Subthrust Reservoir Appraisal

SUBTHRUST RESERVOIR EVALUATION

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SUBTHRUST RESERVOIR APPRAISAL

François ROURE, Institut Français du Pétrole; RudySWENNEN, Katholieke Universiteit Leuven;David G. HOWELL, US Geological Survey

Abstract. Foreland fold-and-thrust belts comprise the world’s largest petroleum reserves. However,due to their structural complexity, foothills areas constitute a frontier for the exploration. Apart ofimaging the structural closures, reservoir quality constitutes the dominant risk for the exploration infoothills areas and deep subthrust prospects.Main processes involved in the damaging of individual reservoirs and specific conditions resulting inthe preservation or secondary development of reasonably good porosity/permeability characteristics insubthrust environments have been documented. They will be compared and discussed here on the basisof representative case studies in Venezuela and Colombia for sandstone reservoirs, and in Albania,Mexico and Pakistan for carbonate reservoirs.Thus, during sedimentary and tectonic burial, both carbonate and sandstone reservoirs are rapidelymodified by compaction, pressure solution and fluid-rock interactions, and the understanding andprediction of these issues require careful multidisciplinary approach. Regional structural cross-sectionsare first compiled and balanced, the resulting restored sections being subsequently used as initialsections for forward kinematic and thermal modeling. This modeling approach in turn provides realisticburial-temperature curves for individual reservoirs, the later curves being then compared with thetrapping temperatures measured on fluid inclusions, resulting in a good dating of the main diageneticcements. Knowing the time span and temperature conditions which prevailed during specificcementation events of subthrust prospects, it becomes now possible touse forward diagenetic modelsbased on the kinetics of fluid-rock interactions to provide realistic estimates on the residual porosity ofdeep reservoirs.The main damaging episode of subthrust sandstone reservoirs indeed relates to layer parallel shortening(LPS) and coeval quartz cementation developing in the footwall of the frontal thrust. Althoughpaleoenvironment remains the main control for carbonate reservoir, synkinematic hydrothermal brinesare also likely to develop secondary porosity in relation to coeval dissolution and/or dolomitisation.Finally, overpressures account also for a delay in the compaction and helps preserving good reservoircharacteristics, hydraulic fracturing episodes ultimately predating the onset of LPS and tectoniccompaction.

1- INTRODUCTION

Foreland fold-and-thrust belts (FFTB) housethe world’s largest petroleum reserves.Because of their structural complexity,however, many FFTB’s remain frontierexploration plays. Historically, initial drillingtargets in the mid or late 1800’s and early1900’s were based on the distribution ofsurficial seepages; these settings correspondedeither to shallow anticlines near the thrust frontor to stratigraphic traps, up-slope along theregional foreland flexure. Beginning in the1950’s, seismic surveys and other geophysicaltechniques imaged sub-surface geometries,leading to major discoveries in deep structuralprospects. Numerous subthrust hydrocarbon-bearing reservoir horizons have been identifiedin the Austrian Alps, in the Carpathians (at

Lopushnia), and in the Southern Apennines (atTempa Rossa ; 3). More recently, majordiscoveries in the Sub-Andean foothills,Cusiana and Caño Limon in Colombia, ElFurrial in Venezuela and Camisea in Peru,have renewed interest in the exploration ofFFTB (7, 8 & 9).Methane and liquid hydrocarbons arepartitioned at depth largely as a consequenceof varying pressure – temperature (P-T)stabilities and modes of origin. Commercialproduction of methane gas occurs as deep as 6km, and future giant hydrocarbon discoveriesare certainly expected to be deeper. Themaximum depth of commercial targets iscontrolled by diminishing economic returnsowing to increasing drilling costs anddecreasing reservoir’s porosities anduncertainty of preservation stabilities for


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hydrocarbons. Thus, in most FFTB explorationstrategies, the reservoirs quality plays a majorpart in assessing exploration risk. Duringsedimentary and tectonic burial, bothcarbonate and sandstone reservoirs are rapidlymodified by compaction, pressure-solution andfluid-rock interactions. To evaluate theseissues, careful multidisciplinary team work isessential.

