1

Origin of Dolomite in the Arab-D Reservoir from the Ghawar Field, Saudi Arabia: Evidence

from Petrographic and Geochemical Constraints

Swart, P.K.1, Cantrell, D.2, Westphal, H.1,3, Handford4, R. and Kendall5, C.

1-MGG/RSMAS, 4600 Rickenbacker Causeway, University of Miami, Miami, Fl 33149

2- Saudi Arabian Oil Company, Dharan, Saudi Arabia.

3- Geologisches Institut, Universität Hannover, Callinstr. 30 , 30167 Hannover, Germany

4- Robert Handford, Strata-Search, Austin, Texas 787265- Christopher George St Clement Kendall , University of South Carolina , Columbia, SouthCarolina 29208

Abstract

A significant proportion of oil production from the Ghawar field, Saudi Arabia originates from

dolomitized rocks. Stratigraphic, petrographic, and geochemical data suggests that at least four

episodes of dolomitization affected these sediments. The lower portion of the Arab-D, Zone 3, is

only partially dolomitized, with the dolomite frequently being associated with firmgrounds. We

propose that these dolomites were formed on an outer ramp setting with a maximum water depth

of 50 m during a period of non-deposition with the dolomitization process being promoted by

the oxidation of organic material and the diffusion of Mg2+ from the overlying seawater. The

dolomites in Zone 2B are geochemically distinct compared to those in Zone 3 in that they have

relatively positive oxygen isotopic compositions (-1 to -2 ‰ compared to -7 ‰). This relatively

positive oxygen isotopic composition and the geochemical similarity to the dolomites in Zone 1,

which are intimately associated with the overlying evaporites, have led us to conclude that the

Zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments.

These hypersaline fluids bypassed Zone 2A by moving rapidly through the grain dominated

sediments. Early cementation and dolomite formation made these units more susceptible to later

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fracturing which affected the entire Arab-D formation. This fracturing allowed higher

temperature fluids to leach the dolomites, thereby removing remaining calcite and partially

resetting the oxygen isotopic composition of the dolomites. As a result of this later

dolomitization event, rocks which were only partially dolomitized were leached creating units

with extremely high permeability and porosity. Dolomites in the lower Zone 3 were

recrystallized during burial by the normal geothermal gradient leading to the present negative

oxygen isotopic values.

Introduction

The Arab-D reservoir in the Ghawar Field of Saudi Arabia contains a significant amount of

dolomitized carbonate rocks. The occurrence of dolomite in the Arab-D has a pronounced

impact on reservoir quality, in a number of ways. In some places, dolomite is responsible for

producing zones of very high flow (super-K) in the reservoir (Meyer et al., 2000), whereas in

other places these rocks act as permeability barriers. There are important differences between

dolomites and limestones in the reservoir, both in terms of their impact on reservoir quality as

well as in terms of the reservoir management strategies that are needed to successfully solve

production problems in these rocks. Dolomite typically has a higher grain density and acoustic

velocity than limestones, which affects the response of most porosity wireline logging tools to

dolomite rocks. In addition, dolomite reacts more slowly to the presence of acids than does

limestone, resulting in uneven responses to acidization treatments. Surface chemistry

differences may also result in dolomite and limestone differing in their wettablity and adsorption

characteristics within a reservoir. Finally, limestone and dolomite differ greatly in their strength

and ductility. Abundant experimental (Handin et al., 1963; Hugman and Friedman, 1979) and

field (Stearns, 1967) data suggest that dolomite is generally stronger and more brittle than

limestone at the same burial depth. The preferential development of fractures in dolomite beds

often enhances their permeability (without greatly affecting their porosity) and potentially

influences the overall quality of the reservoir.

