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Halite cementation and carbonate diagenesis of intra-salt reservoirs from the Late Neoproterozoic to Early Cambrian Ara Group (South Oman Salt Basin) JOHANNES SCHOENHERR* 1 , LARS REUNING  , PETER A. KUKLA  , RALF LITTKE à , JANOS L. URAI*, MICHAEL SIEMANN§ and ZUWENA RAWAHI *Lehr- und Forschungsgebiet Geologie – Endogene Dynamik, Lochnerstrasse 4-20, D-52056 RWTH Aachen, Germany (E-mail: [email protected])  Geologisches Institut, Wu ¨ llnerstrasse 2, D-52056 RWTH Aachen, Germany àLehrstuhl fu ¨ r Geologie, Geochemie und Lagersta ¨ tten des Erdo ¨ ls und der Kohle, Lochnerstrasse 4-20, D- 52056 RWTH Aachen, Germany §Technische Universita ¨ t Clausthal, Institut fu ¨ r Mineralogie und Mineralische Rohstoffe, Adolph-Roemer Str. 2A, 38678 Clausthal–Zellerfeld, Germany Petroleum Development Oman LLC, P.O. Box 81, P.C. 113, Muscat, Sultanate of Oman Associate Editor: Daniel Ariztegui ABSTRACT Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form important intra-salt ‘stringer’ reservoirs in the South Oman Salt Basin. Differential loading of thick continental clastics above the six carbonate to evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a predominantly self-charging hydrocarbon system within partly highly overpressured carbonate bodies (‘stringers’). These carbonates underwent a complex diagenetic evolution, with one stage of halite cementation in a shallow (early) and another in a deep (late) burial environment. Early and late halite cements are defined by their microstructural relationship with solid bitumen. The early phase of halite cementation is post-dated by solid reservoir bitumen. This phase is most pervasive towards the top of carbonate stringers, where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics. Bromine geochemistry revealed significantly higher bromine contents (up to 280 p.p.m.) in the early halite compared with the late halite (173 p.p.m.). The distribution patterns and the (high) bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval. Later in burial history, relatively small quantities of early halite were dissolved locally and re-precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements. Late halite cement also seals fractures which show evidence for repeated reopening. Initially, these fractures formed during a period of hydrothermal activity and were later reopened by a crack-seal mechanism caused by high fluid overpressures. Porosity plugging by early halite cements affects the poroperm characteristics of the Ara carbonates much more than the volumetrically less important late halite cement. The formation mechanisms and distribution patterns of halite cementation processes in the South Oman Salt Basin can be generalized to other petroliferous evaporite basins. Keywords Bromine geochemistry, carbonate diagenesis, dolomite, evapor- ites, halite cementation, solid bitumen. 1 Present address: ExxonMobil Production Deutschland GmbH, Riethorst 12, 30659 Hannover, Germany. Sedimentology (2009) 56, 567–589 doi: 10.1111/j.1365-3091.2008.00986.x Ó 2008 The Authors. Journal compilation Ó 2008 International Association of Sedimentologists 567

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Halite cementation and carbonate diagenesis of intra-saltreservoirs from the Late Neoproterozoic to Early Cambrian AraGroup (South Oman Salt Basin)

JOHANNES SCHOENHERR*1, LARS REUNING� , PETER A. KUKLA� , RALF LITTKE� ,JANOS L. URAI*, MICHAEL SIEMANN§ and ZUWENA RAWAHI–*Lehr- und Forschungsgebiet Geologie – Endogene Dynamik, Lochnerstrasse 4-20, D-52056 RWTHAachen, Germany (E-mail: [email protected])�Geologisches Institut, Wullnerstrasse 2, D-52056 RWTH Aachen, Germany�Lehrstuhl fur Geologie, Geochemie und Lagerstatten des Erdols und der Kohle, Lochnerstrasse 4-20, D-52056 RWTH Aachen, Germany§Technische Universitat Clausthal, Institut fur Mineralogie und Mineralische Rohstoffe, Adolph-RoemerStr. 2A, 38678 Clausthal–Zellerfeld, Germany–Petroleum Development Oman LLC, P.O. Box 81, P.C. 113, Muscat, Sultanate of Oman

Associate Editor: Daniel Ariztegui

ABSTRACT

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group form

important intra-salt ‘stringer’ reservoirs in the South Oman Salt Basin.

Differential loading of thick continental clastics above the six carbonate to

evaporite cycles of the Ara Group led to the formation of salt diapirs, encasing a

predominantly self-charging hydrocarbon system within partly highly

overpressured carbonate bodies (‘stringers’). These carbonates underwent a

complex diagenetic evolution, with one stage of halite cementation in a shallow

(early) and another in a deep (late) burial environment. Early and late halite

cements are defined by their microstructural relationship with solid bitumen.

The early phase of halite cementation is post-dated by solid reservoir bitumen.

This phase is most pervasive towards the top of carbonate stringers, where it

plugs nearly all available porosity in facies with initially favourable poroperm

characteristics. Bromine geochemistry revealed significantly higher bromine

contents (up to 280 p.p.m.) in the early halite compared with the late halite

(173 p.p.m.). The distribution patterns and the (high) bromine contents of early

halite are consistent with precipitation caused by seepage reflux of highly

saturated brines during deposition of the overlying rock salt interval. Later in

burial history, relatively small quantities of early halite were dissolved locally

and re-precipitated as indicated by inclusions of streaky solid bitumen within

the late halite cements. Late halite cement also seals fractures which show

evidence for repeated reopening. Initially, these fractures formed during a period

of hydrothermal activity and were later reopened by a crack-seal mechanism

caused by high fluid overpressures. Porosity plugging by early halite cements

affects the poroperm characteristics of the Ara carbonates much more than the

volumetrically less important late halite cement. The formation mechanisms

and distribution patterns of halite cementation processes in the South Oman Salt

Basin can be generalized to other petroliferous evaporite basins.

Keywords Bromine geochemistry, carbonate diagenesis, dolomite, evapor-ites, halite cementation, solid bitumen.

1Present address: ExxonMobil Production Deutschland GmbH, Riethorst 12, 30659 Hannover, Germany.

Sedimentology (2009) 56, 567–589 doi: 10.1111/j.1365-3091.2008.00986.x

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INTRODUCTION

Carbonates of the latest Neoproterozoic to earliestCambrian Ara Group in the South Oman SaltBasin (SOSB) are fully encased by 3 to 6 kmburied diapirs of Ara salt and can be pervasivelycemented by halite (Fig. 1). These carbonatesrepresent an intra-salt self-charging petroleumsystem with substantial hydrocarbon accumula-tions in dolomitized laminites and thrombolites(the so-called Ara carbonate ‘stringer’ play),which show good poroperm characteristics(Mattes & Conway Morris, 1990; Schroder et al.,2005). Some of the stringers, however, failed to beproduced at significant rates because of extensivehalite cementation of pore space, which repre-sents one of the greatest unknown risks forhydrocarbon exploration in this area (Al-Siyabi,2005).

Although halite cementation forms a majorrisk for hydrocarbon production in many evap-orite basins, only little work has been publishedthat focuses on the characteristics and theformation mechanisms of halite cements. Mostprevious information on halite cementationcomes from reservoir core studies of the terrige-neous Triassic Bunter Formation (Bifani, 1986;Laier & Nielsen, 1989; Dronkert & Remmelts,1996; Purvis & Okkerman, 1996; Putnis &Mauthe, 2001) and from intra-salt clasticreservoirs from the Shabwa Basin in Yemen(Seaborne, 1996).

Studies of halite cementation from marineevaporite basins are reported in only a few cases(Sears & Lucia, 1980; Gill, 1994; Strohmengeret al., 1996; Kendall, 2000; Warren, 2006). Inthese settings, thick evaporites (mainly rocksalt) sandwich shallow marine carbonates, which

Fig. 1. Location map showing the three Infra-Cambrian subsurface evaporite basins (shaded in grey) of interiorOman (after Loosveld et al., 1996). Cross-section A–B shows the simplified geometry of the SOSB. Differentialloading of thick continental clastics (Haima pods) led to diapirism of the Ara Group halite, which encloses isolatedcarbonate ‘stringers’ (from A1C to A5C), which are potential hydrocarbon reservoirs (modified after Peters et al.,2003). The studied halite-cemented carbonates originate from two exploration areas (marked by crosses) in the south-westernmost part of the SOSB.

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show pervasive halite cementation in their upper-most parts.

The scope of this paper is to provide the basicconcepts for halite cementation in the intra-saltAra carbonate stringers to better predict zoneswith deteriorated reservoir quality. It will bedemonstrated that detailed petrographic investi-gations and correlated bromine geochemistry areeffective tools for separating different phases ofhalite cementation in the Ara stringers which, inturn, places constraints on timing, distributionand formation mechanisms of halite cementation.The results can be generalized readily to otherpetroliferous evaporite basins.

GEOLOGICAL SETTING

General

The study area is located in the south-western-most part of the SOSB, which is one of three deepsubsurface salt basins of interior Oman (Fig. 1).The Ara salt forms part of a restricted Infra-Cambrian salt basin system, which stretches fromthe Arabian Shield to central Iran and probablyfurther east (Mattes & Conway Morris, 1990). Theformation of the SOSB started with the sedimen-tation of the Huqf Supergroup from the LateNeoproterozoic until the Early Cambrian on crys-talline basement (Gorin et al., 1982). The HuqfSupergroup comprises four groups, which aremainly constituted by continental clastics andmarine deposits (see ‘Chronostratigraphy’ inFig. 1). Carbonate cores studied in this papercome from the Ara Group, which spans the Pre-Cambrian/Cambrian boundary. The age of the AraGroup is constrained by absolute U–Pb ages of542Æ0 ± 0Æ3 Ma, and by the discovery of the fossilsCloudina and Namacalathus, which is consistentwith other terminal Proterozoic occurrences(Amthor et al., 2003).

