Reservoir characteristics of fault-controlled hydrothermal dolomite bodies: Ramales Platform case...

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published June 27, 2012; doi 10.1144/SP370.1 v.370, firstGeological Society, London, Special Publications

Esestime and R. SwennenThurmond, J. Verges, E. Saura, N. Fernandez, I. Romaire, P. J. Dewit, M. Huysmans, Ph. Muchez, D. W. Hunt, J. B. studyhydrothermal dolomite bodies: Ramales Platform case Reservoir characteristics of fault-controlled

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Geological Society, London, Special Publications published online June 27, 2012 as doi: 10.1144/SP370.1

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Reservoir characteristics of fault-controlled hydrothermal dolomite

bodies: Ramales Platform case study

J. DEWIT1*, M. HUYSMANS1, Ph. MUCHEZ1, D. W. HUNT2, J. B. THURMOND2,

J. VERGES3, E. SAURA3, N. FERNANDEZ3, I. ROMAIRE3, P. ESESTIME3 & R. SWENNEN1

1Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E,

B-3001 Heverlee, Belgium2Statoil Research Center Bergen, Statoil A.S.A., Sandsliveien 90, 5020 Bergen, Norway

3Dynamics of the lithosphere – GDL, Institute of Earth Sciences ‘Jaume Almera’, CSIC,

Lluıs Sole i Sabarıs s/n, 08028 Barcelona, Spain

*Corresponding author (e-mail: [email protected])

Abstract: Hydrothermal dolomite (HTD) bodies are known as high-quality hydrocarbon reser-voirs; however few studies focus on the geometry and distribution of reservoir characteristics.Across the platform-to-basin transition of the Ramales Platform, fault-controlled HTD bodies arepresent. Three kinds of bodies can be distinguished based on their morphology, that is, elongatedHTD corridors, a massive HTD body (Pozalagua body) and an HTD-cemented breccia body. Thedifferences in size and shape of the HTD bodies can be attributed to differences in local structuralsetting. For the Pozalagua body, an additional sedimentological control is invoked to explain thedifference in HTD geometry.

A (geo)-statistical investigation of the reservoir characteristics in the Pozalagua body revealedthat the HTD types (defined based on their texture) show spatial clustering controlled by theorientation of faults, joints and the platform edge. Porosity and permeability values are distributedin clusters of high and low values; however, they are not significantly different for the three HTDtypes. Two dolomitization phases (i.e. ferroan and non-ferroan) can be observed in all HTDbodies. In general, the HTDs resulting from the second non-ferroan dolomitization phase havelower porosity values. No difference in permeability is found for the ferroan and non-ferroandolomites.

Hydrothermal dolomites (HTD) have receivedrenewed attention over recent years because oftheir potential as hydrocarbon reservoirs or aszones of increased porosity and permeabilitywithin reservoirs. Next to the controversy concern-ing the genetic definition of these authigenic dolo-mites, studies especially focused on the genesis ofHTD (e.g. Mallone et al. 1996; Nielsen et al.1998; Al-Aasm 2003; Machel 2004; Vandeginsteet al. 2005; Gasparrini et al. 2006; Lopez-Horgueet al. 2010; Sharp et al. 2010; Ronchi et al. 2011).The reservoir characteristics of HTD have alsobeen addressed by a number of authors. However,in the latter case, the results are either limited tooverall porosity and permeability values withoutspecific reference to their distribution (Wilsonet al. 2007), or focused on the HTD body geometryin terms of field observations and/or seismic datainterpretation (Cantrell et al. 2004; Hopkins 2004;Davies & Smith 2006). Quantitative analysis ofthe geometry of geobodies is often performed onsiliciclastic geobodies (e.g. Geehan & Underwood1993; Fabuel-Perez et al. 2009), while dimensions

of carbonate depositional systems have rarely beenstudied quantitatively. Quantitative data regardingthe distribution and geometry of HTD bodies are,nonetheless, of primary importance to constrainsubsurface reservoir models of HTD bodies.

This contribution presents quantitative data onthe dimensions and spatial variation of rock proper-ties (texture, porosity and permeability) of HTDhosted in platform edge to slope Albian carbonatesof the Ramales Platform (Basque–CantabrianBasin, northern Spain). The aim is to bridge thegap between (1) subsurface studies with limitedwell information to predict the distribution of reser-voir characteristics of HTD and (2) ‘classic’ quali-tative approaches used to describe the diagenesisand reservoir characteristics of HTD.

The objective of this paper is not to performa detailed structural or sedimentological analysis.Instead, detailed observations and quantitativecomputations, resulting from the use of a DigitalOutcrop Model (DOM) and (geo)-statistics, areused (1) to describe HTD bodies and to constrainthe factors controlling their characteristics and

From: Garland, J., Neilson, J. E., Laubach, S. E. & Whidden, K. J. (eds) 2012. Advances in Carbonate Explorationand Reservoir Analysis. Geological Society, London, Special Publications, 370,http://dx.doi.org/10.1144/SP370.1 # The Geological Society of London 2012. Publishing disclaimer:www.geolsoc.org.uk/pub_ethics


(2) to investigate the distribution of reservoir char-acteristics and infer parameters controlling thisdistribution.

First, the distribution of the HTD bodies in back-reef and slope platform carbonates is assessed witha DOM. HTD bodies are easily recognized in theDOM since the HTD and country rock (i.e. lime-stone) have distinct aspects (alteration, vegetationand colour). Tracing the dolomite–limestone con-tact in the DOM allows 3D visualization and charac-terization of the HTD bodies’ outline (shape, size,orientation of main axis, etc.). The limits of thedolomite bodies are fracture and fault controlledand hence linear across a wide range of scales. Inorder to assess the control of depositional and struc-tural setting on the orientation and extent of the frac-tures controlling the dolomite bodies, several keyareas were selected for structural analysis. Jointsets can be traced in the DOM as well, since jointsare weathered in the limestone host-rock, wherethey can be recognized as dark lineaments. HTDhave a different response to weathering and there-fore joints are rarely recognizable.

Second, the most extensive dolomite body inproximity to the platform margin, here referred toas the Pozalagua reservoir analogue, was selectedfor a detailed study. Developed in a back reefarea, approximately 1 km2 of the Pozalagua HTDbody has been densely sampled (n ¼ 262). Coreplugs were extracted both randomly as well as insections perpendicular and parallel to the PozalaguaFault, which acted as hydrothermal fluid conduit(Lopez-Horgue et al. 2010). In order to tighten the

information mesh, dolomite characteristics weremapped directly in the field (n ¼ 1018). The result-ing dataset forms an excellent base for geostatisticalinvestigation of the distribution of rock propertieswithin an HTD body. The parameters controllingthe spatial distribution of the porosity and perme-ability of the HTD’s were inferred by a combinationof field observations, petrographical investigations,petrophysical analyses and (geo)-statistics.

Geological setting

Study area

The study area is located in northern Spain atthe border of the Basque Country and Cantabria(Figs 1 & 2). Mesozoic rocks deposited in theBasque–Cantabrian Basin (BCB) are widelyexposed in the study area (Garcıa-Mondejar 1990).The area is the subject of numerous detailed stud-ies and, for a comprehensive review of the tectonicsetting, sedimentology and sequence stratigraphicevolution of the area, readers are pointed to theseminal works of Garcıa-Mondejar, (1989, 1990),Garcıa-Mondejar and Fernandez-Mendiola (1993),Garcıa-Mondejar et al. (1996) and Garcıa-Mondejaret al. (2005). The main features of this basin and itssedimentary fill and diagenesis pertinent to thisstudy are summarized here.

The BCB originates from post-Variscan exten-sional tectonism, but is now inverted and is con-sidered as the western extension of the Pyrenees(Fig. 1). The sedimentary record of the BCB from

Fig. 1. The study area is part of the Basque–Cantabrian Basin and lies in the western extension of the Pyrenees, NEof the Cantabrian Mountains. Modified from Verges et al. (2002).

J. DEWIT ET AL.


the Jurassic until the end of the Cretaceous reflectsthe rifting and evolution of the Bay of Biscayand the North Atlantic. The Aptian–early Albianwas characterized by a marine transgression causedby a progressive basin subsidence in combinationwith a global sea-level rise. The marine transgres-sion marked the onset of deposition of shallowmarine carbonates with a typical Urgonian facies.

In the BCB the Aptian–Albian carbonate plat-forms have linear and steep margins, showing evi-dence of repeated collapse (megabreccias), as theirposition was controlled by active faulting, foldingand salt diapirism (e.g. Garcıa-Mondejar 1989,1990; Garcıa-Mondejar & Fernandez-Mendiola1993; Garcıa-Mondejar et al. 1996; Martın-Chiveletet al. 2002). Accordingly, thickness and facieschanges are abrupt. Differential subsidence mainlyoccurred along NW–SE and NE–SW basementlineaments. The latter were reactivated by the sinis-tral strike-slip regime induced by the opening of theBay of Biscay and the counter-clockwise rotation ofIberia relative to Europe (Garcıa-Mondejar 1989;Garcıa-Mondejar et al. 2005).

Aptian–Albian sedimentation is characterizedby reciprocal carbonate and clastic deposition, withtransgressive and highstand carbonates depositedover tectonically/salt-controlled highs, and low-stand clastics in the basin centres (Garcıa-Mondejar1990; Garcıa-Mondejar & Fernandez-Molina 1993).One of these margins, the southward prograding andeast–west to ENE–WSW trending Aptian–Albianmargin of the Ramales carbonate platform, is thesubject of this study (the so-called ‘Ramales bar-rier’ of Rat, 1959). This margin is exposed nearlycontinuously for approximately 16 km along-strike,except where incised by valleys (Fig. 3) and is thesubject of several recent sedimentological, biostrati-graphic and diagenetic studies that have significantlyrefined understanding of the Ramales Platform(Lopez-Horgue 2000; Lopez-Horgue et al. 2010;Rosales & Perez-Garcıa 2010).