Previous characterization work on dolomites from the Arab-D in the Ghawar field

(Cantrell et al., In Press) has identified three types of dolomite which have distinctive

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petrographic, stratigraphic, and geochemical characteristics. These three dolomite types include,

fabric-preserving , non-fabric-preserving, and baroque dolomite. Fabric preserving dolomite is

very finely crystalline in which details of the original limestone fabric are usually well

preserved. Fabric preserving dolomite contains low concentrations of Fe (average 247 ppm Fe)

and relatively positive oxygen isotope values (average δ18O= -2.08 ‰). Beds of fabric

preserving dolomite typically occur as thin, sheet-like or stratigraphic layers that are always

intimately associated with the Zone 1 transition from carbonates below to anhydrite above. Non-

fabric preserving dolomites are medium crystalline, non-baroque dolomite in which all traces of

the original sedimentary fabric have been obliterated. The non-fabric preserving dolomite has

low concentration of Fe (152 ppm) and moderately negative oxygen isotope values (average δ18O

= -2.66 ‰). This dolomite also typically occurs as stratigraphic beds, although it is present

throughout the Arab-D and is not restricted to the carbonate-evaporite transition in Zone 1. The

third dolomite type, baroque, is a coarsely crystalline dolomite with ‘saddle-shaped’ crystals

displaying undulose extinction in thin section. Geochemically, this dolomite is distinct from the

fabric preserving and non-fabric preserving dolomite types, in that it typically contains high

concentrations of Fe (31,500 ppm) and fairly negative oxygen isotope values (average δ18O =

-7.37 ‰). Baroque dolomite is rare in the reservoir and appears to be limited to areas that

contain abnormally thick dolomites; in extreme cases, it is vertically pervasive. In this study we

have used a different approach to that employed by Cantrell et al. (In Press) which examined

only the selected dolomitized intervals from a large number of wells scattered throughout the

Ghawar field. We examined the petrography and geochemistry of three wells in detail from the

Haradh region (Figure 1).

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Background

During middle Jurassic time the Arabian platform was situated at about 10o S and faced the

Tethys Ocean which lay to the present-day east and northeast. By that time a large intrashelf

depression, called the Lurestan basin, had developed in the Arabian platform. From the middle

Jurassic to the Turonian (Cretaceous) the Lurestan basin differentiated into the Gotnia (Basrah

basin) and Rubal Khali basins (Murris, 1980). A structural arch (Rimthan arch), which separates

the two basins, stood high relative to the basins on either side, as suggested by the thick

grainstones deposited there during Kimmeridgian time (Wilson, 1975). This grainstone belt and

the northern side of the Rimthan arch probably formed a fairly well defined shelf-margin that

faced the Gotnia basin to the present-day north and into the tradewind belt. The shelf-margin

passed toward the interior of the Arabian platform (present-day south) and into the Rubal Khali,

where Ghawar Field is located. The southward transition from the Rimthan arch and shelf-

margin into the Rubal Khali intrashelf basin, including Ghawar Field, was gradual. This is

indicated by (1) a southward reduction in the volume of grainstones in the Arab-D carbonates

and (2) a southward thinning of the Arab-D carbonates and thickening of the evaporite section

(Mitchell et al., 1988). Mitchell et al. (1988) explained these relationships as a record of a lateral

facies change and shallowing from shoal-water carbonates in the north to coeval sabkha

evaporites in the south. More recent work has suggested that the evaporites are not coeval with

the carbonates and that the evaporites record shallow subaqueous, or salina and desiccated

salina, deposition rather than supratidal sabkha deposits (Handford et al., In Press). This is a

fundamental paradigm shift from previous hypotheses because it proposes that evaporite

deposition occurred later than the carbonates and that the evaporites filled the remaining

accommodation space in the Rub’al Khali intrashelf basin during an evaporative drawdown as

part of a lowstand systems tract.

Arab-D lithofacies classification schemes are largely based on the work of (Mitchell et

al., 1988). These workers divided the Arab-D into five major zones (Zones 1, 2A, 2B, 3A, and

3B) based mainly on porosity log pattern correlations. The top of the Arab-D (Zone 1) is

characterized by thin subtidal to intertidal carbonates with evaporites. Carbonates within this

unit are frequently completely dolomitized. Underlying the evaporites are a series of grainstones

