incidence and importance of tectonics and natural fluid ......foreland fold-and-thrust belts (fftb),...

Oil & Gas Science and Technology – Rev. IFP, Vol. 60 (2005), No. 1, pp. 67-106Copyright © 2005, Institut français du pétrole

Incidence and Importance of Tectonicsand Natural Fluid Migration on Reservoir Evolution

in Foreland Fold-and-Thrust BeltsF. Roure1, R. Swennen2, F. Schneider1, J.L. Faure1, H. Ferket2, N. Guilhaumou3,

K. Osadetz4, P. Robion5 and V. Vandeginste2

1 Institut français du pétrole, 1 et 4, avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex - France2 Afdeling Fysico-chemische Geologie, KU Leuven, Celestijnenlaan 200C, 3001 Heverlee - Belgium

3 CNRS-Museum national d’Histoire naturelle, Laboratoire de Minéralogie, 61, rue Buffon, 75005 Paris - France4 Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, Alberta T2L 2A7 - Canada

5 Université de Cergy-Pontoise, Département des Sciences de la Terre, 5, Mail Gay-Lussac,Neuville/Oise, 95031 Cergy-Pontoise Cedex - France

e-mail: [email protected] - [email protected] - [email protected] - [email protected]@geo.kuleuven.ac.be - [email protected] - [email protected] - [email protected] - [email protected]

Résumé — Impact de la tectonique et de la migration naturelle des fluides sur l’évolution desréservoirs dans les chaînes plissées et leurs avant-pays — Nous avons utilisé ici les résultats d’étudesde cas réalisées dans plusieurs chaînes plissées et leurs avant-pays au moyen d’une méthodologieintégrant la géologie structurale, la pétrographie, la fabrique magnétique et les modélisations de bassin.Cette méthode permet d’identifier les paramètres critiques et les processus contrôlant l’évolution desréservoirs depuis la fin du développement de la marge passive jusqu’aux épisodes postorogéniques decollapse de l’édifice tectonique.

Les interactions fluides-roche sont exacerbées pendant les périodes de crise tectonique. La compactiontectonique est déjà active dans l’avant-pays, tandis que le développement et la réouverture de réseaux defractures permettent de remobiliser les fluides dans les unités allochtones, d’expulser les fluidespréalablement rééquilibrés des roches-réservoirs, mais aussi d’injecter de nouveaux fluides exotiques, endéséquilibre chimique, dans les drains carbonatés ou gréseux. Dans les bassins subandins, la cimentationsiliceuse est ainsi dominée par les effets de la compression tectonique (raccourcissement parallèle auxcouches ou LPS) dans les unités sous-charriées à proximité du front de la déformation. Le LPS est aussisuspecté de contrôler la recristallisation des mésodolomites dans l’avant-pays des montagnes Rocheusesau Canada. En revanche, la dolomitisation secondaire par processus hydrothermaux des niveauxcarbonatés permet souvent une migration latérale des fluides diagénétiques le long d’horizonsstratigraphiques peu pentus dans l’avant-pays, mais aussi l’échappement vertical de fluidesminéralisateurs dans les réseaux de fractures ouvertes de l’allochtone. Inversement, le développement deporosité vacuolaire dans les séries carbonatées allochtones de la Cordillère nord-américaine peuts’expliquer par un refroidissement progressif du réservoir dans un système relativement clos, en liaisonavec la surrection tectonique et l’érosion progressive des séries superficielles.

Les modélisations de bassin permettent d’obtenir des estimations réalistes des paléo-températures et despaléo-enfouissements atteints par un réservoir donné, valeurs qui peuvent être ensuite comparées à despaléo-thermomètres tels que les inclusions fluides ou les isotopes stables, permettant ainsi de dater de

Gas-Water-Rock Interactions ... / Interactions gaz-eau-roche ...

IFP International WorkshopRencontres scientifiques de l’IFP

http://ogst.ifp.fr/http://www.ifp.fr/http://ogst.ifp.fr/index.php?option=toc&url=/articles/ogst/abs/2005/01/contents/contents.html

Oil & Gas Science and Technology – Rev. IFP, Vol. 60 (2005), No. 1

CONTENTS

Introduction1 Methods Used

1.1 Structural Analysis and Sampling of Oriented Coresand Plugs

1.2 Description of the Paragenetic Sequence and Study ofPaleo-Thermometers

1.3 Reconstruction of Burial Curves and Coupled Kine-matic and Thermal Modeling

1.4 Fluid Flow and Pore Fluid Pressure Reconstruction1.5 Forward Diagenetic Modeling

2 Principal Structural Elements and Deformational EventsRecorded by Sandstone and Carbonate Reservoirs in FFTB2.1 Vertical Compaction and Bedding-Parallel Stylolites

(BPS)2.2 Early versus Late Hydraulic Fractures2.3 Pre-Thrusting Layer-Parallel Shortening (LPS) and

Development of Conjugate Vertical Joints2.4 Syndeformational Reservoir Alteration

2.4.1 Selective Synkinematic Reactivation of ForelandStructures

2.4.2 Outer Arc Fractures2.4.3 Synfolding Pressure-Solution

68

manière indirecte les épisodes de cimentation ou de dissolution. Ces modèles de bassins fournissent aussides vitesses d’écoulement des fluides, qui peuvent être ensuite utilisées comme conditions aux limitespour des modèles diagénétiques à l’échelle du réservoir.

Dans la nature, les interactions fluides-roche induites par l’injection tectonique de fluides exotiques sontde courte durée, ne dépassant pas le million d’années. Ces mouvements de fluides contrôlés par latectonique constituent donc de bons analogues pour étudier les effets à long terme d’injection et destockage de gaz acides comme le CO2 ou l’H2S dans les réservoirs naturels. Les mêmes méthodesquantitatives, validées ici dans des études régionales, dont le but principal est l’évaluation pétrolière,constituent également des prototypes permettant déjà de fournir des valeurs de référence pour les vitessesde transfert de fluides en systèmes ouverts, qui seront de fait fort utiles pour le stockage et la surveillancede gaz acides confinés dans des réservoirs naturels.

Abstract — Incidence and Importance of Tectonics and Natural Fluid Migration on ReservoirEvolution in Foreland Fold-and-Thrust Belts — Integrated structural-petrographic-magnetic-basinmodeling case studies in numerous foreland fold-and-thrust belts provided key information on the criticalparameters and processes controlling reservoir evolution from the end of the passive margin phase to thepost-orogenic collapse of the tectonic pile.

Fluid-rock interactions in reservoir rocks are intensified during tectonic events, as tectonic compactionin the foreland and development and re-opening of fracture systems in the allochthon help remobilizingbasinal fluids, to squeeze-out host-rock buffered fluids as well as to reinject exotic fluids in reservoirsandstone or carbonate layers. For instance, quartz cementation in Sub-Andean foothills is dominantlycontrolled by Layer Parallel Shortening (LPS/tectonic compaction) in the footwall of frontal thrusts. LPScan also be inferred to cause in situ recrystallisation of mesodolomite in the Canadian CordilleranForeland Belt. In contrast, secondary hydrothermal dolomitization of limestone strata usually accountsfor lateral migration in stratigraphic conduits in the foreland and for vertical migration of mineralizingfluids in open fractures in the allochthon, respectively. Alternatively, vuggy porosity observed inallochthonous carbonate strata in the North American Cordillera can also be interpreted to result fromreservoir cooling operating in a dominantly closed fluid system during tectonic uplift and coeval erosion.

Basin models can provide realistic estimates of burial-temperature history that can be compared topaleo-thermometers, such as fluid inclusions or stable isotopes, and thus provide a means to determinethe relative age of cementation or dissolution episodes. Basin models can also provide fluid velocities,that can be subsequently used as critical constraints on diagenetic models at reservoir scale.

