleum system elements from the initial extensional tectonic historyand of the later compressional tectonic history which commonly creates new, or modifies oldtrap configurations.
The term ‘inversion’ to describe an inverted basinwas first used by Glennie & Boegner (1981)although inverted basins had been recognizedmany years before (e.g. Lamplugh 1920; Stille1924). The definition of inversion presented byCooper et al. (1989) relied on the concept ofregional elevation. The regional elevation of amarker horizon is the structural elevation of thehorizon where it is undeformed. The ‘regional’can be curved in three-dimensions due to isostatic,thermal and flexural processes affecting the litho-sphere. Contractional deformation will elevatebeds above regional elevation and extension willlower them below regional elevation (Fig. 1).
Inversion can be easily recognized when thehanging wall of a fault has marker horizons thatdisplay different elevations above and/or belowtheir respective regionals. In the classical invertedhalf-graben model of Bally (1984) the upper partof the fault causes elevation of beds above regionalwhilst the lower part of the fault shows beds belowregional elevation (Fig. 2). The change-overpoint (where beds are at regional in the hangingwall) has been termed the null point (Williamset al. 1989).
Extensional faults create accommodation spacein their hanging walls which act as foci for sedimentdeposition (e.g. Gupta et al. 1999; Gawthorpe &Leeder 2000). Many workers have also described
how extensional faults are characterized by segmen-tation and that sediments often enter the hangingwall accommodation space where the segmentsinteract at transfer faults, accommodation zones orrelay ramps (e.g. Gawthorpe & Hurst 1993). Whencomplex extensional fault systems are inverted,understanding the initial extensional geometry andits relationship to syn-rift sediment distributionbecomes critical in developing both the inversionstructures that result from the compressional defor-mation phase as well as understanding petroleumprospectivity and risks.
A key stratigraphic characteristic that is a conse-quence of these relationships is that a syn-rift (orpassive infill) sequence should be recognizable inall inversion structures (Cooper et al. 1989). If asyn-rift package (sedimentation synchronous withextension) cannot be positively identified theninversion cannot be unequivocally interpreted. Thepre-rift sequence has been extended and has anoriginal pre-extension length of X (Fig. 2); the post-rift sequence is longer (Fig. 2, length Y) as it isdeposited on the extended strata. When inversionoccurs the post-rift section must shorten whilst thepre-rift can remain in net extension depending onthe amount of inversion.
The geometry of inversion structures is highlydependent on the geometry of the original fault sys-tem. In the simple case of inverting a half-graben,
From: LAW, R. D., BUTLER, R. W. H., HOLDSWORTH, R. E., KRABBENDAM, M. & STRACHAN, R. A. (eds) ContinentalTectonics and Mountain Building: The Legacy of Peach and Horne. Geological Society, London, Special Publications,335, 827–846. DOI: 10.1144/SP335.33 0305-8719/10/$15.00 # The Geological Society of London 2010.
the inversion fold produced will be an asymmetricmonocline facing the footwall with the crestlocated above the syn-rift depocentre; this is acharacteristic geometric feature of an inversionstructure (Figs 3 & 4). When the inverted exten-sional faults dip towards the hinterland then thefault may act as a rigid buttress generating back-thrusts on the roll-over crest or complex folding ofthe syn-rift fill (Gillchrist et al. 1987; Hayward &Graham 1989; de Graciansky et al. 1989; Colpronet al. 1998; Yamada & McClay 2004). Commonly,however, footwall shortcut thrusts develop thatlower the gradient of the fault to create a fault trajec-tory that is more favourably orientated to the com-pressional stress and thus can allow efficient
shortening to occur (Fig. 4). When the extensionfaults dip towards the hinterland then there is poten-tial for the development of backthrusts, perhapswith related footwall shortcuts (Fig. 3).
A comprehensive discussion of inversion waspresented in Cooper et al. (1989) which includedcontributions from key workers in the field. In thisdiscussion Ziegler noted that grabens that evolvedunder tensional stress regimes are characterized bythinned crust that is prone to inversion under com-pressional stress regimes. Ziegler also suggestedthat inversion should only be applied in intraplatesettings; however this view was disputed by deGraciansky as being too restrictive because manyfold and thrust belts deform pre-existing passivemargins with extensional faults that invert duringorogenesis, for example, the Alps and the Apen-nines (Butler et al. 2006). Since this debate inCooper et al. (1989) a large number of papershave been published based on hydrocarbon explora-tion in the external zones of many orogenic systemsand it is now clear that inversion is a widespreadand well documented phenomenon (Hill 1991;Uliana et al. 1995).
Coward (pers. comm. 1989) observed that inver-sion appears to be favoured by a short time intervalbetween the extensional and compressional phasesbecause attenuated lithophase requires time to ther-mally re-equilibrate (McKenzie 1978) and whenrelatively weak is prone to reactivation underany compressional stress (Ziegler 1989). Lowell(1995) has also noted that inversion selects riftbasins where weakening of the lithosphere occurredby thinning and where mechanical detachmentsurfaces are available for reactivation.
Another factor in controlling compressionalinversion of older extensional faults versusthin-skinned detachment is relative plate motion as
Extensional Fault.Marker bed is loweredbelow regional in HWand has been extended.Omission of stratigraphy.
Contractional Fault.Marker bed is elevatedabove regional in HWand has shortened.Repetition of stratigraphy.
Top WhiteRegionalElevation
Fig. 1. Using the concept of regional elevation to differentiate extensional and compressional faults; after Cooper et al.(1989).
Extensional half-graben
Partial Inversion
X
Asymmetricinversion anticline
Null point
Y
Post-rift
Syn-rift
Pre-rift
Basement
Fig. 2. The concept of basin inversion; after Bally(1984).
