Diagenetic Geobodies:Fracture-Controlled Burial Dolomite in

Outcrops From Northern OmanVeerle Vandeginste, Cedric M. John, and Julia Beckert, Imperial College London

Summary

Diagenetic heterogeneities are difficult to predict in the subsur-face. Nevertheless, such heterogeneities can be crucial in hydro-carbon exploration. Diagenetic processes can significantly alterpetrophysical properties of reservoir rocks, especially in carbonaterocks because of the reactive nature of the carbonate minerals.Dolomitization [i.e., the transformation of calcite (limestone:Lmst) into dolomite] is a common diagenetic process in carbonaterocks. Resulting dolomite bodies have a different pore networkthan the original Lmst and respond also differently to tectonicstress causing different fracture networks than in the originalLmst. The paper presents an overview of the learning outcomesgained by studying fracture-related dolomite in outcrops of Omanand subsequent laboratory analysis conducted over the last 4years. A combined structural, petrographic, and geochemicalapproach was taken to study three dolomite systems occurring indifferent stratigraphic host-rock (HR) intervals. Structurally con-trolled dolomitization (i.e., dolomitization along faults and frac-tures) typically occurs in burial conditions, and the resultingstrong permeability anisotropies caused by the dolomite texturescan cause major challenges for hydrocarbon exploration.

Dolomite bodies in the Precambrian Khufai formation arerelated to N/S to NNE/SSW fractures, whereas dolomite bodiesthat mainly occur in the Jurassic HRs occur along reactivatedWNW/ESE normal faults. These fracture-related dolomite bodiesare generally less than 15 m wide, but can be up to a few hundredmeters long. Late-diagenetic dolomite bodies were also recog-nized in Permian HRs, where they occur at or close to the contactbetween Permian Lmst and early-diagenetic dolomite. This late-diagenetic dolomite system can be traced laterally for at least hun-dreds of meters and occurs in wadis that are approximately 40 kmapart. Our data indicate that there were several dolomitizationevents in the geological history of the succession, generating do-lomite bodies with different characteristics. This paper highlightsthe need to understand timing and structural setting of dolomitebodies in the subsurface to improve reservoir management.

Introduction

Reservoir heterogeneity refers to variations in porosity, perme-ability, and/or capillarity, laterally and/or vertically (Alpay 1972;Morad et al. 2010). Predicting the distribution of reservoir hetero-geneities is important for the prediction of reservoir performance,because the heterogeneity controls fluid flow and recovery factors.The predicted reservoir performance greatly influences the plan-ning of hydrocarbon production. A better understanding of howreservoir heterogeneity is controlled is thus of major economicimportance. Heterogeneity in reservoirs is affected by the natureand distribution of depositional facies, diagenetic products, andstructural elements, and thus, the (sequence) stratigraphic frame-work, thermal burial history, paleofluid flow and deformation his-tory of the reservoir (Weber 1982; Major and Holtz 1990; Moraesand Surdam 1993; Shipton et al. 2002; Guidry et al. 2007; Morad

et al. 2010). An additional complexity is the scale of observation,because one can observe heterogeneity from the microscalethrough to the reservoir scale and it was reported as particularlychallenging to determine diagenetic heterogeneity at the outcropor reservoir scale (Milliken 2001; Bowen et al. 2007).

Heterogeneity in reservoirs is modeled by identifying geobod-ies in the subsurface. These geobodies are typically definedmainly by depositional facies (Barnaby and Ward 2007; Jung andAigner 2012; Amour et al. 2013). The impact of diagenesis onreservoir heterogeneity was reported in siliciclastic rocks (Moraesand Surdam 1993; Bowen et al. 2007; Morad et al. 2010;Deschamps et al. 2012; Eugenia Arribas et al. 2012), as well as incarbonate rocks (Kerans 1988; Tyler et al. 1992; Eisenberg et al.1994; Wang et al. 1998; Hulea and Nicholls 2012) and mixed sili-ciclastic/carbonate settings (Borer and Harris 1991). Variations indiagenetic alteration may be linked to variations in depositionalporosity and permeability, which led Morad et al. (2010) to pro-pose a predicting tool by correlating the types and distribution ofdiagenetic processes to the depositional facies and sequence strati-graphic framework in clastic successions. Carbonates have morevaried facies and diagenetic patterns than siliciclastics and maythus be more challenging for reservoir evaluation. Carbonaterocks are petrophysically very complex because of the wide diver-sity of pore types (Lucia 1995), partly caused by the relative insta-bility of carbonate minerals and the impact of diageneticoverprint (Hulea and Nicholls 2012). In this respect, understand-ing the interplay of depositional and diagenetic controls on rockproperties is essential for reservoir modeling (Hulea and Nicholls2012). Previous work on diagenetic heterogeneities in carbonatesfound that petrophysical properties can be better grouped by rockfabrics than by strict depositional facies and that stacking of rock-fabric units within a high-frequency cycle can be used to approxi-mate fluid flow and recovery efficiency (Kerans et al. 1994). Post-depositional diagenetic alteration, such as karst development anddolomitization, overprints depositional heterogeneity (controlledby facies composition and architecture), resulting in additionallateral and vertical heterogeneities (Tyler et al. 1992; Eisenberget al. 1994) and significant reservoir compartmentalization [forexample, related to localized karst collapse structures (Kerans1988)].