Tectonostratigraphy of the Ara Group

Based on stratigraphic and seismic reconstruc-tions, a geological model for the tectonostrati-graphic evolution of the Ara Group has beenestablished (Fig. 2; Al-Siyabi, 2005; Amthoret al., 2005). During the Late Neoproterozoic,rapid subsidence of the SOSB controlled evapo-rite to carbonate sedimentation of the Ara Group,comprising six third-order cycles (A0/A1 to A6from bottom to top) (Mattes & Conway Morris,1990). Deposition of the first cycle (A0/A1) began

with Ara salt sedimentation on the Buah carbon-ate ramp at very shallow water depths duringbasin restriction, followed by the deposition ofcarbonate platforms (A1C to A6C) developedduring transgressive to highstand conditions(Mattes & Conway Morris, 1990; Schroder, 2000;Al-Siyabi, 2005). Subsequent, strong differentialsubsidence of the SOSB led to fracturing of thecarbonate platforms in the underlying sequences.The depositional architecture (growth of isolatedplatforms and bioherms) and post-depositionalfracturing promoted, in places, the early forma-tion of isolated (and allochthonous) carbonate‘stringers’ which started to ‘float’ in the Ara saltduring progressive burial.

Halite varies laterally in thickness from a fewmetres to more than 2 km. This variation resultsfrom strong halokinesis triggered by differentialloading of the up to 2 km thick continentalHaima clastics above the Ara Group during theMiddle Cambrian (Heward, 1990; Loosveldet al., 1996). The Haima clastics began to saginto the mobile substrate of the Ara salt, leadingto a period of strong diapirism and the forma-tion of pronounced clastic ‘pods’ until the endof the Ordovician (Fig. 1). This ‘downbuilding’mechanism had a large impact on the structuralstyle leading to further fragmentation andfolding of the stringers, and on the hydrocar-bon trapping potential of the Ara carbonatestringer.

The majority of the stringers are strongly over-pressured, most probably a result of conversion ofkerogen to oil and gas, under compaction, mineraltransformation (gypsum fi anhydrite) (Mattes &Conway Morris, 1990) and the external inflow ofhydrothermal fluids (Schoenherr et al., 2007a).Organic geochemical data support in situ matu-ration, with the majority of the oils deriving fromwithin the stringers. The oil later underwent oilto gas cracking and thermochemical sulphatereduction in stringer reservoirs which have beenburied to depths greater than 4 km (Terken et al.,2001; Taylor et al., 2007).

Facies

Increasing sea water salinity because of a decreasein sea-level led to the depositional successioncarbonate–sulphate–halite which, in turn, chan-ged into the succession halite–sulphate–carbon-ate during the following sea-level rise. Generally,the shallow-platform (peritidal) carbonate faciesof the Ara Group consists of grainstones, flatlaminites and laminated stromatolites, while the

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platform barrier is formed by stromatolites andopen-marine thrombolites (Fig. 3). Slope faciesincludes laminated dolostones and the basinalfacies is dominated by sapropelic (crinkly andflat) laminites and mudstones (Al-Siyabi, 2005;Schroder et al., 2005). The total thickness ofindividual carbonate stringers varies from 20 to250 m. This self-sourcing hydrocarbon system isthought to have accumulated organic-rich sourcerocks during sea water highstands in the‘deeper’, periodically anaerobic to dysaerobicparts of the basin (mainly slope and basinalmudstones in Fig. 3). Density stratification ofsea water allowed preservation of a sufficientamount of organic matter in the bottom layers

derived from the highly productive algal mate-rial in the upper water layers (Mattes & ConwayMorris, 1990).

The Ara evaporites include halite and an-hydrite, which replaced primary gypsum (Mattes& Conway Morris, 1990; Schroder et al., 2003).Anhydrite overlying the carbonate is called ‘roofanhydrite’, while that underlying the carbonate isreferred to as ‘floor anhydrite’. Both horizons canbe up to 20 m thick. The thickness of the Ara saltis 50 to 200 m in the A1 to A4 cycles and canexceed 1000 m in the A5 and A6 sequences(Schroder et al., 2003). However, there is a largeuncertainty regarding syn-depositional salt thick-ness because of strong halokinesis.

Fig. 2. Schematic tectono-stratigraphic evolution of the Ara Group cycles during Infra-Cambrian times. Ara Groupsedimentation started with flooding of the precursor Buah topography (cycle A0/A1), which was followed by growthof the Birba platform until A3. The A1 to A3 carbonate platforms are extensive laterally, whereas the A4 platformshave a limited spatial extent. Strong subsidence of the south-western part of the SOSB led to the formation offractures in the underlying strata (adapted from Amthor et al., 2005).

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SAMPLES AND METHODS

Samples

One hundred and ninety thin sections of carbon-ate core plugs from 21 stringers and 24 wells wereinvestigated. One hundred and seventy-five sam-ples were derived from the Greater Harweelexploration area and 15 from the Greater BirbaArea (Fig. 4). The sampling strategy aimed atcovering a wider range of facies in halite-cemented stringer intervals between A1C andA4C and at studying the general diageneticevolution of the Ara carbonates with depth(2800 to 5800 m). The vertical distribution andcharacteristics of halite cementation were inves-tigated in greater detail in two selected stringercores: (i) A1C of well A; and (ii) A2C of well B.The spatial distribution of the carbonate stringers,the sampled wells and their stratigraphic rela-tionship are illustrated in Fig. 4.

Methods

MicroscopyTransmitted and reflected light microscopy wasperformed on polished 3 · 5 cm sized thinsections. Special attention was paid to thethin-section preparation of the halite-cementedcarbonate plugs, which were treated using themethod described by Schleder & Urai (2005). Inaddition, thin sections of selected samples havebeen investigated by scanning electron micro-scopy (SEM) combined with energy-dispersiveX-ray (EDX) spectroscopy, performed on a ZeissDSM-962 (Carl Zeiss AG, Oberkochen, Germany).

Gamma irradiationSelected halite-cemented carbonate plugs weregamma-irradiated for three months at 100 �C witha dose rate between 4 and 6 kGy h)1 to a totaldose of about 4 MGy at the research reactor of theresearch centre Julich, Germany, to highlightotherwise invisible microstructures of the halitecements. The colour intensity observed in mostrock salt samples is heterogeneous, reflecting anirregular distribution of solid-solution impuritiesand crystal defects within halite grains (vanOpbroek & den Hartog, 1985; Schleder & Urai,2005).

X-ray diffraction analysisX-ray diffraction (XRD) bulk-rock analysis of 30selected carbonate samples was carried out usinga D 5000 diffractometer (Siemens AG, Karlsruhe,Germany). In addition, all halite cements analy-sed for bromine geochemistry (n = 21 samples)were measured by XRD. After crushing thehalite-cemented carbonate plugs, the small halitecrystals were picked under a binocular andsubsequently powdered for XRD analysis.

Stable isotopesOxygen and carbon isotope analyses were per-formed at the laboratory of IFM-GEOMAR (Kiel,Germany). Carbonate powder was dissolved in100% H3PO4 at 75 �C in an online, automatedcarbonate reaction device (Kiel-Device) con-nected to a Finnigan Mat 252 mass spectrometer(ThermoFinnigan MAT GmbH, Bremen, Ger-many). Isotope ratios are calibrated to the ViennaPee Dee Belemnite (V-PDB) standard using theNBS-19 carbonate standard. Average standard

Fig. 3. Generalized facies distribution within one Ara carbonate platform with interpreted reservoir and source rockoccurrences (Al-Siyabi, 2005).

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deviation based on analyses of a reference stan-dard is < 0Æ07& for d18O and < 0Æ03& for d13C.

Bromine geochemistryThe bromine content of each sample wasdetermined from two separate weightings by ionchromatography at the Department of Mineralogy,Geochemistry and Salt Deposits at TechnicalUniversity of Clausthal-Zellerfeld (Germany) afterthe method described by Siemann & Schramm(2000). The halite (< 10 mg) was dissolved indistilled water at a ratio of 1:1000. The Cl concen-tration in the calibration standards was adaptedstrictly to that of the crystals and solution. Thestandard deviation of the two weightings variedbetween ±19Æ6 and ±0Æ1 for all samples measured.

RESULTS

Petrography

Based on petrographic observations of 190 thinsections of core plugs from various stringers, aregional paragenetic sequence was established(Fig. 5), which constitutes the basic framework

for processes leading to halite cementation in theAra carbonate reservoirs. Although these obser-vations are derived from different explorationareas (Fig. 4) and different carbonate stringerintervals (A1C to A4C), the diagenetic evolutionof most samples shows a number of similarities.However, because different reservoir attributes(e.g. facies, fluid pressure conditions, poropermcharacteristics) from different depths are proba-bly diagenetically controlled and are prone toshowing a diverging diagenetic evolution, twowells with different attributes were selected for adetailed case study. Ninety of the 190 thinsections studied were from these two wells. WellA targeted the basal A1C reservoir, a hydrostati-cally pressured stringer, at a shallow depth range(3365 to 3443 m), whereas well B tested the A2Cstringer, an overpressured reservoir at depthsgreater than 5000 m. For these two wells, adetailed study of halite cements and their rela-tionship to other diagenetic phases has beenundertaken.