M. COOPER & M. J. WARREN828
discussed by Lowell (1995). For example, in theFrench Pyrenees foothills, thick-skinned mid-to-Late Cretaceous inversion of Triassic to EarlyCretaceous extensional structures between Iberia
and Europe pre-dates rather than post-dates theMiocene thin-skinned thrusting (e.g. Canerot et al.2005; Biteau et al. 2006; Serrano et al. 2006).This may be because the early transpression washighly oblique due to the relative plate motions atthe time, so steep faults were favoured for largestrike-slip motions as well as compressional dip-slip motions. The later main Pyrenean deformationwas caused by more orthogonal plate motionswhich favoured early thin-skinned deformation fol-lowed by thick-skinned inversion in the ‘normal’late-orogenic sequence (Cooper et al. 1989).Another excellent example of this is in Moroccowhere Beauchamp et al. (1996) attribute the inver-sion of the High and Middle Atlas Triassic–Jurassicrift systems to Late Cretaceous-Early Oligocenedextral strike slip. However, inversion is probablyunder-recognized in many known strike-slip set-tings, as illustrated below in the central Africa casestudy. This is because of superficial similarities bet-ween transpressional ‘flower’ structures and inv-erted grabens if the syn-rift sedimentary intervaland early extensional geometry are not correctlyidentified.
In this paper our objective is to present some welldocumented examples of inversion structures fromhydrocarbon exploration areas and illustrate thegeometric and hydrocarbon system characteristicsof inversion structures.
Asymmetric anticline in post-rift sequence which faces towardsthe footwall is a characteristic indicator of inversion
Upper part of fault beds are elevated above regionalLower part of fault beds are depressed below regional
Regional Top Blue
Regional Base Green
Growth package in the HWof the extensional fault
0
3
1
2
0 5 km
TW
Tse
cs
Fig. 3. An example of an inversion structure illustrating the key geometric characteristics from the Cuyo Basin ofArgentina after (Uliana et al. 1995).
Extensional half-graben
Inverted half-graben
Incipient footwallshortcut fault
Footwallshortcut fault
s 1
q 1 q 2
Fig. 4. Characteristic geometry of an inversion structureand the development of footwall shortcut faults afterCooper et al. (1989). Note that the angle, u1 of theoriginal extensional fault to the compressional stress s1
is much larger than the angle of s1 to the footwallshortcut u2 and as a result the footwall shortcut will bethe surface of maximum resolved shear stress and willpreferentially fail.
CHARACTERISTICS OF INVERSION STRUCTURES 829
Inversion in Western Newfoundland
Western Newfoundland provides an excellent casestudy of inversion where the data outline the diag-nostic geometry of inversion with:
(1) an extensional basin history with thicknessand facies changes;
(2) the compressional inversion of the extensionalfaults and the development of footwall short-cut structures;
(3) excellent petroleum prospectivity due to thepresence of a reservoir in traps developedduring inversion that were on migration path-ways from a mature, high quality source rockkitchen during and after the inversion.
The case study is documented by Cooper et al.(2001) and shows how the integration of regionaldata and the drilling of a single well proved theinversion model to be valid.
Newfoundland’s west coast lies in the deformedLaurentian cratonic foreland of the Canadian Appa-lachians (Williams 1979). Shallow wells drilled inthe 19th and early 20th centuries and oil seeps onthe Port au Port Peninsula and to the north producedsmall quantities of oil used locally (Department ofEnergy 1989) and indicate a working petroleumsystem.
The Port au Port #1 well was drilled to test asmall onshore anticlinal closure identified withlimited seismic data (Fig. 5) (Cooper et al. 2001).The play concept involved a Lower Ordoviciancarbonate platform reservoir (Knight et al. 1991),with source rocks in the time equivalent basinalfacies that had undergone thin-skinned thrustingonto the platform during the Middle OrdovicianTaconic Orogeny, the Humber Arm Allochthonin Figure 5 (James & Stevens 1986; Fowler et al.1995). The top seal was provided by tight lime-stones of the Table Head Group and shales of theGoose Tickle Group (Cooper et al. 2001). The struc-tural hydrocarbon traps were the result offootwall shortcuts developed during DevonianAcadian inversion of early Middle Ordovicianextensional faults that result from the flexuralbulging of the carbonate platform caused by theload of the Taconic foreland basin to the east(Knight et al. 1991) which included the Bay ofIslands Ophiolites.
The Port au Port #1 well penetrated 615 m ofgranitic Grenville basement before intersecting theRound Head Fault (Fig. 5) and proved that thicknessand facies changes in the Llanvirnian (Early–Middle Ordovician) occur across the Round HeadFault (Stenzel et al. 1990; Stockmal et al. 1993;Cooper et al. 2001). The significant observationsare as follows.
Fig. 5. (a) Surface geology map of the Port au PortPeninsula, West Newfoundland showing key wells andthe location of the cross section (b) and seismic line (c);after Cooper et al. (2001).
M. COOPER & M. J. WARREN830
(1) The Llanvirnian Cape Cormorant Conglomer-ate submarine fan is absent.
(2) The Llanvirnian marine shales and limestonesof the Goose Tickle and Table Head groupsare thinner (185 m and 15 m respectively)than on the west coast of the Port au PortPeninsula (at least 1000 m, Quinn 1995;Stenzel et al. 1990; Fig. 5).
These thicknesses suggest the Round Head Faultoriginally had an extensional offset of about1000 m by the Late Llanvirnian. Significant earlyOrdovician extensional relief on the Round Headfault controlling Cape Cormorant Conglomeratedeposition was postulated by Stenzel et al. (1990)and Waldron et al. (1993). The Port au Port #1well confirms the early extensional history and con-strains the stratigraphic thickness of these units inthe footwall (Fig. 5). Thick late Llanvirnian turbi-dite fan sands in the immediate Round Head faulthanging wall represent later infill of the starved,fault-bounded basin (Quinn 1995).
The early extensional history of the fault iscritical in the development of reservoir qualityin the inherently low porosity and permeabilityrocks of the Cambro-Ordovician carbonate platform(Cooper et al. 2001). The palaeo-highs on the foot-walls of the extensional faults were the foci of minorkarst development in the early Middle Ordovicianand later hydrothermal dolomitization during theDevonian (Cooper et al. 2001). In Early to Mid-Devonian time, the carbonate platform had beenburied under the Taconic and later foreland basins(Fig. 6), and Acadian tectonism was occurring tothe east. Hydrothermal fluids moved up majorfaults bounding the platform blocks in the develop-ing foreland basin, an interpretation corroborated byisotopic evidence from Pb–Zn mineralization ofEarly–Mid-Devonian age in the Ordovician plat-form (Lane 1990). The hydrothermal fluids migratedupwards until impeded by a significant permeabilitybarrier, the Goose Tickle Group shales. As a result,hydrothermally altered rocks are located on thepalaeo-highs that existed prior to the Acadian
inversion (the fault block crests) where the hydro-thermal fluids preferentially attacked the porousand permeable units in the platform (Cooper et al.2001; Fig. 6).