Other than being linked to the depositional facies, diageneticheterogeneities can also relate to structural heterogeneities (Da-vies and Smith 2006; Guidry et al. 2007; Vandeginste et al. 2013,2014) that should be captured in realistic geological reservoirmodels. Generally, fracture networks originate early in the historyof the host rock, suggesting that structurally related diageneticheterogeneities can evolve during early diagenesis (Guidry et al.2007) and can influence fluid flow for a large part of the burialhistory of the rock. A reservoir-productivity study on the Subangas field in Indonesia has shown that bulk-reservoir performancerelates to local stress acting on existing faults and fracture-dam-age zones (Hennings et al. 2012). The reservoir potential wasshown to be most enhanced in areas that have a large amount offractures with high ratios of shear-to-normal stress (Henningset al. 2012). This paper discusses a type of diagenetic heterogene-ity that is clearly templated by structures (i.e., structurally con-trolled dolomite geobodies). We investigate outcrop examplesfrom several stratigraphic horizons in the central Oman

Copyright VC 2015 Society of Petroleum Engineers

This paper (SPE 173176) was revised for publication from paper IPTC 17328, first presentedat the International Petroleum Technology Conference, Doha, Qatar, 20–22 January 2014.Original manuscript received for review 3 October 2013. Paper peer approved 4 September2014.

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84 February 2015 SPE Reservoir Evaluation & Engineering

Mountains, focusing thereby on the dimensions, the structural set-ting, and the origin of the fluids. All these factors are important inunderstanding the structurally controlled dolomitization processthat causes diagenetic heterogeneities. Importantly, this type ofdolomite body is generally characterized by high porosity (Daviesand Smith 2006), and therefore is potentially filled with hydrocar-bons (Sagan and Hart 2006; Smith 2006).

Geological Setting

The geological setting of fracture-controlled burial dolomitebodies is of key interest because it forms the main control on thedimensions of these bodies. The tectonic history determines sev-eral aspects of the fracture network that is associated with the bur-ial dolomite bodies. Understanding the link between thediagenetic bodies and the geological setting can help with predic-tions of analogous diagenetic heterogeneities in reservoirs. Thestudy area is in the central section of the Al Hajar Mountains (orOman Mountains), in the Jebel Akhdar tectonic window that is ageological structure in which erosion or normal faulting produceda hole in the Semail ophiolite and Hawasina nappes (i.e., largesheetlike bodies of rock that have been moved more than 5 kmabove a thrust fault from their original sites of formation), andthus the underlying autochthonous (i.e., nontransported) Precam-brian to Mesozoic rocks crop out (Fig. 1). During Late Precam-brian extension (i.e., tectonic stress associated with stretching ofthe crust or lithosphere) in the Arabian-Nubian Shield, Gondwanaformed (Stern 1994) marking the Arabian plate by a passive conti-nental margin on the southern side of the Paleotethys Ocean formost of the Paleozoic (Stampfli et al. 1991). The breakup ofGondwana started in the Late Permian with the formation of theNeotethys Ocean (Stampfli and Borel 2002; Stampfli and Kozur2006). Northern Oman was on a NW-trending passive continentalmargin of the Arabian plate during most of the Mesozoic, but itevolved into a compressional margin during the Late Cretaceousas a result of Cenomanian intraoceanic subduction (i.e., part ofoceanic plate moving under another part of oceanic plate) andMiddle Turonian to Early Campanian SW-directed obduction(i.e., overthrusting of oceanic lithosphere onto continental litho-sphere) of the adjacent oceanic crust of the Neotethys during theconvergence of Gondwana and Laurasia (Glennie et al. 1974;Hacker 1994; Stampfli and Borel 2002; Breton et al. 2004). ThisAlpine Phase I obduction led to regional downbending of the con-tinental plate with NE/SW extensional strain and the formation ofW- to NW-trending normal and transtensional faults (Loosveldet al. 1996; Filbrandt et al. 2006), but also led to Semail ophiolite

obduction associated imbrication and predominantly SW-vergingthrusting of the Arabian platform margin that peaked in the EarlyCampanian. The Alpine Phase II (Eocene to Pliocene), related tothe separation of the Arabian from the African plate, involvedNE/SW compression leading to overprinting of earlier extensionalstructures in the Oman Mountains by folding and thrusting (Loos-veld et al. 1996). Doming of Jebel Akhdar may have started earlyafter the emplacement of the Hawasina and Semail nappes duringlatest Cretaceous and Paleocene time and was probably enhancedby later Tertiary compression (Searle 1985; Michard et al. 1994).

Methodology

We explored the whole of the Jebel Akhdar tectonic window forthe occurrence of late-diagenetic dolomite bodies, mainly by driv-ing through all wadis and mountain roads and observing cliffs, aswell as by hiking the mountain flanks. Although early-diagenetic,stratabound dolomite bodies were also recognized, these geobod-ies are not discussed in detail in this paper, because our focus ison late-diagenetic, structurally controlled dolomite geobodies.The latter were identified in the field, and their dimensions weremeasured on the accessible outcrops with a surveyor’s tape. Theiroccurrence was mapped by recording their global-positioning-sat-ellite locations. In addition, late-diagenetic dolomite geobodies oninaccessible outcrops were recorded on photopanoramas, and forcliff faces less than 1 km away, dimensions were calculated fromangle and distance measurements with a tripod, compass, andSwarovski 8� 30 monocular rangefinder. Several fracture analy-ses were carried out by measuring strike and dip of each fracturealong scan lines with a Brunton geological compass. Additionalinformation, such as the width, infilling, geometry of the fracture,and distance between fractures, was recorded also. Limestone(Lmst) and dolomite samples were collected in beds of Neopro-terozoic age in Wadi Bani Awf, of Permian age in both Wadi Sah-tan and Wadi Mistal, and of Jurassic age in Wadi Mistal, asindicated in Fig. 1. Hand samples were cut, finely polished, etchedwith 1-M hydrochloric acid, and stained with Alizarin Red S andpotassium ferricyanide to distinguish calcite and dolomite andtheir ferroan equivalents (Dickson 1966). Thin sections were pre-pared, halves were stained, and they were studied with a ZeissAxioskop 40 polarization microscope and a CITL Cathodolumi-nescence Mk5-2 stage mounted on a Nikon Eclipse 50i micro-scope. Dolomite and Lmst samples were also investigated fortheir geochemical elemental composition and stable carbon, oxy-gen, and strontium isotopic composition, following proceduresexplained in Vandeginste et al. (2013).

