ration 89 (2006) 27–32www.elsevier.com/locate/jgeoexp

Journal of Geochemical Explo

Fluid flow record from fracture-fill calcite in the Eocene limestonesfrom the South-Pyrenean Basin (NE Spain) and its

relationship to oil shows

M.A. Caja a,⁎, A. Permanyer a, R. Marfil b, I.S. Al-Aasm c, T. Martín-Crespo d

a Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, Martí i Franquès, s/n,08028-Barcelona, Spain

b Departamento de Petrología y Geoquímica, Facultad de Geología, Universidad Complutense de Madrid, Avd. Jose Antonio Novais, s/n,28040-Madrid, Spain

c Department of Earth Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4d ESCET, Universidad Rey Juan Carlos, Edificio Departamental I, 28933-Madrid, Spain

Received 30 July 2005; accepted 25 November 2005Available online 15 March 2006

Abstract

The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin andevolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 andC2) were distinguished in the fractures of the Eocene Armàncies platform carbonates; C1 is characterised by fence-like crystals,exhibits dull red luminescence and contains abundant twin planes, inclusions and δ18O values that range from −6.2‰ to −4.8‰VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands,and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120°C. δ18O values range from−9.6‰ to −8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115°C. This wouldseem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing wascoeval with C2 and continued after calcite cementation was completed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Fracture-fill calcite; Geochemistry; Oil shows; Pyrenees

1. Introduction

Though the presence of many oil shows and theexistence of a good source rock (e.g., the ArmànciesFormation) have attracted interest from oil companies in

⁎ Corresponding author.E-mail addresses: miguelangel.caja@ub.edu (M.A. Caja),

albert.permanyer@ub.edu (A. Permanyer), marfil@geo.ucm.es(R. Marfil), alaasm@uwindsor.ca (I.S. Al-Aasm),tomas.martin@urjc.es (T. Martín-Crespo).

0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.gexplo.2005.11.009

exploring the southeastern Pyrenees (Martínez et al.,1997; Permanyer, 2004), they have yet to discover anyactual reservoirs. The Alpine Pyrenees consist of anarrow, asymmetrical, double-wedge mountain rangethat includes a relatively wide southern proforelandbasin and a northern retroforeland basin; the combina-tion of complex tectonic structure and basin evolutionmakes it difficult to apply successful explorationstrategies.

It has been widely accepted that large-scale fluid flowoccurs in sedimentary basins during tectonic deformation

28 M.A. Caja et al. / Journal of Geochemical Exploration 89 (2006) 27–32

(Moore and Vrolijk, 1992). Orogenic compression drivesfluid migration directly, and significant fluid flow maymigrate towards the foreland coeval with thrusting (Qingand Mountjoy, 1992). Fluids play an important role in theevolution of sedimentary basins because they transportheat, control diagenesis, induce mineral reactions; formore deposits and drive the migration and entrapment ofhydrocarbons (Dewaele et al., 2004). The faults and zone-faults in the foreland basins play a major role in draininglocal fluids that are expelled from the sediments. Thethrust-fault zones in the studied area are connected overlarge distances and also serve to drain external fluidsoriginating in more internal parts of the belt (Travé et al.,1997).

The presence of oil shows associated with fracturesprovides a significant opportunity to unravel the type,origin and evolution of fluids involved in the formationof fracture-fills and their relationship to oil migration. Inthis study, the timing of calcite precipitation and oilmigration will be considered within the geological con-text of the evolution of the Pyrenees Basin.