In the Ramales Platform, the Urgonian lime-stones are considered to have formed rudist-richcarbonate ramps during the Aptian, but are replacedby steep carbonate platforms rimmed by micriticmounds during the Albian (Lopez-Horgue 2000;

Fig. 2. Simplified geological map after Robador Moreno et al. (2000) and Lopez-Horgue (2000). The studiedhydrothermal dolomite (HTD) bodies of the Ramales Platform are located in Ranero, El Moro and El Mazo. They arehosted by Albian rudist-rich limestones of the Ramales Formation.

RESERVOIR CHARACTERISTICS OF HTD


Lopez-Horgue et al. 2010). The margin of theRamales Platform is relatively undeformed and, inthe study area, characterized by very abrupt lateraland vertical changes between platform strata andmargin/foreslope strata (Figs 4–6). The dolomitesof interest here are developed across the platform-to-basin transition of the Ramales Platform in prox-imity to the NW–SE-trending Pozalagua Fault(Figs 2 & 6).

Throughout the Late Cretaceous and early Ceno-zoic, uniform regional subsidence led to burial ofthe Mesozoic deposits (Garcıa-Mondejar 1989).The Cenozoic Alpine compression resulted in theinversion of the basin. The main faults within theRamales Platform are the north–south trendingRamales Fault, the east–west trending CabuernigaFault and the NWW–SEE trending RuahermosaFault (Garcıa-Mondejar et al. 1996; Lopez-Horgueet al. 2010) (Fig. 2). The Pozalagua Fault is inter-preted as a step-over between the Ruahermosa andCabuerniga Fault by Lopez-Horgue et al. (2010).

Hydrothermal dolomite bodies

In the Basque–Cantabrian Basin, the Urgonian lime-stones host different HTD bodies (Lopez-Horgue

et al. 2010; Shah et al. 2010) and ore depositswith dolomite as gangue mineral or host rock(Velasco et al. 1994, 1996; Grandia et al. 2003).The genesis of the HTD bodies occurring in theRamales Platform has been studied by Nader et al.(2009), Swennen et al. (2009), Lopez-Horgueet al. (2010) and Shah et al. (2010). This paperfocuses on the HTD bodies at Ranero, El Mazoand El Moro (Figs 2 & 6). Lopez-Horgue et al.(2010) demonstrate the hydrothermal nature of theHTD bodies by comparing the burial and thermalhistory of the Urgonian limestones with the hom-ogenization temperatures of fluid inclusions meas-ured in dolomites by means of micro thermometry.The dolomitization process pre-dates the Alpinecompression and occurred associated with sinistralstrike-slip faulting. This can be inferred by crosscut-ting relations of the dolomite with stylolites andfault kinematic indicators. The dolomitization pro-cess post-dates a first set of burial (bed parallel) sty-lolites, but pre-dates a second set of burial stylolitesand a set of tectonic stylolites (perpendicular tobedding). The tectonic stylolites can be related tothe Alpine compression. The relation between sinis-tral strike-slip faults and the dolomite bodies as wellas the occurrence of sheared dolomite veins indicates

Fig. 3. Overview of the different fault-controlled HTD bodies in the Ramales Platform. The HTD bodies occur inthree outcrop areas separated by valleys: Ranero, El Moro and El Mazo. Numbering of the HTD bodies is from the SW tothe NE (see also Table 1). Joints were mapped and measured in four different areas indicated by blue squares andnumbered in the order discussed in the text.

J. DEWIT ET AL.


that the dolomitization occurred in a sinistral strike-slip regime, which influenced the BCB until the LateCretaceous. Hydrothermal vents (with dolomite pre-cipitation) along regional faults of the BCB havebeen dated as Late Cretaceous (Agirrezabala2009). Lopez-Horgue et al. (2010) suggest that thedolomitization process occurred at the transition ofthe Albian and Cenomanian, since this was a

period of high tectonic activity in the BCB. Basedon field observations, petrographic investigations,stable (d18O and d13C) and Sr isotope analysis,Swennen et al. (2012) conclude that the HTDbodies are formed by two major hydrothermal dolo-mitization phases. The aim of this paper is to deter-mine the reservoir properties associated with thesetwo dolomitization phases in the Ranero HTD body.

Fig. 4. (a) View of the Ranero Area with the Digital Outcrop Model (DOM). (b) View of the Ranero Area afterinterpretation of the geological features based on the DOM analysis and fieldwork.



Methodology

Geometry of HTD bodies

The outline of HTD bodies occurring in the platformslope and back-reef carbonates deposited adjacent

to the margin was analysed to study the influenceof the precursor limestone facies in a basin-to-platform transition zone in the Ramales Platform.An area of approximately 10 km2 was covered bythe 3D DOM, including a total of 15 fault-controlledHTD bodies and host limestones of the study area.

Fig. 5. (a) Uninterpreted DOM view of the clinoforms and back reef of the area of Ranero. (b) Interpreted DOM view ofthe clinoforms and back reef of the area of Ranero with georeferenced points of identified geological features. (c) DOMview of the clinoforms and back reef in the area of Ranero with strip maps representing the different identifiedgeological features. (d) Panoramic view and corresponding line drawing of geological features (bedding, fracturesand dolomite bodies (pink surfaces)) of the distal platform deposits in Ranero. (e) DOM map view of the clinoformcrests in Ranero. The presence of spur and groove is characteristic for clinoforms. ( f) Field observations indicatethe occurrence of slumps at the foot of the clinoforms in Ranero.

J. DEWIT ET AL.


This DOM was built by combining a digitalelevation model (DEM) and aerial photographs,and has a resolution of 0.5 m. The fit between theDEM and the aerial photographs is excellent, onlyin areas of steep topography, such as verticalcliffs, the fit between the image and the DEM ispoor, resulting in smearing of the image data. TheDOM was analysed using proprietary softwaredeveloped by Statoil. The 3D visualization of theoutcrops was achieved by a stereoscopic projection.

The software allows the tracing and marking of sedi-mentary units, stratigraphic horizons, faults, jointsand the dolomite–limestone contact. Each geologicalfeature is saved as a separate set of geo-referencedpoints. The set of points can subsequently be usedto define localized surfaces along the outcrop facein GoCadw. These surfaces, or ‘strip maps’, whichare limited to the immediate vicinity of the datapoints, are characterized by the general strike anddip of the data points defining the concerned geo-logical feature. This generally provides a betterresult than gridding algorithms, as it allows directgeologic input into the creation of the surface, anddoes not require discontinuities in the surface tobe smoothed over. These surfaces can be importedinto the DOM and permit a better appreciation ofthe features’ extent, shape and orientation in 3D,which can be vital in the correlation of boundariesbetween outcrop faces. An example of the differentsteps from uninterpreted DOM view to DOM withstrip maps of the identified geological featurescan be found in Figure 5a–d. All interpretationsbased on the 3D model have been systematicallyground-truthed.

The use of a DOM to track the dolomite–lime-stone contact is particularly suitable since dolomiteand limestone outcrops are easily distinguished onaerial photographs. The contrasting patina of lime-stone and dolomite is the most important diagnosticfeature, that is, limestones have a grey-white patina,while dolomites display a brown patina. Moreover,limestones and dolomites have different weatheringcharacteristics. In general, limestones are respon-sible for abrupt changes in the landscape withsteep cliffs and intensely developed fracture pat-terns (e.g. Fig. 7c). Dolomites form more gentlesurfaces, partially covered by low vegetation andlichen. The carbonates of the study area sufferedfrom recent meteoric alteration and are karstified.This karstification leads to enlarged fracture setsand faults. In addition, dolines can sometimes befound in fault zones.

The geometry of the fault-controlled HTDbodies was assessed by measuring the width perpen-dicular to the body’s main axis every 10 m. Thespacing between the HTD bodies was measuredfor the areas of Ranero, El Moro and El Mazo indi-vidually, since they are separated by deep and largevalleys. The orientation of the main axis of theHTD bodies was measured as well.

Analysis of joint sets

On the DOM, joints are easily observed in the lime-stone host rock, while rarely visible in the HTDowing to the difference in weathering character-istics. Only the joints strike was taken into consider-ation because it is not always possible to accurately

Fig. 6. Illustration of omnidirectional variogram andvariogram map calculation for a fictitious dataset.(a) Spatial distribution of the studied dataset. (b) Anomnidirectional semivariogram is computed andmodelled to explore the overall spatial continuity ofthe dataset. With increasing spatial separation (h), thecorrelation between two data points decreases. The rangeis the distance at which two data points becomeindependent. The sill corresponds to the variance of thedata points. (c) A semivariogram map is calculated tovisualize the spatial variability in all directions. In thiscase spatial anisotropy can be observed along theNW–SE and east–west axes.



measure the dip of the joints. The joint sets weremeasured in four areas. Two areas were selectedalong the Pozalagua Fault in the Ranero area, onebetween a small and a large HTD corridor inRanero, and one between HTD corridors in ElMazo (Fig. 3). Frequency rose diagrams of thejoints’ strike were calculated for each selected area.

Porosity and permeability

Investigation of the reservoir characteristics of HTDwas performed in the area of Ranero since a largeHTD body developed in the exposed back-reef car-bonates. The HTD body present in Ranero is relatedto the Pozalagua Fault and covers an area ofapproximately 1 km2, which is large enough toobserve patterns in the HTD characteristics at sub-reservoir scale. Equally, this Pozalagua HTD bodyis small enough to be sampled and mapped in

detail to investigate the distribution of the HTDcharacteristics.

In order to analyse the standard He-porosityand Klinkenberg permeability and to define HTDtypes based on their texture, plug samples weredrilled in the field with a core-drill (PomeroyEZ Core Drill, Stihl D026-C). Sampling was per-formed randomly and along sections perpendicularand parallel to the Pozalagua Fault. To increasethe density of observation points the study areawas mapped and different dolomite textures wererecorded in the field. This resulted in 1018observation points.

All samples (n ¼ 262) were subjected to macro-scopic petrographic analysis consisting of stainingfor mineralogy with alizarine red S and K ferrycya-nide and texture classification into three types ofdolomite. A selection of 120 plugs were analysedfor standard petrophysic characteristics.