The extensional history of the faults also pro-vided a migration pathway for generated hydrocar-bons by juxtaposing the source rocks in theCambro-Ordovician basinal shales of the HumberArm Allochthon against the reservoir rocks of theplatform. Modelling of the thermal history of thesource rocks suggests that they entered the oilwindow during the Late Ordovician as a result ofdeposition of the Taconic and later foreland basinsediments, just prior to the onset of the Acadianorogeny (Cooper et al. 2001). The thermal historymodelling also implies that the source rocks werestill within the oil window during late DevonianAcadian inversion with oil source kitchens stillactive where not elevated during the inversion(Cooper et al. 2001). The inversion of the exten-sional faults during the Acadian orogeny createdcompressional footwall shortcut structural trapsthat have been the target of exploration drillingto-date (Cooper et al. 2001).
Inversion in NE Thailand
NE Thailand provides an excellent example of howthe various tectono-stratigraphic components ofinversion may produce a working petroleum system.The Permian pre-rift stratigraphy contains the reser-voir units, the Triassic syn-rift stratigraphy containsthe source rocks (Sattayarak et al. 1989), theJurassic-Cretaceous post-rift stratigraphy providesthe seal and the late Cretaceous inversion createdthe structural traps. As most of the Triassicsyn-rift basins are in the subsurface this examplealso highlights how the regional elevation conceptcan be applied to recognize ‘concealed’ invertedbasins. This case study also illustrates how the orig-inal basin morphology strongly controls the geome-try of the inversion structures that result from thecompressional deformation (Cooper et al. 1989a).
Fig. 6. Model of Early–Mid Devonian dolomitization of Ordovician carbonate reservoirs in the extensional faultfootwalls, after Cooper et al. (2001). Key as for Figure 5.
CHARACTERISTICS OF INVERSION STRUCTURES 831
In NE Thailand, an extensive outcrop of post riftMesozoic rocks occurs on the Khorat Plateau whichis somewhat misleadingly named, as the ‘plateau’ isonly 100–200 m above sea level (Fig. 7). It isrimmed by higher ground, notably the Loei-Phetchabun Foldbelt to the west in which thepre-rift Permian strata are exposed. Seismic dataand exploration wells from the Khorat Plateaudemonstrate the existence of a number of syn-riftTriassic basins and there are also eroded remnantsof uplifted Triassic basins in the Loei-PetchabunFoldbelt (Cooper et al. 1989a).
The Permian and Mesozoic stratigraphy of theregion is well documented (e.g. Sattayarak 1985;Booth 1998) and records the transition from anactive margin in the Permian to subsequent collision
in the Early Triassic (the Indosinian Orogeny).In the Late Triassic an extensional event occurred,followed by thermal subsidence for the remainderof the Mesozoic.
The Triassic sequence frequently rests with amarked angular unconformity on the Permian seq-uence. The Triassic was deposited in a series offault controlled extensional basins as demonstratedby seismic data and field relationships and beneaththe relatively undeformed part of the Khorat Plateausome basins are visible on seismic data (Cooperet al. 1989a). To the west of the Khorat Plateau,extensive outcrops of Triassic rocks suggest that acomplex system of Triassic basins developed(Fig. 7) in the Loei-Petchabun Foldbelt. TheTriassic continental extensional basins are filled
0 200 km
IndosinianBlock
Nan River Suture
Chi
ang
Mai
Sut
ure
Khorat FrontalMonocline
Line of sectionin Figure 8
ShanThaiBlock
Andaman
ThreePagodas
Fault Zone
Triassic MarineTriassic Continental
SutureGulf of
Mai Ping Fault Zone
Sea
Thailand
Myanm
arThailand
CambodiaThailand
Extensional FaultStrike slip Fault
N
β = 1.33
β = 1.13
Fig. 7. Simplified tectonic map of Thailand showing major structural features and the interpreted Triassic basins of NEThailand after Cooper et al. (1989a).
M. COOPER & M. J. WARREN832
by fluvial, alluvial and lacustrine deposits whichinclude potential source rocks in the Huai Hin LatFm (Chonglakmani & Sattayarak 1978).
During the Jurassic and Cretaceous the red bedKhorat Group was deposited over much of centraland eastern Thailand. Maranate & Vella (1986)noted that the subsidence curve for the JurassicCretaceous sediments was consistent with thermalsubsidence following the extensional developmentof the Triassic basins. The flat, undeformedJurassic-Cretaceous sediments which occur oneither side of the Phu Phan Uplift on the KhoratPlateau (Fig. 8) are considered to be at regionalelevation. Projecting this regional elevation acrossthe section it becomes obvious that for much ofthe section the Khorat Group is elevated aboveregional (Fig. 8). Post-Khorat Group compressionaldeformation produced large wavelength folds in
the western part of the Khorat Plateau, the KhoratMonocline and the Phu Phan Uplift (Fig. 8).
These compressional structures cannot bethin-skinned as the amplitude and wavelength ofindividual folds are only compatible with the invol-vement of basement in the deformation. Some ofthe thick-skinned contractional structures appearto be spatially related to the extensional faults thatcontrolled the development of the Triassic basinsand are thus true inversion structures. For examplein the Phu Phan Uplift, the Kuchinari-1 explorationwell penetrates the Triassic at an elevation aboveregional suggesting that it is an inverted Triassicdepocentre (Cooper et al. 1989a) and the samerelationship is suggested by the Phu Phra-1 well(Lovatt Smith et al. 1996). Immediately to the eastof the Khorat Monocline the Nam Phong structurehas been a producing gas field since 1991 and can
Fig. 8. (a) Regional structural cross-section through NE Thailand; see Figure 7 for line of section after Cooper et al.(1989a). (b) Surface geology map and (c) detailed cross-section through the Nam Phong Gas Field, a footwallshortcut structure.