Regional paragenetic sequenceThe paragenetic sequence of the Ara carbonatereservoirs (Fig. 5) is supported by microstructures

B

B

A

Fig. 4. Map (A) shows the palaeo-geography of the SOSB during theA4 cycle of the Ara Group (fromSchroder et al., 2005) and the loca-tion of the two study areas withinthe Southern Carbonate Domain,namely the Greater Harweel andGreater Birba areas. Map (B) showsthe stratigraphy and spatial distri-bution of the Ara carbonate ‘string-ers’ from A1C to A4C. Only thesampled wells are indicated. Arasalt intervals between the stringerintervals A1C to A4C are not shown.

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(Fig. 6). In the case of the intra-salt carbonatestringers, the term ‘shallow burial’ refers todiagenetic processes that occurred until the car-bonate stringers were completely sealed by theAra salt, presumably at a burial depth of around30 m for rock salt (Casas & Lowenstein, 1989).Consequently, the term ‘deep burial’ refers todiagenetic alterations after the carbonate stringerswere fully encased by the Ara salt.

One of the earliest major diagenetic alterationsis attributed to early reflux dolomitization. Ingrainstones and laminites, organic-rich laminaeare replaced partly by euhedral dolomite rhombo-hedrons, a process which results in a majorincrease of intercrystalline mesoporosity tomacroporosity (Fig. 6A). Some thrombolite sam-ples show millimetre-sized fibrous crystals with afan-like texture, defining dolomitized botryoidalaragonite cements. A few grainstones show well-preserved ooids, which exhibit pendant (gravita-tional) cements, most probably formed in thevadose zone (Fig. 6B). Furthermore, most samplesshow drusy euhedral dolomite cements andmicritic cements, which substantially occludeprimary porosity.

Secondary vuggy pores up to 6 mm in diameter,which are connected to irregular-shaped channelsoutlined by strongly corroded dolomite cements,characterize most grainstones and laminites. Inmany cases, this fabric reflects solution-enlargedfracture porosity, creating the pore space avail-able for early anhydrite and halite cementation.

Apart from halite, anhydrite is volumetricallyone of the most porosity-destroying phases in thecarbonate stringers. Its formation occurred inmultiple stages from the shallow to deep burialenvironment. Earliest anhydrite fabrics are pres-ent in the dolomite matrix, mostly as severalsingle laths showing a decussate arrangement,which suggests growth in poorly consolidatedcarbonate. The latest anhydrite phase occurs intapering microfractures which followed andprobably opened solid bitumen-bearing stylolites.Syntaxial growth of this cement is suggested byblocky and euhedral anhydrite crystals up to4 mm in size (Fig. 6C).

Numerous grainstones show a high primaryporosity which is completely plugged by halite.This effect is also seen in stromatolites showingwell-preserved fenestral pores and in thrombo-lites exhibiting growth-framework porosity. Incontrast to the dolomite matrix, these halite-cemented pores generally are devoid of solidbitumen, thereby suggesting that oil migrationpost-dated halite cementation. This observation issupported by the presence of solid bitumen-impregnated microcracks within the halitecement (Fig. 6D). On the other hand, the presenceof black solid bitumen in halite cement of milli-metre-wide to centimetre-wide veins indicatestheir late cementation (Figs 6E and 7). Mineral-ogically, the pore-cementing and fracture-cement-ing halite cements show a subordinate presenceof anhydrite, sylvite, quartz, dolomite and calcite.

Fig. 5. Regional parageneticsequence of Ara stringer carbonatesdeduced from 190 samples from theA1C to A4C of 24 wells in theGreater Harweel and the GreaterBirba Area. HT refers to hydro-thermal alteration (Schoenherret al., 2007a). CS1 and CS2 refer tocase study 1 (well A) and case study2 (well B), respectively. Thick barsindicate major diagenetic processes,thin bars indicate minor diageneticprocesses; stippled lines displayuncertainty of process duration.‘+’ denotes enhancement and‘)’ denotes degradation of theporoperm quality.

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A B

C D

E F

G H

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Calcite shows a patchy occurrence in the Aracarbonates and some wells show widespreadcalcite replacement of dolomite. Its microstruc-tural relationship to most other phases remainsunclear, although sparry calcite appears to post-date halite in the A2C of well B (Fig. 6F).

A widespread diagenetic phase is that ofmicrofracture-lining and matrix-replacive saddledolomite, characterized by curved crystal facettesand undulose extinction (Fig. 6G). In many cases,straight microfractures and vuggy pores are out-lined by saddle dolomite, often showing stronglycorroded crystal faces.

Almost all samples are characterized by solidbitumen, which has been characterized by meansof petrography and geochemistry (Schoenherret al., 2007a). Solid bitumen generally occurs inpores, covering euhedral dolomite rhombo-hedrons and saddle dolomites or bridging thepores as meniscus-shaped cement and, in manycases, is associated with microfractures. In theexample provided in Fig. 6C, solid bitumen isaligned along stylolites; this may indicate thatpressure solution pre-dates the formation of solidbitumen and that stylolites acted as flow path-ways for oil (Esteban & Taberner, 2003; Mallonet al., 2005). A rare but very important observa-tion is the presence of coke-like pyrobitumen(Fig. 6H) which indicates palaeo-temperatureshigher than 350 �C (Schoenherr et al., 2007a).

Generally, the dolomite matrix of most samplesis recrystallized strongly, which is indicated byinequigranular, tightly packed, anhedral to sub-hedral crystals with lobate and straight grainboundaries. Cathodoluminescence investigationsdid not reveal distinct patterns in the matrix orclear zonations in euhedral dolomite cements.In most cases, a homogeneous orange to reddull luminescence characterizes the dolomitematrix, indicating complete recrystallization andchemical homogenization.

Case study 1: Reservoir properties in the A1Cstringer of well AThe A1C stringer of well A is 80 m thick (3450 to3370 m) and completely surrounded by the Arasalt. Seventy thin sections have been selected forthis study in order to investigate the reservoirproperties of a non-producing carbonate stringerwith a special emphasis on the zone with theworst reservoir quality (3420 to 3380 m). Thelithofacies changes from rock salt and anhydrite(primary gypsum) to mudstones and crinklylaminites at the bottom towards grainstones andthrombolites at the top of the A1C (Fig. 7). Theporosity increases continuously from ca 1% at thetop to ca 13% at the base of A1C, whereaspermeability is developed only in the lower part.The zone of low porosity coincides with the zoneof near-zero permeability which, in turn, coin-cides with the halite-plugged section of the core.This trend is strongly controlled by an early stageof halite cementation in growth-related frame-work porosity of the thrombolites and vuggypores of the flat laminites in the upper 40 m ofthe A1C. In this zone, halite is volumetrically thedominant cement (up to 32%), while solid bitu-men occurs in amounts of ca 3%, on average.

The following section describes microstruc-tures from the top to the middle section of theA1C core (3380 to 3421 m). Grainstones andlaminites of the top section show a number of 1to 3 cm wide fractures oriented almost perpen-dicular to the lamination. These fractures arecemented by black halite. The microstructure ofone halite-cemented fracture was investigated asan example (Fig. 8) by microstructure-correlatedXRD and EDX analyses. The host rock consistsmainly of strongly corroded dolomite, which islocally cemented by halite, anhydrite, whewellite(CaC2O4ÆH2O), barite, sphalerite and traces ofsolid bitumen. The vein wall is lined by euhedralcalcite, solid bitumen and, in places, by whitish

Fig. 6. Diagenetic products of the early and deep burial realm. (A) Alternating lamination of euhedral dolomiteswith intercrystalline porosity (black arrows) and tightly packed anhedral dolomite (white arrows) (transmitted plane-polarized light; well B, A2C). (B) Detail view of an ooid showing a pendant cement (black arrow), which issurrounded by a micritic cement (red arrow); ‘p’ is open porosity (transmitted light micrographs, · nicols; well D,A2C). (C) Transmitted light micrograph (· nicols) showing syntaxial vein anhydrite, which trends parallel andoblique to solid bitumen-bearing (Bi) stylolites (well F, A2C). (D) Thin section of peritidal carbonate with high vuggyporosity, which is cemented by halite (decorated blue by gamma irradiation) (image width is 3 cm; well G, A2C).Inset shows a solid bitumen-impregnated microcrack; Do is dolomite. (E) Polished core plug showing halite veins(gamma-irradiated), crosscutting the lamination. Inset (from white arrow) shows streaky solid bitumen within halite(well E, A3C). (F) Effect of dedolomitization: Staining with Alizarin Red S highlights calcite (in pink) replacingdolomite (transmitted plane-polarized light; well B, A2C). (G) Vuggy pores, outlined by strongly corroded saddledolomites; p = open porosity (transmitted light, · nicols; well D, A2C). (H) Reflected light micrograph (underimmersion oil) showing coke-like solid bitumen formed at temperatures of ca 380 �C (Schoenherr et al., 2007a).

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aggregates of whewellite (Fig. 8A and B). Most ofthe vein is composed of up to 0Æ5 cm sized halitecrystals with grain boundaries oriented more orless perpendicular to the vein wall and withinternal structures such as slip lines and sub-grains decorated by gamma irradiation (Fig. 8C).This halite shows a number of intracrystallineblack particles (Fig. 8A) composed of dolomite,anhydrite and pyrite fragments which are agglu-tinated by sulphur-rich solid bitumen. The centreof the vein shows a black 0Æ5 cm thick bandwhich consists mainly of corroded dolomite,euhedral calcite, whewellite and sulphur-richsolid bitumen. The solid bitumen-impregnatedwhewellite is crossed partly by a 5 mm widehalite vein (Fig. 8C). The whewellite is brecciatedintensively with fragments of dolomite, anhydriteand fine-grained barite which are cemented bysulphur-rich solid bitumen (Fig. 8D).