2. Geological setting

The intense tectonic deformation that affected thePyrenees is a consequence of the collision between theIberian and European plates. The south-eastern sector ofthe Pyrenees consists of a stack of south-vergent units thatinclude both sedimentary cover and basement (Martínez

Fig. 1. Schematic structural map showing the main units of the southeastern P

et al., 1997). Two distinct thrust systems can bedistinguished; the upper thrust is comprised of mainlyMesozoic sediments with the detachment fault in theKeuper format group, and the lower thrust comprises themajor Eocene units. The study area is located in the Cadíthrust sheet (Fig. 1); this thrust sheet consists of a verythick Lower–Middle Eocene series and Paleocene sedi-ments that are discordant on the basement (Martínez et al.,1997). The Lower–Middle Eocene sediments can bedivided into several transgressive/regressive cycles (Ver-gés et al., 1998). The increase in bathymetry of the basin isrepresented by up to 700mof carbonates,marls and shalesof the Armàncies Formation (Cuisian). These sedimentscrop out over a strike length of 100km, from Terrades inthe east to Bagà in the west (Fig. 1). The lower part of theArmàncies Formation (up to 220m) is characterisedby an important anoxic event (Permanyer et al., 1988;Permanyer, 2004) and the upper part by a carbonateplatform facies (Martínez et al., 1997; Vergés et al., 1998).The upper part of the Armàncies Formation crops out inthe Terrades area; there are many oil shows there that arerelated to fractures and vug porosity.

3. Sampling and methodology

A total of 25 limestone samples were collected fromareas characterized by both fractures filled with calcitecements and the presence of oil shows. These samplescome from three quarries in the Terrades area.

yrenees (modified after Vergés and Martínez, 1988) and the study area.

29M.A. Caja et al. / Journal of Geochemical Exploration 89 (2006) 27–32

Doubly-polished thin sections were prepared from eachsample and were examined using both conventionallight microscopy and cathodoluminescence petrographyunder the operating conditions of 12–16kVacceleratingvoltage, 300–350μA beam current and 0.2–0.1 Torrvacuum pressure. The chemical composition of thecalcite cements was determined by electron microprobeanalysis; the operatng conditions were 20kV accelerat-ing voltage, 15nA beam current and 10μm beam size.Fluid inclusion microthermometry was performed onthe second-stage calcite cement of 3 representativesamples using a Linkam THMSG 600 heating-coolingstage ( over a temperature range of −196 to 600°C) thatwas attached to an Olympus BH-2 microscope equippedwith a long focal distance lens (80×). The apparatus wascalibrated at low temperature by inclusion of pure CO2

in topaz (−56.6°C) and synthetic inclusions of purewater in quartz (0.015°C), and at high temperature bythe recognized standard melting temperatures providedby Merck. Stable oxygen and carbon isotope ratios wereobtained for thirteen samples of calcite extracted fromrock sections using a microdrill attached to a binocularmicroscope. The analytical procedure is described byAl-Aasm et al. (1990). C and O isotope ratios are givenin permil with respect to the Vienna Pee Dee Belemnite(VPDB) standard. Precision was better than 0.05‰ forboth isotopes. The seven oil shows present in thecollected samples were recovered by means of dichlor-omethane. The C15+ saturated fraction was analysed bygas chromatography and gas chromatography–massspectrometry in order to determine steranes isomerisa-tion degree and to calculate the temperature.

Fig. 2. (A) Plot of MgCO3 versus FeCO3 content of carbonate phases. Noteoxygen and carbon stable isotopic composition of calcite phases showing thecement filling fractures. Oxygen depletion calcite cement suggests higher preclimestones (after Hudson and Anderson, 1989; Veizer et al., 1999).

4. Calcite-cemented fractures

4.1. Studied tectonic structures

The upper carbonates of the Armàncies Formation areaffected by a network of fractures; these fractures arefilled with calcite cement. Cross-cutting relationshipsbetween cemented fractures are complicated and preventdistinction between the different generations. As thestudied quarries are currently being exploited, it was notpossible to perform a detailed structural study; however,most of the widest fractures are N–NE trending, dipsteeply from 55° to 72°, and are characterized by athickness of up to one cm, a length of several meters, andshow indications of the presence of oil. Other structures,such as small fractures (mm in thickness), en-echelontension gashes and vugs are filled with similar carbo-nates, albeit to a lesser degree.