CHARACTERISTICS OF INVERSION STRUCTURES 833
be interpreted as an inversion footwall shortcut to aTriassic half-graben located beneath the reservoir(Fig. 8).
Late Cretaceous–Early Cenozoic deformationcaused the post-Triassic folding and elevation seenin the Loei-Petchabun Foldbelt and the Phu PhanUplift (Fig. 8). Booth (1998) suggested that a lateTriassic deformation event inverted the Triassicbasins, however this does not fit with the clear defor-mation of the Khorat Group by these structures.Lovatt Smith et al. (1996) have suggested that theage of the inversion may be mid Cretaceous basedon revision of the age of some of the Cretaceousstratigraphic units.
The extensional history of the faults createdthe accommodation space for the Triassic lacustrinesource rocks. The overlying post rift strata providethe top seal to the Permian carbonate reservoirsthat were charged by gas generated from the Triassicsource rocks during the Late Cretaceous andCenozoic. The inversion of the extensional faultsin the Late Cretaceous and Cenozoic created com-pressional anticlines and footwall shortcut structuraltraps. NE Thailand offers an excellent exampleof how to use regional elevation to constrain astructural cross-section.
Inversion in the BC Foothills
This example of inversion is located in a fold andthrust belt that has been extensively explored forhydrocarbons for over 80 years. The work of Ballyet al. (1966) and Dahlstrom (1970) has resulted ina strongly rooted paradigm that the fold and thrustbelt is purely thin skinned and this paradigm isstill influencing papers written on the region. Thiscase study illustrates a spectacular example of aninversion structure that is supported by seismicand well data in addition to the observations thatcan be made from the surface geology. The CameronRiver structure also provides an excellent exampleof an inversion structure where an asymmetric folddeveloped initially, but as compressionally drivenslip on the old extension fault continued this faultlocked up. The result of continued compressionwas a large inversion anticline that developed asthe additional shortening was accommodated bythe amplification of the inversion anticline.
The development of the fold and thrust belt com-menced in the middle Late Jurassic and terminatedin the Eocene, due to the accretion of a series ofexotic terranes to the Pacific Margin of NorthAmerica (Monger et al. 1982). As the foothills aretraced to the NW from Alberta into British Colum-bia the amount of displacement on the thrusts gradu-ally decreases, thrusts at surface are less commonand folds predominate (McMechan & Thompson
1989). This can be at least partially attributed tochanges in the mechanical stratigraphy of theDevonian to Cretaceous section in the BC Foothillswhich becomes progressively more shale dominatedfrom south to north.
The BC foothills can be divided from south tonorth into three zones (Cooper 2000) each with acharacteristic structural style (Fig. 9). The boundarybetween the Detachment Fold and Thick-skinnedprovinces is located at Williston Lake and coincideswith the extrapolated locations of the Hay RiverFault Zone and the Fort St John Graben into thefoothills (Fig. 9). This has been proposed as amajor Late Palaeozoic depocentre (O’Connellet al. 1990), and probably represents a major trans-fer zone that shifts the basin margin westwards.There is a major change in the lowest detachmentlevel from base Mississippian south of the boundaryto intra-basement to the north of the boundary achange also noted by Stockmal (2001). The thick-skinned province is characterized by the following.
† Basement-involved faults in the foothills whichaffect structural geometry by evolving intoinverted extensional faults and by acting as trig-gers for thin-skinned thrusts.
† A major regional detachment in lower Mississip-pian shales (Cooper 2000), above which all stratadeform as a single, coherent tectonostratigraphic
Fort St. John
57°
58°
56°
55°
124° 122° 120°
0 50km
Detachment Fold ProvinceThin-Skinned Thrust Province
Thick-Skinned ProvincePlains
Front Ranges
Figure 10
Fig. 9. Structural zonation map of the foothills of BritishColumbia; after Cooper (2000).
M. COOPER & M. J. WARREN834
unit creating detachment folds described by Fitz-gerald (1968) and Thompson (1979).
† The timing of interaction between thin- andthick-skinned deformation is variable.
In the external part of the foothills the structuralrelief is lower and the folds are due to ramp anti-clines that have a lower detachment in the earlyMissippian Shales and an upper detachment inlower Triassic Shales. There are also deeper detach-ing faults in this part of the foothills that penetratethe basement (Fig. 10) and often have trendsoblique to those of the thin-skinned Laramidestructures. These faults have a long and complexhistory; some of them originated as Devonianextensional faults whilst others originated ascompressional features related to earlier Palaeozoicorogenic events. During the Late Palaeozoic andMesozoic some faults were reactivated as exten-sional faults across which thickening of the stratacan be seen and at Cameron River the depositionof Triassic sandstone reservoirs was controlled bythe extensional fault (Fig. 10). Many of thesefaults were reactivated compressionally during theLate Cretaceous deformation to produce broadwavelength inversion anticlines (Fig. 10). A fewof these structures have significant amplitudes but
the majority only show subtle evidence of compres-sional reactivation.
The differences in structural style in the Detach-ment Fold and Thick-skinned province are duepartially to the more extensive development of apre-Laramide Devonian basin margin fault systemnorth of Williston Lake. However, another veryimportant factor in controlling the structural styleis the nature of the deformed mechanical strati-graphy (Cooper 2000). To the south of the lateralramp the Mesozoic sequence is much more hetero-geneous with, for example, rapid alternations ofsand and shales seen in the Lower Cretaceous. Tothe north the whole sequence becomes dominatedby shales and in addition many of the Triassicunits subcrop the Cretaceous. It is also probablethat the facies changes that control the mechanicalstratigraphy are an indirect product of Devonianextensional geometry and palaeogeography, thatis, more subsidence to the north in the Mesozoicover previously attenuated lithosphere, resultingin more deep-water shales than sands. Thus bothbasement fabric and lithological distribution arecontributory factors to the style changes.