Fig. 7. (a) The Pozalagua HTD body has a lobate outline in the platform slope. Dolomite ‘tongues’ developedwest of the Pozalagua Fault along N70E joints. (b) The Pozalagua HTD body has a more important lateral extentin the back-reef than in the slope carbonates. In addition, a small HTD patch can be observed within the clinoforms(arrow). (c) On the DOM joints can be traced in the limestone host rock. The HTD body in the platform carbonatesis generally controlled by N30W and EW joints. (d) Frequency rose diagram of the orientation of the border of theHTD body (shown in c).

J. DEWIT ET AL.


Geostatistical analysis

Several statistical and geostatistical tools are usedfor spatial analysis of HTD-type occurrence and pet-rophysical parameters. Given the large dataset,(geo)-statistical tools can detect and quantify spatialtrends and anisotropy in HTD type, occurrence,and porosity and permeability variability. Three(geo)-statistical tools were selected for this study:transition probability calculation, exploratory vario-graphy and ANOVA. Transition probability calcu-lation reveals patterns of succession of differentHTD types. Exploratory variography is used toexplore the overall spatial continuity of the datasetand to detect directional anisotropy. ANOVA (analy-sis of variance) reveals significant porosity andpermeability differences in different HTD types.

Transition probability calculation. Transition prob-ability calculation can be used to analyse patterns ofsuccession of different HTD types. Transition prob-ability is the probability that a variable changesfrom one state to another. In this study, it is the prob-ability that a certain dolomite type or limestone hostrock sample is neighboured by a certain dolomitetype or limestone host rock further away from thePozalagua Fault. Transition probabilities are cal-culated by analysing HTD type succession in 10 sec-tions approximately perpendicular to the PozalaguaFault. Every successive sample couple (251 couplesin total) is registered and counted to determinetransition probabilities.

Exploratory variography. Geostatistics provides aset of tools to describe the spatial continuity thatis, an essential feature of many natural phenomena.Definitions of geostatistical estimators are coveredextensively by Isaaks and Srivastava (1989) andDeutsch and Journel (1998). The most familiargeostatistical estimator is the semivariogram. Thesemivariogram, or simply the variogram, is calcu-lated as half of the average squared differencebetween variable values separated by a lag vector h:

gii(h) = 1

2N(h)

∑N(h)

a=1

(zi(xa + h) − zi(xa))2

where N(h) is the number of pairs and zi(x) is aregionalized variable. The semivariogram can beunderstood as the sample variance described as afunction of spatial separation. Low semivariogramvalues indicate a high degree of correlation betweenvariable values separated by the lag vector, whilehigh semivariogram values indicate a low degreeof correlation (Fig. 6).

In this study, an exploratory variogram analysisis carried out for HTD occurrences to investigate

the overall spatial continuity of the dataset and todetect directional anisotropy. First, an experimentalomnidirectional variogram is computed and mod-elled to explore the overall spatial continuity ofthe dataset. An omnidirectional variogram presentsvariogram values at increasing lag distances withoutspecifying the direction of the lag vector h, meaningthat only the magnitude of the lag vector (| h |) istaken into account. An omnidirectional variogramserves as a useful starting point for detecting thepresence of spatial continuity in a sample dataset.Second, a variogram map or variogram surface iscalculated to visualize spatial variability in all orien-tations (Fig. 3). This analysis allows determinationof whether the dataset displays directional aniso-tropy and the axes of minimum and maximumspatial continuity.

In this study, the data consist of qualitativecharacteristics (three HTD types: matrix, zebraand cement dolomite) and quantitative data (e.g.porosity values). The qualitative data have to bebinary transformed to 0 and 1 (indicators). The indi-cator omnidirectional semivariograms are calcu-lated for each HTD type separately. In order tohave a good dataset to compute (indicator) omnidir-ectional semivariograms, the dataset should consistof sufficient data points randomly distributed overthe study area. Therefore, the indicator omnidirec-tional semivariograms are not only based on thedata obtained from the drilled plug samples,but also on data obtained from mapping HTDtypes (n ¼ 1018) in the field. All experimental var-iograms were calculated and modelled using theVARIOWIN 2.2 interface (Pannatier 1996). Theexperimental variograms were modelled using anindicative goodness-of-fit criterion, which is aweighted least squares measure of the agreementbetween the experimental and model variograms,where the weights are a function of the number ofpairs and mean pair distances for each lag interval(Pannatier 1996). The shapes of the variogramssuggest fitting by a spherical model (Fig. 6). Aspherical variogram is defined by three variogramparameters: nugget, sill and range. The nugget valuereflects the very small-scale variability. The sill valueindicates the total variability within the dataset. Thevariogram range is a measure for the distance untilwhich sample values are spatially correlated.

ANOVA. ANOVA is a general statistical techniquethat can be used to test the hypothesis that themeans among two or more independent groups areequal. The purpose is to test for significant differ-ences between class means, this is done by analy-sing the variances. ANOVA is here used to checkwhether porosity and permeability distribution aresignificantly different in different HTD types. Thenull hypothesis assumes no difference in the



porosity or permeability means of the different HTDgroups. If the p-value corresponding to this nullhypothesis is below 0.05, the null hypothesis isrejected. In that case, different HTD groupsdisplay significantly different porosity or per-meability means. This would then mean that under-standing the spatial distribution of lithologies anddolomite types is crucial for understanding thespatial distribution of porosity and permeability.

Results

Occurrence and geometry of HTD bodies

The studied HTD bodies are hosted along the edgeof the Ramales Platform. The architecture of thisrimmed platform has been studied by severalauthors (Garcıa-Mondejar1990;Fernandez-Mendiolaet al. 1993; Rosales 1999; Martın-Chivelet et al.2002; Garcıa-Mondejar et al. 1996, 2005; Lopez-Horgue et al. 2010). The platform slope depositshave been found cropping out in the south and SEof the study area (Fig. 2). DOM analysis combinedwith fieldwork gives the possibility of defining anddescribing the main parts of the platform, thatis, platform slope, back-reef and inner-platform.An example of a DOM view of the Ranero Areabefore and after interpretation is given in Figure 4.The slope deposits are characterized by steep clino-forms, spur and groove clinoform crests, olistolithsand slumps (Fig. 5). The edge of the Ramales Plat-form is built up by microbial mudstone withrudists, chondrodonta and to a smaller extent bycorals and orbitolinids (Lopez-Horgue et al. 2010).The back-reef deposits are characterized by muddylimestones with variable bed thickness. The inner-platform deposits are recognized as well-beddedlimestone successions of alternating bioclast-richbeds and reef mounds.

Two types of HTD bodies are distinguished inthe Ramales Platform based on their dimensions andgeometry, that is, fault-controlled and stratabound.

Fault-controlled bodies are defined as HTD bodiesassociated with a fault or fault-zone and with amaximum width of 1 km. The fault-controlledHTD crosscuts stratigraphic horizons along thefault strike. Stratabound HTD bodies are describedas large HTD bodies confined to specific strata andlaterally pinching out. This study is focussed onthe fault-controlled HTD bodies that are present inthe slope deposits of the Ramales Platform, that is,at Ranero, El Moro and El Mazo (Fig. 3).

The fault-controlled HTD bodies occur in threedistinct forms: corridors, massive bodies (PozalaguaHTD body) and dolomite-cemented breccias. HTDcorridors are elongate and rather straight HTDbodies up to 1 km long and several metres to deca-metres in width. The massive HTD body has awidth much larger than 100 m and is the longest(.1000 m) HTD body. The breccia body consistsof limestone clasts embedded in coarse crystallinedolomite cement. The breccia body is bordered bythe Breccia Fault to the north and east (Fig. 3).The HTD corridors and the massive HTD bodyconsist solely of dolomite with the exception ofrare, meter-sized relict limestone lenses within theHTD bodies. The dolomite–limestone contact istypically sharp.

HTD body dimensions and outline. In the areas of ElMazo and El Moro the slope deposits are well pre-served; however, the platform deposits have beeneroded. Furthermore, since the northern hill side issituated in the shade, the contrast between limestonehost-rock and HTD is unclear on the DOM. InRanero the younger deposits cover the platformcarbonates east of the Pozalagua Fault. Therefore,except for the Pozalagua HTD body (number 12,Fig. 3), the measurements of the width of the HTDbodies are mainly from the platform slope. For thePozalagua HTD body, a distinction is madebetween widths measured in the platform slopedeposits, that is, 12 S (Table 1), and in the back-reefcarbonates, that is, 12 P (Table 1).

Table 1. Geometry of HTD bodies. Overview of the geometrical parameters of the fault-controlled HTDbodies (as numbered in figure 3). All data are expressed in metres. Three areas are considered, that is, ElMazo, El Moro and Ranero, which are separated from each other by deep valleys. The widths measured in thePozalagua body (number 12) are separated in two columns, 12 S represents data measured in the slope and12 P data measured in the platform

HTD body El Mazo El Moro Ranero

1 2 3 4 5 6 7 8 9 10 11 12 S 12 P 13 14 15

Mean width 5 1 1 39 38 38 36 17 66 35 18 120 450 7 15Max width 16 1 1 44 97 72 72 32 116 11 46 253 920 137 28 41Min width 1 1 1 1 1 10 5 8 5 1 1 5 20 6 1 5Length 520 340 226 226 517 350 380 170 1270 960 590 455 760 250 244 1000Aspect 0.01 0.003 0.004 0.2 0.07 0.1 0.1 0.1 0.05 0.04 0.03 0.3 0.6 0.03 0.02Interdistance 0 150 60 255 170 270 180 80 0 860 650 0 475 325 120

J. DEWIT ET AL.


Only the widths measured in the slope depositsare taken into account to compare the outline ofall HTD bodies in the study area. The averagewidth of the HTD bodies varies from 1 to 120 m(Table 1), the highest average width being foundin the Pozalagua HTD body (12 S), which also hasthe most irregular outline. With the exception ofthe Pozalagua and the breccia bodies, the HTD bod-ies have a rather rectilinear and elongated outline.The width is generally constant over a large partof the HTD body. Some HTD bodies envelop largelimestone lenses, 40–200 m long and 6–40 m wide.The limestone lenses always have their main axisparallel to the HTD body’s border. The averagewidth of the breccia body is 91 m, which is largecompared with its length of 250 m. Other HTDbodies with similar lengths have average widthsranging between 1 and 39 m. In the Pozalagua HTDbody the width can increase by 100 m between twomeasurement points. Dolomite tongues developedwest of the Pozalagua Fault along N70E joints(Fig. 7a). This HTD body is also the largest bodyoccurring in the platform slope. The lowest pointat which HTD has been observed in the PozalaguaHTD body is in the Pozalagua Quarry at 480 m alti-tude. The highest point at which HTD can beobserved is at an altitude of 660 m at the transitionfrom foreslope to platform deposits. The verticalthickness of the dolomitized sequence is thus atleast 180 m. In addition, there seems to be a relationbetween the size of the HTD bodies and theirspacing (Table 1). The HTD bodies with lowaverage width systematically occur closer to neigh-bouring bodies, with the spacing varying from 60 to150 m for narrow bodies (average width 1–7 m). Incontrast HTD bodies with a higher average widthcan be up to 860 m apart.