0 10

Wadi Sahtanoutcrop

Wadi BaniAwf outcrop

Wadi Mistaloutcrops

km Road

IRAN

Samail

Naki

Al Awabi

Al Rustaq

Nazwa

Birkat al Mawz

Izki

JebelAkhdar

OMAN

200 km

U.A.E.

SAUDIARABIA

Fault

Upper Proterozoic -Cambrian Huqf Group

Late Permain-TriassicAkhdar Group

Jurassic Sahtan Group

Upper Jurassic-middleCretaceous Kahmah Group

Middle Cretaceous WasiaGroup

Middle-Upper CretaceousMuti Formation

Fig. 1—Geological map of the Jebel Akhdar tectonic window with inset figure of Oman. The different outcrops are indicated. Thefigure is modified after Vandeginste et al. (2013).

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Results

Dolomite Geobody Types and Dimensions. Three differenttypes of dolomite bodies were recognized. The first type (Type 1)has a dark-reddish weathering color and is hosted in the Neopro-terozoic Khufai formation in Wadi Bani Awf (Fig. 2a). These do-lomite bodies, which are high angle to perpendicular to bedding,follow NNE-trending fractures (Fig. 3) along hundreds of metersand are up to approximately 15 m wide. The second type (Type 2)has a brownish-red weathering color and occurs typically at thecontact between limestone (Lmst) and overlying early-diageneticdolomite in the lower part of the Khuff equivalent formation(Lower Permian). This type was observed in Wadi Mistal, Wadi

Sahtan (Fig. 2b), and in several smaller wadis. These dolomitebodies are laterally more extensive (one can trace them for nearlya kilometer in outcrops) and are up to approximately 60 m thick(vertically). The third type (Type 3) of dolomite body was identi-fied in both Permian and Jurassic host rock (HR) in Wadi Mistal(Fig. 2c). This type is characterized by a reddish-to-rusty weather-ing color and is strongly affected by dedolomitization (Vande-ginste and John 2012). This dolomite type occurs along WSW-trending fractures and normal faults (Fig. 3). These geobodies areup to approximately 100 m long and a few meters wide. The dolo-mite body is several meters wide where it has replaced Lmst HR,whereas its lateral extent is very limited where it cuts early-

(a)

Lmst

Lmst

Lmst

DT1

DT2

DT3

ED

ED

PHR

(b)

(c)

Fig. 2—Field outcrop photographs. Trees (2 to 3 m high) for scale. Late-diagenetic dolomite bodies are outlined by white transpar-ent line on the pictures. (a) DT1 in Lmst HR of the Neoproterozoic Khufai formation in Wadi Bani Awf. (b) DT2 at the contactbetween Lmst and overlying ED dolomite HR of Early Permian age, unconformably overlying Precambrian siliciclastic host rock(PHR) in Wadi Sahtan. (c) DT3 in Lmst and stratabound ED dolomite HR of Jurassic age in Wadi Mistal.

330 300

180n = 62

Neoproterozoic, Wadi Bani Awf Permian, Wadi Mistal Jurassic, Wadi Mistal

300 60

90270

240 120

210 150

330 300

180n = 82

300 60

90270

240 120

210 150

330 300

180n = 77

300 60

90270

240 120

210 150

Fig. 3—Rose diagrams of strike of fractures analyzed in transects along stratigraphic beds crosscutting both Lmst and dolomite inthe Neoproterozoic Khufai Formation in Wadi Bani Awf, in the Lower Permian Khuff equivalent formation in Wadi Mistal, and in theJurassic Sahtan Group in Wadi Mistal.

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diagenetic (ED) dolomite HR beds; in the latter case, the late-dia-genetic dolomite occurs in small veins rather than meter-scalereplacement bodies. Dolomite-to-Lmst contacts are sharp for alltypes. However, the Type-2 dolomite contact with ED dolomite inPermian HR is gradational.

Dolomite bodies of both the second and third type occur inPermian HR in Wadi Mistal. On the basis of field observations, itis clear that the third dolomite type (i.e., reddish-to-rusty WNW-trending dolomite) crosscuts the second dolomite type (i.e., later-ally extensive brownish-red dolomite).

Fractures. Fracture-density histograms are presented for transectsacross late-diagenetic dolomite Type 1 (DT1) and Type 3 (DT3)and their HR (Fig. 4). The majority of the fractures are less than 0.5cm wide, and most are completely cemented with calcite, dolomite,or both. The fractures are up to several meters or tens of meterslong (laterally), and they generally do not extend vertically beyondthe thickness of the bed and are thus bed-bound. The results of thefracture analyses show that the highest density of fractures and ofopen fractures occurs close to the main fracture within the late-dia-genetic dolomite body Type 1. There is no clear trend in terms of

fracture width along this transect in Type 1 dolomite and its HR. Ahigher abundance of open fractures (as well as wider fractures)close to the main fracture is also observed in DT3 in Permian HR,but this is not confirmed for the same dolomite type in Jurassic HR.For DT3, a higher density of fractures occurs at the edges of the do-lomite body rather than in the center, as was the case for Type 1.The most-significant difference in fracture density is measured inJurassic HR with a higher density of fractures (approximately sixtimes more) in stratabound ED dolomite than in Lmst [i.e., approxi-mately 15 to 95 fractures/m in stratabound dolomite vs. 1 toapproximately 15 fractures in Lmst (for scanlines of the same ori-entation in Jurassic HR)]. In all cases, the strike and dip of the openfractures are similar to those of the cemented fractures. The mainfracture set of the open fractures has an orientation similar to that ofthe main dolomite body, and an additional minor fracture set couldalso be separated. Details of the statistical analysis of the structuralorientation of the open fractures are presented in Table 1.