Natural fluid-rock interactions induced by exotic tectonic fluids are short, no longer than one millionyears. As such, they constitute very good models of the long term effects of CO2 and H2S injection andstorage in natural reservoirs. The integrated quantitative appraisal approach proposed here forpetroleum evaluation and reservoir prediction, also provides useful information on the overall changesin fluid flow regime and fluid velocities trough time in natural open systems, that should be used asregional boundary conditions for future reservoir storage and monitoring of acid gases in naturalreservoirs.

F Roure et al. / Incidence and Importance of Tectonics and Natural Fluid Migration on Reservoir Evolution

2.4.4 Fore-Bulge versus Synkinematic Karstification2.5 4D Evolution of the Fracture-Porosity/Permeability

Characteristics from the Foreland to Frontal Anticlines3 Major Trends of the Regional Fluid Flow and Pore Fluid

Pressure Evolution in FFTB3.1 Vertical versus Horizontal Fluid Migration in the

Foreland3.2 Hydraulic Heads and Development of Overpressures3.3 Changes in Lateral Velocity of the Fluid Flow as a

Response to Squeegee Pulse and Thrust Closure4 LPS Controls on Reservoir Cementation

4.1 Multiphase Quartz Cementation in Sandstone Reser-voirs of Sub-Andean Basins in Colombia and EasternVenezuela4.1.1 Origin of Silica4.1.2 AMS and Fluid Inclusion Studies: Dating the

Main Cementation Episodes4.1.3 Stable Isotope Data and Origin of the Circulating

Fluids4.2 Mesodolomite and Syn-LPS Recrystallisation of

Dolostone Reservoirs in the Foreland Belt and Basinof the Canadian Cordillera4.2.1 Petrographic, Isotopic and Magnetic Character-

istics of Mesodolomites4.2.2 Origin of the Mesodolomite

5 Late Carbonate Dissolution and Dolomitization in an OpenSystem5.1 Hydrothermal Karst and Mississippi Valley Type Ore

Deposits5.2 Synkinematic Fluid Escapes and Development of the

Zebra Dolomite5.2.1 Petrographic, Isotopic and FI Data of the Zebra

Dolomites5.2.2 Origin of the Zebra DolomitesSyn-Compressional OriginSyn-Extensional, Post-Compressional Hypotheses

5.3 Fluid Conduits, Lateral and Vertical Mg Transfer andDevelopment of Saddle Dolomite

6 Thrust-Related Development of Vuggy Porosity7 Impact of Hydrocarbon Charge on Reservoir Charac-

teristics, and How 1D and 2D-Modeling can PredictReservoir Performance7.1 Oil-Anhydrite Interactions and Thermo-Sulfate Reduc-

tion in FFTB7.2 Hydrocarbon Circulation and Secondary Dissolution

Along Stylolitic Planes7.3 Successive Trapping Episodes, Deasphalting and

Permeability Reduction7.3.1 Migration Pulses and Deasphalting7.3.2 Tectonic Uplift, Erosion and Deasphalting7.3.3 How to Evaluate Deasphalting Risk in FFTB?

7.4 Burial Increase and the Pyrobitumen Risk in FFTB7.5 Oil-Water Interactions and Biodegradation in FFTB

7.5.1 Foreland Tar Belts7.5.2 Occurrence of Biodegradated Hydrocarbon Pro-

ducts in the Foothills7.5.3 How to Predict the Occurrence of Biodegradated

Products in FFTB Prospects?Conclusions

INTRODUCTION

Recently, improved seismic imaging methods have led to thediscovery of giant petroleum fields in deep underthrustprospects of the Sub-Andean basins and other compressionalsystems, renewing the corporate interest in the petroleumpotential of mobile belts, where the risk of reservoir damagedue to diagenesis remains the main exploration challenge. Inorder to identify and understand the critical parameterscontrolling the evolution of reservoir characteristics inforeland fold-and-thrust belts (FFTB), we have developed anintegrated approach involving structural geology, petro-graphical description and basin modeling, thus providing acontrol on the burial, thermal and diagenetic evolution ofindividual sandstone and carbonate reservoirs, in relation tothe global dynamics, tectonics and deformational history,regional fluid flow and petroleum systems.

This methodology was applied successively, and success-fully, to both sandstone reservoirs in the Sub-Andean basinsof Venezuela and Colombia (Figs. 1a and 1b; Bordas-LeFloch, 1999; Roure et al., 2003; Toro et al., 2004), andcarbonate reservoirs in the inverted Ionian Basin in Albania(Fig. 1c; Van Geet et al., 2002; Roure et al., 2004), the SierraMadre foothills and foreland in Mexico (Fig. 1d; Ferket etal., 2003 and 2004; Ortuño-Arzate et al., 2003), in theHimalayan foothills in Pakistan (Fig. 1e; Grelaud et al.,2002; Benchilla, 2003; Benchilla et al., in press), and theCanadian Cordilleran Foreland (Fig. 1f; Faure et al., 2004).However, because both the deformation and thrust frontspropagate from the hinterland toward the foreland, carbonatereservoirs initially deposited within the former passivecontinental margins are progressively buried in the foredeepbasin, and then accreted tectonicaly into the allochthonousunits of the thrust belt where they are progressively upliftedand unroofed by erosional processes (Fig. 2). Therefore, theparagenetic sequence in carbonate reservoirs currentlyexposed in the foothills usually record a wide range ofcements, accounting for successive episodes of fluid-rockinteractions in very distinct P-T conditions, but also withdifferent types of fluids (i.e., formation and meteoric watersor hydrothermal brines). The oldest cements and deformationfeatures such as normal faults and bedding-parallel styloliticplanes (BPS) still account for burial compaction during theformer passive margin evolution in an extensional regime,

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Location map of the main SUBTRAP regional transects: a) Eastern Venezuela SUBTRAP transect accross the foothills of the Serrania, the Maturin Basinand adjacent Orinoco foreland, aimed at the study of the Oligocene Merecure sandstone reservoir. b) Northern and southern SUBTRAP transects across theEastern Cordillera of Colombia, aimed at the study of the Eocene Mirador sandstone reservoir in the Llanos and eastern foothills of the Cordillera (e.g., inthe Caño Limon, Cusiana and Chichimene fields and Medina and Coporo wells), as well as the study of Cretaceous sandstone reservoir analogues in theMagdalena Basin and Sabana de Bogota. c) Kremenara transect accross the inverted Ionian Basin in Albania, aimed at the study of Late Cretaceouscarbonate turbidite reservoir analogues. d) SUBTRAP transect across the Cordoba Platform and adjacent Veracruz Basin in the Sierra Madre foothills ansadjacent Coastal Plain of the Gulf of Mexico in Mexico, aimed at the study of Late Cretaceous platform carbonate reservoir analogues and productive LateCretaceous deep water breccias. e) SUBTRAP transects across the Salt-Range and Potwar basins in the Himalayan foothills in Pakistan, aimed at the studyof Eocene Chorgali platform carbonate reservoir in the Dakhni field and surface analogues, as well as Jurassic Datta sandstone reservoir in the Toot fieldand surface Cambrian Khewra sandstone reservoir analogue. f) Calgary-Banff SUBTRAP transect in the foothills of the Rocky Mountains and adjacentAlberta foreland in Canada, aimed at the study of Carboniferous carbonate reservoirs.

F Roure et al. / Incidence and Importance of Tectonics and Natural Fluid Migration on Reservoir Evolution 71

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whereas only the most recent cements and deformationfeatures such as layer-parallel stylolitic planes (LPS) aredirectly significant for the thrust belt evolution in acompressional regime (Figs. 2 and 3; see also Fig. 8 forcomplements). In contrast, most sandstone reservoirs in theSub-Andean basins were deposited at the end of the passivemargin stage, or even at the onset of the development of theforedeep basin, adjacent to the thrust front. Therefore, mostdiagenetic evolution of the sandstone reservoirs studied here,account mostly, apart of their early depositional history thatwas controlled by the paleo-environment, for synflexural andthen synkinematic episodes of foreland and foothillsevolution in a dominantly compressional regime.