In some of the thrombolites in the middlesection (Fig. 7), solid bitumen lines the walls ofhalite-plugged pores (Fig. 9C). This observationmisled the authors to the interpretation thathalite cementation in the Ara carbonates wouldpost-date oil migration and hence would be a lateburial process. Using transmitted light and SEM,

it becomes clear that thin films of solid bitumenrather occur as an intercrystalline phase of thehalite cement (Fig. 9A) and between the eu-hedral saddle dolomite cement and pore-fillinghalite (Fig. 9B). The saddle dolomites are asso-ciated with the emplacement of oil, as suggestedby solid bitumen, which is aligned along thegrowth facets of the saddle dolomites. In addi-tion, solid bitumen fills irregular-shaped vugs inthe halite cement (Fig. 9C) which points todissolution of the halite prior to solid bitumenemplacement. Analysis by SEM of this micro-structure shows that the halite-cemented poresare surrounded by a ca 1 mm wide halo showinga high microporosity and intensively corrodeddolomite rhombohedrons, whereas the surround-ing dolomite matrix is tight (Fig. 9D). The lattercontains numerous ca 10 lm sized cubic halitecrystals, which incorporate ca 2 lm sized dolo-mite grains (Fig. 9E). The boundary betweenhalite cement and porous dolomite is eitherdefined by open porosity (dissolved halite) orthin films of solid bitumen (Fig. 9F). Thesurfaces of those halite cements are covered bynumerous droplets of solid bitumen (Fig. 9G),suggesting impregnation by oil. In some cases,

Fig. 7. Poroperm data and vertical distribution of halite and solid bitumen in the A1C of well A. Core from 3385 mshows a halite-cemented and whewellite-cemented vein. Microstructures of this vein are shown in Fig. 8 (indicatedin the facies column). Core from 3399 m is a solid bitumen-cemented and halite-cemented thrombolite. Micro-structures of this core are presented in Fig. 9.

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the pore-cementing halite of thrombolites is pre-dated by subhedral to euhedral barite cementscontaining intracrystalline displacive halitecubes (Fig. 9H). Close to the barite crystals, afew finely dispersed aggregates of celestite occurwithin the halite cement.

Case study 2: Reservoir properties in the A2Cstringer of well BThe A2C stringer of well B is 70 m thick and wasstudied because it is one of the deepest producingintra-salt reservoirs in the SOSB (5570 to5500 m). The facies of the lowermost part of theA2C is constituted by rock salt and anhydrite(5568 to 5562 m), which is overlain by mudstonesand laminites (5562 to 5546 m, Fig. 10). Themiddle part of the section essentially consists of

open-marine thrombolites (5546 to 5525 m)which are overlain by laminites and basinalturbidites (5525 to 5500 m). Point-count analysisshows a negative correlation between halite andporosity (Fig. 10). An abrupt increase in halitecontent is caused by the almost complete cemen-tation of the large pores in the thrombolite facies(middle part), while solid bitumen is more abun-dant in the laminites of the lowermost anduppermost sections.

Twenty thin sections from the middle sec-tion (5546 to 5525 m) were studied (Fig. 10).In this zone, with the worst reservoir quality,halite clearly replaced anhydrite which, inturn, replaced dolomite along solution-enlargedpores and fractures (Fig. 11A). The anhydritecrystals are corroded heavily in places, creating

A B

C D

Fig. 8. Microstructures of black halite vein from carbonate core of well A shown in Fig. 7. (A) Core plug showinghalite-cemented fracture with a black median band, consisting of dolomite and whitish whewellite, cemented byblack solid bitumen. View is perpendicular to vein orientation in Fig. 7. Letters B to D refer to location of followingfigure panels. (B) SEM image (BSE mode) shows a highly porous dolomite matrix of the host rock in the lower part,cemented by halite (H). Note local occurrence of sphalerite (SPH). The vein wall is cemented by whewellite (W),lined by solid bitumen (SB) which, in turn, is post-dated by vein-filling halite. (C) Solid bitumen-impregnatedaggregates of whewellite (W) are fractured and cemented by halite (transmitted plane-polarized light; long side ofmicrograph is 1Æ6 mm). (D) SEM image (BSE mode) shows solid bitumen-filled (SB) microcrack in whewellite (W)layer (see Fig. 8A), which contains fragments of dolomite (D) and anhydrite (A).

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A B

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E F

G H

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additional pore space which was subsequentlycemented by halite.

The growth-related framework porosity andvuggy pores of the thrombolites are outlined byeuhedral drusy dolomite cement which is impreg-nated by solid bitumen. In contrast to the halite–solid bitumen relationship in case study 1, solidbitumen pre-dates halite cementation. In addition,there are three clear indications that halite pre-cipitated in the pores from a two-phase solution,containing NaCl-supersaturated brine and oil:(i) in a number of halite-cemented pores, solidbitumen forms globules which adhere to euhedraldrusy dolomite cements (Fig. 11B); (ii) some halitecements show intracrystalline 5 to 100 lm sizeddroplets and streaky-shaped solid bitumen; and

(iii) the halite cements incorporate ca 15 lm sizedparticles of solid bitumen, which are cubic inshape (Fig. 11C). These inclusions are filled par-tially with solid bitumen whereas the remnantspace of the inclusion is probably filled with gas.The walls of the gas-filled parts have a lightbrownish appearance, suggesting impregnation byoil, which was later converted into a thin film ofsolid bitumen (Fig. 11C). In one example, aninclusion shows an external light brownish haloof solid bitumen. Additional evidence for a lateorigin of the halite in the A2C is given by halite,post-dating euhedral saddle dolomites, whichshow the typical criteria of curved crystal faces,twins, a wavy extinction and a fluid inclusion-richcore (Fig. 11D).

Fig. 9. Microstructures of early halite cements from 3400 m to 3420 m in the A1C of well A. Micrographs C to H arefrom the same sample (see Fig. 7 for sample location). (A) Early halite grains (Ha) infilling pores of dolomite (Do).Black solid bitumen occurs at grain boundaries of halite (plane-polarized transmitted light). (B) Halite cements aremarginally rimmed by solid bitumen (SB), which occurs in crystal facettes of euhedral saddle dolomites (SD) (noteinset) (plane-polarized transmitted light). (C) Vuggy pores are outlined by corroded dolomite, which is impregnatedby solid bitumen. The halite cement shows truncations as a result of dissolution (plane-polarized transmitted light).(D) Overview SEM image (SE mode) focuses on the contact dolomite–solid bitumen–halite, similar to the micro-structure shown in (C). (E) SEM image (BSE mode) focuses into the tight dolomite matrix marked by the frame inpanel (D), showing displacive halite cubes with inclusions of dolomite (arrow). (F) SEM image (SE mode) of thecontact between the pore-cementing halite and the dolomite host rock indicated by the frame in panel (D). Micro-structures marked by ‘1’ represent open porosity (epoxy), and ‘2’ marks solid bitumen-impregnated dolomite. (G)SEM image of a broken piece shows crystal faces of halite cements, covered by droplets of solid bitumen. (H) SEMimage (BSE mode) of thin section shows a thrombolite pore, cemented by subhedral barite cements (Ba) and halite,which incorporates fine-grained celestite (Ce).

Fig. 10. Poroperm data and distribution of halite and solid bitumen in the A2C of well B. Letters A to D in the faciescolumn refer to sample location of microstructures shown in Fig. 11.

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Geochemical data

Stable isotope dataStable oxygen and carbon isotopes of 94 intra-saltcarbonate stringer samples were analysed fromboth matrix dolomite and pore-lining and frac-ture-lining saddle dolomite cements (seeFigs 6B,G and 11D). The samples have d18O valuesthat range between )4Æ15& and 2Æ39& V-PDB(mean = )2Æ38, r = 1Æ27), and d13C values between)3Æ54& and 3Æ60& V-PDB (mean = 1Æ84, r = 1Æ65).The majority of the samples plot in the d18O rangebetween )1Æ00& and )5Æ00& V-PDB and in thed13C range between 0Æ50& and 3Æ50& V-PDB, withsome outliers plotting in the positive quadrant(Fig. 12). Eight samples show d18O values thatrange between )3Æ00& and )5Æ00& V-PDB and

d13C values between )1Æ50& and )3Æ50& V-PDB,from which six samples are from the A4C stringerinterval of the wells J, K, L and M and two from theA2C stringer interval of well N (Fig. 4).

Saddle and matrix dolomites on average havesimilar d13C values but saddle dolomite onlyshows negative d18O values and thus is charac-terized by slightly lighter oxygen isotope values()2Æ58&) than matrix dolomite ()2Æ19&). How-ever, where saddle dolomite and adjacent matrixdolomite were analysed from the same sample(n = 8), their isotopic values are not significantlydifferent.

Bromine dataBromine is often used as a genetic indicator insubsurface brines and chemical sediments to

A B

C D

Fig. 11. Transmitted light micrographs of halite-cemented samples from the A2C cycle of well B. See Fig. 10 forsample location. (A) Replacement of dolomite (d) by anhydrite (a) in a solution-enlarged fracture. Remnant porosityis cemented by halite (h), partly replacing anhydrite (plane-polarized light). (B) Droplets of solid bitumen adhere todolomite pore walls within the halite cement (h) (plane-polarized light). (C) Halite-cemented pores, showing anumber of cube-shaped inclusions partly filled with solid bitumen (arrows). Inset highlights an inclusion, showing abrownish halo of solid bitumen around the inclusion (plane-polarized light). (D) Detailed view of a pore wall exhibitsa euhedral saddle dolomite cement surrounded by a halite cement (h) (· nicols).