In the Cameron River inversion structure theextensional fault creates accommodation space inwhich the Triassic sands that form an important
Fig. 10. Regional and detailed structural cross-sections and the seismic line through the Cameron River structure, seeFigure 9 for location. The coloured boxes surrounding the text in the key indicate the seismic horizons on the seismicline. Seismic line reproduced by kind permission of Sigma Explorations, Calgary.
CHARACTERISTICS OF INVERSION STRUCTURES 835
reservoir are deposited; these sands are not presenton the footwall of the fault to the east. The develop-ment of the inversion anticline creates the trappinggeometry for the gas charge. Elsewhere in the regionother more subtle extensional faults that appearto have a similar history have not inverted but setup trigger points for thin skinned structures. Theregional extensional palaeogeography thus influ-ences the initial thin-skinned deformation as wellas the thick-skinned inversion by controlling mech-anical stratigraphy and therefore detachment leveland structural style. Recognizing the early exten-sional history regionally is important both for inter-preting other inversion structural traps and regionalplay fairways where compression has overprintedextension.
Inversion in the rift systems of central
Africa
The intracratonic rift system of western and centralAfrica (Fig. 11) provides an opportunity to explore aspectrum of relationships between initial extension
and later compressional inversion over a widespreadand genetically linked system, made up of multiplebasins with variable orientations and basin fillhistories. Examples from several basins highlightthe degree of compressional inversion as a functionin part of the orientation of compressional stresswith respect to original rift structures (Warren2009). In some examples it is difficult in hangingwall anticlines to distinguish the compressionalfolding component from true extensional rolloverbecause the compressional inversion is so subtlecompared to the extension. However, the use ofregional elevation usually allows recognition ofeven small amounts of inversion. Several otherexamples illustrate how geometries traditionallyinterpreted as ‘flower’ structures in areas of knowntranspression/strike slip can be interpreted as inver-sion structures when stratigraphic and structuralgeometry are examined critically. Finally, much ofthe western and central African rift system containsrecent or currently active hydrocarbon explorationand production, providing additional insights intothe implications of even very subtle inversion forpetroleum systems and hydrocarbon prospectivity.
500 km
RedSea
AtlanticOcean
T-LC
LB
BG
DB
DS
MG
BN ML
Nigeria
Ethiopia
Cameroon
Niger
Central AfricanRepublic
Sudan
Algeria
Chad
Kenya
CASZSL
20oN
0o
40o E
13b
13c
12a
12d
12b13a
12c
0o
20oN
20o E
20o E
40o E
Fig. 11. Mesozoic–Cenozoic rift system of western and central Africa. Individual basins referred to in text: T-LC,Termit/Lake Chad; LB, Logone Birni; BN, Benue Trough; BG, Bongor; DB, Doba; DS, Doseo; SL, Salamat; MG,Muglad; ML, Melut; CASZ, Central African Shear Zone (bold solid line). Bold dashed lines, inferred subsidiary shearzones. Locations of cross sections shown in Figures 12 and 13 are approximate. Modified after Genik (1993) and Mangaet al. (2001).
M. COOPER & M. J. WARREN836
The main segments of the Mesozoic–Cenozoicrift system (Fig. 11) coincide primarily with Pre-cambrian crustal suture zones within the Africancraton (e.g. Fairhead 1988; Daly et al. 1989). Themost significant Phanerozoic intracratonic riftingoccurred in the Early Cretaceous, in associationwith south Atlantic opening and regional NE–SWextension (e.g. Fairhead 1988; Fairhead & Binks1991). NNW–SSE orientated extensional basinsdeveloped extensively in two systems in westernAfrica (e.g. Niger) and in east-central Africa (e.g.Sudan), linked by the Central Africa Shear Zone(Fairhead 1986, 1988; Genik 1993) dextral strike-slip fault system and related transtensional basinsin central Africa. Fairhead (1986) has argued thatnearly 50 km of dextral strike-slip motion documen-ted on the Central African Shear Zone is roughlyequivalent to the amount of extension in the riftsystems linked to the northwest and southeast ofthe Central African Shear Zone.
Several kilometres of clastic sediments, primar-ily lacustrine shales and arkosic sandstones, weredeposited in complexly faulted extensional/trans-tensional basins (Schull 1988; Giedt 1990; Genik1993 and references therein). By Late Albian time,active rifting had given way to thermal subsidence,allowing progradation of widespread fluvial, deltaicand, locally in the west, marine facies into many ofthe rift basins (Schull 1988; Genik 1993 and refer-ences therein). Compressional inversion of manyearlier features occurred in Santonian time (Petters& Ekweozor 1982; Genik 1993; Reynolds & Jones2004), most likely related to change in relativeplate motions (Fairhead & Binks 1991) and speci-fically convergence between Africa and Europe(Ziegler 1989; Guiraud & Maurin 1992). Followinga widespread Santonian unconformity, there weretwo cycles of renewed regional extension and subsi-dence in latest Cretaceous and early Cenozoic time,followed finally by widespread regional upliftin Miocene time, particularly in western Africa(Fairhead 1988).
It is important to note that according to mostsources the Santonian compressional stress wasnearly orthogonal to the Early Cretaceous extensiondirection (Genik 1993 and references therein).The resulting inversion is best documented in theENE–WSW orientated Benue, Logone Birni,Bongor, Doba and Doseo basins (Figs 11 & 12a,b; Petters & Ekweozor 1982; Fairhead 1988;Genik 1993; Manga et al. 2001), perhaps becausetheir basin-controlling faults were more favourablyorientated with respect to later compressional stress.Inversion is least pronounced in the WNW–ESETermit-Lake Chad, Muglad, Melut and other basins(Figs 11 & 12c, d), where extensional geometriesare commonly fully preserved with little or nodirect evidence of compressional reactivation, and
latest Cretaceous and early Cenozoic subsidence ismore significant (Schull 1988; Giedt 1990; Genik1993; Mohamed et al. 2000; Idris & Yongdi2004). For example, the Niger Termit Basinprofile (Fig. 12d) shows a ‘classic’ rift geometrywith what appear to be simple extensional rolloveranticlinal geometry and depression below regionalelevations at the top Lower Cretaceous and top base-ment horizons. In fact renewed extension and growthfaults are documented in the Late Cretaceous (EarlySenonian) in Sudan (Schull 1988; Giedt 1990), con-temporaneous with compression in basins to thewest in Chad (Genik 1993). This is again consistentwith Sudan basin orientation at a low angle toregional Santonian compressional stress. Althoughseen in these examples over a wide geographicarea, these relationships highlight how even in asingle basin, early structures of different orientationsmay experience different degrees of inversion due torelative orientations to the compressional stress.