2- GEODYNAMIC SETTING andECONOMIC PERSPECTIVES

Geochemists have spent many yearsdeveloping techniques that correlate sandstonereservoir characteristics with maturation stagesof the organic matter (porosities versus Rocurves). Recent exploration in a forelandsetting - North Slope, Alaska - and in thefoothills foldbelt itself - Cusiana in theColombian Andes - have shown that thetexture and composition of the detritus playsalso a major role in controlling the nature ofdiagenesis. The effects of maximum horizontalstress (σ1) compacts strata in footwallreservoirs in areas where oil could possiblydrain from a reservoir- i.e. along décollementlevels and frontal ramps in thrust systems.Ironically, in some areas where fluid is unableto escape, secondary porosity will increasefollowing overpressuring and fluid-fracturing.

Basin-scale fluid circulations can also controlthe diagenesis of carbonate reservoirs. Theeffects of these fluids reflect distinct stages asthe sedimentary basin is transformed into aFFTB, evolving from passive margin (pre-orogenic stage) through episodes of flexuralsubsidence of the foredeep basin, and duringtectonic uplift in the foothills area. Recentinvestigations and « in situ » measurementsevince fluid flow and dewatering processes in

active accretionary wedges, e.g. convergentsubmarine margins settings of Barbados andCascadia, in emerged FFTBs of the WestCanada Basin and adjacent foothills, and theLlanos of Colombia and Eastern Venezuela.The study of calcite and quartz veins andcements provides direct evidence of past fluidcirculations. Hydraulic, crack-and-seal featuresdocument episodes of overpressuring andperiods of pressure release. Additionalinformation on the history of fluid circulation,timing of successive diagenetic phases, anddevelopment or destruction of porosity can beachieved by studying diagenetic phases bycathodoluminescence and fluorescencepetrography, oxygen and carbon isotopegeochemistry, trace element analyses and fluidinclusion microthermometry (11). Theintegrated research described below shoulddecrease economic risk associated withexploration in foreland fold-and-thrust belts.

3- INTEGRATED APPROACH forSUBTHRUST RESERVOIR

APPRAISAL

Prior to their tectonic accretion in theallochthon, most rock units were either part ofthe passive margin sequence of the foreland orpart of a synflexural sequence within theforedeep. Potential reservoirs are likely torecord successive episodes of compaction anddiagenesis, related to their pre-orogenic,synflexural or synkinematic stages,respectively.

Figure 1: Structural section compiled from subsurface data across a reference Sub-Andean Basin.


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3.1- Forward kinematic modelling andreconstruction of the burial curves

For selected basins located in forelandand foothill setting, surface and subsurfacedata (wells and seismic) are used to compileregional structural sections. Initial passivemargin geometries are restored by means of2D-balancing techniques (Figs. 1 & 2).Forward balanced cross-sections are thenreconstructed with the Thruspack tool to showinstantaneous geometries along transects,representing times from the onset of thedeformation to the Present (8, 9 & 12; Fig. 2).This structural modelling helps to identifypotential migration pathways for both meteoricand diagenetic fluids. The models displayphysical connections between major thrustplanes and the geometries of potentialreservoirs and seals. By integratingsynkinematic erosion and sedimentationprocesses, coupled with various paleo-thermometers (Tmax, Ro, apatite fissiontracks), temperature-depth burial curves aredeveloped for prospective reservoir intervals.These reconstructions are valid for horizonslocated in the autochthonous foreland, insubthrust horizons or in the allochthon itself.

Figure 2 : Structural modeling of thesame Sub-Andean Basin. Structure sectionsa (present day) and e (initial, pre-orogenic)

3.2- Petrographic, petrophysic, fluidinclusion and magnetic anisotropy

studiesSandstone and carbonate horizons in

the foreland provide a reference for reservoirstrata prior to their underthrusting beneath theallochthon. Reservoirs sampled in theallochthon provide the other end-memberreference - reflecting fluid-rock interactionsand effects of maximum burial in the footwallof the frontal thrust. Petrographic analysis(including cathodoluminescence), stableisotope and fluid inclusion studies constrainreservoir characteristics - primary andsecondary porosities, microfractures, and howfluid-rock interactions control the timing anddegree of diagenetic alteration.

In the foothills, reservoirs are studiedat different depths and in different structuralpositions - along the conjugate flanks and crestof a single anticline or at discrete distancesfrom a major fault or thrust plane - in order tocompare the respective effects of gravitationalcompaction and stress gradients. Part of thisenquiry is to identify the presence or absenceof natural conduits for the dewatering. In theforeland, reservoir are also studied at differentdepths, at discrete distances from the thrust-front - in the footwall of the frontal thrusts andin areas sufficiently far from the foothills toprovide a reference for the pre-compressionalhistory - to document the long term effects oftectonic loading and the development of LayerParallel Shortening (LPS).