Only the Pozalagua HTD body crops out in bothslope and back-reef carbonates (number 12 S &P, Fig. 3). The outline of the Pozalagua HTD bodyis different in the slope and back-reef deposits(Fig. 7a, b). In the slope deposits the averagewidth of the HTD body is 120 m and the maximumwidth 253 m. In the back-reef carbonates the HTDbody has a more important lateral extent. Theaverage width is 450 m and the maximum width920 m. In general, the margins of the HTD bodyare rather straight and dominantly possess N30W,N40E and EW orientations (Fig. 7c, d). In additionto the large Pozalagua HTD body, a small HTDbody occurs at the platform edge between clino-forms (black arrow, Fig. 7b).

Joint set orientation. Joints were measured in fourareas (Fig. 3) to investigate whether local differencesin structural setting occur to which the size andoutline of the HTD bodies could be related. Alongthe Pozalagua Fault, two areas were selected to

verify whether the joint pattern is similar in platformslope and back-reef carbonates. Two additionalareas have been selected in the northeastern andsouthwestern extremities of the study area aroundHTD corridors to study regional variations.

Between the Pozalagua and breccia body (Fig. 3,square 1) three sets of main joints were determinedin the DOM owing to their continuous nature andaperture of several pixels. The first set of mainjoints is characterized by a N30W orientation.This set is the dominant joint set in terms of occur-rence. The second set of main joints has a N75Eorientation. The third set of main joints with aN40E orientation represents the smallest numberof joints (Fig. 8).

Small joints are recognized by the presenceof aligned pixels with contrasting colours and thusas short lineaments (,50 m). Two sets of thesejoints were recognized in the slope deposits alongthe Pozalagua Fault. The first set has a N30W–N60W orientation. The second has a N10E–N70Eorientation (with N40E as the main orientation).This latter set of joints represents the major set ofsmall joints (Fig. 8). Although both sets of smalljoints are characterized by scattered orientations,the second set has the largest range. The range cor-responds to the orientations of the second and thirdset of main joints with a dominant N70E orientation.

The platform carbonates in Ranero (Fig. 3, square2) are also characterized by the presence of threejoint sets (Fig. 9). The most important set is charac-terized by a N30W–N60W strike. The second set ischaracterized by c. EW-oriented joints. The leastimportant set of joints recognized in the platformcarbonates has a N40E orientation.

Two joint sets (Fig. 10) are observable adjacentto the dolomite corridors (Fig. 3, square 3). Thefirst set is characterized by a rather scattered strikebetween N20W and N60W. The second set com-prises joints that are longer and have a N40E–N70E strike. Joints with an approximately EWstrike are lacking. In El Mazo (Fig. 3, square 4)two main joint sets can be identified, NW–SE andNE–SW (Fig. 11).

In the three latter cases no distinction betweenmain and small joints was made because the differ-ence in aperture and length of the joints was not con-siderably different. In summary, joint sets withsimilar orientation can be found in the slope andback-reef deposits along the Pozalagua Fault. Inaddition, the joint density is also similar in theslope and back-reef carbonates. A difference inlocal structural setting can thus not explain thedifference in lateral extent of the Pozalagua body.

Around the HTD corridors, two joint sets havebeen observed with c. N30W and NE–SW strike.This is the case for both selected areas in Raneroand El Mazo. There is thus no difference between



the two extremities of the study area. However,around the HTD corridors only two joint sets areobserved, while along the Pozalagua Fault threesets are present. In general, joints with a N30Wstrike are dominant in the four areas. This orien-tation corresponds to the orientation of the mainaxis of all HTD bodies and the orientation of the

sinistral strike-slip Pozalagua Fault. This is inter-preted to indicate that the flow of the dolomitizingfluids was mainly controlled by joints and faultswith a N30W orientation.

Faults and fault displacement. The Pozalagua HTDbody is the largest of all HTD bodies occurring in

Fig. 8. Analysis of joint sets between the Pozalagua HTD body and the Breccia body (Fig. 3, square 1). Only the majorjoints are shown on the DOM in order to keep a good overview. The strikes of the joints are plotted in frequencyrose diagrams.

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the platform slope of the Ramales Platform. In thePozalagua Quarry, the Pozalagua Fault can beobserved as well as kinematic indicators such assinistral shear bands and associated oblique veinsand striae.

The Pozalagua Fault runs through the slopedeposits of the Ramales Platform, which are builtup by clinoforms. These clinoforms can be observed

on both sides of the Pozalagua Fault and were digi-tized independently. West of the Pozalagua Fault,the crest of the clinoforms can be digitized basedon the spur and groove morphology of the clino-forms and the difference in weathering of thefacies deposited at the beginning and at the endof a clinoform formation. The clinoforms’ crestsare less vulnerable to weathering. The outcrop

Fig. 9. Analysis of joint sets in platform carbonates hosting the Pozalagua HTD body (Fig. 3, square 2). A frequencyrose diagram displays the orientation of all the mapped joints.

Fig. 10. Analysis of joint sets in the slope carbonates hosting the dolomite veins in the area of Ranero (Fig. 3, square 3).A frequency rose diagram shows the distribution of the joints strike. The dashed purple lines show the digitized limitsof different clinoforms.



conditions east of the Pozalagua Fault do not allowan easy identification of the clinoforms since thecrest is eroded and no cliff is present. Thereforethe digitization of the different clinoforms wasbased mainly on the presence of ridges. Nonethe-less, the thicknesses of the different clinoformseast and west of the Pozalagua Fault correspondrather well, except for clinoform 4 (Table 2).Based on this observation, the horizontal displace-ment caused by the Pozalagua Fault can be inferredto be approximately 500 m (Fig. 12).

In contrast, no large-scale displacement wasobserved along the HTD corridors in the DOM.In the field, however, fault planes with sinistralstrike-slip indicators were observed in the HTDcorridors, especially in those formerly mined forPb–Zn mineralizations (Coto Txomin, Simonet al. 1999).

The breccia body is delineated by a bendinglineament (Fig. 3). In the field this lineament canbe observed as a deeply eroded joint separating anintensely veined limestone to the east and a brec-ciated limestone to the west. No lateral displace-ment can be observed on the DOM or in the field.The limestone clasts are made from slope sedimentand are typically angular and often fit together. Ingeneral the breccia clasts display the typicalarrangement of a crackle breccia (Morrow 1982)and are only separated by HTD cement.

Structural analysis. During fieldwork, differentfractures with dolomite cement infill have beenobserved in the area of Ranero (Fig. 13). One sethas a N30W70N orientation, is 3–10 mm thick,about 60 cm long and generally regularly spaced.A second set is characterized by an EW70N

Table 2. Clinoforms west and east of the Pozalagua Fault. Thicknesses (expressed in metres) of the differentclinoforms observable west and east of the Pozalagua Fault in the area of Ranero. The thickness was measuredat three points within each clinoform. The thicknesses of the different clinoforms on both sides of thePozalagua Fault match well, except for clinoform 4

Clinoform West East Difference of average

Measure point Average Measure point Average

1 2 3 1 2 3

1 110 76 80 89 98 103 73 91 32 93 60 87 80 81 99 61 80 03 90 96 125 104 93 101 87 94 104 81 82 67 77 101 92 115 103 265 117 81 105 101 115 83 92 97 4

Fig. 11. Analysis of joint sets in the area of El Mazo (Fig. 3, square 4). A frequency rose diagram summarizes thedistribution of the orientation of the joints’ strike.

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orientation, is 3–15 mm thick, 30 cm long onaverage and occurs in clusters. Finally, a third sethas been observed with N50E90 orientation, 1 mmthick, 50 cm long and clustered as well. The latterdolomite vein orientations correspond to the differ-ent joint sets observed in the DOM. In addition,NS90-trending joints can be found. NS-oriented

brecciating veins cemented by HTD cement canbe observed west of the breccia body. Within thebreccia body clasts have been observed to bealigned in a NS90 direction. In the vicinity ofthe HTD corridors in Ranero (Fig. 3, square 3)two joint sets were identified. The first set concernsdolomite veins with a N30W70N orientation.

Fig. 12. The clinoform surfaces digitized on the DOM are highlighted with distinctive colours and numbered from 1 to5 (starting with the youngest one in the south). The clinoforms are recognized on both sides of the Pozalagua Fault,allowing estimation of the fault’s throw. The relative displacement of clinoforms along the Pozalagua Fault in Ranero isc. 500 m.

Fig. 13. Occurrence and orientation of dolomite veins along the Pozalagua Fault. The black rose diagrams representdolomite vein orientations measured in dolomite. The grey rose diagrams represent dolomite vein orientations measuredin the limestone host rock.



The second set of joints presents a N60–70E90orientation.

The good match between the joint sets identifiedby means of the DOM and the dolomite veinsobserved in the field shows that the different jointsets are related to the dolomite emplacement.

Distribution of HTD characteristics

Dolomite types. Based on their texture, HTD sam-ples are classified into three types: (1) matrix dolo-mites (fine crystalline, grey, replacive dolomites),(2) massive cement dolomites (coarse crystalline,white-pink, fabric destructive, saddle dolomites)and (3) zebra dolomites (dolomites built up fromalternating layers of fine crystalline, grey matrixdolomite and coarse crystalline white-pink saddledolomite; Fig. 14). Matrix dolomites make up 12%of all HTD samples, while massive cement dolo-mites represent 32%. The major part of the HTDsamples (56%) has zebra dolomite features.