Even where most pronounced in favourablyorientated basins, the compressional overprint isgenerally subtle compared to the extensional faultgeometry generated during Early Cretaceousrifting, for example in Bongor and Doseo basins(Genik 1993; Figs. 12a, b). Critical examination ofthe geometry reveals the inversion component. Forexample, inversion can be demonstrated easily forthe seismic interpretation shown in Figure 12bwith the following observations:
(1) thickening of Lower Cretaceous isochronsfrom footwall to hanging wall across faultsshowing net extension, yet elevation of topLower Cretaceous in hanging wall anticlinerelative to footwall;
(2) thinning of uppermost Cretaceous intervalfrom FW to HW anticline;
(3) thinning and onlap of Cenozoic sedimentarysequence onto a frontal monocline.
However, some structures (Fig. 13) are far lessclearly the result of inversion without more carefulscrutiny. The section in Figure 13a has beeninterpreted as purely extensional (Mohamed et al.2000), but close examination reveals subtle yetconvincing evidence for inversion. The Heglig oilfield is placed within an asymmetric graben that isdominated by a series of southwest-dipping exten-sional faults. From NE to SW there is thickeningof both Lower Cretaceous and the lower UpperCretaceous intervals across the faults, and the topLower Cretaceous shows net extension across thefaults. Yet where the thickening across faults ismost apparent in the lower Upper Cretaceous, theintermediate Upper Cretaceous horizon (topreservoir sands) is also structurally elevated inhanging wall relative to footwall, indicating slight
CHARACTERISTICS OF INVERSION STRUCTURES 837
inversion during Late Cretaceous time. There isalso arguably a footwall shortcut splay at the NEedge of the Heglig field graben and inversionfairway, where compressional faulting has cutthrough the basement into the adjacent half-grabenat the northeast edge of the basin. In this case thefootwall shortcut can be recognized because thebasal Upper Cretaceous and especially Lower Cre-taceous intervals are notably thinned above a tiltedbasement high in the hanging wall, but horizonsare very subtly elevated in the hanging wall, particu-larly the top Lower Cretaceous. Finally, the inter-preted fault geometry of Mohamed et al. (2000)suggests there may be an incipient footwall shortcutdeveloped in the basement at the NE edge of thebasin, although there is no clear evidence for inver-sion along the basin-bounding extensional faults.
Once inversion is recognized in profiles such asoutlined above, other ‘extensional’ examples inthe rift basins may perhaps be questioned, includingtwo cited above (Fig. 12c, d). The Unity oil field(Fig. 12c) is located in the same area of the Muglad
basin as the Heglig basin and has also been inter-preted as the product of trapping extensional faultsand rollover anticline geometry (Giedt 1990).However it is possible that there is slight structuralcompressional folding and elevation displayedat horizons near the top Cretaceous, although allhorizons appear to be depressed below regionalelevation.
Inversion can also be recognized in structuresoriginally interpreted simply as transpressional or‘flower’ structures on seismic data. The seismicinterpretation example in Figure 13b clearly showsa syn-rift growth sequence although it was notrecognized by the original authors (Manga et al.2001), revealed by steeper dips into the basin-bounding fault at depth. It is not possible to tellwhether the top of the syn-rift section is above, ator below regional elevation relative to the footwallof the master fault, but the top syn-rift has beengently folded and at least one of the antitheticfaults shows compressional offset. Therefore evenwithout confident seismic correlations, geometric
Fig. 12. Structural styles in the western and central Africa rift system: (a) Cross-section showing inversion in BongorBasin, more notable in northern portion of basin. After Genik (1993). (b) Inversion structure and petroleum trap inDoseo basin. Redrawn from interpreted seismic section in Genik 1993. (c) Cross-section showing preserved extensionalgeometry and setting of the Unity Field, Muglad Basin, Sudan. After Giedt (1990). (d) Cross-section showing preservedextensional geometry, Termit Basin, Niger. After Genik (1993). See text for discussion of (a–d) and Figure 11for locations.
M. COOPER & M. J. WARREN838
relationships alone indicate a subtly invertedsyn-rift basin. The second example (Fig. 13c)shows an asymmetric graben, originally describedas a ‘half-flower’ structure (Manga et al. 2001).Two syn-rift growth sections are suggested by dipchanges with depth beneath the sub-Upper Cretac-eous unconformity: an older interval that thickensto the south, associated with initial motion on thesouthern basin-bounding fault, and a youngerinterval that thickens to the north associated withlater motion on the northern basin-bounding fault.In this case the southern basin-bounding fault hasbeen noticeably compressionally reactivated,causing uplift and erosional truncation of LowerCretaceous sediments. The ‘flower’ geometry isfor the most part the preserved synthetic and anti-thetic fault geometry of the original extensionalbasin, with the exception of the immediate hangingwall of the southern basin-bounding fault where theminor faults clearly show compressional reactiva-tion. In both of these examples, the structures mayhave a strong transpressional component, but theimportant point is that an early extensional historyis also recognized.
The distinction between simple extension andpartial inversion, or between simple transpressionand transpressional inversion, is of significant prac-tical value when considering the impact on pet-roleum system risks and opportunities. The mainsource rocks in the western and central African riftsystem are contained in the thick, primarily lacus-trine Lower Cretaceous syn-rift sequences (Schull1988; Genik 1993; Mohamed et al. 1999, 2000;Tong et al. 2005), although source rocks mayoccur also in Upper Cretaceous to Cenozoic lacus-trine or marine shales (Petters & Ekweozor 1982;Genik 1993; Tong et al. 2005). The most importantreservoirs are fluvial and locally shallow marinesandstones in the Upper Cretaceous to Cenozoicpost-rift sequences, with lesser lacustrine reservoirsandstones in the Lower Cretaceous (e.g. Schull1988; Genik 1993 and references therein). Sealsare Upper Cretaceous and Cenozoic terrestrial orlacustrine shales, or intraformational lacustrineshales in the Lower Cretaceous (e.g. Schull 1988;Genik 1993 and references therein).