Lmst and Dolomite Textures. The Lmst HR and the respectivedolomitized equivalent are presented as pairs in Fig. 5 [i.e., Pre-cambrian HR and dolomitized equivalent (Fig. 5a, 5b), Permian

Neoproterozoic, Wadi Bani Awf Premian, Wadi Mistal

Jurassic, Wadi Mistal (limestone host rock)

–8

40 30

25

20

15

10

5

0

35

30

25

20

15

10

5

0

20 100

80

60

40

20

0

15

10

5

00 5 10

Distance From Main Fracture (m) Distance From Main Fracture (m) Distance From Main Fracture (m) Distance From Main Fracture (m)

Distance From Main Fracture (m)

Num

ber

of F

ract

ures

Num

ber

of F

ract

ures

Num

ber

of F

ract

ures

Num

ber

of F

ract

ures

Distance From Main Fracture (m) Distance From Main Fracture (m) Distance From Main Fracture (m)

15 0 5 10 15 20 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16

–6 –4 –2 0 2 –6 –4 –2 0 2 –8 –6 –4 –2 0 2 4 –6 –4 –2 0 2 4

Jurassic, Wadi Mistal (early diagenetic dolomite host rock)

Lmst Lmst LmstopenDT1 DT1 DT3

DT3DT3

DT3filled

openfilled

openED EDfilled

openfilled

Lmst2–6 cm1–2 cm0.5–1 cm0–0.5 cm

1–2 cm0.5–1 cm0–0.5 cm

3–4 cm2–3 cm1–2 cm0.5–1 cm0–0.5 cm

3–10 cm2–3 cm1–2 cm0.5–1 cm0–0.5 cm

LmstLmst

Fig. 4—Histograms of fracture density in function of the distance from the main large fracture (with negative values for distancesat the left side of the main fracture). Histogram intervals are 1 m. For each fracture analysis, the data are separated for fracturewidth ranges and for open vs. completely cemented fractures. The fracture width is defined here by the width of the zone inbetween HR rims (i.e., open space and/or cement filling). The shading in the background indicates the position of the late-diage-netic dolomite along the transects. The HR is, in most cases, Lmst, but ED dolomite in the fourth case. DT1 is the late-diageneticdolomite hosted in Neoproterozoic HR and sampled in Wadi Bani Awf, whereas DT3 is the late-diagenetic dolomite hosted in bothPermian and Jurassic HRs and sampled in Wadi Mistal.

Standard DeviationMean DipStandard DeviationekirtSnaeMrebmuNteSerutcarFegARH

Neoproterozoic 9 201 8 62 10

Neoproterozoic 7 291 11 89 17

2196726821naimreP

5498316435naimreP

645790101cissaruJ

188621945cissaruJ

Jurassic (early dolomite) 10 110 28 47 14

Jurassic (early dolomite)

Main

Minor

niaM

roniM

niaM

roniM

Main

Minor 6 20 21 74 13

Table 1—Statistics on strike and dip of open fractures.

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HR and dolomitized equivalent (Fig. 5c, 5d), and Jurassic HR anddolomitized equivalent (Fig. 5e, 5f)]. The Lmst texture variesbetween stratigraphic beds, but is generally difficult to recognizebecause of microsparitization. The original texture of the Precam-brian HR is particularly faint because of thick (up to 25 mm)cleavage twin planes, but is identified as a peloidal grainstone(Fig. 5a). The Permian HR is a bioclastic packstone with benthicforaminifera, small gastropods, shell and coral fragments (Fig.5c). The Jurassic HR consists of mudstone, wackestone, and pack-stone with one or more of the following components: shellfragments, crinoids, gastropods, sponge fragments, benthic fora-minifera, or peloids (Fig. 5e). There are also a few siliciclasticbeds and some stratabound dolomite beds in the Jurassic outcrop.

Both the first dolomite type (NNE-trending dark-reddishweathered dolomite bodies in Precambrian HR) and the third type(WSW-trending reddish-to-rusty dolomite bodies in Permian andJurassic HR) are mainly composed of fabric-destructive zebra do-

lomite in which gray medium-crystalline (150 to 400 mm) andwhite coarse-crystalline (500 to 2000 mm) dolomite bands alter-nate (Fig. 5b, 5f). In the center of white bands, calcite cementoccupies former pore space on top of saddle dolomite crystals.Also, the second dolomite type (laterally extensive dolomitebodies at the Lmst-to-ED dolomite contact in Permian HR) con-tains zebra dolomite at several sites. However, zebra dolomite isnot predominant in this case, but gray medium-crystalline dolo-mite prevails and both fabric-destructive (Fig. 5d) and fabric-pre-serving textures are recognized.