The comparison of both carbonate and sandstonereservoirs paragenetic evolution in various thrust belts aroundthe world was necessary to know whether or not tectonics(i.e., rock deformation and tectonically induced fluidmigration) could exert a significant control on reservoircharacteristics in FFTB, and whether or not these effectscould be predicted and quantified. Because thermodynamics(i.e., P-T conditions) and regional fluid flow (open versusclosed system, composition of the circulating fluids) have adirect impact on fluid-rock interactions, we tried also toreconstruct the complete burial and temperature history ofindividual reservoir units, and to characterize the mainevolutionary stages of the regional fluid flow and pore fluidpressure regimes. Surprisingly, major diagenetic episodes inboth carbonate and sandstone reservoirs in FFTB are usuallydirectly linked to short-living tectonic episodes, inconnection with either tectonic compaction (pressure-solution in a closed system) or fracturing (allowing rapidfluid migration and fluid-rock interactions in an opensystem). Results of these various case studies aresummarized below, and used to illustrate the input suchintegrated structural-diagenetic-basin models can bring toreservoir appraisal, qualitatively (i.e. basin scale processoriented) and quantitatively (i.e. forward diageneticsimulations at the scale of individual prospects andreservoirs), for a specific time interval and a given diageneticepisode. However, because tectonically controlled episodesof fluid-rock interactions are:– limited in time;– localized to the vicinity of the deformation/thrust front;– result from the rapid injection of exotic fluids into a given

reservoir horizon, they are also likely to provide directnatural analogues for studying the long term effects (i.e.,in the range of 100 000 to 106 y) of acid gas injection andstorage in natural reservoirs.

1 METHODS USED

Our objective is not only to describe the present, finalarchitecture and characteristics of a given reservoir, but also

to understand its long term evolution through geologicaltimes, in relation to the global dynamics and the regionalgeological-tectonic evolution, as well as during successivefluid migration and fluid-rock interaction episodes.Therefore, multidisciplinary team-work is required, involvingdifferent techniques, including field observations andsampling, laboratory measurements and numerical modeling,to be applied at different scales, i.e., from the thin-section andthe core, up to the prospect and even basinal scale.

1.1 Structural Analysis and Samplingof Oriented Cores and Plugs

Direct observation of deformation features is necessary todefine the 3D architecture and relative chronology of varioussets of fractures and stylolites. This is not always easy oncore material, because of both recovery and orientationproblems. Instead, surface reservoir analogues can usually bestudied in great detail, each set of fractures or microstructuresbeing carefully measured (both strike and dip), analysed interm of kinematic criteria (either normal, versus reverse orstrike-slip motion) and a relative chronology with respect toother sets of deformation features (Fig. 3).

Oriented plugs are also sampled in individual veins andcemented fractures or stylolitic planes, thus allowingsubsequent laboratory analyses such as:– Anisotropy of magnetic fabric (AMS; Averbuch et al.,

1992; Aubourg, 1999) measurements on oriented plugshelps to unravel the deformation pattern of the rockmatrix, even in weakly deformed sandstones, and toprovide oriented thin-sections parallel to the mainprincipal stress axes;

– Cathodoluminescence and isotope studies on specific setsof cemented fractures and microstructures, in order todetermine the nature of paleo-fluids circulating within thereservoir during a given tectonic event (Swennen et al.,2003a).

1.2 Description of the Paragenetic Sequenceand Study of Paleo-Thermometers

Cathodoluminescence is helpful in documenting the suc-cessive episodes of cementation of a given fracture or thegrowth of cements within the rock matrix.

As for the description of the fracture system, it is alsoimportant to properly describe the nature of the parageneticsequence in the rock matrix, to propose a relative chronologyamong its various cements, and to characterize both thecomposition and temperature of trapping of the paleo-fluidsaccounting for each episode of fluid-rock interaction. Paleo-thermometers such as maturity ranks of the organic matter(i.e., vitrinite reflectance measurement/Ro and Rock-Evalpyrolysis/Tmax), fluid inclusion, stable isotopes and apatite

72


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fission tracks (AFT) are combined during the same study, inaddition to bottom hole temperatures (BHT), in order to getindependant calibrations of the paleo-temperatures duringsubsequent thermal modeling (Roure and Sassi, 1995).

Fluid inclusion studies provide also direct information onthe salinity of fluids circulating in the reservoir duringcementation, thus providing critical information on whether agiven cementation episode was coeval with the circulation ofmeteoric (low salinity, fresh) water, basinal/formation(marine) water, or hydrothermal brines (salinity higher thanmarine water) within the reservoir (Guilhaumou et al., 1994,1996; Larroque et al., 1996).

1.3 Reconstruction of Burial Curvesand Coupled Kinematic and Thermal Modeling

The relative chronology among cements and fractures isprovided by microtectonic analysis and description of theparagenetic sequence. However, the absolute dating of agiven episode of fluid-rock interaction is never obvious, dueto the overall composition of the mineralogical assemblage inthe sediments, which usually prevents the use of radiometricgeochronology.

Therefore, we rely on the combination of paleo-thermo-metric measurements in the cemented reservoirs and onresults of thermal modeling calibrated against the maturityranks of organic matter. Dates are obtained directly bycomparing the paleo-temperatures measured in a givenmineral with the temperature versus time curves computed forthe overburden (Bordas-Le Floch, 1999; Roure et al., 2003).

Classic backstripping techniques allow burial historyreconstruction of the former passive margin succession, butforward kinematic modeling coupling thrust motion withsynorogenic sedimentation and erosion processes is requiredto provide more realistic burial curves for reservoir unitslocalized in such tectonically complex areas as foothills andfold-and-thrust belts. Thermal modeling is then performed,assuming purely conductive heat transfers with 1 or 2D basinmodeling tools such as Genex and Thrustpack. The detaileddescription of such coupled Thrustpack kinematic andthermal modeling performed along these Venezuelan,Colombian, Albanian, Mexican, Pakistani and Canadiantransects is published elsewhere (Grelaud et al., 2002;Ortuño-Arzate et al., 2003; Roure et al., 2003, 2004; Faureet al., 2004; Toro et al., 2004).

1.4 Fluid Flow and Pore Fluid PressureReconstruction

The kinetics of a given cementation episode due to fluid-rockinteraction is dependent on temperature. Alternatively, thevolume of cement generated during a given time interval (i.e.,a given tectonic episode) is also dependent on the chemistryand velocity of the circulating fluids when the reservoir acts

as an open system. Seemingly, values of the pore fluidpressure have a direct impact on the mode of deformation,thus allowing either the coupling or decoupling ofsedimentary layers with respect to the underlying substratumwhen subjected to horizontal tectonic stress, overpressureultimately accounting for a drecrease or delay of compaction.

Fortunately, computation of the velocity of circulatingfluids and reconstruction of the pore fluid pressure evolutionof the reservoir can now be performed in 2D, not only acrosspassive margins and extensional basins using commercialtools such as BasinMod or Temis, but also in tectonicallycomplex areas such as foreland fold-and-thrust belts, usingnew model prototypes such as CERES. For the currentproject, we used the resulting sections derived from the 2Dforward kinematic modeling (templates of the intermediategeometries of the fold-and-thrust structures and erosionalsurfaces taken from the Thrustpack sections) as an input forfurther fluid flow modeling with CERES. Detailed descriptionof the CERES fluid flow modeling along these Venezuelanand Canadian transects has been already published elsewhere(Schneider, 2003; Faure et al., 2004; Schneider et al., 2004).