Trap geometries and risks are dependent on thedegree of inversion. Lack of significant inversion
Fig. 13. Recognizing inversion versus simple extension or strike-slip, central Africa: (a) Preserved extensionalgeometry, interpreted inversion fairway and setting for the Heglig oil field, Muglad basin, Sudan. Redrawn frominterpreted seismic section in Mohamed et al. (2000). (b) Previously interpreted ‘flower structure’ in Logone Birnibasin, Cameroon, revealed as partly inverted syn-rift growth section. Bold dashed line at base of section indicatesproposed extension of basin-bounding fault at depth; more compatible with the growth section present than thesouth-dipping fault shown on the original. Redrawn from interpreted seismic section in Manga et al. (2001).(c) Previously interpreted ‘half-flower’ structure in Logone Birni basin, Cameroon, showing syn-rift growth sectionsassociated with both basin-bounding faults, and inversion on the southern fault. Redrawn from interpreted seismicsection in Manga et al. (2001). See text for discussion of (a–c) and Figure 11 for locations.
CHARACTERISTICS OF INVERSION STRUCTURES 839
overprint favoured preservation of original exten-sional fault trap geometry, without hydrocarbonre-migration or leakage. In these cases most of theextensional geometry also was in place early rela-tive to maturation of oldest source rocks and socould trap the earliest migrated hydrocarbons.Finally, structures remained fully buried ratherthan uplifted and partly eroded, thus preservingseals. These favourable conditions may be partlyresponsible for several large established hydro-carbon fields in Sudan for example, Muglad andMelut basins (Schull 1988; Mohamed et al. 2000;Idris & Yongdi 2004; Tong et al. 2005), wherebasins and largest extensional structures are orien-tated NNW–SSE and therefore least favourablefor reactivation during the Santonian compressionalevent. However, simple tilted extensional faultblocks require lateral fault seal and therefore thisis a key exploration risk (Idris & Yongdi 2004).
Where significant enough to elevate the top ofthe syn-rift sequence above regional elevation,compressional/transpressional inversion createdthe opportunity for four-way dip closed structuresthat did not depend on fault juxtaposition or shalesmear seal. Inversion anticlines form the mainhydrocarbon traps in the more east–west orientatedbasins (Genik 1993; Reynolds & Jones 2004), incontrast to the more north–south basins whereextensional fault blocks are the main traps.However, inversion anticlines may post-date initialmigration from syn-rift source rocks, introducing acharge risk, although in general renewed subsidencein latest Cretaceous time allowed continued hydro-carbon generation and migration (Genik 1993).Because the compression modified the originalextensional geometries there is risk of loss of hydro-carbons during re-migration (Petters & Ekweozor1982), for example upward along non-sealingfaults (e.g. Giedt 1990) that may be, as outlinedabove, younger inversion faults even if they stilldisplay a net extensional offset. Perhaps the mostsignificant potential exploration risks result fromthe fact that inverted basins have been uplifted asmuch as 2 km (Genik 1993; Manga et al. 2001),potentially eroding seals and eroding the bestUpper Cretaceous to Cenozoic fluvial reservoirsbeneath the Santonian unconformity (Figs 12a &13b, c). Significant uncertainty also adds to riskbecause the erosional basin edge is no longer coinci-dent with the original depositional edge of the basin(e.g. Doba & Bongor Basins; Genik 1993), so thatseismic or other data may not detail original basin-bounding extensional fault geometry and relatedsediment entry pathways to the basin. Uplift alsopotentially exposed light hydrocarbon accumu-lations to meteoric waters and cooler temperatures,resulting in a biodegradation risk (Petters &Ekweozor 1982; Genik 1993; Manga et al. 2001).
Recognizing inversion in settings dominated
by thin-skinned and strike-slip structures
The presence of inverted extensional faults may beoverlooked initially in basins with well documentedthin-skinned fold-thrust belts or in transpressionalsettings with abundant anticlinal positive flowerstructures. This is especially true when deep (i.e.near basement) subsurface data is lacking. Forexample in the Canadian Rockies the work ofBally et al. (1966) and many others established thedominantly thin-skinned nature and evolution ofthis orogenic belt. However more recent and abun-dant drilling, seismic and surface geological datahave revealed the presence of inversion both in theemergent fold-thrust belt and in the adjacent partof the foreland basin (e.g. Colpron et al. 1998;Lemieux 1999; Cooper 2000). In many other foldand thrust belts initial pure thin-skinned modelshave been updated to hybrid models involvingboth thick and thin-skinned compressional struc-tures, for example, compare the cross-sectionsthrough the Papuan fold belt by Hobson (1986)with those of Hill (1991) and the numerousexamples given by Coward (1996) and Ulianaet al. (1995).
Similarly in some current transpressional set-tings, an inversion component of transpressionalanticlines may not be recognized if there are notadequate seismic data or deep well data to identifyan earlier syn-extensional stratigraphic section.However with critical examination and integrationof well data, seismic data isochrons and/or equival-ent surface lithofacies and thickness data, theinterpreter in either tectonic setting should be ableto able to recognize inversion readily as illustratedby the case studies presented in this paper. Coward(1996) and Cooper (1996) also have presented someexcellent examples of how this approach can facili-tate the recognition of inversion structures.
Recognizing inversion in addition to simplethin-skinned or transpressional tectonics is criticalfor complete understanding of potential petroleumsystems, as discussed in the next section. Examplesgiven by Vann et al. (1986) and Cooper (1996) illus-trate the impact of thin-skinned versus thick-skinned structural interpretations on hydrocarbonprospectivity at mountain fronts (Fig. 14).