Geochemical Signature. Each of the three dolomite types has adistinctive iron (Fe) vs. manganese (Mn) content (Fig. 6). The Fecontent is lowest (up to approximately 2%) in Type 2 dolomite,slightly higher (2 to 4%) in Type 1, and highest (3 to 9%) in Type3 dolomite. Despite the differences in Fe content, the Mn contentis more similar in the three types and ranges from approximately

500 µm 500 µm

500 µm 500 µm

500 µm200 µm

(a) (b)

(c) (d)

(e) (f)

Fig. 5—Microphotographs of stained thin sections. Black bars indicate scale with labeled length. (a) Recrystallized Lmst from Neo-proterozoic Khufai formation in Wadi Bani Awf outcrop. (b) DT1 in Neoproterozoic HR in Wadi Bani Awf. Note the coarse crystalson the left and medium-sized crystals on the right, as part of the zebra dolomite texture. (c) Bioclastic packstone of Lower PermianLmst strata in Wadi Sahtan. Note the presence of some small dolomite rhombs and stylolitic seams. (d) DT2 in Permian HR in WadiSahtan. Note the stylolite. (e) Wackestone to packstone of Jurassic age in Wadi Mistal. (f) DT3 in Jurassic HR in Wadi Mistal. Notethe coarse dolomite crystals with some pore space in the center of the coarse band and medium-sized crystals at the top of thepicture representing the dark-gray band in the zebra dolomite.

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0.2 up to 0.8% for Types 1 and 3 and up to 1.1% for Type 2 dolo-mite (Fig. 6).

All three dolomite types have a d13C signature that is similarto that of original marine Lmst of the respective HR age (Fig. 7).In contrast, the d18O signature of Type 1 dolomite is approxi-mately 10% more depleted than the original signature of the Neo-proterozoic marine Lmst, and also Type 3 dolomite has a more-

depleted d18O signature than the original Permian and JurassicHRs in which they occur (Fig. 7). However, Type 2 dolomite hasa d18O signature similar to that of Permian marine Lmst, andlower d18O values seem to correlate with lower d13C valueswithin this range for marine Lmst.

The 87Sr/86Sr isotopic signatures in all three dolomite typesdisplay a 87Sr enrichment compared with the signature expectedfor marine Lmst of their respective HR age. Nevertheless, thereare clear discrepancies in the significance of this enrichmentbetween the different dolomite types. Type 1 dolomite shows anincrease of approximately 0.001 for the 87Sr/86Sr ratio and Type 2dolomite an increase of approximately 0.002, whereas Type 3 do-lomite shows an increase of 0.005 up to 0.012 (Fig. 8). It is clearthat Type 3 dolomite follows a 87Sr/86Sr vs. d18O trend that doesnot change with the formation in which is occurs (i.e., Permian orJurassic strata).

Discussion

Dolomite Types and Spatial Distribution. Detailed fieldworkled us to distinguish three types of late diagenetic dolomite in theJebel Akhdar tectonic window in northern Oman. These dolomitetypes differ in dimension or structural orientation and have aslightly different weathering color. The fracture-related dolomitetype in Neoproterozoic host rock (HR), defined as Type 1, hasNNE-trending bodies that are hundreds of meters long and up toapproximately 15 m wide. The dolomite bodies, which we definedas Type 3, occurring in both Permian and Jurassic HR, are up toapproximately 100 m long and a few meters wide. Type 3 dolo-mite bodies are also mainly controlled by structures, such as areactivated normal fault (Vandeginste et al. 2013), and their orien-tation is WNW, approximately perpendicular to the Type 1 dolo-mite trend. For dolomite Types 1 (DT1) and DT3, it is thus clearthat dolomitization took place mainly along the faults or fractures,forming a dolomitization halo around the respective structure, andthat the length of the dolomite body along the structure is at leastfive times greater than the width for the majority of these bodies.This indicates that fluid flow focused along the fault withoutmuch penetration into the HR matrix, which, in turn, may suggestthat the matrix had a low permeability at the time of dolomitiza-tion. High length/width ratios are typical for dimensions of struc-turally controlled hydrothermal dolomite bodies. Most of the

10

8

6

4

2

00

Fe

(%)

Mn (%)

0.2 0.4 0.6 0.8 1.0 1.2

Type 1 dolomite in Precambrian HRType 2 dolomite in Permian HRType 3 dolomite in Permian HRType 3 dolomite in Jurassic HR

Fig. 6—Iron (Fe) vs. manganese (Mn) crossplot of dolomitetypes. Dashed lines indicate trendlines representing the Fe vs.Mn ratio in each of the three different dolomite types. The Fe/Mn ratio decreases from Type 3 to Type 1 to Type 2 dolomite.Data from Type 3 dolomite in Jurassic HR are from Vandeginsteet al. (2013).

7Precambrianmarine Lmst

Permianmarine Lmst

Jurassicmarine Lmst

6

5

4

3

2

1

0

δ13 C

(‰

VP

DB

)

δ 18O (‰VPDB)

0–5–10–15

Type 1 dolomite in Precambrian HRType 2 dolomite in Permian HRType 3 dolomite in Permian HRType 3 dolomite in Jurassic HR

Fig. 7—Stable carbon and oxygen isotope crossplot of dolo-mite types. Shaded boxes indicate isotopic signature for ma-rine Lmst of the respective HR age (Veizer et al. 1999). Type 3dolomite in Jurassic HR data are from Vandeginste et al. (2013).