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constrain the brine sources (Schreiber & ElTabakh, 2000) and to determine the concentrationof marine-derived brines throughout deposition(Holser, 1979; Taberner et al., 2000). The finalbromine content of a halite crystal is influenced bysalinity variations of the fluid from which itprecipitated, by solution and re-precipitation pro-cesses, by recrystallization, and depends on thelevel of other ionic species present in the solution(Siemann & Schramm, 2000). Bromine studies ofthe pure Ara rock salt suggest a marine origin(Schroder et al., 2003). Twenty-one samples ofpetrographically early and late halite cements wereanalysed, to compare their bromine contents with12 samples from pure Ara rock salt intervals. Thehalite cements come from seven wells and threecarbonate stringers (A1C to A3C). The rock saltsamples, which are composed of halite and sub-ordinate anhydrite, sylvite and polyhalite, are fromfive wells. Figure 13A shows that most of the halitecements in the carbonates have higher brominecontents compared with samples from pure Arasalt intervals (mean Brpure halite = 79 p.p.m.; meanBrhalite in carbonate = 213 p.p.m.). Eight of 10 sam-ples from the A1C cycle of well A belong tothe early, porosity-plugging halite phase of thethrombolite facies (Fig. 9A to C). The brominecontents of these samples consistently clusteraround 260 p.p.m. (Fig. 13B). The remaining twosamples with relatively low bromine contents of223 p.p.m. (r = 5 p.p.m.) and 244 p.p.m. (r =8 p.p.m.), come from the upper and lower part ofthe vein (Fig. 8A). The two samples from the pure

Ara salt interval directly below the A1C show anaverage bromine content of 145 p.p.m. whichprobably represents the latest stage of Ara saltprecipitation in this cycle.

RESERVOIR PROPERTY EVOLUTION – ADISCUSSION

Diagenetic framework for halite cementation

Diagenetic processes in the shallow burial realmare controlled strongly by early dolomitizationwhich is attributed to the seepage-reflux modelproposed by Mattes & Conway Morris (1990) andSchroder (2000). Carbon isotope values of thesyn-depositional dolomite fabrics are very similarto other well-preserved Late Neoproterozoic toEarly Cambrian carbonates (Jacobsen & Kaufman,1999). The negative carbon isotope values fromthe A4C interval record the distinct negativecarbon isotope excursion generally attributed tothe Precambrian–Cambrian boundary (Amthoret al., 2003). The fact that the carbon isotopevalues of the dolomite reflect the secular changesin the composition of Neoproterozoic to EarlyCambrian sea water indicates that early dolomi-tization occurred from marine waters before thecarbonate stringers were encased fully in the Arasalt. During progressive burial this isotopic signalwas preserved largely in a rock-buffered systemwith a high rock/fluid ratio. However, twosamples from the A2C interval of well N plot

Fig. 12. Stable isotope data (d18Ovs. d13C) of saddle and matrix dol-omites from intra-salt Ara carbon-ates. Data within frame representthe short-lived negative d13C excur-sion in the A4C stringers at thePrecambrian–Cambrian boundary(see Amthor et al., 2003).

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close to the A4C field (Fig. 12) and might indicateassimilation of organic carbon-derived d13C fromthermochemical sulphate reduction (TSR) in thisdeeply buried (> 5100 m) stringer (Machel et al.,1995).

In general, samples with the best primary fabricpreservation show the highest oxygen isotopevalues (e.g. Fig. 6B: 2Æ39&). Hence, the positiveoxygen isotope values are probably close to theoriginal oxygen isotope signal of sea water. Thebulk of samples with negative isotope values aredue to thermal fractionation under elevated tem-peratures in the burial realms (McCrea, 1950).Saddle dolomite shows solely negative oxygenisotope values and hence was formed at relativelyhigh temperature. The fact that their oxygenisotope signal is only slightly depleted comparedwith matrix dolomite points to a burial ratherthan a hydrothermal origin (Machel & Lonnee,2002).

Anhydrite cementation is pervasive andoccurred in multiple stages, i.e. it pre-dates andpost-dates early halite cementation. Some of theearly (pore-cementing) anhydrite is likely to berelated to brine reflux; however, this anhydritepartially underwent strong recrystallization. Late

anhydrite is related to fracturing which post-datespressure solution and the formation of solidbitumen within stylolites (Figs 5 and 6C). Therelationship between anhydrite and coarse calciteclearly points to dedolomitization (Figs 6F and10A), most probably triggered by TSR in thedeeply buried stringers (some 5000 m), whichcontain sour gas of up to 6 mol.% hydrogensulphide (Taylor et al., 2007). Because hydro-carbons reduce sulphate due to TSR at reservoirtemperatures of 100 to 180 �C (Machel et al.,1995), all halite cements post-dating the dedolo-mites (Fig. 6F) are formed under deep burialconditions.

All diagenetic phases post-dating solid bitu-men were formed late in the burial history, i.e.at least after the stringers entered the oilwindow, which corresponds to ca 100 to150 �C and burial depths of 2Æ5 to 4Æ5 km,respectively (MacKenzie & Quigley, 1988).Therefore, early and late halite cements aredefined by their microstructural relationshipwith solid bitumen, i.e. whether halite pre-datesor post-dates solid bitumen. Therefore, thisdefinition represents an important diagenetic‘marker’ in the present study (Fig. 6D and E).

A B Bromine (p.p.m)Bromine (p.p.m)

Fig. 13. Distribution of Br contents in pure Ara salt intervals and halite cements in various Ara carbonate stringerswith depth. (A) Most Br contents of the pure Ara salt (bold symbols and crosses) are markedly below the Br values ofthe halite cements from carbonate stringers. (B) Detail of frame in Fig. 13A shows that Br content of halite cements incarbonate stringer A1C of well A are up to 150 p.p.m. higher than Br contents of the underlying Ara salt. The twostratigraphically uppermost data points derive from one halite vein shown in Fig. 8A. The crystals in the upper partof the vein have a Br content of 21 p.p.m. lower than halite crystals in the lower part. The remaining data points(n = 8) cluster at around 260 p.p.m. and are from early pore-cementing halite.

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Halite cementation

The fact that halite cementation occurred in poresand in (late) fractures complicates the interpreta-tion of the origin of sodium chloride-supersatu-rated fluids and the timing of halite cementationduring basin evolution. Based on a combinationof petrography, mineralogy and geochemistry, thefollowing discussion demonstrates that the bulkof halite cementation occurred early rather thanlate (Mattes & Conway Morris, 1990; Schroder,2000) in burial evolution.

Early halite cementation (Fig. 14A)The vertical distribution of halite cements in casestudy 1 reflects lithofacies-controlled cementa-tion, which a priori suggests a fluid source fromabove (Fig. 14A), similar to the scenario de-scribed by Kendall (2000) for (early) halitecementation in salt-encased carbonates. Tounderstand the fluid volume required to plugthe pores of stringer A1C (well A) with halite, arough calculation, including the parameters,porosity, temperature and supersaturation (Mul-lin, 2001), reveals a fluid volume correspondingto a cube of 0Æ83 km3 in size. It seems unrealisticthat this amount of sodium chloride-supersatu-rated fluid might be derived from any sourceother than sea water. The fact that brominecontents of early halite cements from case study1 are higher than in the underlying pure rock saltinterval (Fig. 13), points to very high salinities ofthe mother fluid (sea water brine). Chemostrati-graphic studies have shown that, from the earlystages of halite precipitation from sea water,bromine concentrations can increase from ca 60to 270 p.p.m. at late-stage halite formation (Hol-ser, 1979; Hardie, 1984; Land et al., 1995) and theonset of bittern salt precipitation (Valyashko,1956). Hence, the high bromine contents suggesthalite precipitation from a highly evolved brine atthe onset of deposition of the overlying rock saltcycle (A2E interval, Fig. 14A) down to a burialdepth of around 30 m. The presence of potassiumin these (petrographic) early halite cements andsubordinate sylvite (KCl) in most halite cementsstrongly supports this interpretation.

Bittern salts which overlie halite commonly arecomposed of sylvite, a mineral not expected toprecipitate during the evaporation of present-daysea water (Land et al., 1995). The origin of potashdeposits is still controversial, because the evap-oration of present-day sea water producesmagnesium and potassium sulphates, whereasmany potash deposits of ancient sedimentary

basins consist of sylvite rather than potassium–magnesium sulphates (Ayora et al., 2001; seeWarren, 2006, p. 793 for a full discussion). Theformation of sylvite is generally favoured by:

(1) Sea water with ionic proportions differentfrom today (Hardie, 1996; Kovalevich et al.,1998), or depletion of sulphate in the brine on abasin-scale because of diagenetic brine–rockreactions such as dolomitization (Ayora et al.,2001). Interestingly, Brennan et al. (2004) showedthat primary brine inclusions of the Ara saltindicate sulphate depletion with respect to mod-ern sea water and that they contain highly con-centrated brines in equilibrium with sylvite andcarnallite (KMgCl3Æ6H2O). Thus, it seems proba-ble that the early dolomitization of the Aracarbonates created suitable conditions forprimary sylvite precipitation in the SOSB.