Inversion structures and petroleum
system elements
The review presented above of some examplesof inversion structures shows that such structurescan be important hydrocarbon plays in bothmature and under-explored areas. Previous papershave discussed the importance of inversion on the
M. COOPER & M. J. WARREN840
understanding of hydrocarbon prospectivity. Forexample, Charlton (2004) presents some well-documented examples of inversion structuresin the Banda Arc and discusses at length therelationship of the petroleum system elements tothe tectonic evolution of the inversion structures.Macgregor (1995) reviews the characteristics ofhydrocarbon provinces that have suffered differingdegrees of inversion and discusses how mildinversion can create large simple anticlinal trapswith migration from the surrounding synchronouslyactive source kitchens. Sibson (2003) has arguedthat large amplitude fluid-pressure cycles due tofault-valve action during inversion of extensionalfaults may contribute to hydrocarbon migration.This is supported by evidence that hydrocarbonmigration into inversion traps is synchronous withtrap development (Macgregor 1995; Butler 1998).
The pre-cursor extensional fault commonly con-trols reservoir distribution in the hanging wall ofthe fault system (Fig. 15) for example, late Jurassicsand distribution in the UK Central North Sea(Roberts et al. 1990; Roberts 1991; Schmitt &Gordon 1991) and in some circumstances in thefootwall as illustrated by the West Newfoundlandcase study and by the onlap and erosional truncationof sands onto the footwall crests of tilted fault blocksin the North Sea, for example, Farquharson &Gibson (2005). The accommodation space created
during extension can also control source rocks dis-tribution (Fig. 15) as illustrated by the NE Thailandand central Africa case studies and the deposition ofthe Kimmeridge Clay in the UK North Sea. Exten-sional faulting can also control source maturationin the fault controlled depocentre, for example, theWessex Basin of southern England (Buchanan 1998)and numerous examples in Africa. The extensionalfaulting can also create structural trap geometriesin both the footwall and hanging wall of the faultthat can be preserved following inversion of thestructure as is well illustrated in the Wessex Basin(Butler 1998) and the Doseo Basin (Genik 1993;& Fig. 12b).
The compressional inversion of the extensionalfault system will principally affect structural trapconfigurations, creating new trap geometries suchas footwall shortcut structures and the inversionanticline (Fig. 15). These features are a commontheme through all of the case studies presented.The inversion can also modify older structural trapgeometries which could result in the re-migrationof pre-inversion hydrocarbon accumulations forexample, the Logone Birni Basin in Central Africaand possibly some of the Sudanese rifts (Mangaet al. 2001; Giedt 1990). The geometric changesresulting from inversion can also create new trapgeometries, for example, by turning a facieschange from reservoir to seal in a down dip direction
Fig. 14. An example of a thin-skinned paradigm cross-section from the northern Magallanes Basin in Argentina afterRamos (1989) with an alternative thick-skinned model after Cooper (1996).
CHARACTERISTICS OF INVERSION STRUCTURES 841
into a viable stratigraphic trap that occurs in anupdip direction (Fig. 15). However, the upliftabove regional elevation that is a product of inver-sion, if significant, can result in erosion of one ormore critical components of the petroleum system.
Conclusion
Based on the geometric criteria and illustrative casestudies described in this paper it has been shownthat inversion structures have easily recognizablegeometric characteristics, the key ones being:
1. a syn-rift (or passive infill) sequence should berecognizable in all inversion structures;
2. the hanging wall of an inverted fault has markerhorizons that display different elevations aboveand/or below their respective regionals;
3. an asymmetric monocline faces the footwallwith the crest located above the syn-riftdepocentre;
4. footwall shortcut thrusts commonly developthat create a fault trajectory more favourablyorientated to the compressional stress for effi-cient shortening to occur.
Footwall shortcuts and anticlines above invertedmaster extensional faults can create potential hydro-carbon traps that offer attractive exploration targetsas illustrated by the four case studies presented.However, whilst inversion can create structuraltraps it can also destroy earlier structural trap geo-metries developed during the extensional faultingor modify this earlier trap geometry and causere-migration of earlier hydrocarbon accumulationsinto the new, modified traps. The degree of inver-sion that develops will be significantly influencedby the relative magnitudes of the extensional andcompressional stresses and their durations. The geo-metry and degree of inversion will also be controlledby the orientation between the original extensionalstructures and the subsequent compressional stres-ses. If the compressional stress is at a low obliqueangle to the trend of the extensional fault then strike-slip motion will dominate and the degree of inver-sion due to dip slip will be limited. As the anglebetween the compressional stress and the trend ofthe extensional fault increases the proportion ofdip-slip motion and hence inversion will increase.
The extensional phase of fault development in aninversion structure can control the following pet-roleum system elements.
Extensional half-graben
Inverted half-graben
Incipient footwallshortcut fault
Footwallshortcut fault
Source depositedin starved half-graben
Reservoir derivedfrom HW
Reservoir facies inimmediate HW
Inversion createsstructural/strat trap Inversion
creates traps
Fig. 15. The impact of inversion structures on petroleum system elements; see text for discussion.
M. COOPER & M. J. WARREN842
1. Source rock distribution, for example, theTriassic lacustrine source rocks in NEThailand, particularly in situations where theevolving basin is starved of sediment andanoxic conditions develop due to restrictedcirculation, for example, the KimmeridgeClay in the UK Central North Sea.
2. Reservoir distribution and deposition; asaccommodation space is created during exten-sional faulting the extensional fault geometriesand in particular transfer zones and relay rampswill strongly control sediment dispersionpathways into the developing basin. This is amajor factor in controlling where reservoirfacies will be deposited (Gawthorpe & Leeder2000).
3. Seal distribution, for example, intraformationalshales that commonly separate reservoir sandsin extensional basins (Gawthorpe & Leeder2000).
Inversion structures in some fold and thrust beltshave not always been recognized due to the domi-nance of thin-skinned structures and interpretations,for example, in the Canadian Rockies, or due toconfusion with strike-slip ‘flower’ structures, forexample, in the African rift systems. This hasoften resulted from the lack of sufficient data andthe failure to apply critically the geometric andstratigraphic criteria outlined in this paper.
We would like to acknowledge G. Rait, A. Ferster andE. Trevisanut for comments on the manuscript;R. Graham, A. Gibbs and R. Law for insightful reviewsand Sigma Explorations for permission to publish theseismic line in Figure 10.
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