1.5 Forward Diagenetic Modeling

Once a given diagenetic event has been dated and its durationestimated, it becomes possible to compute the amount ofcement generated in the reservoir during this episode (or theincrease of porosity in case of a dissolution event), providedwe know the kinetics of this specific fluid-rock trans-formation, the average temperature of the reservoir duringthis time interval, and the composition and velocity of thecirculating fluids, in the case where the reservoir is an opensystem (Le Gallo et al., 1998).

Because 1D numerical tools such as Diaphore can workwith a significant number of mineralogic phases, forwarddiagenetic simulations are currently used to test a number ofhypotheses regarding the boundary conditions of fluid-rockinteractions at reservoir scale (i.e., closed versus opensystem, chemical composition of the exotic fluids versusmineralogical composition of the rock matrix). The results ofthese simulations and further iterations are then comparedwith the actual petrographic observations in thin-sections.

Instantaneous temperatures and fluid flow velocitiesderived from basin-scale Thrustpack or CERES modeling fora given tectonic episode have been used as input data formore local reservoir-scale diagenetic modeling of quartz-cementation in eastern Venezuela, and hydrothermaldolomitization in the Canadian Rockies. Note that theincremental deformation applied to basin-scale models suchas Thrustpack and CERES is dependent on the duration ofthrust belt development, and also on the duration of previouspassive margin evolution, i.e., from the time of deposition ofthe reservoir up to Present (i.e., 300 Ma for Mississippiancarbonates in Rockies, 60 Ma for the Mirador reservoir

74


in Colombia, against only 30 Ma for the Merecure reservoirin Venezuela). Most modeling effort is focused onsynkinematic episodes. Typicaly, time intervals between 1and 5 Ma have been used between two successive steps ofthe kinematic simulation in both Venezuela and Canada,which is a maximum value for the duration of individualthrust controlled fluid flow and diagenetic episodes. Instead,forward diagenetic simulations were performed with muchshorter incremental time intervals, starting from 1000 and10 000 years, up to 100 000 years and 1 Ma, keeping bothtemperature and fluid flow velocities constant for theserelatively short intervals compared to the ones used for thebasin-scale modeling.

2 PRINCIPAL STRUCTURAL ELEMENTSAND DEFORMATIONAL EVENTS RECORDEDBY SANDSTONE AND CARBONATE RESERVOIRSIN FFTB

Sandstone reservoirs are commonly located within, or at thebase of, the foredeep basin succession, with the best reservoirlithologies being the coarse quartz dominant clastics within aforeland craton provenance, as opposed to the arc-derivedclastics, which usually contain more feldspar and lithicgrains. Aside from their initial detrital mineralogy, sandstonereservoir characteristics result from successive episodes ofburial in the foredeep, and their subsequent deformation anddiagenesis during tectonic accretion in the foothills.

In contrast, most carbonate reservoirs preserve a muchlonger and complex diagenetic history, as they werecommonly deposited during the former passive margininterval. Consequently, early diagenetic episodes linked tochanges in sedimentary setting, i.e. facies controlled as wellas changes in sea-level, rift related extensional deformation,uplift and exposures, can usually still be identified in theselithologies, in addition to the overprint of secondarydeformation and diagenetic episodes related to theirsyntectonic burial in the foredeep and subsequent tectonicaccretion in the foothills (Sassi and Faure, 1996).

The study of cemented fractures is informative regardingthe nature of the fluids circulating during a given tectonicepisode. Alternatively, knowing when fractures were openand acted as vertical conduits to fluids is also critical for pre-dicting when fluid-rock interactions could occur. Ourdiscussion below focuses only on the synflexural andsyncompressional deformation features of the reservoirs(Fig. 3).

2.1 Vertical Compaction and Bedding ParallelStylolites (BPS)

Bedding parallel stylolites (BPS) are common structures incarbonate reservoirs, but they are also locally present in

sandstones, e.g., in the Cretaceous siliciclastic succession atMoose Mountain in the Canadian Foothills. BPS planesdamage the porosity-permeability of the rock matrix bycausing both compaction and cementation. BPS are alsopotential conduits for subsequent fluid migration, whichcommonly is the cause of dissolution in nearby strata and ofthe development of secondary porosity along the stylolites.Several processes are likely to enhance the opening of BPSafter their development, i.e.:– Maturation of organic material concentrated along the

seams and generating acidic fluids which may dissolve theadjacent matrix (Harrison and Thyne, 1991).

– Bedding-parallel shear during folding and thrusting andthe inherent roughness of these surfaces.

– Splitting due to pressure reduction after tectonic uplift anderosion, the latter mainly affecting outcrop examples.The development of BPS and the subsequent eventual

evolution of enhanced matrix porosity along these stylolitesare lithology-dependent:– In the upper Cretaceous to Eocene pelagic formation of

the Ionian basin in Albania, only the dominantly micriticcarbonate facies displays well-developed stylolites andgood secondary porosities related to BPS. Pressure-solution related to vertical compaction is less well-developed in coarser grained turbidite beds and remainsmore diffuse in debris flows where each individual clastsbecome the site of indentation and stylolitization.However, the secondary porosity developed along thestylolites may contribute to bulk matrix porosity (VanGeet et al., 2002; Swennen et al., 2004).

– In the Salt Range-Potwar basin in Pakistan, BPS canconstitute important conduits to fluids in marine Permianlimestones where individual stylolitic peaks up to 10 cmlong develop around coarse lithoclasts where they are thesource of good secondary porosity. In contrast to thePermian carbonates, Eocene reservoirs in Pakistan have ahigh shale content and shaley interbeds are common. Thisprevents the development of penetrative pressure-solutionat the boundaries of carbonate beds in these strata.The minimum burial depth for BPS to develop is assumed

to be 800-1000 m (Machel, 1990; Railsback, 1993). Onlyrelatively pure carbonate formations are likely to developsecondary porosity along these deformation structures, whichresult in good permeabilities along bedding.

2.2 Early versus Late Hydraulic Fractures

Paleo-hydraulic fracturing, although difficult to recognize,can be inferred locally from the occurrence of breccia-veins,sedimentary dykes, clustered cyclic microveins and/orcompound veins. Fluid inclusion pressure-calculations give astronger indication, but unfortunately vein-cements do notalways possess suitable inclusions (Ferket et al., 2004).Hydraulic fracturing and brecciation have been documented

75


locally, in the vicinity of major normal faults, where theyprovide a conduit for the progressive vertical escape ofoverpressured fluids within a dominantly extensional stressregime when the maximum principal stress is vertical.

In compressional stress regimes, when the maximumstress is horizontal, it is also possible to recognize hydraulicfractures along decollement surfaces. Examples include thesole thrust of the accretionary wedges in both Sicily and theSouthern Apennines (Roure et al., 1991; Larroque, 1993;Larroque et al., 1996). Breccia-veins and indications foroverpressures during the compressional stage are found alsoin Cretaceous carbonate reservoirs in eastern Mexico (Ferketet al., 2004). Hydraulic fractures can develop parallel to thebedding planes, indicating pore fluid pressures that possiblyexceeded the lithostatic load. Unfortunately, these hydraulicfractures are generally cemented and they are no longerconduits for modern fluid migration. Other featuresindicating episodic fluid release are “crack-seal” fractures(Fig. 3) recognized in all the foreland fold-and-thrust beltsettings studied. This cyclic behavior can be caused byvariations in local stresses or by oscillations in fluid pressure(Simpson and Richards, 1981).