Table 3 shows probabilities that a certain HTDtype or limestone host rock (left column) is followedby a certain HTD type or limestone host rock(top row) when moving away from the PozalaguaFault. For example, if a massive cement sample isobserved in a section perpendicular to the PozalaguaFault, there is a 45.6% chance that the next sample isalso a massive cement sample, a 5.9% chance that itis limestone, a 5.9% chance that it is a matrix and a42.6% chance that it is a zebra dolomite sample.These numbers should be compared with the rela-tive occurrence of each dolomite type or limestonehost rock (bottom row). For example, if a massivecement sample is observed in a section perpendicu-lar to the Pozalagua Fault, there is a 45.6% chancethat the next sample is also a massive cementsample, although massive cement samples only rep-resent 27.9% of all samples. Increased chances orprobability–occurrence ratios can be observed forcement–cement (probability 45.6% and occurrence27.9%), matrix–matrix (probability 16.7% andoccurrence 7.2%), zebra–zebra (probability 64.7%and occurrence 52.6%), matrix–limestone (prob-ability 16.7 and occurrence 12.4%) and lime-stone–matrix (probability 12.5% and occurrence7.2%) pairs. These results indicate that HTD typesoccur in clusters of the same type. In addition, itcan be concluded that matrix dolomite and thelimestone host rock often occur at neighbouringsites. Moreover, Figure 15 shows that cementdolomites preferentially occur in the vicinity ofthe Pozalagua Fault and along the platform edge.Zebra dolomites are mainly present in the centralpart of the HTD body.

To investigate directions of spatial continuity andanisotropy in datasets, indicator omnidirectionalsemivariograms are used. The latter semivariograms

are all modelled with a spherical model. The range,sill and nugget vary for the different HTD types. Therange of the indicator omnidirectional semivario-gram is the highest for cement dolomites (397 m).Matrix and zebra dolomites have much smallerranges, respectively 157 and 116 m. Clusters of thesame HTD type with diameters approximately cor-responding to the indicator omnidirectional semi-variogram ranges of the HTD types can indeed be

Fig. 14. Three types of HTD are distinguished based ontheir texture. (a) Matrix dolomite. (b) Cement dolomite.(c) Zebra dolomite.

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observed in Figure 15c. Massive cement dolomites,for instance, occur in large clusters parallel and per-pendicular to the Pozalagua Fault.

The indicator omnidirectional semivariogramsmaps of the different HTD types (Fig. 16) revealdirectional anisotropy in the dataset. The N30–35W and N60E directions mark a more gradualincrease in the variogram values with distance andcorrespond to the direction of maximum spatial con-tinuity. The N30–35W strike corresponds to thegeneral orientation of the Pozalagua Fault. TheN60E orientation either relates to structural fea-tures, that, is a set of joints and dolomite veins, orto the orientation of the platform edge.

Ferroan and non-ferroan dolomites. The paragen-esis of the HTD can be studied in the PozalaguaQuarry. A detailed study of the genesis of theHTD is beyond the scope of this contribution; never-theless, it should be specified that HTD formed bythe succession of two main dolomitization phases(Fig. 17). The first phase resulted in ferroan dolo-mites, while the second phase generated non-ferroandolomites. These two phases have been observed inall the HTD bodies. In general, dolomite samples arebuilt up by ferroan or non-ferroan dolomites. Bothphases rarely occur together.

After staining for mineralogy, ferroan and non-ferroan dolomite are respectively found in 47 and53% of the samples. The ferroan and non-ferroanHTD are unequally distributed over the matrix,cement and zebra dolomite types. Matrix andzebra dolomites are ferroan dolomites in respect-ively 52 and 60% of the samples. Cement dolomitesare mainly non-ferroan dolomites (76%). Figure 18shows that zones that are dominated by cementdolomites correspond to zones where non-ferroandolomites are prevailing and vice versa.

Indicator omnidirectional semivariograms werecalculated for the ferroan and non-ferroan HTD.

The semivariogram maps display a direction ofanisotropy along N50E, indicating that the resultsare biased by the main sampling orientation. There-fore they cannot be used to infer anisotropy orien-tations in the distribution of the ferroan andnon-ferroan dolomites.

Porosity. One-hundred and twenty samples wereanalysed for standard He-porosity and Klinkenbergpermeability. The porosity values range between0.01 and 12.1%. Low porosity values (,1%) char-acterize the limestone host rock. Porosity values ofthe HTD range between 1.8 and 12.1% (Table 4).Based on the diagenetic history, burial curve andthe inferred timing of hydrothermal dolomitizationof the area (Lopez-Horgue et al. 2010), the limestonehost rock can be regarded as tight at the momentof dolomitization. Since HTD have significantlyhigher porosities than their limestone host rock,the dolomitization process has a positive effect onreservoir characteristics. The spatial distribution ofthe porosity values seems to be characterized byclusters of high and low values (Fig. 19). Averageporosity of the different HTD types is similar forcement, matrix and zebra dolomites with 5.8, 6.8and 5.5%. ANOVA also indicates that the porosityvalues are not significantly different for each HTDtype. The boxplots in Figure 20a illustrate the poros-ity distribution for each HTD type.

The relation between porosity values and ferroanv. non-ferroan dolomite is analysed by classic stat-istical analysis and ANOVA. The former showsthat the lowest median of standard porosity is obser-ved in non-ferroan dolomites (Fig. 20b). ANOVAresults indicate that porosity values are significantlydifferent for ferroan and non-ferroan dolomites. TheHTDs resulting from the first, ferroan dolomitiza-tion phase have the highest porosity values andthe lower porosities characterize the second, non-ferroan dolomitization phase (Table 5).

Table 3. Transition probability calculation. The calculation is based on datafrom sections sampled perpendicular to the Pozalagua Fault. The transitionprobabilities are expressed as percentages and should always be compared tothe relative occurrence of each HTD type or limestone host rock (bottom row).The table shows the probability to encounter a certain HTD type or limestonehost rock (top row) as next sample when moving away from the PozalaguaFault, given the information that a certain HTD type or limestone host rock(left column) is found

Cement Matrix Zebra Limestone

Cement 45.6 5.9 42.6 5.9Matrix 16.7 16.7 50.0 16.7Zebra 23.3 5.3 64.7 6.8Limestone 15.6 12.5 25.0 46.9Relative occurrence 27.9 7.2 52.6 12.4



https://www.researchgate.net/publication/223473357_Structurally_controlled_hydrothermal_dolomites_in_Albian_carbonates_of_the_Ason_Valley_Basque_Cantabrian_Basin_Northern_Spain?el=1_x_8&enrichId=rgreq-f8adebb7-91a5-4bf6-afd4-70fcd5cb420a&enrichSource=Y292ZXJQYWdlOzI1ODY0NjMxNTtBUzoxODE1NTE4ODMxMDQyNTZAMTQyMDI5Njc1MTA0OQ==

Permeability. Klinkenberg permeability valuesrange between ,0.01 and 17.4 mD (Table 6).Figure 21 shows the spatial distribution of the per-meability values. Low permeability values are notsolely restricted to the limestone host rock. A few

outliers presenting values remarkably higher thanthe rest of the samples occur. The map displayssome spatial correlation with clusters of higherand lower values. ANOVA results show that thepermeability values of the HTD types are not

Fig. 15. (a) Distribution map of HTD types in the sampled sections. (b) Distribution of the HTD types mapped in thefield. (c) HTD types occur in clusters of the same type.

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significantly different. Matrix dolomites are charac-terized by the highest average permeability, that is,2.3 mD compared with about 1 mD for both cementand zebra dolomites (Fig. 22a).

The highest average permeability value isobserved in non-ferroan dolomites (Table 7). Thedifference between the average permeabilities issmall and ANOVA results show no significantlydifferent permeabilities for ferroan and non-ferroandolomites.

Discussion

HTD bodies’ dimensions and controlling

parameters

Different HTD bodies are hosted by the slope depos-its of the Ramales Platform. Three kinds of bodiescan be distinguished, that is, elongated HTD corri-dors (numbers 1–11 and 14–15 in Fig. 3), amassive HTD body (Pozalagua body) and an HTDcemented breccia body. The HTD corridors arecharacterized by rectilinear outlines with averagewidths between 1 and 66 m and a main axis with aN30W strike. Narrow HTD bodies generally arelocated close to a neighbouring HTD corridor. ThePozalagua body (12 S) and breccia body have adifferent outline and a much higher average widthof respectively 120 and 91 m. Furthermore, thePozalagua HTD body (12 S) is characterized bythe presence of dolomite ‘tongues’, which devel-oped along N70E joints. This suggests a differencein mode of emplacement and local structuralsetting for the Pozalagua and breccia body com-pared with the HTD corridors. Parameters control-ling the geometry of the HTD bodies can be bothsedimentological and structural.

Since no important facies differences have beenobserved in the slope carbonates, a sedimentologicalcontrol cannot be invoked to explain this differencein size of the HTD bodies and in content regardingthe breccia body. Two structural features distinguishthe Pozalagua HTD body and the breccia body fromthe HTD corridors.