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and packstones (Zones 2A and 2B). The grainstones in Zone 2A are frequently undolomitized

while the grainstones and packstones in Zone 2B are pervasively dolomitized and compose the

majority of the reservoir. Both skeletal and non-skeletal grains are common, with the skeletal

material being composed of benthic foraminfers, dasycladacean green algae, corals, red algae,

bivalves, and stromatorporoids. The stromatorporoid, Cladocorposis is particularly important

within Zones 2A and 2B where it is frequently associated with zones of extremely high porosity

and permeability (Super-K zones; Meyer et al. 2000). Super-K or super permeability covers all

unpredictable and anomalous rates of production or injection from confined intervals. In this

paper we have used it to refer intervals whose production or injection rate equals or exceeds 500

barrels of fluid per day per foot (bf/d/ft). (Meyer et al., 2000). Zone 3A and 3B are composed

primarily of micritic limestones interbedded with grain-supported carbonates. The rocks are

frequently partially dolomitized, although the amount can vary substantially over short distances

and is frequently common in burrow-fills associated with marine non-depositional surfaces.

Sample Selection

In contrast to the study by Cantrell et al. (In Press), which examined samples of dolomites from

selected wells throughout the entire Ghawar field, this study concentrated on three wells from

the Haradh region and analyzed bulk rocks and separated dolomites from samples at intervals of

between 0.3 to 1 meter from zones 3, 2, and 1 of the Arab-D (Figure 1).

Methods

For an integrated study of the dolomites in the Arab-D formation, petrographic and

geochemical studies were performed and linked with the sequence stratigraphic framework.

Standard petrographic analyses of thin sections were performed to characterize the lithofacies.

Petrographic analysis include the description of non-dolomitized constituents, amount of

dolomite, dolomite type (fabric-preserving mimetic, fabric destructive sucrosic; crystal shape

and size), porosity type (intercrystalline, intergranular etc.), late diagenetic features such as

pressure solution and dedolomitization. Petrography examinations were performed on about 850

thin sections from three wells (Wells A, B, and C) (Figure 1).

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All materials were ground to finer than 63 :m. For XRD analyses, powdered samples

were smear mounted on glass slides. Peak areas for aragonite, calcite and dolomite were

determined using a Scintag XDS-2000 diffractometer. Mineral concentrations were calculated

from peak area ratios assuming that each sample was composed only of calcite, aragonite, and

dolomite. Peak area ratios were calibrated and the concentrations calculated using calibration

curves prepared from results using a series of pure mineral standards (verified by XRD analysis).

Duplicate analyses indicate reproducibility of ±3%. In order to isolate the dolomite contained in

the samples, selected sieved samples with greater than 5% dolomite were treated with buffered

acetic acid for a period of 2 hours (Swart and Melim, 2000). This procedure selectively leaches

the less stable minerals leaving the dolomite behind.

For δ13C and δ18O analyses, all samples were reacted for 10 minutes using the common

acid bath method at 90oC and the CO2 produced analyzed using a Finnigan-MAT 251. Standard

isobaric corrections were applied, but no correction has was applied for the differences in the

fractionation of δ18O as a result of the dissolution of dolomite and calcite by phosphoric acid

(Land, 1980; Vahrenkamp and Swart, 1994). Data are reported relative to Vienna Pee Dee

Belemnite (V-PDB) using the conventional notation. The strontium concentration of the pure

dolomite separates was determined using atomic absorption (Perkin-Elmer 4500). In this method

approximately 100 mg of dolomite separate was dissolved in 10% nitric acid solution, filtered,

and the filtrate diluted to 25 cm3. Corrections were made for the percentage of insoluble residue.

Standards were made using specpure CaCO3 and MgCO3 (Johnson-Matthey) weighed out in

approximately the same concentrations as contained in the samples. Standards were then spiked

with 1000 ppm Sr, Mn, and Fe standard solutions to provide standards with similar intensities

and matrix interferences to the analyzed samples. Reproducibility of this method is

approximately +/-5%. No corrections were made for possible contributions from Sr leached

from insoluble residues, and it is believed that this source of Sr is low compared to that derived

from the dolomite.

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Results

Petrography

The intervals examined cover Zones 3 to 1. Zones of high permeability (super-K zones) are present in

all three cores, but in different stratigraphic positions within Zone 2. Dolomitization is a dominant

feature in the three cores, the vast majority of the samples being partly or completely dolomitized.