Where possible, strata from a singlereservoir are sampled beneath and above theoil-water contact. This helps to demonstratehow hydrocarbon migration affected thediagenesis and residual porosity of thereservoir. Where possible the composition offormation waters is compared with thediagenetic phases and fluid inclusions, both inthe foothill and foreland parts of the FFTB.This allows assessing the state of present-dayequilibrium between rock constituents and theformation waters. Finally, analyzing themagnetic fabric (anisotropy of magneticsusceptibility or AMS) and studying thin-sections oriented parallel to the main transportdirection help to document the deformationhistory of the rock matrix in clastic reservoirs(1 & 2).

reflect classic section balancing techniques.Intermediate geometries (b, c and d) come

from forward kinematic modelling using theThruspack numerical tool.3.1- Forward

kinematic modelling and reconstruction ofthe burial curves


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3.3- Thermal modelling

Analysis of heat flow data insedimentary basins generally assumes simple

conductivity, constrained by maturity data(e.g. vitrinite reflectance) from source rocksand the present distribution of the isotherms(BHT, bottom hole temperatures; Fig. 3).

Figure 3: Results of thermal (a) and source-rock (b) maturation modelling applied to setof cross-sections from the same Sub-Andean Basin.

Applying this technique, however, insome instances produces large discrepanciesbetween the inferred palaeotemperatures andthe palaeotemperatures measured from thefluid inclusions of quartz and carbonate(calcite, saddle dolomite, ankerite) cements.This indicates to us that the conductive modelis too simple and that locally advection of heatis caused by fluid migrations (2, 4 & 6). Thus,it is important to model basin architectures inlight of fluid circulations and how the lattercan modify hydrocarbon stabilities duringmaturation, migration and entrapment.

4- CRITICAL PARAMETERSCONTROLLING the OVERALL

RESERVOIR CHARACTERISTICS inFORELAND FOLD-and-THRUST

BELTSBoth sandstone and carbonate

reservoirs have been studied in foreland andfoothills settings.

4.1- Sandstone reservoirs

Sandstone reservoirs reflecting anoriginal environment of depositioncorresponding to a former passive margin aswell as synflexural and syntectonic clasticreservoirs have been studied in Venezuela(Oligocene series) and in Colombia(Cretaceous, Paleocene and Eocene series).The magnetic fabric (AMS) and thin-sectionsoriented parallel to the tectonic transportdirections indicate that parts of the El Furrial(Venezuela) and Cusiana (Colombia)anticlines preserved a layer parallel shortening(LPS) component that was produced during anearly episode of deformation. This horizontalpressure-solution formed while the reservoirswere still a part of the foreland architecture inthe footwall of the FFTB (2; Fig . 4).


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Figure 4

We have also learned frommicrothermometric studies of fluid inclusionsand thermal modelling that quartz cementationoccurred in the footwall of the frontal thrustwhile the strata still belonged to theautochthon. Nonetheless, strata in thisconfiguration were also subjected to horizontalcompaction (LPS) within an open system andexposed to regional fluid flow, thus accountingfor the transport of silica (Figs. 5 & 6).

4.2- Carbonate reservoirsBoth platform and basinal carbonates

of the former passive margin have beenstudied in Pakistan (Eocene series), Albania(late Cretaceous and Paleogene), and Mexico(Middle and late Cretaceous). Unlikesandstone reservoirs, in carbonate strata thenature of the depositional environment is themost critical parameter controlling thecharacteristics of the reservoir rock matrix.Thus, our objective was to document theevolution of the FFTB and to determine theprocesses which may improve or degradereservoir characteristics during thrusting. Forexample, secondary dolomitization,hydrothermal karst development, anddeformation features such as stylolites, jointsand faults can enhance porosity or secondaryporosity development along stylolites (11),while cementation, burial and tectonicaccretion of carbonate units can also damagethe rock matrix, reducing porosity.

Fgure 5 : Schematic diagram documenting the burial curve of Sub-Andean sandstonereservoirs, paleo-temperatures derived from thermal modelling, as well as trappingtemperatures measured from fluid inclusions, for the foreland autochthon, the frontalanticline and the allochthon, respectively.


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Figure 6 : Schematic diagram documenting the source of silica and the parameterscontrolling quartz-cementation of sandstone reservoirs in foreland fold-and-thrust belts

In this context, the effect ofoverpressuring during tectonic deformation isalso important since it may delay porositydestruction and maturation. The existence oftemporary overpressuring is attested by thepresence of hydraulic and crack-and-sealfractures.