Nonetheless, cross-cutting relationships at outcrop scale orin thin-sections demonstrate that these structures maydevelop prior to BPS (Ferket et al., 2004), but that theydevelop more generally after BPS and before the formationof the LPS structures (Van Geet et al., 2002). As such,hydraulic fractures are very informative regarding dewateringprocesses operating in FFTB, and they demonstrate thatoverpressures can reduce the effects of both vertical andhorizontal compaction in sandstone and carbonate reservoirs.Due to increasing tectonic stress and lowered differentialstress in the early compressional stage, increased fluidpressures are required to explain extensional fracturing(hydraulic fracturing). The veins, formed before LPS-stylolite development, are interpreted to originate from acaterpillar-type of LPS-deformation. With the onset oftectonic contraction, formation waters are first driven outand, if impermeable barriers are present, overpressures buildup, which may lead to hydraulic fracturing. With increasingtectonic stress, the reservoir rock is compacted and LPSstructures may develop. The deformation is progressive fromhinterland to foreland and may result in a cyclic process ofdewatering with hydraulic fracturing and subsequentpenetrative LPS deformation (Ferket et al., 2004).

2.3 Pre-Thrusting Layer Parallel Shortening (LPS)and Development of Conjugate Vertical Joints

Conjugate strike-slip faults with incipient pressure-solutionalong slickensides and stylolitic planes perpendicular tobedding are well documented in most carbonate foothillsoutcrops, especially where the carbonates are fine grained(Alvarez et al., 1976, 1978; Geiser and Sansone, 1981). In an

example of salt-detached folds from the South Pyrenees,Sans et al. (2003) derive a LPS shortening estimated atbetween 16 and 23% greater than the folding relatedshortening. Late-stage secondary porosity commonlydevelops along sub-vertical LPS features, making themefficient conducts for vertical fluid migration in mostcarbonate reservoirs (Van Geet et al., 2002; Ferket et al.,2004).

Joints and related LPS structures are generally interpretedto develop prior to folding, when the bedding is stillsubhorizontal. However, some authors (Averbuch et al.,1992) have shown that LPS occurs more frequently near faultramps, suggesting either a peculiar concentration of the LPSwhere the structure will develop subsequently, or that theformation of LPS extends into the early stages of folding.Some models, like the trishear folding model (Erslev, 1991;Hardy and Ford, 1997; Allmendinger, 1998; Grelaud et al.,2002), assume that a thickening resulting from a beddingparallel pure shear occurs in the early stages of folding in theforelimb as well as above the ramp (backlimb) of fault bendfolds (see Fig. 8 for schematic description).

These are eventually reactivated during subsequentfolding episodes where they can contribute to the formationof secondary porosity. LPS surfaces are likely to act aslocalized vertical conduits to fluids, and are frequently filledwith hydrocarbons (Ferket et al., 2003a and 2003b, 2004;Dewever et al., in press). Although such LPS structuresremain relatively diffuse in massive platform carbonates,they are commonly associated with orthogonal tension jointsthat form parallel to the main horizontal stress. These jointscan also contribute to vertical fluid migration, if they escapecementation, provided that their present orientation isfavourably oriented with respect to the modern stressregime.

Craddock and Van der Pluijm (1989) have shown the largescale effects of the deformation in the Paleozoic Appalachian-Ouachita foreland, where the mechanical twinning in calciterecorded a horizontal shortening due to the development ofthe Alleghanian thrust system. This result was confirmed bySun et al. (1993), Cioppa et al. (1999, 2000, 2001, 2003 and2004), Lewchuk et al. (1998), who measured typical LPSmagnetic fabric and suggested spatial and temporal relation-ship between the development of internal deformation, theacquisition of chemical remagnetization and the circulation oforogenic fluids.

2.4 Syndeformational Reservoir Alteration

2.4.1 Selective Synkinematic Reactivationof Foreland Structures

The current aperture and permeability of microstructures thatdeveloped during the foreland evolution are controlled bynumerous parameters, including:

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– their initial cementation and secondary porositycharacteristics;

– their current position in the folded structures (i.e., in theouter arc or instead in an inner arc position, in theforelimb or in the backlimb of the fold; Sassi and Faure,1996; Mercier et al., 1997; Frizon de Lamotte et al., 1997,1998; Saint-Bezar et al., 1998; Grelaud et al., 2000);

– their orientation and spacing relative to the modernprincipal stresses (Guiton, 2001; Guiton et al., 2003aand 2003b);

– the amount of tectonic uplift and coeval erosion, resultingin a cooling of formation waters and local developmentof secondary porosity (i.e., synkinematic developmentof vuggy porosity, Swennen et al., 2002; see alsoSection 6 of this paper).

Figure 4

Late Cretaceous carbonate outcrop along the eastern flank ofthe Kremenara anticline (Ionian Basin, Albania, see locationin Figure 1c), outlining the selective reactivation of LPSjoints in the outer arc of the fold.

Among a specific family of microstructures, characterizedby a given strike and dipping atitude and spacing, only a

fraction of the pre-existing structures are commonlyreactivated, indicating that the density and spacing ofreactivated structures are more diffuse than those inheritedfrom the foreland autochthon. This is well documented forthe synfolding reactivation of LPS planes in the upperCretaceous carbonates of the Kremenara anticline in theIonian basin in Albania (Van Geet et al., 2002; Fig. 4):

– the initial spacing of inherited structures is approximately5 cm. Most of the LPS planes remain cemented anduseless in terms of fluid transfer and secondary porosity;

– in contrast, the spacing of reactivated planes is approxi-mately 20 cm. A quarter of the currently open andreactivated LPS planes are open outer arc high-anglefissures with a vertical offset of 2 to 5 cm.

2.4.2 Outer Arc Fractures

Reactivated LPS structures observed in Albania and Pakistanmimic the expected distribution of incipient outer arcstructures (van Geet et al., 2002; Benchilla, 2003), whichchallenges the previous interpretation of similar open fracturesets in other foothills settings, especially where no nearbyfootwall analogue was studied to determine the pre-foldingdistribution of microstructures.

Therefore, a good understanding of the kinematics andmodes of folding is required to specify or predict the timingand distribution of individual outer arc structures, especiallywhen there are indications for a progressive migration of foldhinges (Mercier et al., 1997; Saint-Bezar et al., 1998).

Regardless of whether these outer arc features are initiatedduring folding episodes (Frizon de Lamotte et al., 1997,1998; Grelaud et al., 2000) or progressively reactivated pre-existing LPS joints (this paper; Fig. 4), they contribute to thesecondary porosity and enhanced vertical permeability, thusallowing rapid vertical fluid migration near the crest ofanticlines. However, this is not always a benefit in terms ofthe total-production characteristics of the reservoir, as it oftenresults, as in Albania, a geometry that contributes to earlywater production and lower petroleum recovery factors.

2.4.3 Synfolding Pressure-Solution

Another point relates to the occurrence and distribution oflocalized pressure-solution surfaces that develop in an innerarc position, at a high-angle to the bedding, in response to thecurvature of folded beds (Mercier et al., 1997; Saint-Bezar etal., 1998; Fig. 3, stage 6). Unlike the bedding-parallel andLPS stylolitic planes, which developed in the foreland, thesemicrostructures are very localized in the inner arc portion ofkinks and folds. Often, they are not interconnected at thescale of a bed, which isolates them from any further role invertical or lateral fluid migration. In contrast, they contributeto reservoir damage due to increased compaction andcementation.