(2) Dissolution of potassium-rich bittern saltssuch as carnallite, deposited during the lateststage of sea water evaporation (Lowenstein &Spencer, 1990; Land et al., 1995). The presence ofseveral corrosion surfaces within the Ara evap-orites records repeated flooding of the SOSB(Schroder et al., 2003), which may explain theoccurrence of trace amounts of diagenetic sylviteby incongruent dissolution of carnallite inslightly undersaturated waters.

(3) Cooling of highly saline near-surface brines,which is a major mechanism to achieve super-saturation with respect to sylvite as a primaryprecipitate (Lowenstein & Spencer, 1990).

(4) A highly restricted continental environ-ment (Lowenstein & Spencer, 1990; Cendon et al.,2003). However, the high bromine content of thesylvite-bearing Ara salt contradicts its formationunder continental conditions (Risacher & Fritz,2000).

In summary, each of the processes (1) to (3), or acombination of these can account for the traceamounts of sylvite and the bromine-rich nature ofthe halite cements. Most importantly, these pro-cesses indicate early precipitation from highlyevolved sea water.

The early origin of halite is supported addi-tionally by displacive growth of micrometre-sizedhalite cubes (Fig. 9E) into poorly consolidatedcarbonate mud and its relationship to baritecements and celestite (Fig. 9H), which are inter-preted as the earliest diagenetic products. Bariteis found commonly in marine sediments inregions of high primary productivity (Monnin &Cividini, 2006; Riedinger et al., 2006). In theAra Group, high productivity is indicated by

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A

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C

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sediments rich in kerogeneous organic matter andthe abundance of stromatolites throughout thebasin (Mattes & Conway Morris, 1990). Theintracrystalline celestite probably formed as aprimary precipitate from brines which dissolvedaragonite (Swart & Guzikowski, 1988; Reuninget al., 2002) and/or gypsum or anhydrite (Olaus-sen, 1981; Fig. 6C).

Secondary porosity and oil generation(Fig. 14B)Porous zones in the dolomite matrix around thehalite-cemented pores (Fig. 9D and E) indicate achange in pore fluid chemistry, which is mostprobably associated with the generation of oil, assuggested by solid bitumen filling this newlycreated pore space (Fig. 9B and C). Esteban &Taberner (2003) have shown that corrosive dia-genetic fluids can be created prior to hydrocarboncharge under late burial conditions in carbonatereservoirs. The maturation of organic mattercreates weak, carboxylic acids (Giles & de Boer,1989), which may have shifted the pH of the porewaters towards acidic values, thereby marginallydissolving the dolomite matrix and early halitecement (Fig. 14B).

This additional porosity was cemented later by‘saddle-like’ dolomite cements during or directlyafter oil migration, as indicated by solid bitumenaligned in growth facettes of the dolomitecements (Fig. 9B). It is more probable that oilrather than solid bitumen was incorporated dur-ing growth of the dolomite cements, because of itshigher mobility. In the same stage, the dissolutionvugs at the margins of the halite cements wereflushed and filled by oil (Fig. 9G). The oil waslater transformed in situ into solid bitumen. Thispetrographic relationship could lead at firstglance to the misinterpretation that halite post-dates solid bitumen. This selective dissolution atthe halite–dolomite interface probably suggeststhat a kind of ‘roll front’ of acidic fluids precededhydrocarbon migration, as described by Mazzullo& Harris (1991), could explain the sporadic

occurrence of solid bitumen in the alreadyhalite-cemented upper part of the A1C stringer.

Solid bitumen and late halite cementation(Fig. 14C)This section firstly outlines the formation condi-tions of solid bitumen and secondly those of latehalite post-dating solid bitumen in fractures ofthe A1C stringer of well A. This outline isfollowed by an interpretation of the complexpetrographic relationships of (late) halite andsolid bitumen in pores of stringer A2C fromwell B.

Solid bitumenThe mineral assemblage in the fractures of well A(Figs 7 and 8) represents a typical hydrothermalparagenesis, which is difficult to explain either asan already existing syn-sedimentary paragenesisor a late burial diagenetic product. The abun-dance of the rare mineral whewellite (CaC2O4ÆH2O) supports this interpretation. It is the best-known crystalline organic mineral which canform as a low-temperature primary hydrothermalmineral in carbonate-sulphide veins by oxidationof organic material in the surrounding rock.Whewellite often shows an association withbarite, sphalerite, pyrite, bitumen and siderite(Zak & Skala, 1993; Hofmann & Bernasconi, 1998)which have all been observed in the veins (Fig. 8).The detailed microstructural analysis of the veinsuggests that the solid bitumen and whewelliteprecipitated almost at the same time (Fig. 14C).This hydrothermal origin of the fracture fill isconsistent with the findings of Schoenherr et al.(2007a), who postulated that the Ara Group wasaffected by the far-reaching fractures associatedwith the external inflow of hydrothermal fluids.Their interpretation was based on the highlyheterogeneous distribution of thermal maturitywith depth (A1C; well A) and the presence ofcoke-like solid reservoir bitumen (Fig. 6H; A3C,well C), indicating palaeo-temperatures of up to380 �C.

Fig. 14. Schematic cartoon illustrating the evolution of early and late halite cementation in the A1C stringer of wellA with time. Facies distribution of the A1C is adapted from Al-Siyabi (2005). Black frames in burial environment ofFig. 14A to C represent corresponding micro-scale processes of Fig. 14A to C. (A) Early halite cementation in the A1Cinterval is due to infiltration of hypersaline brines, seeping from above during deposition of the overlying evaporiteinterval A2E. (B) This stage is marked by oil generation from the organic-rich mudstones and laminites. Because theupper part of the A1C was cemented by halite (dashed zone), oil migration is restricted to the lower portion. Oilmigration was preceded by a ‘roll front’ of acidic brines which caused slight dissolution of the dolomite and thehalite cements at their immediate contact. (C) The formation of fractures in the Ara Group was accompanied bystrong overpressuring and the presence of hydrothermal fluids, causing solid bitumen formation. A second phase ofoverpressure (OP) build-up led to the formation of halite-cemented (crack-seal) veins.

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Late halite in the A1C of well AThe orientations of solid inclusions in the halite-cemented vein (Figs 8A and 14C) suggest aprocess of repeated fracturing and sealing – theso-called ‘crack-seal mechanism’ (Ramsay, 1980;Hilgers & Urai, 2005). This vein-opening mech-anism commonly is described as extensionalfailure, initiated by a strong increase in fluidpressure under low differential stresses (Sibson,2003). The first phase of fracture opening prob-ably was triggered by hydrothermal fluids, asindicated by the mineral assemblage includingwhewellite. During a later stage this fracture wasreactivated, probably several times, by hydro-fracturing or salt tectonics (Mattes & ConwayMorris, 1990; Schoenherr et al., 2007b). Duringthe reactivation of the fracture, the whewelliteand associated inclusions were incorporated intothe halite, which cemented the reopened frac-ture. The upper part of the halite vein showsincreased bromine values compared with thelower part of the halite vein (Figs 8A and 13B).This observation might suggest precipitationfrom fluids of different salinities during twoseparate phases of fracture reopening and halitecementation.

Late halite in the A2C of well BThe distribution of halite in the A2C stringer ofwell B generally is very similar to the distributionof early halite in the A1C of well A (Fig. 10).Halite cements are concentrated in the upper two-thirds of the A2C and are most abundant in themiddle part of the section, where halite occludesthe initially high primary porosity of the thromb-olite facies. This distribution suggests an earlyorigin from highly saline brines, infiltrating theA2C stringer from above during deposition of theA3E. However, microstructures in the thrombo-lites clearly document a late origin for the halitecements (Fig. 11). Saddle dolomites pre-datingthe late halite are not necessarily diagnostic forhydrothermal activities, which is a favouredinterpretation for their formation conditions(Davies & Smith, 2006), but rather indicate rela-tively high formation temperatures ‡ 60 �C (Radke& Mathis, 1980). Hence, the temperature of 60 �Cserves as a rough proxy for the minimumtemperatures of late halite precipitation.

The best explanation is local redistribution ofthe early halite cements into (microstructurally)late halite by solution and re-precipitation; this issuggested by bromine content which is signifi-cantly lower (depleted) than in the early halite

cements of well A (Fig. 13). Wardlaw & Watson(1966) stated that solution and subsequent re-deposition of halite lead to bromine depletion inhalite because the distribution coefficient frombromine in halite is less than one. However, thelate halite bromine content of well B is still veryhigh with respect to that of the pure Ara salt; thisindicates only slight depletion of bromine withinthe closed system of the salt-encased stringer.

The source of the fluid triggering dissolution ofearly halite in the A2C of well B remains spec-ulative and may have several causes. There is nodirect evidence for hydrothermal activity in theA2C interval of well B. Salt tectonics may havetriggered remobilization of interstitial pore watersand dissolution/re-precipitation of the earlyhalite by fluid cross flows. Seismic interpretationshows that interval A2C is folded strongly. TheTSR, as deduced from corrotopic anhydrite,dedolomitization (Fig. 6F) and presence of sourgas in this stringer, probably provided water torecycle the early halite in interval A2C. Althoughthe amounts of water released during TSR arethought to be small, it can be of local significance(Machel, 2001).