Precambrianmarine LmstPermian

marine Lmst

0.720

0.718

0.716

0.714

0.712

0.710

0.708

87S

r/86

Sr

δ18O (‰VPDB)

0–5–10–15

Type 1 dolomite in Precambrian HRType 2 dolomite in Permian HRType 3 dolomite in Permian HRType 3 dolomite in Jurassic HR

Fig. 8—Stable strontium vs. oxygen isotope crossplot of dolo-mite types. Shaded boxes indicate isotopic signature for ma-rine Lmst of the respective HR age (Veizer et al. 1999). Note thatthe 87Sr/86Sr for Jurassic marine Lmst is approximately 0.707and thus falls outside the plot. Data from Type 3 dolomite in Ju-rassic HR are from Vandeginste et al. (2013).

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documented hydrothermal dolomite reservoirs have larger dimen-sions than Type 1 and Type 3 dolomite bodies in northern Oman,though. Some of the largest, the Albion-Scipio reservoir in the Or-dovician Trenton-Black River formation and the Clarke Lake res-ervoir in the Devonian Slave Point formation, are 45 and 35 kmlong and up to 1 and 7 km wide, respectively (Davies and Smith2006). The Albion-Scipio hydrothermal dolomite field is localizedat sags linked to a major strike-slip fault, and dolomitization wasmainly constrained by the damage zone of this fault because fluidsdid not penetrate the HR matrix much (Davies and Smith 2006).The location and dimension of the hydrothermal dolomite bodiesthus give information on paleofluid pathways, which one couldextrapolate to extract reservoir implications. In a similar way,because paleoconditions generated massive fluid flow resulting indolomitization, current stress fields and structures can cause majorwellbore instability and a loss of drilling-fluid circulation in theannulus. Natural fractures can be “stress-sensitive” when they arehydraulically conductive because of their suitable orientation forfailure (shear or tensile) in the present-day stress tensor (Fink-beiner et al. 1997). A study by Haghi et al. (2013) concluded thatboth stress-sensitive and stress-insensitive fracture sets have sig-nificant influence on hydraulic conductivity.

The late-diagenetic dolomite occurring mainly at the contactbetween limestone (Lmst) and overlying early-diagenetic (ED)dolomite in Lower Permian HR, which we defined as Type 2 do-lomite, is laterally extensive along at least approximately 1 kmand seems to be more controlled by lithology or ED contact,whereas the control by fractures is less clear in this case. Thisimplies that dolomitizing fluid flow for this Type 2 dolomite didexplore a matrix that must have been permeable enough to allowsufficient fluid flow and input of magnesium for dolomitization totake place. We hypothesize here that, given a source of dolomitiz-ing fluids and a driving mechanism for fluid flow, it is permeabil-ity that plays a key role in determining the sites where late-diagenetic dolomite bodies form. However, in the case of ED do-lomite bodies, where dolomitization at low temperature is moreprone to kinetic inhibition, the grain size and organic content inthe Lmst may play an important role as well, in addition to perme-ability. The impact of organic matter was demonstrated for dolo-mite-formation experiments at low temperature (Roberts et al.2013), and also the impact of the crystal size of the reactant wasshown in laboratory experiments (Sibley et al. 1987).

Origin of Dolomite Types and Timing of Dolomitization

Events. The distinction between the three dolomite types is alsoclear on the basis of their geochemical signature, because eachdolomite type shows a distinct signature in Fe-Mn, d18O-d13C,and 87Sr/86Sr (Figs. 6 through 8). This indicates that each of thedolomite types was formed by different dolomitizing fluids, andthus likely formed at a different time. Crosscutting relationshipsshowed that dolomite Type 3 (DT3) is younger than DT2. DT1 isinterpreted to be the earliest phase, and certainly predates DT3,because (1) it was found in Precambrian HRs and thus has thepotential to be the oldest phase; (2) the fracture-related dolomitebodies are folded, and thus dolomitization must have precededsome folding; (3) dolomitization occurred in between the onsetand termination of bedding-parallel stylolitization, and hence,most likely before the deep burial related to the Alpine Orogeny;and (4) DT1 was observed only in Precambrian rocks, and theserocks were affected by intense deformation related to the Hercy-nian Orogeny during Carboniferous time (in contrast to the Per-mian and Jurassic HRs) (Vandeginste et al. 2014). The 87Sr/86Srratios show that DT1 is least enriched in 87Sr, whereas DT3 showsvery high 87Sr/86Sr ratios (Fig. 8). Assuming that the derivedchronology of the three dolomite types is correct, this shows thenan increasing 87Sr enrichment in the dolomitizing fluids throughgeologic time (i.e., more interaction of the dolomitizing fluidswith siliciclastic units or Precambrian basement). However, thistrend is not confirmed by the iron content, which is lower in Type2 than in Type 1 dolomite (Fig. 6). The average d13C signaturedecreases from Type 1 to Type 2 to Type 3, but these signatures

relate to the d13C signature of the original marine limestone(Lmst) of the respective HR age (Fig. 7), and thus indicate HRbuffering for the stable carbon isotope signature recorded in thedolomite. In contrast, the d18O values measured in DT1 and DT3are much more depleted than the signature of marine Lmst of therespective HR age (Fig. 7). This is most likely related to dolomiti-zation at high temperature, but also the d18O signature of the dolo-mitizing fluid will affect the d18O signature measured in thedolomite. Most of the stable oxygen isotopic values of DT2 plotwithin the signature for marine Lmst of Lower Permian age (Fig.7). On the basis of the different mineralogy and thus differentfractionation of oxygen isotopes between seawater and carbonate,the dolomite d18O values are still more depleted than the expectedmarine dolomite signature of Lower Permian age, which would beapproximately 3% higher than that of Lmst, on the basis of equa-tions from Kim and O’Neil (1997) for calcite and Vasconceloset al. (2005) for dolomite. Thus, even for DT2, formation atslightly elevated temperature is expected assuming a seawater-derived formation fluid.