PreexistingLPS planes

ReactivatedLPS planes

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2.4.4 Fore-Bulge versus Synkinematic Karstification

Karstification of carbonates commonly occurs at the end ofthe passive margin stage, when the foreland is uplifted andexposed in a fore-bulge position, prior to its renewedsubsidence and burial in the foredeep basin (i.e. in the Potwarbasin in Pakistan; Benchilla, 2003; and in the CordobaPlatform Mexico, Ferket et al., 2003, 2004; Ortuño-Arzateet al., 2003; Fig. 2). Alternatively, karstification can occursynchronously with folding and thrusting at the crest ofgrowth folds, thus post-dating the episodes of maximumburial in the foredeep basin (i.e., in the Kremenara anticlinein the inverted Ionian Basin in Albania, van Geet et al., 2002;Fig. 2). Both early and late meteoric karsts develop alongformer rock heterogeneities such as paleo-sols, paleo-karsts,bedding planes, stylolites and fractures. Its intensity isdependent on the relief created and the time-period duringwhich meteoric water infiltration may occur. Many of thesekarsts conduits become infilled or cemented and are thus nolonger important for porosity and permeability enhancement.However, since these strata are often reactivated by late stagediagenesis (e.g., hydrothermal karstification; Nielsen et al., inpress), they may become important fluid conduits. In theCordoba Platform-Veracruz Basin (Mexico), exposureduring the Campanian led to erosion and karstification of theupper Cretaceous platform strata. Some of these pre-Laramide karst systems show oil-stains indicating that theyserved as conducts for oil migration (Ortuño-Arzate et al.,2003; Ferket et al., 2003, 2004).

2.5 4D Evolution of the Fracture-Porosity/Permeability Characteristicsfrom the Foreland to Frontal Anticlines

Major tear faults are likely to control the current com-partimentalization of both sandstone and carbonate reservoirsat seismic and regional scales, i.e. at the scale of an anticlineseveral kilometers long. Frequently, tear fault systems resultfrom a selective reactivation of foreland structures, thatoriginated as pre-existing normal faults, or eventually one ofthe sets of LPS conjugate strike slip systems. As such, tearfaults provided a protracted control on fluid migration, whichis likely to progressively modify the overall reservoircharacteristics of nearby strata, from their initial developmentwithin the foreland autochthon, until their currentremobilization within the allochthon. However, their preciseinfluence of the hydrodynamics of a given prospect is noteasily documented without access to production data.

In contrast, interconnections between the rock matrix andfracture systems are easier to document at outcrop andreservoir scales. We have summarized the main results of ourfield observations in the block diagrams of Figure 3. Thisfigure displays the progressive evolution of the total porosityand permeability of fracture systems and stylolitic planes,

starting from the foreland, to the footwall of the frontalthrust, and finally in the accretionary wedge carbonatereservoirs and anticlinal prospects.

These features help understanding which type of micro-structure is likely to enhance horizontal or vertical fluidmigration during each of the successive tectonic episodes.Clearly, only the last, current stage would be of direct interestfor the modeling of production and injection behaviour, butother features, formed in earlier time intervals are alsoimportant in order to understand and predict the petrophysicalproperties of the reservoir rocks, as these can have influencedfluid migration and diagenesis in the vicinity of previouslyopen microstructures. Furthermore they may becomereactivated. In a number of studies, we could document theevolution of fluid migrations through the fracture system,using crosscutting relationships between these different setsof structural elements (Van Geet et al., 2002; Ortuño-Arzateet al., 2003; Ferket et al., 2003a and 2003b, 2004).

3 MAJOR TRENDS OF REGIONAL FLUID FLOWAND PORE FLUID PRESSURE EVOLUTION IN FFTB

Local core or outcrop observation and the distribution ofcemented versus open fractures provide information onwhether the reservoir acted as a closed or an open system tofluids. However, basin-scale modeling is also required to geta more regional understanding of fluid migration, and toknow both at what time tectonic stress or rising topographyof the foothills are likely to have reorganised previouscirculation regimes inherited from the former passive margin,and the composition of the circulating fluids (marine fluidsversus meteoric water or hydrothermal brines).

Numerous papers have described the fluid flow regimeassociated with dewatering processes in modern accretionarywedges and seismically active areas (Moore et al., 1991; MuirWood, 1994; Henry, 2000), as well as paleo-fluid flow inPaleozoic and Alpine orogens (Oliver, 1986; Yao andDemicco, 1995, 1997; Muchez et al., 1995, 2000, 2002; Travéet al., 1998, 2000 and 2004; Roure and Swennen, 2002;Swennen et al., 2003a, 2004). Numerical simulations havebeen used recently to account for the topographic gravitydriven regional fluid flow currently observed in foreland fold-and-thrust belts (Garven, 1985; Ge and Garven, 1989, 1992and 1994; Nesbitt and Muehlenbachs, 1994; Bachu, 1999).

For the first time however, we could use a coupledkinematic-basin modeling tool developed at IFP, namelyCERES (Schneider et al., 2002, 2004; Schneider, 2003), toreconstruct both the fluid flow and pore fluid pressureregimes along two regional transects across the EasternVenezuelan foothills and the Canadian Rockies betweenBanff and Calgary, taking into account not only the moderntopography, but also the former thrust kinematics andcompeting sedimentation and erosion processes.

78


When reconstructing the pressure regime and the fluidflow history in the Oligocene sandstone reservoir of theEl Furrial structure in Eastern Venezuela and in theMississippian dolomites of the Canadian Foothills, twocritical periods can be identified, the first at the beginning ofthe flexuring deformation, and the second time whenthrusting was active (Schneider, 2003).

Thrust kinematics are no longer active in the CanadianFoothills, the frontal thrusts being dated as late Cretaceousuntil Paleocene-Early Eocene in age, whereas folding andthrust deformation lasted from the Neogene until Plio-Quaternary times along the El Furrial-Pirital thrusts inEastern Venezuela. Noteably, current fluid flow regimes arealso quite distinct in Eastern Venezuela and the CanadianRockies, fluid flow being exclusively topographic gravitydriven in the North American Cordillera, whereas it is stillpartly controlled by thrust motion and layer parallelshortening in the still currently tectonically active Caribbeanthrust belt (Roure et al., 1994).

3.1 Vertical versus Horizontal Fluid Migrationin the Foreland

Before the onset of thrust loading and the development of theforeland basin, fluids within reservoir rocks are usually closeto chemical equilibrium with the sediments. Basinal fluidsare continuously expelled toward the surface duringsedimentary burial, coeval compaction and dewatering,unless a local disequilibrium occurs between the rates ofsedimentation and compaction, and the resulting values of

vertical permeability. As mentioned previously, overpressureand hydraulic fracturing can develop locally beneath efficientseals during these early stages of basin evolution, even beforethe onset of thrust belt development.

Subsequent episodes of thrust loading will induce aregional tilting and progressive development of the forelandmonocline with the deposition of synflexural sediments.Fluids are then rapidly expelled laterally from the foredeeparea toward the foreland during a so-called “squeegee”episode (Machel and Cavell, 1999). In the footwall of thefrontal thrust and in the foredeep basin, fluid flow ischannelized along the stratification of reservoir conduitswhere it can reach velocity of tens of kilometers per Ma.The squeegee episode of fluid expulsion stops when thegiven reservoir unit is detached from the foreland autochthonand becomes tectonically accreted into the frontal anticline(Fig. 5).

3.2 Hydraulic Heads and Developmentof Overpressures

From the time of deposition to the onset of reservoir tilting,overpressure is mainly controlled by the dewatering processassociated to compaction, the maximum principal stress beingthen vertical. During subsequent episodes of compression,maximum principal stress becomes horizontal in the foredeepbasin. Layer parallel shortening (LPS), increase ofsedimentation rate in the foreland and tectonic loading in thehinterland may change the previous equilibrium toward anincreased overpressure.

79

Run: startFile.cereAge: 0 MaX: length in m Y: depth in mLithology

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CERES fluid flow simulations along the Banff-Calgary SUBTRAP transect in the Canadian Rockies and adjacent foreland. See Figure 1f for location.Present-day fluid flow pattern along the transect, as computed by CERES. Notice the forelandward fluid escape in the footwall autochthon (a); thereverse flow observed in shallow Cretaceous sandstones of the foreland (b); and convection of meteoric water in the hinterland (c).


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Fluid flow velocity versus time diagram in Mississippian reservoirs for two model wells (PW1 and PW2) currently located in the foothills.Notice the rapid changes inferred in fluid velocity in Mississippian reservoirs when entering in the foothills area (squeegee episode), andthe subsequent closure of the system after thrust displacement (for further detail, see Schneider, 2003, and Faure et al., 2004).