Streaky-shaped intracrystalline solid bitumendroplets indicate two-phase (oil and sodiumchloride-brine) fluid flow during recycling ofearly into late halite. A brownish halo aroundcube-shaped solid bitumen inclusions (Fig. 11C)further indicates decrepitation and leakage ofoil inclusions (Roedder, 1984), most probably asa result of increasing temperatures (burial),which finally led to thermal cracking of theoil inclusion into gas and solid bitumen. Thisinterpretation means that halite crystallizationoccurred before the so-called ‘oil-deadline’ atca 150 �C (Dahl et al., 1999) and well below230 �C, the maximum palaeo-temperature ofthis stringer (Schoenherr et al., 2007a). There-precipitation of the late halite cements, there-fore, took place during burial in a temperaturerange between 60 and 150 �C. This dissolutionand re-precipitation of halite cements seem tohave acted only on a local scale, as the halitecements show the same distribution as could beexpected from a reflux mechanism.

The general sequence of the processes, leadingto the formation of late halite and solid bitumenin interval A2C of well B can be summarized as inthe following: early halite fi ‘oil in place’ firecycling of early into late halite fi temperatureincrease (deep burial) causing formation of solidbitumen (Fig. 5).

586 J. Schoenherr et al.

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CONCLUSIONS

Detailed petrographic work and bromine geo-chemistry of halite cements in the Ara carbonatestringer reservoirs provided significant new infor-mation on: (i) the relative timing; (ii) the forma-tion mechanisms; and (iii) the impact of halitecements on reservoir quality.

1 The relative timing of early and late halitecementation is constrained by its microstructuralrelationship with solid bitumen and brominegeochemistry. Case study 1 (A1C) showed thatearly halite formed in the shallow burial realm(down to a depth of ca 30 m). Late halitecementation is restricted to fractures formed inthe deeper burial realm, in which halite post-dates solid bitumen. Late halite in case study 2(A2C) formed at a burial temperature range of 60to 150 �C, as deduced by its microstructural rela-tionship with saddle dolomites and decrepitatedoil inclusions.

2 Mass balance constraints, the sylvite-richnature and the high bromine content of the halitecements indicate their origin from reflux of seawater-derived highly saline brines. The crack-sealmicrostructures associated with fracture-fillinglate halite cements (case study 1) were formed byoverpressure-generated hydrofracturing. Porosity-occluding late halite cement (case study 2) is aproduct of dissolution and re-precipitation ofearly halite as indicated by the depleted brominecontents compared with early halite cements ofcase study 1.

3 Pervasive cementation of early halite is themain poroperm-degrading process in the Aracarbonate stringers. Early halite predominantlyoccurs in the uppermost parts of the reservoirsand led to an inversion of reservoir quality. Thesolution and re-precipitation processes of earlyhalite only led to a local redistribution of halitecements. However, this process may producevariable (re)distribution patterns which compli-cate reservoir quality prediction with respect tolate halite cementation. The impact of late frac-ture-cementing halite on reservoir quality isinterpreted as low.

ACKNOWLEDGEMENTS

The authors are grateful to Petroleum Develop-ment Oman LLC (PDO) and to the Ministry of Oiland Gas Oman for granting permission to publishthis study. We thank PDO for sponsoring and for

providing the samples and supporting data. UweWollenberg is acknowledged for conducting scan-ning electron microscopy and XRD measure-ments, Manfred Thome from the ResearchCentre Julich, Germany is thanked for gammairradiation of the halite-cemented samples, Lul-zim Haxhiaj from IFM-GEOMAR for stable-isotope analyses and Werner Kraus for thinsection preparation. Data on modal analysis ofthe two case studies were kindly provided byBadley Ashton & Associates Ltd. The final man-uscript benefited from thorough reviews by twoanonymous reviewers and Sedimentology editorDaniel Ariztegui.

REFERENCES

Al-Siyabi, H.A. (2005) Exploration history of the Ara intrasalt

carbonate stringers in the South Oman Salt Basin. Geo-

Arabia, 10, 39–72.

Amthor, J.E., Grotzinger, J.P., Schroder, S., Bowring, S.A.,Ramezani, J., Martin, M.W. and Matter, A. (2003) Extinc-

tion of Cloudina and Namacalathus at the Precambrian-

Cambrian boundary in Oman. Geology, 31, 431–434.

Amthor, J.E., Ramseyer, K., Faulkner, T. and Lucas, P. (2005)

Stratigraphy and sedimentology of a chert reservoir at the

Precambrian-Cambrian Boundary: the Al Shomou Silicilyte,

South Oman Salt Basin. GeoArabia, 10, 89–122.

Ayora, C., Cendon, D.I., Taberner, C. and Pueyo, J.J. (2001)

Brine-mineral reactions in evaporite basins: implications for

the composition of ancient oceans. Geology, 29, 251–254.

Bifani, R. (1986) Esmond Gas Complex. In: Habitat of Palaeo-

zoic Gas in N. W. Europe (Eds J. Brooks, J.C. Goff and B. Van

Hoorn), Geol. Soc. Spec. Publ., 23, 209–221.

Brennan, S.T., Lowenstein, T.K. and Horita, J. (2004) Seawater

chemistry and the advent of biocalcification. Geology, 32,473–476.

Casas, E. and Lowenstein, T.K. (1989) Diagenesis or saline pan

halite: comparison of petrographic features of modern,

Quaternary and Permian halites. J. Sed. Petrol., 59, 724–739.

Cendon, D.I., Ayora, C., Pueyo, J.J. and Taberner, C. (2003)

The geochemical evolution of the Catalan potash subbasin,

South Pyrenean foreland basin (Spain). Chem. Geol., 200,339–357.

Dahl, J.E., Moldowan, J.M., Peters, K.E., Claypool, G.E.,Rooney, M.A., Michael, G.E., Mello, M.R. and Kohnen, M.L.(1999) Diamondoid hydrocarbons as indicators of natural oil

cracking. Nature, 399, 54–57.

Davies, G.R. and Smith, L.B. Jr (2006) Structurally controlled

hydrothermal dolomite reservoir facies: an overview. AAPG

Bull., 90, 1641–1690.

Dronkert, H. and Remmelts, G. (1996) Influence of salt struc-

tures on reservoir rocks in Block L2, Dutch continental

shelf. In: Geology of Gas and Oil under the Netherlands (Eds

H.E. Rondeel, D.A.J. Batjes and W.H. Nieuwenhuijs),

pp.159–166. Kluwer, Dordrecht.

Esteban, M. and Taberner, C. (2003) Secondary porosity

development during late burial in carbonate reservoirs as a

result of mixing and/or cooling of brines. J. Geochem.Explor., 78-79, 355–359.

Late Neoproterozoic to Early Cambrian carbonates of the Ara Group 587

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists, Sedimentology, 56, 567–589

Giles, M.R. and de Boer, R.B. (1989) Secondary porosity:

creation of enhanced porosities in the subsurface from the

dissolution of carbonate cements as a result of cooling for-

mation waters. Mar. Petrol. Geol., 6, 261–269.

Gill, D. (1994) Niagaran reefs of Northern Michigan. 1.

Exploration portrait. J. Petrol. Geol., 17, 99–110.

Gorin, G.E., Racz, L.G. and Walter, M.R. (1982) Late Pre-

cambrian-Cambrian sediments of Huqf Group, Sultanate of

Oman. AAPG Bull., 66, 2609–2627.

Hardie, L.A. (1984) Evaporites: marine or non-marine? Am. J.

Sci., 284, 193–240.

Hardie, L.A. (1996) Secular variation in seawater chemistry:

an explanation for the coupled secular variation in the

mineralogies of marine limestones and potash evaporites

over the past 600 m.y. Geology, 24, 279–283.

Heward, A. P. (1990) Salt removal and sedimentation in

southern Oman. In: The Geology and Tectonics of the Oman

Region (Eds A.H.F. Robertson, M.P. Searle and A.C. Ries)

Geol. Soc. London Spec. Publ., 49, 637–651.

Hilgers, C. and Urai, J.L. (2005) On the arrangement of solid

inclusions in fibrous veins and the role of the crack-seal

mechanism. J. Struct. Geol., 27, 481–494.

Hofmann, B.A. and Bernasconi, S.M. (1998) Review of

occurrences and carbon isotope geochemistry of oxalate

minerals: implications for the origin and fate of oxalate in

diagenetic and hydrothermal fluids. Chem. Geol., 149, 127–

146.

Holser, W.T. (1979) Trace elements and isotopes in evaporites.

In: Marine Minerals (Ed. R.G. Burns), Reviews in Miner-

alogy, 6, 295–346. Mineralogical Association of America,

Washington, DC.

Jacobsen, S.B. and Kaufman, A.J. (1999) The Sr, C and O

isotopic evolution of Neoproterozoic seawater. Chem. Geol.,161, 37–57.

Kendall, A.C. (2000) Compaction in halite-cemented carbon-

ates-the Dawson Bay formation (Middle Devonian) of Sas-

katchewan, Canada. Sedimentology, 47, 151.

Kovalevich, V.M., Peryt, T.M. and Petrichenko, O.I. (1998)

Secular variations in seawater chemistry during the

Phanerozoic as indicated by brine inclusions in halite.

J. Geol., 106, 695–712.

Laier, T. and Nielsen, B.L. (1989) Cementing halite in Tri-

assic Bunter Sandstone (Tonder, southwest Denmark) as a

result of hyperfiltration of brines. Chem. Geol., 76, 353–

363.

Land, L.S., Eustice, R.A., Mack, L.E. and Horita, J. (1995)

Reactivity of evaporites during burial – an example from

the Jurassic of Alabama. Geochim. Cosmochim. Acta, 59,3765–3778.

Loosveld, R.J.H., Bell, A. and Terken, J.J.M. (1996) The tec-

tonic evolution of interior Oman. GeoArabia, 1, 28–51.