(1) Three sets of joints can be observed in thelimestone between the Pozalagua and thebreccia body (N30W, N40E and N75E–EW). In the limestone surrounding HTD corri-dors in Ranero and El Mazo, only two jointsets were observed (N30W and N40–70E).As previously mentioned, the study area hasbeen subjected to a sinistral strike-slipregime (Garcıa-Mondejar 1989) and theN30W oriented Pozalagua Fault is known tobe a sinistral strike-slip fault (Lopez-Horgueet al. 2010). In association with strike-slipfaults Riedel shears may develop (Twiss &

Moores 1992). The most common mode ofoblique joint development occurs up toapproximately 308 to the direction of strike-slip faulting (Fig. 23). These fractures areknown as R1 Riedel shears and can

Fig. 16. Indicator omnidirectional semivariogram mapsas calculated and modelled for each HTD type. Twodirections of anisotropy can be inferred from theindicator omnidirectional semivariogram of the differentHTD types, that is, N60E and N30–35W. (a) Cementdolomite. (b) Matrix dolomite. (c) Zebra dolomite.



correspond to the joint sets with a N30W–N60W strike observed in the slope sedimentsof the Ramales Platform. Antithetic Riedelshears, R2, generally have an angle of approx-imately 758 to the faults’ strike (Wilcox et al.1973). The joint set with an average N75Estrike could correspond to these R2 Riedelshear fractures. Secondary synthetic P1 andantithetic P2 shear fractures can be associatedwith strike-slip faults as well (Sylvester 1988).These P1 and P2 Riedel shear fractures occurunder the same angles to the strike-slip fault asthe R1 and R2 Riedel shears, but the strike-slipfault bisects the angle between R1 and P1 andR2 and P2. Joints with an approximatelyN40E strike, which are only observed alongthe Pozalagua Fault, could be related to P2secondary shear fractures. Field observationsrevealed the HTD also displayed a set ofjoints with a NS strike. This set of jointscould correspond to P1 Riedel shear fractures.The presence of an additional set of jointsbetween the Pozalagua HTD and the brecciabody points towards a local difference instructural setting and fault activity.

(2) A horizontal throw of approximately 500 mwas estimated for the Pozalagua Fault, whileno displacement was observed in the DOMalong the HTD corridors. Fault planes with asinistral strike-slip character have, neverthe-less, been observed in some of the HTD corri-dors in the field. The striae in these faultplanes are made from dolomite. The HTD cor-ridors are thus also related to fault activity andcould be mineralized throughgoing fractures.Throughgoing fractures are important fluidconduits in the subsurface and have tracesgoing through several beds or crossing entireoutcrops (Finn et al. 2003). These fracturesgenerally form by the coalescence of pre-existing joints and can be reactivated to formfaults and indicate a locally increased strain(Gross & Eydal 2007). In this case, the HTDcorridors span the whole outcrop, have recti-linear and elongated outlines and a consistentorientation, and are rather systematicallyspaced. Fault planes can be observed in theHTD bodies and are mineralized by HTD. Inaddition, relict limestone lenses occur insome of the HTD corridors. These lenses

Fig. 17. Along the Pozalagua Fault a non-ferroan dolomite vein crosscuts ferroan dolomite.

Fig. 18. (a) Distribution of ferroan and non-ferroan HTD. The HTDs occur in clusters of the same mineralogy, that is,ferroan and non-ferroan dolomite. (b) Zones dominated by cement dolomites correspond to zones where non-ferroandolomites are prevailing and vice versa.

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https://www.researchgate.net/publication/240379213_Kinematics_of_throughgoing_fractures_in_jointed_rocks?el=1_x_8&enrichId=rgreq-f8adebb7-91a5-4bf6-afd4-70fcd5cb420a&enrichSource=Y292ZXJQYWdlOzI1ODY0NjMxNTtBUzoxODE1NTE4ODMxMDQyNTZAMTQyMDI5Njc1MTA0OQ==

have their main axis parallel to the outline ofthe HTD body. This is possibly related to theamalgamation of different dolomitized faultsand fractures of the throughgoing fractures.

A lineament bordering the breccia body to thenorth and NE can be observed. In the field this linea-ment occurs as a deeply eroded joint separatingintensely veined limestone and brecciated lime-stone. This configuration is similar to typicaldamage zones caused by faults (Billi et al. 2003).Even though no direct indications for displacementcan be found along this lineament, it can be inter-preted as a fault. The occurrence of a relativelylarge breccia body along a bending fault in a strike-slip regime suggests that the breccia body resultsfrom the implosion of the limestone host rock in adilational bend, as described by Sibson (1994).

The area surrounding the Pozalagua Fault andbetween the Pozalagua Fault and the Breccia Fault

has probably been influenced by important sinistralstrike-slip faulting, while outside this area the sinis-tral strike-slip deformation was more restricted.This interpretation is based on the absence oflarge-scale fault displacements and the differencein the present joint sets. This difference in localstructural setting in the Ramales Platform slopecould have resulted in HTD bodies of differentsizes, shapes and contents.

The Pozalagua HTD body (12 P) is hosted byslope deposits and back-reef carbonates. Only forthis HTD body a sedimentological control on thesize and outline of the body can be considered. Inthe platform slope the HTD body has a maximumwidth of 253 m and an average width of 120 m.The outline of the HTD body is lobate and dolomite‘tongues’ (Fig. 17) occur along N70E joints. In theback-reef carbonates, the HTD body’s extent isbroader and reaches up to 920 m with an averagewidth of 450 m. The contact of the HTD and host

Fig. 19. Standard porosity distribution of the HTD and limestone host rock. Samples with low and high porosity occurin clusters. The radius of the circles is proportional to the porosity.

Table 4. Porosity distribution for HTDs and limestone

Porosity Cement Matrix Zebra LimestoneN 31 9 78 2

Average 5.8 6.8 5.5 0.2Median 5.7 6.4 5.3Minimum 1.9 5.0 1.8Maximum 11.8 11.8 12.1



https://www.researchgate.net/publication/222570302_The_damage_zone-fault_core_transition_in_carbonate_rocks_Implications_for_fault_growth_structure_and_permeability?el=1_x_8&enrichId=rgreq-f8adebb7-91a5-4bf6-afd4-70fcd5cb420a&enrichSource=Y292ZXJQYWdlOzI1ODY0NjMxNTtBUzoxODE1NTE4ODMxMDQyNTZAMTQyMDI5Njc1MTA0OQ==

limestone in the back-reef carbonates is straight andthe orientation of the contact corresponds to jointorientations, that is, N30W, N40E and EW. Thedensity of joints is not remarkably higher in theback-reef carbonates compared with slope carbon-ates. Therefore, in addition to the structuralcontrol on the HTD bodies’ size and outline, a sedi-mentological control can be inferred as well.

In summary, the studied HTD bodies can berelated to faults and joints with a N30W strike,which control the orientation of the main axis ofthe HTD bodies. In addition, joints with EW andN40E strike control the lateral extent of the Pozala-gua HTD body. During dolomitization, these jointswere permeable pathways and channelized thehydrothermal fluids away from the feeder fault.

HTD characteristics and controlling

parameters

The Pozalagua HTD body (12 P) is highly suitablefor geostatistical analysis of HTD characteristics.It is large enough to identify trends and at thesame time it is small enough to be sampled andmapped in detail. HTDs were simply classifiedbased on their texture as matrix, zebra and cementdolomite. In the Pozalagua body, the HTDs cropout in clusters of the same textural dolomite type.Moreover, matrix dolomite and limestone occur atneighbouring sites. Zebra dolomite is found in thevicinity of the Pozalagua Fault and in the central

part of the Pozalagua body. Cement dolomitesmainly occur along the Pozalagua Fault and the plat-form edge, where they are the dominant HTD type(Fig. 15). The anisotropy directions in the distri-bution of the HTD types were investigated bymeans of indicator omnidirectional semivariogramsand were found to be N30W and N60E. These orien-tations correspond to the strike of the PozalaguaFault, the orientation of the platform margin andjoints. The presence of zebra dolomite is describedby Davies & Smith (2006) as related to low per-meability precursor limestones. In the fault-relatedHTD bodies of the study area, no relation couldbe established between the precursor limestone,which is tight and generally consists of bioclasticwackestone, and the dolomite texture. In the rela-tively small fault-related HTD bodies the spatial dis-tribution of dolomite textures is controlled bystructural features as described above.

This distribution of HTD types indicates ageneral trend that can be explained by the migrationof the dolomitizing fluids in Ranero. Matrix dolo-mite, which is fine crystalline and replacive, occursfar away from the fault near the limestone hostrock contact. Zebra dolomites are present in thePozalagua Fault Zone and west of it. It is generallyaccepted that zebra dolomites are the result offocussed and high-pressure fluid flow near tofaults (Nielsen et al. 1998; Vandeginste et al.2005; Davies & Smith 2006). Cement dolomitesare coarse crystalline, fabric-destructive and specifi-cally originate from the second dolomitization

Table 5. Porosity distribution of ferroan andnon-ferroan dolomites

Porosity Ferroan Non-ferroanN 64 60

Average 5.5 5.9Median 5.4 5.3Minimum 1.9 1.8Maximum 12.1 11.8

Fig. 20. (a) Boxplot of the porosity values found in different HTD types. (b) Boxplot of the porosity values observed inthe ferroan and non-ferroan dolomites.

Table 6. Permeability distribution of HTDs andlimestone

Permeability Cement Matrix Zebra LimestoneN 31 9 78 2

Average 1.0 2.3 1.0 0.01Median 0.2 0.2 0.1Minimum 0.01 0.03 0.01Maximum 10.3 17.4 16.9

J. DEWIT ET AL.


phase. Therefore, the cement dolomites could be theresult of high water–rock ratios and/or the productof recrystallized dolomites from the first dolomitiza-tion phase. Discontinuities between clinoformscould have been utilized by hydrothermal fluids asa pathway from the basin to the carbonate platform.Although joints with a N60E strike occur in the area,the control by discontinuities in the platform edge ispreferred because of the dominant occurrence andthe large portion of cement dolomites present as amajor cluster along the platform edge, and the pres-ence of a small HTD body within clinoforms (blackarrow, Fig. 7b).

Porosity of the HTDs is distributed in clusters ofhigh and low values in the Pozalagua body. No sig-nificant difference in porosity could be computedfor the different HTD types, but in general HTDs

resulting from the first dolomitization phase have ahigher porosity than those resulting from the seconddolomitization phase, which commonly have thecement dolomite texture. This could be relatedto a certain degree of overdolomitization (Lucia2004) and/or cementation and recrystallization.Permeability also occurs in clusters of high andlow values in the Pozalagua body. The permeabilityvalues cannot be related to the different HTD typesdefined in the Pozalagua body, since it was notsignificantly different for the different HTD types.No significant difference in permeability valueswas observed for ferroan and non-ferroan dolo-mites. Therefore, dolomitization resulting from thesecond non-ferroan phase resulted in partial orlocal pore cementation (lower porosity values), butdid not significantly lower the permeability values.

Fig. 21. Map of the permeability values of the HTD. Notice that samples with low and high permeability values occurin clusters.