Figure 5b

Present distribution of overpressures across the transect.


Figure 6

CERES fluid flow simulations along the Eastern Venezuela SUBTRAP transect. See Figure 1a for location. a) Present-day distribution of HCaccumulations. CERES petroleum maturation and migration modeling accounts for the southward escape of HC toward the Orinoco foreland,trapping in the El Furrial field, but also for HC occurrences in the Prital allochthon as well as in shalloow prospects of the Serrania delInterior. Seemingly, results of CERES simulation outline the possibility for trapping HC in the lower Cretaceous Barranquin sandstoneformation beneath the Querecual source rock horizons of the El Furrial trend. b) Fluid flow velocity versus time diagram in the OligoceneMerecure reservoir of the El Furrial field. Velocity values above zero indicate a transient circulation episode toward the north, at a time theEl Furrial structural closure was already achieved in the south, but when the reservoir was still connected lateraly with the Pirital fluidconduits. Notice the main changes observed in the velocity of the fluids circulating toward the south, with a first squeegee episode followedby a recharge of the reservoir from the lower Cretaceous Barranquin aquifer of the Pirital allochthon (for further detail, see Schneider, 2003;Schneider et al., 2004).

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Such episode of syncompressional pore fluid pressurebuildup can account for the development of hydraulicfracturing observed in the Neogene Carapita seal in theEastern Venezuelan section. After structural closure, thehydraulic system can become completely closed in reservoirunits of the frontal anticline and adjacent foothills. Thefossilized overpressure regime is still preserved in the ElFurrial trend in Eastern Venezuela (Fig. 6).

Alternatively, regional fluid flow can evolve again towardan open system after the end of thrust emplacement, beingthen controlled by the topography, as observed in theCanadian Rockies and adjacent foreland (Fig. 5).

3.3 Changes in Lateral Velocity of the Fluid Flowas a Response to Squeegee Pulseand Thrust Closure

Although CERES computations are only performed in 2D,we consider the resultant fluid flow velocity values aresignificant, assuming here that the overall structural grain inthe Rockies and Eastern Venezuela is grossly cylindrical.

During the so-called squeegee episodes, fluids are expelledlaterally from the footwall autochthon (subthrust units)toward the foreland with velocities ranging from a fewkilometer per Ma in the Canadian foreland to up to 10 km/Main Eastern Venezuela (Figs. 5 and 6; Schneider, 2003).

In Eastern Venezuela, deeper sedimentary units, namelythe lower cretaceous Barranquin Formation of the Piritalallochthon came locally in contact with shallower units, e.g.,Oligocene and late Cretaceous reservoirs of the adjacentforeland (the latter being currently tectonically accreted in theEl-Furrial unit), giving rise to a second squeegee episodewith velocities up to 20 km/Ma.

In contrast, the intensity of the lateral fluid flow remainslow before and after the squeegee episodes.

4 LPS CONTROLS ON RESERVOIR CEMENTATION

Compaction and interaction between adjacent quartz orcarbonate minerals can considerably modify reservoirproperties.

Fine-grained pelagic limestone layers constitute the bestlithofacies to study syn-compressional deformation structuressuch as Layer Parallel Shortening and coeval stylolitic joints,as the deformation is usually localized along regularly spacedsurfaces oriented perpendicular to bedding. However,sandstone and dolostone reservoirs can also be affectedduring compression by more penetrative deformation, i.e.more diffuse pressure-solution and recrystallisation at grainscale, causing drastic changes in their total reservoircharacteristics (Renard et al., 1997). Anisotropy of themagnetic fabric (AMS) proves to be a very efficient tool inboth sandstone and dolostone for tracing the records of LPS

deformation on core plugs collected from both wells andoutcrops. The same samples can be subsequently prepared asthin-sections oriented parallel to the greatest principal stress(σ1), thus allowing visualization and quantification of theamount of tectonic compaction.

4.1 Multiphase Quartz Cementationin Sandstone Reservoirs of Sub-Andean Basinsin Colombia and Eastern Venezuela

Roure et al. (2003) studied the influence of tectonicprocesses on total sandstone reservoir quality in EasternVenezuela and Colombia. In the Llanos and eastern foothillsof the Eastern Cordillera in Colombia, e.g., in the Cusianafield, the main reservoir is an Eocene fluvial quartz-arenite(Mirador Formation; Cazier et al., 1995; Graham et al., 1997;Warren et al., 1998; Warren and Pulham, 2001; Roure et al.,2003; Toro et al., 2004). In the Monagas fold belt in EasternVenezuela, e.g., in the El Furrial field (Carnevali, 1988;Roure and Sassi, 1995; Gallango and Parnaud, 1995), themain reservoir is a quartz-arenite (Oligocene MerecureFormation), lacking both feldspar and lithic clasts.

4.1.1 Origin of Silica

Oriented thin-sections in these quartz-arenites show pressure-solution structures at the suture between adjacent detritalgrains. The spatial distribution of pressure relative to thebedding and to the main horizontal stress σ1 solution canalso be described (Fig. 7). Most silica derived from pressure-solution is likely to have been redeposited immediatelyin adjacent pores of the reservoir, generating quartz-overgrowths that progressively decrease both reservoirporosity and permeability.

However, recent studies of the aluminium content of thequartz-overgrowths within single sandstone layers of theOligocene Merecure sandstone away from the shaleyinterbeds, show a significant decrease of Al from the outerpart toward the central part of sandstone layers, thus pointingtoward a possible secondary source of silica derived from thediagenesis of shaley strata (Pagel et al., in press).

4.1.2 AMS and Fluid Inclusion studies:Dating the Main Cementation Episodes

AMS studies were first interpreted to record layer parallelshortening (LPS) in the Oligocene Merecure reservoir at theEl Furrial Carrito anticline in Eastern Venezuela (Aubourg,1999). In most wells of the foothills in Colombia, LPS hasbeen also observed in the Eocene Mirador reservoir, i.e., inthe Coporo well, and in the Cusiana, Chichimene and Apiayfields, but not at Tame (Robion et al., 1997).

Fluid inclusion studies and thermal modeling also helpedto date the main quartz-cementation episodes, whichoccurred mostly at the time of maximum burial (i.e. 4 km or

82


more) in the foredeep, when temperatures were between 110and 130°C (Bordas-Lefloch, 1999).

These sandstone reservoirs from the Sub-Andean basinswere compacted both vertically, by the load of the forelandsequence, and horizontally by tectonic stress (LPS) prior tobeing tectonically accreted to the allochthon. Layer-parallelshortening by pressure solution is the major source of silicain the underthrust foreland. Quartz cementation developsduring a relatively limited time interval that is infered to beno more than a few million years, when the reservoir isdamaged by the dual effects of LPS and deep burial (Fig. 8).

These Venezuelan and Colombian sandstones still havereasonably good reservoir characteristics, although they wereburied to great depths. Overpressure that developed in thesereservoirs as a result of rapid foredeep sedimentationprobably delayed compaction. Early carbonate cements mayhave also contributed locally to prevent compaction untilsecondary porosity developed as a result of the dissolution ofthis early diagenetic phase. Finally, development of structuralclosures and hydrocarbon trapping has resulted progressivelyin the modification of the hydraulic system, and theprogressive restoration of an overpressured regime, thussuppressing any local or exotic source of silica.

Most reservoirs display a dominant, single episode ofquartz-cementation characterized by a single homoge-neization temperature (Th) maximum that can be correlatedwith the onset of deformation (i.e., 110-130°C in Cusianaand El Furrial, representative of the late Andean-Neogeneburial and deformation), the Mirador reservoir in the Medinafield still records an earlier episode of cementation, at lowertemperatures close to 80°C, which is likely related to Eo-Andean-Paleogene burial and deformation (Fig. 9; Bordas-Lefloch, 1999; Roure et al., 2003).