The distribution of most fractures andstylolitic joints in carbonates is controlled bythe architectural development of the foreland.These early formed features can control thegeometry of folding and thrusting duringtectonic accretion and uplift (10 & 11).Understanding of the nature of these events atboth reservoir and regional scales is crucial forexploration strategies in the FFTB as fluidmigrations and the trapping of hydrocarbonsare likely to be coeval phenomena.

5- PERSPECTIVES andCONCLUSIONS

Analytical recordings (Tmax, Ro,stable isotopes, apatite fission tracks and fluidinclusions) and 1 or 2D modelling are thecalibrating tools that allow an explorationist toevaluate the burial and thermal history ofreservoirs (Fig. 5). However, severalparameters - specifically fluid flow and its rolein fluid-rock interactions and the effect ofoverpressuring - need to be constrained beforewe can reliably predict the characteristics ofprospective reservoirs.

5.1- Fluid-rock interactions

The following parameters need to bemore carefully studied:- The regional patterns of fluid flow. This

characterizes the efficiency of fluid-rockinteractions, and controls whether the

reservoirs behave as a closed, i.e. rock-buffered, or an open system.

- The elemental composition of thecirculating fluid is needed to calculate theequilibrium state with respect to the hostrock. In sandstone reservoirs, this isespecially critical for the Si-content that isso important in quartz-cementation. Incarbonates, pCO2, temperature and Ca-content are key parameters needed toassess whether dissolution or cementationwill occur.

- Regional pressure regimes and thedistribution of overpressured domains,which control the occurrences and natureof pressure-solution, hydraulic fracturesand sealing cements.

- Where overpressure zones can counteractthe negative effects of overburdencompaction.

Quantification of the water-flow

Thermal modelling generally only considersconductive heat transfers (Thrustpacksimulations). However, new numerical tools(e.g. the Ceres software) also incorporatecompaction and fluid flow in modellingstructurally complex domains. Basin-scalefluid flow values derived from 2D studies willalso be compared with results of 1D forwarddiagenetic models in order to control theregional mass-balance, and to calculate watervolumes necessary to account for observedfluid-rock interactions (e.g. the volume ofquartz-cements, high-temperaturedolomitization or the dissolution of carbonatein hydrothermal karst setting).


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Paleo-barometers and controls on thepressure regime

Fluid inclusion data provide the principalcontrol for calculating the salinity of paleo-fluids and homogenisation temperatures duringthe diagenesis. Because of a betterunderstanding of the thermodynamics ofcomposite and hydrocarbon-bearing fluidinclusions, it is now possible to determine thepressure conditions at the time of entrapment,provided the paleo-temperatures arereasonably well constrained by means of anindependant thermal modelling. Thus, fluidinclusions data should be integrated in studiesto reconstruct paleo-pressure regimes.

5.2- Deformation of the reservoirs

Magnetic fabric studies and analysis of thin-sections oriented parallel to transportdirections identify those parts of anticlinalstructures that preserve the signature of layerparallel shortening (LPS) - deformation whenthe reservoir strata resided in the footwallconfiguration. During thrusting and foldingany additional pressure-solution is likely tooccur only near the ramp-flat transitions (kink-axes). Most parts of a hangingwall are upliftedpassively during tectonic contraction. In suchcases the matrix characteristics are preservedfrom the earlier burial episodes in the footwall.

Opening of formerly generated joints, fracturesand stylolites can be predicted based on thestyle of deformation.

Reserve estimates

Intergranular porosity and fractures of varioustypes must be part of any equation used toestimate reserves for a basin and evaluate theprospects for any particular reservoir. In orderto calculate the various porosity figures, wehave tried to make the case that it is importantto know the history of the entire FFTB as wellas existing physical attributes of selectedportions of the FFTB. Futhermore, withrespect to reservoir performance, especially incompartimentalised reservoirs, it is importantto evaluate porosity enhancement in terms ofopen (reactivated) joints and stylolitic network.

Acknowledgements

This paper integrates some results of theSubtrap-I project (1996-1998), operated byIFP, which was sponsored by the followingcompanies : BP-Colombia, Chevron, Conoco,EAP, Enterprise, Exxon, Intevep, Lasmo,PetroFina, Premier Oil, Sipetrol, Shell, Total-CFP . In addition, Ecopetrol, IMP-Pemex,OGDC, OGI-Albpetrol have also contributedto this project as associate members.

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Subthrust Reservoir Appraisal