Fig. 22. (a) Boxplots of permeability values per HTD type. (b) Boxplot of permeability values measured in the ferroanand non-ferroan dolomites.



Finally, it should be noted that these resultswere obtained from standard-sized plug samples(2.5 cm diameter and +5 cm long). Previousstudies (Ehrenberg 2007) have demonstrated theimportance of the sample size for porosity and per-meability values, with whole cores having higherporosity and permeability values than standardplugs. Sampling dolomites with vugs larger thanthe plugs diameter is impossible. Ideally, the Repre-sentative Elementary Volume for samples with largevuggy pores should be defined and the sample sizeshould be adapted to that volume. In practice, cover-ing a representative geospatial dataset with largersamples is not achievable; moreover, standard por-osity and permeability measurements can only beperformed on samples of well-defined dimensions.In addition, densely fractured dolomite cannot besampled (since plugs will break along fractures).The permeability created by fractures in the dolo-mite is thus underrepresented in the dataset. The per-meability related to these fractures is, however,important for HTD reservoir exploitation.

Conclusions

In the Ramales Platform, HTD bodies of differentsizes are present. The largest HTD body, the

Pozalagua body, is related to an important strike-slip fault and back-reef deposits of the Ramales Plat-form. The lateral extent and outline of this HTDbody is controlled by structural and sedimentologi-cal parameters. In the first instance the size of theHTD can be related to the Pozalagua Fault witha horizontal throw of approximately 500 m. Theoutline of the HTD in the back-reef carbonates iscontrolled by three sets of joints with a N30W,N40E and EW strike. A sedimentological controlof the HTD body’s outline and shape can be inferredsince the HTD body’s extent is considerably largerin the back-reef than in the slope carbonates. Inthe slope sediments the outline of the HTD bodyis not rectilinear, but HTD ‘tongues’ developedalong joints with N70E strike. The breccia body isrelated to a dilational bend along the BrecciaFault. This fault bends towards the PozalaguaFault to the north. Both HTD bodies formed in azone of localized sinistral strike-slip deformation,possibly related to the Pozalagua step-over Faultbetween two major regional faults.

The elongated HTD corridors formed in thesame sinistral strike-slip regime as the previouslymentioned HTD (cemented) bodies. The HTD corri-dors are interpreted as dolomitized throughgoingfractures, because of their rectilinear outline, verti-cal importance and consistent orientation, which isparallel to a set of joints and faults with N30Wstrike. Small HTD corridors systematically occurclose to neighbouring HTD bodies.

The HTD types (i.e. matrix, cement and zebradolomite) occur in clusters of the same type. Thedistribution of the types is controlled by two direc-tions of anisotropy, that is, N30W and N60E,which correspond to the strike of the PozalaguaFault, joints and the platform edge. Porosity andpermeability values are not significantly differentfor the different HTD types.

Two dolomitization phases have been recognizedin the Ramales Platform: a ferroan and a non-ferroanphase. The influences on the HTD are twofold. On theone hand the texture of the HTD is influenced. Non-ferroan dolomites are dominantly cement dolomites.On the other hand, ferroan dolomites have higherporosity values than non-ferroan dolomites. No sig-nificant difference was found for the permeabilityof ferroan and non-ferroan dolomites. Future workshould be carried out to check the effect of thesamples’ size on the porosity and permeabilityvalues and distribution of HTD.

In the studied area of 10 km2, 15 different HTDbodies are present. All HTD bodies are of sub-reservoir scale. Nevertheless, if HTD corridors ora massive fault-controlled HTD body are encoun-tered during exploratory (field) work, larger HTDbodies can be expected to occur along the mainregional faults and/or in the inner-platform

Table 7. Permeability distribution of ferroan andnon-ferroan dolomites

Permeability Ferroan Non-ferroanN 64 60

Average 0.4 2.1Median 0.1 0.4Minimum 0.01 0.02Maximum 3.7 17.4

Fig. 23. Overview of the Riedel shear fractureorientation, given that the principal displacement zone(PDZ) has a N30W strike.

J. DEWIT ET AL.


https://www.researchgate.net/publication/249897900_Whole_core_versus_plugs_Scale_dependence_of_porosity_and_permeability_measurements_in_platform_carbonates?el=1_x_8&enrichId=rgreq-f8adebb7-91a5-4bf6-afd4-70fcd5cb420a&enrichSource=Y292ZXJQYWdlOzI1ODY0NjMxNTtBUzoxODE1NTE4ODMxMDQyNTZAMTQyMDI5Njc1MTA0OQ==

sediments. Trends and parameters identified as con-trolling factors for the distribution of reservoircharacteristics of HTD by means of (geo)-statisticalinvestigation can most probably be extrapolated toor used in studies of similar sub-surface HTD bodies.

This contribution is the result of a collaboration betweenresearchers of the K.U. Leuven and Statoil A.S.A. Theauthors would like to thank Statoil A.S.A. for the possi-bility to publish the data. S. Homke, J.-C. Embry,F. Lapponi, J. Bernaus and A. Colpaert are acknowledgedfor their direct or indirect contributions to this work. Thisresearch is financed by Statoil. J. Dewit benefits from adoctoral grant of the Institute for the Promotion of Inno-vation through Science and Technology Flanders.M. Huysmans benefits of a post-doctoral grant from theFund for Scientific Research, Flanders. J. Garland andT. Smith are kindly acknowledged for the review of thismanuscript; their constructive comments were helpfuland improved the quality of the manuscript. The authorswould like to thank R. Snellings and M. Dewit for theirassistance during fieldwork.

References

Agirrezabala, L. M. 2009. Mid-Cretaceous hydrother-mal vents and authigenic carbonates in a transformmargin, Basque–Cantabrian Basin (Western Pyre-nees): a multidisciplinary study. Sedimentology, 56,969–996, doi: 10.1111/j.1365-3091.2008.01013.x.

Al-Aasm, I. 2003. Origin and characterisation of hydro-thermal dolomite in the Western Canada SedimentaryBasin. Journal of Geochemical Exploration, 78–79,9–15, doi: 10.1016/S0375-6742(03)00089-X.

Billi, A., Salvini, F. & Stoti, F. 2003. The damage zone-fault core transition in carbonate rocks: implicationsfor fault growth, structure and permeability. Journalof Structural Geology, 25, 1779–1794, doi: 10.1016/S0191-8141(03)00037-3.

Cantrell, D., Swart, P. K. & Hagerty, R. M. 2004.Genesis and characterisation of dolomite, Arab-D Reser-voir, Ghawar Field, Saudi Arabia. GeoArabia, 9, 11–36.

Davies, G. R. & Smith, B. Jr. 2006. Structurally con-trolled hydrothermal dolomite reservoir facies: anoverview. American Association of Petroleum Geol-ogists Bulletin, 90, 1641–1690, doi: 10.1306/05220605164.

Deutsch, C. V. & Journel, A. G. 1998. GSLIB Geosta-tistical Software Library and user’s Guide. OxfordUniversity Press, New York.

Ehrenberg, S. N. 2007. Whole core v. plugs: scale depen-dence of porosity and permeability measurements inplatform carbonates. American Association of Pet-roleum Geologists Bulletin, 91, 835–846.

Fabuel-Perez, I., Hodgetts, D. & Redfern, J. 2009. Anew approach for outcrop characterization and geosta-tistical analysis of a low-sinuosity fluvial-dominatedsuccession using digital outcrop models: Upper Trias-sic Oukaimeden Sandstone Formation, central HighAtlas, Morocco. American Association of PetroleumGeologists Bulletin, 93, 795–827, doi: 10.1306/02230908102.

Fernandez-Mendiola, P. A., Gomez-Perez, I. &Garcıa-Mondejar, J. 1993. Aptian-Albian carbonateplatforms: central Basque–Cantabrian Basin, NorthernSpain. In: Toni, T. J. A., Scott, R. W. & Masse, J.-P.(eds) Cretaceous Carbonate Platforms. AmericanAssociation of Petroleum Geologists, Tulsa, OK,Memoirs, 56, 315–324.

Finn, M. D., Gross, M. R., Eydal, Y. & Draper, G. 2003.Kinematics of throughgoing fractures in jointedrocks. Tectonophysics, 376, 151–166, doi: 10.1016/j.tecto.2003.10.001.

Garcıa-Mondejar, J. 1989. Strike-slip subsidence ofthe Basque–Cantabrian Basin of Northern Spain andits relationship to Aptian–Albian opening of bayof biscay. In: Tankard, A. J. & Balkwill, H. R.(eds) Extensional Tectonics and Stratigraphy of theNorth Atlantic Margins. American Association ofPetroleum Geologists, Tulsa, OK, Memoirs, 46,395–409.

Garcıa-Mondejar, J. 1990. The Aptian–Albian carbon-ate episode of the Basque–Cantabrian Basin (northernSpain): general characteristics, controls and evolution.In: Tucker, M. E., Wilson, J. L., Crevello, P. D.,Sarg, J. F. & Read, J. F. (eds) Carbonate Platforms:Facies, Sequences and Evolution. InternationalAssociation of Sedimentologists, Special Publications,9, 257–290.

Garcıa-Mondejar, J. & Fernandez-Mendiola, P. A.1993. Sequence stratigraphy and system tracts of amixed carbonate and siliciclastic platform-basinsetting: the Albian of Lunada and Soba, northernSpain. American Association of Petroleum GeologistsBulletin, 77, 245–275.

Garcıa-Mondejar, J., Agirrezabala, L. M., Ara-

nburu, A., Fernandez-Mendiola, P. A., Gomez-

Perez, I., Lopez-Horgue, M. & Rosales, I. 1996.Aptian–Albian Tectonic pattern of the Basque–Can-tabrian Basin (northern Spain). Geological Journal,31, 13–45.

Garcıa-Mondejar, J., Lopez-Horgue, M. A., Ara-

nburu, A. & Fernandez, P. A. 2005. Pulsating subsi-dence during a rift episode: stratigraphic and tectonicconsequences (Aptian–Albian, northern Spain).Terra Nova, 17, 517–525.