In terms of absolute timing, DT3 formed around Santoniantime (Late Cretaceous) during tectonic compression before Cam-panian obduction of the Semail ophiolite, as proposed by Vande-ginste et al. (2013). Type 3 dolomitizing fluids were warm andshow evidence of interaction with Precambrian sandstone or thecrystalline basement, mainly on the basis of their high 87Sr enrich-ment and iron and manganese content. These fluids exploited nor-mal faults that developed because of extensive forces in thesubducting slab; these faults thus trend parallel to the continentalmargin (Vandeginste et al. 2013). Type 2 dolomitization occurredbetween Triassic ED dolomitization and Santonian Type 3 dolo-mitization, on the basis of crosscutting relationships. Type 1 dolo-mitization is most likely associated with the Hercynian Orogenyduring Carboniferous time.

Dolomitization and Implications for Reservoir Properties.

Good reservoir properties in dolomite bodies of burial origin aremainly linked to two factors. First, it was shown that burial dolo-mite is more prone to fracturing than its Lmst equivalent becauseof its crystalline nature that enhances a brittle behavior during de-formation (Sinclair 1980). It was reported that dolostone reser-voirs commonly have lower matrix porosities and permeabilitiesthan Lmst reservoirs but higher fracture porosities and permeabil-ities (Schmoker et al. 1985). Amthor et al. (1994) suggested thatpermeability of Lmst vs. dolostone depends on depth and thatdolostone is more permeable than Lmst at depths of more than 2km. The trend of more fractures in dolomite than in Lmst is alsoconfirmed by our results. The ED dolomite has a fracture densitythat is approximately six times higher than the Lmst in the Juras-sic beds studied. Also the late-diagenetic dolomite Type 1 (DT1)and DT3 contain a higher density of fractures and of open frac-tures than their Neoproterozoic and Permian Lmst HR, respec-tively. These results thus show the importance of definingdolomite bodies as geobodies that can contribute to the heteroge-neity in reservoirs, because open fractures will influence perme-ability. Second, the involvement of hydrothermal fluids seems tobe important for the generation of the late-diagenetic dolomitebodies. In this case, hydrothermal dolomitizing fluids migratedalong faults or fractures in Lmst. Several dolomitized carbonatesof hydrothermal origin have good reservoir characteristics (Da-vies and Smith 2006; Sagan and Hart 2006; Wierzbicki et al.2006). In these examples, the development of good reservoirproperties is generally associated with the reactivation of faults ormigration of corrosive fluids. Typical textures in these hydrother-mal dolomites show vuggy porosity, zebra dolomite, and brecciatextures. However, porosity in these dolomitized zones could beoccluded by extensive dolomite cementation (i.e., overdolomitiza-tion). The key to create or preserve good reservoir quality seemsto be to maintain a balance between porosity generation by dila-tional fracturing and/or carbonate dissolution, and a lesser degreeof dolomite cementation (and potentially later, calcite or quartzcementation). These features are intrinsic to the dolomitization

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90 February 2015 SPE Reservoir Evaluation & Engineering

process itself (i.e., the tectonic stress fields and amount, duration,and chemical composition of the fluids exploiting the structures).This shows that understanding the tectonics or structural develop-ment and potential reactivation as well as the chemical composi-tion, temperature, and migration of basinal fluids is critical inpredicting the reservoir properties of burial dolomites, and thatthus a good geological insight is important in well planning andengineering of hydrocarbon-production plans.

To understand the dolomitization process in the context of tec-tonics and fluid flow, an additional challenge with outcrop analogstudies is reconstructing the subsurface equivalent (i.e., strippingthe nonrelevant uplift and surface-weathering history from theresulting outcrops). For example, if calcite occludes pore space inthe center of zebra-dolomite bands, it is of major importance todetermine the origin of this calcite (i.e., whether this calciteformed in deep burial conditions or by meteoric fluids in the near-surface environment upon uplift). Impact of surface weatheringon DT3 is reported in Vandeginste and John (2012). The latterstudy demonstrated significant dedolomitization related to near-surface conditions and resulting instability of the iron-rich dolo-mite, and a clear control of climate on dedolomitization. Evenregions that experience a high degree of aridity today, such asOman, could have experienced a much more active hydrologicalcycle in the past, and this needs to be taken into account whenlooking at outcrop analogs. Similarly, other studies developed a“Standard Property Calculator” to strip back all late-diageneticchanges from outcrop analogs, to approximate conditions that arespecific for the reservoirs and thus only taking into account the(early) diagenetic processes until the burial reservoir conditions(Benson et al. 2014).

The Role of Fractures in Reservoir Heterogeneity. As shownpreviously, fractures and faults play an essential role with respectto reservoir quality of dolostones; both predolomitization andpost-dolomitization fractures are important. Larger structures andpreferentially faults rather than fractures are of interest for chan-neling dolomitizing fluids in the early stages of dolomitization,because fault activity can trigger fluid flow (Sibson 1992) neededfor the delivery of magnesium (Mg) to the site of dolomitization.Hydrothermal dolomite bodies testify to hydrothermal fluid flowin fractures, synchronous structural features, fluid-flow channel-ing, and dolomitization linked to a tectonic control (Iriarte et al.2012). Hereby, fluid circulation concentrated preferentially inmore-fractured areas with increased permeability and in exten-sional chimneys for the Ason Valley dolomite bodies (Iriarte et al.2012). Diagenetic dolomitization reactions resulting from fluidinteraction with the host Lmst along the fault result in dolomitebodies with different petrophysical properties than the host Lmst(Lucia 1995). Within these bodies, the diagenetic textures andchemical composition may be different closer to the fault thanaway from the fault, with a gradual change in diagenetic textures(Wilson et al. 2007; Sharp et al. 2010). This gradual change in tex-tures may be present in large (hundreds of meters to kilometersize) dolomite bodies, but may be minimal in smaller (meter-scalewide) dolomite bodies that are mainly defined by a halo along themain structure (fault or fracture). Characteristics of predolomitiza-tion faults and fractures, such as orientation and transmissivity,can thus act as a template for structurally controlled dolomitebodies. Understanding the predolomitization fracture network thusprovides information on paleofluid-flow processes, from which wecan then derive insight into current fluid-flow processes. A signifi-cant challenge in this workflow is some obliteration of the original(predolomitization) fracture network by the generally fabric-de-structive replacement process of hydrothermal dolomitization.