4.1.3 Stable Isotopes Data and Originof the Circulating Fluids

No formation water has ever been collected in the El FurrialOligocene sandstone reservoirs, due to the very large oilcolumn and great depth of the structure. This prevents directaccess to the water-oil contact. Due to the small size of fluidinclusions in quartz overgrowth (typically less than 3 µm), itwas not possible to characterize directly the salinity ofpaleofluids in El Furrial reservoir using fluid inclusionstudies, as a result of poor control on the melting temperatureof ice crystals (Bordas-Lefloch, 1999). Therefore, it remainsconjectural whether quartz cementation in these Venezuelan

83

Figure 7

Oriented thin-sections in the Oligocene sandstone reservoir of El Furrial, outlining the 3D distribution of deformation structures (pressure-solution, microcracks) with respect to the main principal stress direction (σ1, grossly parallel to K3 axis as defined by AMS measurements):a) Vertical section orthogonal to S0 and to K3 axis. b) Horizontal section parallel to S0 plane, and orthogonal to K1 axis.

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sandstone reservoirs occurred during episodes of lateral fluidmigration involving dominantly basinal fluids, or if itoccurred coeval with the topographically-induced flow ofmeteoric water.

Most quartz overgrowths identified in El Furrialsandstones belong to a single event, but minor secondaryovergrowths can be observed locally. Fluid inclusions in thelatter are characterized by the same Th distribution as thedominant quartz cement. These two generations of quartzcements were also sampled for δ18O isotope studies usinglaser-probe investigations (Pagel et al., in press). This studyshowed that the main quartz cementation episode involved aformation fluid that was equilibrated with the host Oligocene

sandstone, with a signature typical of the basin. δ18O valuesin the second generation of quartz overgrowths weresignificantly shifted from these host-rock buffered values,indicating a distinct origin of coeval circulating fluids,assuming both quartz cementation episodes occurred atsimilar burial-temperatures. CERES software fluid flow hassubsequently simulated the possibility of recharging the ElFurrial Oligocene reservoirs with formation waters fromthe lower Cretaceous Barranquin aquifer that occurs inthe Pirital hanging-wall unit (Fig. 10; Schneider, 2003;Schneider et al., 2004). Therefore, it is assumed that thequartz cementation events in El Furrial occurred during atleast two distinct episodes, the first when only lateral

84

Stage 2(LPS)

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Figure 8

Total distribution of magnetic fabric anisotropy in the El Furrial anticline (Venezuela). Notice that all sites from the anticline record the LPSsignature derived from their previous foreland evolution.


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Figure 9

Burial-temperature curves of Sub-Andean prospects and coeval dating of quartz-overgrowth (after Bordas-LeFloch, 1999, see location of thewells and field in Figure 1a and 1b). El Furrial (Venezuela) and Cusiana (Colombia) record a single quartz-cementation episode, coeval withthe Andean deformation and development of LPS. However, the Medina well (Colombia) also displays an earlier, lower temperaturecementation episode, likely related to Paleogene Laramian deformation, at a time when the Mirador reservoir was less deeply buried, butalready affected by tectonic stress.


12 Ma (the Barranquin aquifer of the Pirital allochthon expells fluids laterally into El Furrial reservoirs)

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Figure 10

Tectonic scenario accounting for the development of LPS andquartz-cementation episodes in the Oligocene reservoirs in EasternVenezuela. a) The main episode damaging reservoir quality occursin front of the frontal thrust, in the foreland that is already affectedby tectonic stress and LPS, and which has reached its maximumburial. The isotopic signature of the main Q-overgrowth suggeststhe fluids where formation waters of the Oligocene succession. b) Alater episode of quartz-cementation is indicated locally by a second,and even locally, by a a third generation of Q-overgrowth, withdistinct isotopic signatures, which suggest an interaction with moreevolved fluids, most likely derived from adjacent Cretaceoussandstones from the Pirital allochthon.


intraformational fluid migration occurred, and the secondwhen tectonic uplift of the Pirital allochthon allowedrecharge of the Oligocene formations in the footwall by olderaquifers in the allochthon.

4.2 Mesodolomite and Syn-LPS Recrystallisationof Dolostone Reservoirs in the Foreland Beltand Basin of the Canadian Cordillera

In major parts of the Foreland Belt and Basin of the CanadianCordillera, some of the best reservoir strata occur indolomitized Devonian and Mississippian strata. However,reservoir characteristics are highly variable in the dolostones,and depend on crystal fabric relative to initial dolomitisationprocesses and/or recrystallisation and secondary porositydevelopment:– Pervasive fine to medium crystalline dolostones can be

inferred to have an eogenetic origin, being controlled bypaleo-environments, and therefore, they are not relevant toFFTB processes and evolution.

– Two other types of dolostones resulting from late burialevolution occur in Cordilleran Foreland Belt, i.e. themesodolomites and zebra dolomites, which are bothdiscussed below.

Numerous wells penetrating the autochthonous forelandhave been sampled, as well as surface outcrops in thefoothills, allowing a comparison to be made betweenPaleozoic carbonate reservoirs in portions of the foreland thatare located at various distances from the thrust front, inreservoir strata that are now tectonically accreted into theallochthon.

4.2.1 Petrographic, Isotopic and Magnetic Characteristicsof Mesodolomites

Mesodolomite is a general term that describes medium tocoarse (50-1000 µm) crystalline dolomitic lithologies. Thesedolomites are clearly less porous than the others, with theexception of lithologies where a secondary macroporosityexists. Typically, an increase in non-planar crystal boundariesoccurs with increasing crystal size (Fig. 11). Based on theirextinction pattern under crossed polarizers, the internalstructure of the mesodolomite crystals is composed ofdifferent sectors, explaining the mosaic extinction pattern.Locally some smaller sucrosic dolomite crystals float withinthe mesodolomite crystals. These fabrics point towards arecrystallisation origin of the mesodolomites, as advocatedby many authors.

However, fabric replacive mesodolomites, as indicated bythe preservation of relict structures from the original strata(e.g. oolites, bioclasts, etc.) also exist. In most of thesedolomites, carbon isotope ratios appear to be buffered by theprecursor carbonates (Cioppa et al., 1999; Al-Aasm andPackard, 2000). δ18O-values are always rather depleted and

Figure 11

Thin-section illustrating the strong recrystallisation ofmesodolomite from the Alberta foreland, in an area whereAMS data clearly document the imprint of LPS.

often plot below –8‰ versus PDB. Often the coarsest fabricspossess the most depleted oxygen isotopic signatures. Theisotopic variations of the mesodolomite often show acorrelated trend between oxygen and carbon isotopes (Al-Aasm and Lu, 1996; Al-Aasm et al., 1996; Cioppa andSymons, 2000). Mesodolomites show higher 87Sr/86Srisotope ratios than coeval seawater.

Cioppa et al. (1999) identified a correlation of magneticgrain size and isotope signature in the mesodolomites.Chemical remagnetization has overprinted the primaryPaleozoic magnetisation of the reservoirs as far as 150 kmaway from the Front Ranges, with polarities reverse in thefoothills, and both reverse and normal in Interior Plains,showing a diachronous acquisition of the magnetization incarbonate rocks (Enkin et al., 2000; Cioppa et al., 2001, 2003,2004; Robion et al., 2002, 2004). This dolomite possesses alate Cretaceous to Paleocene paleomagnetic remanence,pointing towards a dolomitisation and/or recrystallisation

100 µm

87


event during Laramide deformation (Symons et al., 1998,1999; Enkin et al., 2000

incidence and importance of tectonics and natural fluid ......foreland fold-and-thrust belts (fftb),...

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