Gasparrini, M., Bechstadt, T. & Boni, M. 2006.Massive hydrothermal dolomites in the southwesternCantabrian Zone (Spain) and their relation to theLate Variscan Evolution. Marine and PetroleumGeology, 23, 543–568, doi: 10.1016/j.marpetgeo.2006.05.003.

Geehan, G. & Underwood, J. 1993. The use of lengthdistributions in geological modeling. In: Flint, S. S.& Bryant, I. D. (eds) The Geological Modelling ofa Hydrocarbon Reservoir and Outcrop Analogues.Association of Sedimentologists, Special Publications,15, 205–212.

Grandia, F., Canals, A., Cardellach, E., Banks, D. A.& Perona, J. 2003. Origin of ore-forming brines insediment-hosted Zn–Pb deposits of the Basque–Cantabrian Basin, Northern Spain. Economic Geology,98, 1397–1411, doi: 10.2113/gsecongeo.98.7.1397.

Gross, M. R. & Eydal, Y. 2007. Throughgoing fracturesin layered carbonate rocks. Geological Society ofAmerica Bulletin, 119, 1387–1404.



Hopkins, J. C. 2004. Geometry and origin of dolomud-stone reservoirs: Pekisko Formation (Lower Carbon-iferous), western Canada. In: Braithwaite, C. J. R.& Darke, G. (eds) The Geometry and Petrogenesisof Dolomite Hydrocarbon Reservoirs. GeologicalSociety, London, Special Publications, 235,349–366, doi: 10.1144/GSL.SP.2004.235.01.15.

Isaaks, E. H. & Srivastava, R. M. 1989. An Introductionto Applied Geostatistics. Oxford University Press,New York.

Lopez-Horgue, M. A. 2000. El Aptiense–Albiense deKarrantza–Lanstona (Bizkaia y Cantabria). PhDthesis, Universidad del Paıs Vasco, Euskal HerrikoUnibertsitatea.

Lopez-Horgue, M. A., Uriarte, E. et al. 2010. Structu-rally controlled hydrothermal dolomites in Albian car-bonates of the Ason valley, Basque Cantabrian Basin,Northern Spain. Marine and Petroleum Geology, 27,1069–1092, doi: 10.1016/j.marpetgeo. 2009.10.015.

Lucia, J. 2004. Origin and petrophysics of dolostone porespace. In: Braithwaite, C. J. R. & Darke, G. (eds)The Geometry and Petrogenesis of Dolomite Hydro-carbon Reservoirs. Geological Society, London,Special Publications, 235, 141–155, doi: 10.1144/GSL.SP.2004.235.01.06.

Machel, H. G. 2004. Concepts and models of dolomitiza-tion: a critical reappraisal. In: Braithwaite, C. J. R.,Rizzi, G. & Drake, G. (eds) The Geometry and Petro-genesis of Dolomite Hydrocarbon Reservoirs. Geo-logical Society, London, Special Publications, 235,7–63, doi: 10.1144/GSL.SP.2004.235.01.02.

Mallone, M. J., Baker, P. A. & Burns, S. J. 1996.Hydrothermal dolomitization and recrystallisation ofdolomite breccias from the Miocene Monterey For-mation, Tepusquet Area, California. Journal of Sedi-mentary Research, 66, 976–990, doi: 10.1306/D426845A-2B26-11D7-8648000102C1865D.

Martın-Chivelet, J., Berastegui, X. et al. 2002.Cretaceous. In: Gibbons, W. & Moreno, M. T. (eds)The Geology of Spain. Geological Society, London,255–292.

Morrow, D. W. 1982. Descriptive field classification ofsedimentary and diagenetic breccia fabrics in carbon-ate rocks. Bulletin of Canadian Petroleum Geology,30, 227–229.

Nader, F. H., Dumont, C. et al. 2009. From field studyto numerical modelling of hydrothermal dolomiti-zation in Early Cretaceous platform carbonates(Cantabrian mountains, northern Spain). Journal ofGeochemical Exploration, 101, 73.

Nielsen, P., Swennen, R., Muchez, PH. & Keppens, E.1998. Origin of Dinantian zebra dolomites south of theBrabant-Wales Massif, Belgium. Sedimentology, 45,727–743, doi: 10.1046/j.1365-3091.1998.00171.x.

Pannatier, Y. 1996. VARIOWIN: Software for SpatialData Analysis in 2D. Springer, New York.

Rat, P. 1959. Les pays Cretaces Basco-Cantabriques(Espagne). Pub. Univ. Dijon, IV, 525.

Robador Moreno, A., Heredia Carballo, N., Rodrı-

guez Fernandez, L. R. & Marquinez Garcıa, J.2000. Mapa Geologico de Cantabria. Instituto Geolo-gico y Minero de Espana.

Ronchi, P., Jadoul, F., Ceriani, A., Di Giulio, A.,Scotti, P., Ortenzi, A. & Previde Massara, E.

2011. Multistage dolomitization and distribution ofdolomitized bodies in Early Jurassic carbonate plat-forms (Southern Alps, Italy). Sedimentology, 58,532–565, doi: 10.1111/j.1365-3091.2010.01174.x.

Rosales, I. 1999. Controls on carbonate-platform evol-ution on active fault blocks: the lower cretaceouscastro urdiales platform (Aptian–Albian, northernSpain). Journal of Sedimentary Research, 69,447–465.

Rosales, I. & Perez-Garcıa, A. 2010. Porosity develop-ment, diagenesis and basin modelling of a Lower Cre-taceous (Albian) carbonate platform from northernSpain. In: van Buchem, F. S. P., Gerdes, K. D. &Esteban, M. (eds) Mesozoic and Cenozoic CarbonateSystems of the Mediterranean and the Middle East:Stratigraphic and Diagenetic Reference Models. Geo-logical Society, London, Special Publications, 329,317–342, doi: 10.1144/SP329.13.

Shah, M. M., Nader, F. H., Dewit, J., Swennen, R. &Garcia, D. 2010. Fault-related hydrothermal dolo-mites in cretaceous carbonates (Cantabria, northernSpain). Bulletin de la Societe Geologique deFrance, 181, 389–405, doi: 10.2113/gssgfbull.181.4.391.

Sharp, I., Gillespie, P. et al. 2010. Stratigraphic archi-tecture and fracture controlled dolomitization of theCretaceous Khami and Bangestan groups: an outcropcase study, Zargos Mountains, Iran. In: van

Buchem, F. S. P., Gerdes, K. D. & Esteban, M.(eds) Mesozoic and Cenozoic Carbonate Systems ofthe Mediterranean and the Middle East: Stratigraphicand Diagenetic Reference Models. Geological Society,London, Special Publications, 239, 343–396, doi:10.1144/SP329.14.

Sibson, R. H. 1994. Crustal stress, faulting and fluid flow.In: Parnell, J. (ed.) Geofluids: Origin, Migration andEvolution of Fluids in Sedimentary Basins. GeologicalSociety, London, Special Publications, 78, 69–84, doi:10.1144/GSL.SP.1994.078.01.0.

Simon, S., Canals, A., Grandia, F. & Cardellach, E.1999. Estudio isotopica y de inclusions fluidas endepositos de calcita y dolomita del sector oeste delAnticlinal de Bilbao y su relacion con las mineraliza-ciones de Fe–Zn–Pb. Boletın de la Sociedad Espanolade mineralogıa, 22, 55–71.

Sylvester, A. G. 1988. Strike-slip faults. Geolog-ical Society of American Bulletin, 100, 1666–1703,doi: 10.1130/0016-7606(1988)100,1666:SSF.2.3.CO;2.

Swennen, R., Dewit, J. et al. 2009. Evidence ofrepeated hot fluid pulses along the Ranero Faultsystem (Cantabrian Mountains, North Spain). Journalof Geochemical Exploration, 101, 100.

Swennen, R., Dewit, J., Fierens, E., Muchez, PH.,Shah, M., Nader, F. H. & Hunt, D. 2012. Multipledolomitisation events along the Ranero Fault (Pozala-gua Quarry, Basque–Cantabrian Basin): episodicearthquake activity. Sedimentology, doi: 10.1111/j.1365-3091.2011.01309.x.

Twiss, R. J. & Moores, E. M. (eds) 1992. Strike-slip faults.Structural Geology, 113–127.

Vandeginste, V., Swennen, R., Ellam, R. & Schnei-

der, F. 2005. Zebra dolomitization as a result of foca-lized fluid flow in the Rocky Mountains Fold and

J. DEWIT ET AL.


Thrust Belt. Sedimentology, 52, 1067–1095, doi:10.1111/j.1365-3091.2005.00724.x.

Velasco, F., Herrero, J. M., Gil, P. P., Alvarez, L. &Yusta, I. 1994. Mississippi valley-type, sedex andiron deposits in lower cretaceous rocks of the Basque–Cantabrian Basin, Northern Spain. In: Fontbote, L. I.& Boni, M. (eds) Sediment-Hosted Zn–Pb Ores.Society of Geology Applied to Mineral Deposits,Special Publications, 10, 246–270.

Velasco, F., Pesquera, A. & Herrero, J. M. 1996.Lead isotope study of Zn–Pb ore deposits associatedwith the Basque–Cantabrian basin and PaleozoicBasement, Northern Spain. Mineralium Deposita, 31,84–92, doi: 10.1007/BF00225398.

Verges, J., Fernandez, M. & Martınez, A. 2002. ThePyrenean orogen: pre-, syn-, and post-collisional evol-ution. In: Rosenbaum, G. & Lister, G. S. (eds) Recon-struction of the Evolution of the Alpine–HimalayanOrogen. Journal of the Virtual Explorer, 8, 55–74,doi: 10.3809/jvirtex.2002.00058.

Wilcox, R. E., Harding, T. P. & Seely, D. R. 1973. Basicwrench tectonics. The American Association ofPetroleum Geologists Bulletin, 57, 74–96.

Wilson, M. E. J., Evans, M. J., Oxtoby, N. H., Nas, S. D.,Donnelly, T. & Thirlwall, M. 2007. Reservoirquality, textural evolution, and origin of fault associ-ated dolomites. Association of Petroleum GeologistsBulletin, 91, 1247–1272, doi: 10.1306/0507 0706052.



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