Lowenstein, T.K. and Spencer, R.J. (1990) Syndepositional

origin of potash evaporites: petrographic and fluid inclusion

evidence. Am. J. Sci., 290, 43–106.

Machel, H.G. (2001) Bacterial and thermochemical sulfate

reduction in diagenetic settings – old and new insights. Sed.Geol., 140, 143–175.

Machel, H.G. and Lonnee, J. (2002) Hydrothermal dolomite – a

product of poor definition and imagination. Sed. Geol., 152,163–171.

Machel, H.G., Krouse, H.R. and Sassen, R. (1995) Products

and distinguishing criteria of bacterial and thermochemical

sulfate reduction. Appl. Geochem., 10, 373–389.

MacKenzie, A.S. and Quigley, T.M. (1988) Principles of geo-

chemical prospect appraisal. AAPG Bull., 72, 399–415.

Mallon, A.J., Swarbrick, R.E. and Katsube, T.J. (2005) Per-

meability of fine-grained rocks: new evidence from chalks.

Geology, 33, 21–24.

Mattes, B.W. and Conway Morris, S. (1990) Carbonate/evap-

orite deposition in the Late Precambrian-Early Cambrian

Ara formation of Southern Oman. In: The Geology and

Tectonics of the Oman Region (Eds A.H.F. Robertson,

M.P. Searle and A.C. Ries), Geol. Soc. London Spec. Publ.,49, 617–636.

Mazzullo, L.J. and Harris, P.M. (1991) An overview of disso-

lution porosity development in the deep-burial environ-

ment, with examples from carbonate reservoirs in the

Permian Basin. In: Permian Basin Plays – Tomorrow’s

Technology Today (Ed. M.P. Candelaria), West Texas Geol.

Soc. Publ., 91-98, 125–138.

McCrea, J.M. (1950) On the isotopic chemistry of carbonates

and a paleotemperature scale. J. Chem. Phys., 18, 849–857.

Monnin, C. and Cividini, D. (2006) The saturation state of the

world’s ocean with respect to (Ba, Sr)SO4 solid solutions.

Geochim. Cosmochim. Acta, 70, 3290–3298.

Mullin, J.W. (2001) Crystallization. Butterworth Heinemann,

Oxford, 594 pp.

Olaussen, S. (1981) Formation of celestite in the Wenlock,

Oslo region Norway – evidence for evaporitic depositional

environments. J. Sed. Petrol., 51, 37–45.

van Opbroek, G. and den Hartog, H.W. (1985) Radiation

damage of NaCl: dose rate effects. J. Phys. C: Solid State

Phys., 18, 257–268.

Peters, J.M., Filbrandt, J.B., Grotzinger, J.P., Newall, M.J.,Shuster, M.W. and Al-Siyabi, H.A. (2003) Surface-piercing

salt domes of interior North Oman, and their significance for

the Ara carbonate ‘‘stringer’’ hydrocarbon play. GeoArabia,

8, 231–270.

Purvis, K. and Okkerman, J.A. (1996) Inversion of reservoir

quality by early diagenesis: an example from the Triassic

Buntsandstein, offshore the Netherlands. In: Geology of Gas

and Oil under the Netherlands (Eds H.E. Rondeel, D.A.J.

Batjes and W.H. Nieuwenhuijs), pp. 179–189. Kluwer,

Dordrecht.

Putnis, A. and Mauthe, G. (2001) The effect of pore size on

cementation in porous rocks. Geofluids, 1, 37–41.

Radke, B.M. and Mathis, R.L. (1980) On the formation and

occurrence of saddle dolomite. J. Sed. Petrol., 50, 1149–1168.

Ramsay, J.G. (1980) The crack-seal mechanism of rock defor-

mation. Nature, 284, 135–139.

Reuning, L., Reijmer, J.J.G. and Betzler, C. (2002) Sedimen-

tation cycles and their diagenesis on the slope of a Miocene

carbonate ramp (Bahamas, ODP Leg 166). Mar. Geol., 185,121–142.

Riedinger, N., Kasten, S., Groger, J., Franke, C. and Pfeifer, K.(2006) Active and buried authigenic barite fronts in sedi-

ments from the Eastern Cape Basin. Earth Planet. Sci. Lett.,241, 876–887.

Risacher, F. and Fritz, B. (2000) Bromine geochemistry of salar

de Uyuni and deeper salt crusts, Central Altiplano, Bolivia.

Chem. Geol., 167, 373–392.

Roedder, E. (1984) The fluids in salt. Am. Mineral., 69, 413–

439.

Schleder, Z. and Urai, J.L. (2005) Microstructural evolution of

deformation-modified primary halite from the Middle Tri-

assic Rot Formation at Hengelo, the Netherlands. Int. J.

Earth Sci. (Geol. Rundsch.), 94, 941–955.

Schoenherr, J., Littke, R., Urai, J.L., Kukla, P.A. and Rawahi, Z.(2007a) Polyphase thermal evolution in the Infra-Cambrian

588 J. Schoenherr et al.

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists, Sedimentology, 56, 567–589

Ara Group (South Oman Salt Basin) as deduced by solid

bitumen maturity. Org. Geochem., 38, 1293–1318.

Schoenherr, J., Urai, J.L., Kukla, P.A., Littke, R., Schleder, Z.,Larroque, J.M., Newall, M.J., Al-Abry, N., Al-Siyabi, H.A.and Rawahi, Z. (2007b) Limits to the sealing capacity of

rock salt: A case study of the Infra-Cambrian Ara Salt from

the South Oman Salt Basin. AAPG Bull., 91, 1541–1557.

Schreiber, B.C. and El Tabakh, M. (2000) Deposition and early

alteration of evaporites. Sedimentology, 47, 215–238.

Schroder, S. (2000) Reservoir quality prediction in Ara Group

carbonates of the South Carbonate Platform, South OmanSalt Basin. PhD Thesis, University Bern, 232 pp.

Schroder, S., Schreiber, B.C., Amthor, J.E. and Matter, A.(2003) A depositional model for the terminal Neoprotero-

zoic-Early Cambrian Ara Group evaporites in south Oman.

Sedimentology, 50, 879–898.

Schroder, S., Grotzinger, J.P., Amthor, J.E. and Matter, A.(2005) Carbonate deposition and hydrocarbon reservoir

development at the Precambrian-Cambrian boundary: the

Ara Group in South Oman. Sed. Geol., 180, 1–28.

Seaborne, T.R. (1996) The influence of the Sabatayn Evap-

orites on the hydrocarbon prospectivity of the Eastern

Shabwa Basin, Onshore Yemen. Mar. Petrol. Geol., 13,963–972.

Sears, S.O. and Lucia, F.J. (1980) Dolomitization of Northern

Michigan Niagara reefs by brine refluxion and fresh water/

sea water mixing. In: Concepts and Models of Dolomitiza-

tion (Eds D.H. Zenger, J.B. Dunham and R.L. Ethington),

Soc. Econ. Paleontologists Mineralogists, Spec. Publ., 28,215–235.

Sibson, R.H. (2003) Brittle-failure controls on maximum sus-

tainable overpressure in different tectonic regimes. AAPG

Bulletin, 87, 901–908.

Siemann, M.G. and Schramm, M. (2000) Thermodynamic

modelling of the Br partition between aqueous solutions

and halite. Geochim. Cosmochim. Acta, 64, 1681–1693.

Strohmenger, C., Voigt, E. and Zimdars, J. (1996) Sequence

stratigraphy and cyclic development of Basal Zechstein

carbonate-evaporite deposits with emphasis on Zechstein 2

off-platform carbonates (Upper Permian, Northeast

Germany). Sed. Geol., 102, 33–54.

Swart, P.K. and Guzikowski, M. (1988) Interstitial water

chemistry and diagenesis of periplatform sediments from

the Bahamas, ODP Leg 101. In: Proceedings of the OceanDrilling Program, Leg 101, Scientific Results (Eds J.A. Aus-

tin, W. Schlager and P. Comet), 101, 363–380.

Taberner, C., Cendon, D.I., Pueyo, J.J. and Ayora, C. (2000)

The use of environmental markers to distinguish marine vs.

continental deposition and to quantify the significance of

recycling in evaporite basins. Sed. Geol., 137, 213–240.

Taylor, P., Idiz, E., Macleod, G., Al-Ghammari, M. and Ochs,S. (2007) Understanding the geological controls on fluid

properties in the carbonate stringer play of South Oman.

GeoArabia, 12, 196.

Terken, J.J.M., Frewin, N.L. and Indrelid, S.L. (2001) Petro-

leum systems of Oman: charge timing and risks. AAPG

Bull., 85, 1817–1845.

Valyashko, M.G. (1956) Geochemistry of bromine in the pro-

cesses of salt deposition and the use of bromine content as a

genetic and prospecting criterion. Geokhimiya, 6, 570–589.

Wardlaw, N.C. and Watson, D.W. (1966) Middle Devonian salt

formations and their bromide content, Elk Point Area,

Canada. Can. J. Earth Sci., 3, 263–275.

Warren, J.K. (2006) Evaporites: Sediments, Resources and

Hydrocarbons. Springer-Verlag, Berlin/Heidelberg, 1035 pp.

Zak, K. and Skala, R. (1993) Carbon isotopic composition of

whewellite (CaC2O4 * H2O) from different geological envir-

onments and its significance. Chem. Geol., 106, 123–131.

Manuscript received 22 November 2007; revisionaccepted 15 May 2008

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� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists, Sedimentology, 56, 567–589