The second type of fractures (i.e., post-dolomitization frac-tures) can affect the reservoir heterogeneity because dolomitebodies can react differently to applied stress than the surroundingLmst. The crystalline nature of dolostone links to its brittle behav-ior, and it will generally fracture more rapidly than Lmst under acertain applied stress (Sinclair 1980). Our results have clearlydemonstrated this in the comparison between Jurassic ED dolo-

mite vs. Lmst beds, with a fracture density that was approximatelysix times higher. In this case, the dolomite bodies will act as atemplate for fracture patterns, which help define reservoir hetero-geneity. Also, late-diagenetic dolomite showed a higher fracturedensity than the Neoproterozoic and Permian HR counterparts.This trend was not confirmed though by the late-diagenetic dolo-mite in Jurassic HR. We interpret this difference to be linked tothe timing of late-diagenetic dolomitization, and hence, the post-dolomitization history. The late-diagenetic dolomite Type 1(DT1) formed much earlier than the late-diagenetic DT3, and thusunderwent a longer burial and tectonic history after dolomitiza-tion. DT3 occurs both in Permian and Jurassic HRs; in this case,the fracture pattern in the dolomite bodies in the stratigraphicallylower Permian host rock may reflect the impact of the deeper bur-ial compared with that in the Jurassic beds.

Conclusions

The prediction of reservoir heterogeneity is of major importancein the planning of hydrocarbon production (Hamilton et al. 1998;Barton et al. 2004; Sech et al. 2009). This heterogeneity has threemain components (i.e., depositional, diagenetic, and fracture het-erogeneity). Although the depositional lithology forms the foun-dation and is defined from deposition onwards, both diagenesisand fracturing can affect these depositional layers throughout thegeological history of the rocks. Moreover, there is an importantinterplay between fractures and diagenetic products, with multiplefeedback mechanisms between fracturing and dolomitization andlikely several dolomitization “cycles” related to larger regionaltectonics and fluid-flow migration. A thorough understanding ofthe region by a multidisciplinary approach is key to a better pre-diction of reservoir heterogeneity.

This study has illustrated how different diagenetic dolomitegeobodies can be distinguished by their link with structures andtheir geochemical signature. This information helps in the recon-struction of paleofluid migration. The insights gained from thisand similar studies can then be extrapolated to derive more-globalinformation on the interplay between depositional facies, frac-tures, and diagenetic products. Moreover, this study has shownthe clear impact of dolomitization on fracture distribution, andprovided quantitative data on these outcrops that may be analogsto subsurface reservoirs.

Acknowledgments

This research project is funded by Qatar Petroleum, Shell, and theQatar Science and Technology Park. Field logistics was providedby Shuram. We would like to thank Emma Williams for help withinductively coupled plasma-atomic emission spectrometry meas-urements and Simon Davis for help with stable isotopemeasurements.

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Veerle Vandeginste is research fellow in sedimentary andstructural geology and leads the Diagenesis Linked Researchgroup at the Department of Earth Science and Engineering ofImperial College London. Previously, she performed postdoc-toral research at CEREGE (France) and worked approximately2 years for the Geological Survey of Belgium. Vandeginste’sresearch interests include diagenesis and sedimentology, sta-ble isotope and minor element geochemistry, structural geol-ogy, and ore mineralization. She has authored 15 peer-reviewed journal publications and coauthored another two.Vandeginste holds a PhD degree in geology from the K.U.Leuven (Belgium).

Cedric John is a senior lecturer in carbonate sedimentologyand heads a research group focused on carbonate studiesand the development and application of clumped isotopegeothermometry with respect to carbonate systems. He holdsa PhD degree from the University of Potsdam, was a post-doc-toral research fellow at the University of California (Santa Cruz)from 2004 to 2005, and worked 3 years as a research scientistfor the International Ocean Drilling Program based at TexasA&M University. John has authored more than 25 peer-reviewed papers, and he teaches carbonate reservoir geol-ogy and advanced marine stratigraphy to graduate andundergraduate students.

Julia Beckert is a PhD degree candidate at Imperial CollegeLondon. Her research interests include carbonate sedimentol-ogy and diagenesis. In the framework of her PhD degree pro-ject, funded by Qatar Petroleum, Shell, and the QatarCarbonates and Carbon Storage Research Centre, Beckert iscurrently working on the characterization of large-scale dolo-mite bodies hosted in the Lower Permian in the Central OmanMountains. She holds a BS degree in geology and an MSdegree in geoscience and sedimentology, all from the Univer-sity of Mining and Technology, Freiberg (Germany). Beckert’sMS degree thesis—performed in collaboration with the Ger-man University of Technology in Muscat (Oman)—focused onPermian/Triassic facies variations in the Oman Mountains.

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