4.2. Petrology and geochemistry

The carbonate platform facies in the ArmànciesFormation is composed of wackestone with foraminifera(primarily miliolids). The bulk rock micritic matrix dis-plays dark red luminescence and has an average chemicalcomposition of: (Ca0.980Mg0.010Fe0.001Mn0Sr0) (CO3)(n=3) (Fig. 2-A). δ18O values of carbonate host rockrange from −4.4‰ to −3.8‰, and δ13C values rangefrom −1.1‰ to −0.4‰ (Fig. 2-B). Intraskeletal porosityis occluded by a drusy mosaic of clean crystals of calcite(Ca0.977Mg0.020Fe0Mn0.001Sr0.002) (CO3) (n=5), showingdark and bright red luminescence.

the lowest Fe and Mg content of C2 calcite cement. (B) Cross-plot ofpossible fluid evolution from the marine host rock limestones to calciteipitation temperature for C2 than for C1. ⁎Range for the Eocene marine

30 M.A. Caja et al. / Journal of Geochemical Exploration 89 (2006) 27–32

There are two main generations of fracture-fillingcalcite cement (Fig. 3). The first fracture-filling stage (C1)occurs in the contact with the margins of the fracture, andis characterised by fence-like crystals (Ca0.980Mg0.012Fe0.006Mn0.001Sr0.001) (CO3) (n=18) (Fig. 2-A) that showdark red luminescence with disperse bright spots. Theδ18O values range from −6.2‰ to −4.8‰, and δ13Cvalues range from−1.4‰ to−0.4‰ (Fig. 2-B). The mosttypical and distinctive features of C1 are an abundance oftwin planes and inclusions. This cement is cloudy underthe microscope and orange in the field.

The second fracture-filling calcite cement (C2) appearsin the centre of the fracture, and occurs after the cemen-tation of C1. In contrast to C1, the C2 cement appearsclean and clear under the microscope, and is white in thefield. C2 cement is characterised by large blocky equi-dimensional crystals (up to 500μm), and occasionallyshows twin planes (Fig. 3). The mean elemental compo-sition of C2 is (Ca0.994Mg0.003Fe0.001Mn0Sr0.001) (CO3)(n=11). δ18O values range from −9.6 to −8.9, and δ13Cvalues range from −2.5 to −1.9 (Fig. 2-B). C2 crystalshave dark brown-red luminescence that alternates withbright yellow bands towards the outer zone. C2 cementdoes not usually occlude fractures, and the remainingporosity is commonly occupied by liquid oils.

4.3. Fluid inclusions petrography and microthermometry

Two types of fluid inclusions (hydrocarbon-rich andaqueous) were identified in association with the second

Fig. 3. Photomicrographs of fracture-fill calcite cement. (A) Optical microgracement. Note that C2 cement does not occlude the fracture, and the remainingformer view. Notice that C1 and C2 calcite cements display different petrog

generation of calcite cement (C2). Hydrocarbon-richinclusions (15 selected inclusions) have been found inisolation or in groups of fewer than three. They are ofidiomorphic and irregular crystal shape, and are locatedin the inner part of the calcite crystals, and are notassociated with fracture planes. There were no idiomor-phic crystals that followed growth planes parallel to thecrystal faces; however, petrographic and texturalfeatures suggest a probable primary origin. The salinityof the fluid inclusions could not be calculated becausethe last melting temperatures were not observed; again,this was because of their natural and extremely lowsalinity. The wide range of vapor filling (15% to 40%)and homogenisation temperatures (120 to 190°C) ofsome of these inclusions could be attributed to thestretching processes that follow fluid inclusion forma-tion. Even so, a 7 fluid inclusion population displaying avapor filling of 20–25% and a homogenisation tem-perature between 120 and 145°C, mode 120°C can beconsidered as representative values.

Aqueous two-phase fluid inclusions associated withfracture planes that crosscutmajor crystals and postdate theprevious fluid inclusions were observed; these should beconsidered secondary in origin. Eutectic melting tempera-tures (≈−50°C) indicate the H2O–NaCl–(CaCl2) system.Although the last melting of ice was not clearly observed,sudden movements of the bubble between −1 and 0°Cindicated a low salinity (b1.5wt.% eq. NaCl). Homoge-nisation temperatures range between 50 and 65°C, mode50°C, markedly lower than hydrocarbon-rich inclusions.

ph (plane polarized light) showing the two main generations of calciteporosity is filled with oil. (B) Cathodoluminescence photograph of theraphic and luminescence features.

31M.A. Caja et al. / Journal of Geochemical Exploration 89 (2006) 27–32

5. Compositional characteristics of oil shows

Oils in fractures and vugs are characterised by theoverwhelming presence of n-alkanes (C17 to C23); thisis consistent with an algal origin for the source organicmatter. The predominance of even n-alkanes over theodd n-alkanes for the C26 to C30 range can be attributedto the carbonatic character of the source and to theinfluence of the host rock (Permanyer, 2004).

Sterane isomerisation was used to calculate equiva-lent vitrinite reflectance of oil (Bein and Sofer, 1987).Equivalent vitrinite reflectance values range from 0.73%to 0.82%, and average 0.77% (n=7). The estimatedtemperature calculated using empirical calibrations(Barker and Pawlewicz, 1994) for the vitrinite reflec-tance equivalence average of 0.77% is 115°C.

6. Discussion and conclusions

Two types of fluids migrated through fractures in theArmàncies Formation. The first fluid type precipitatedcalcite cement under reducing conditions. In contrast,the second fluid precipitated calcite cement from fluidswith relatively low Fe and Mg content. The relativedepletion in δ18O in the C2 calcite cement (as comparedto C1) can be attributed to an increase in fluid tempera-ture (Emery, 1987); this suggests a deep circulation forthe second fluid. Within the geological context, the lowδ13C values of the calcite cements filling fractures couldbe attributed to the oxidation of organic matter.

The abundance of twin planes in the first calcitecement (C1) indicates intracrystalline deformation(Burkhard, 1993). This phase of calcite cement wasprobably precipitated from fluids that were expelledfrom thrust sheets into the foreland basin; this expulsionprocess can be attributed to tectonic loading during theAlpine orogeny (cf. Buschkuehle and Machel, 2002).The lack of twin planes in the second calcite cementsuggests precipitation in a different tectonic setting.Migration of fluids through the thrusts in the south-Pyrenean fold-and-thrust belt are regarded as multiepi-sodic (Travé et al., 2000).

The homogenisation temperatures reported for hydro-carbon fluid inclusions clearly match the temperaturesestimated using sterane isomerisation and equivalentvitrinite reflectance. In addition, the presence of oil in-clusions in late-stage calcite cement (C2) provides evi-dence of movement of liquid hydrocarbons through theArmàncies Formation fractures. This suggests that hydro-carbon migration was coeval with late calcite cementprecipitation (cf. Sellwood et al., 1989; Munz et al., 1999)and continued after calcite cementation. The similarity

between the temperatures from fluid inclusions andsterane isomerisation suggests a unique maturationphase of the source rock that resulted in the free oil andthe fluid inclusions oil originating from the same migra-tion pulse.

Acknowledgements

This work was financed by the DURSI of the Cata-lonian Government (2001SGR00075 “Grup de GeologiaSedimentaria”), Project BTE2003-06915, Programme“Juan de la Cierva” from the Spanish Ministry of Educa-tion and Science, and by Project 01-LEC-EMA10F(REN2002-11404-E) of the European Science Founda-tion. Ihsan Al-Aasm would like to acknowledge thecontinual support of theNatural Sciences and EngineeringResearch Council of Canada (NSERC). S. Morad andM.I. Benito were of great help in their improvement of themanuscript. We are indebted to M. Sibila for her assis-tance with the organic analyses.

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