Undolomitized samples occur mainly in the grainstones in Zone 2A. Dolomitization generally is

fabric-destructive. In Wells-A and C, unimodal dolomite predominates, while in Well C, polymodal

dolomite is common. The polymodal dolomite could result from initial inhomogeneities in the

sediment (the substrate) or from multiple dolomitization events. In Zones 3 and 2, dolomites are

predominantly fabric-destructive sucrosic, whereas in Zone 1 fabric-preserving (mimetic) also

dolomite occurs. In addition to differences between stratigraphic units, there are variations between

the different wells. Visible porosity includes primary porosity (interparticle pores), molds and vugs,

intercrystalline pores in sucrosic dolomites, intracrystalline pores in dedolomites, and fractures. The

most abundant type of porosity is intercrystalline porosity. Baroque dolomite has been reported

elsewhere (Cantrell et al., In Press), but was not observed in the samples examined here.

Zone 3

Most samples in Zone 3 are partially dolomitized mud- to wackestones and contain considerable

amounts of preserved calcite (Figure 2a and 2b). Dolomite crystals are sucrosic reaching about 100

:m in size. In samples with burrows or borings, these are preferentially dolomitized. Stylolites

surround and cut dolomite rhombs, indicating post-dolomitization chemical compaction. Visible

porosity is low in these micritic rocks. Occasionally molds are observed, especially in the upper part

of Well A. In Well A, a 4 m interval of pure sucrosic dolomites with intercrystalline porosity is

present. In this interval, dolomite crystals reach 200 :m in size, and skeletal dedolomite, forming

intracrystalline pores, is abundant (Figure 2c). Occasionally molds are observed, especially in the

upper part in Well A. Dedolomite is rare to absent in Well B, but common in the lower part of Well C

(calcite replacement dedolomite) and Well A (leaching dedolomite (Figure 2d). The dedolomite in

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these samples that still contains calcite is interpreted to indicate that the dolomite was formed early as

a non-stoichiometric dolomite which was later recrystallized to a more stoichiometric dolomite.

Zone 2

Zone 2 samples are primarily coarser-grained than in Zone 3. Wacke- to grainstones occur that show

varying portions of dolomite. Similarly to Zone 3, Zone 2 exhibits sucrosic dolomites (Figure 3).

However, crystal sizes are larger and increase upwards from about 150 :m to up to 400 :m before

they decrease again to less than 100 :m. This shift in crystal size roughly parallels the portion of

dolomite in the samples. Zone 2B is characterized by partly dolomitized wacke- to packstones.

Moldic porosity and inter-granular (primary) porosity dominate. Pre-lithification (mechanical)

compaction as well as chemical compaction (stylolites) are observed. In Well A, also pure sucrosic,

dedolomitized dolomites occur. In Well C, a zone of high permeability is located at the base of this

lower part of Zone 2b. This zone is composed of sucrosic dolomite with low amounts of calcite.

Inter-crystalline porosity predominates. Post-dolomitization solution seams are common. In Well A, a

super-K interval is situated higher in the stratigraphic succession than in Well C, however, due to lost

recovery no petrographic descriptions can be given. The middle part of Zone 2 consists of a low-

porosity interval in Well C (moderate porosity in Well A), but the lower few meters in Well B

compose a super-K interval. The super-K interval in Well B is composed of sucrosic dolomite (300

:m) with inter-crystalline porosity and locally tight dolomite. Post-dolomitization solution seams are

occasionally observed. Above the super-K interval in Well B, tight dolomite with lower porosity and

crystal sizes reaching 400 :m occurs (Figure 3b). This interval is characterized by sucrosic dolomite

euhedral to anhedral shapes. Zonation, cloudy core-clear rim, can be distinguished in the dolomite

crystals. Polymodal dolomite could point to multiple-event dolomitization in this interval. In some

samples, dolomitization seems to be related to fractures (6583.7' and 6587.7'), where subhedral to

anhedral dolomite occurs adjacent to fractures in samples otherwise dominated by euhedral sucrosic

dolomite. Replacive silica is frequently observed in this interval. In Well C, partly dolomitized

packstones to grainstones occur. Dolomite crystals are generally fabric destructive. Porosity consists

of moldic and inter-granular pores and is generally low. Chert also occurs as a minor component in

this interval (Figure 3c).

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Zone 2A is characterized by regions with low dolomite (with the exception of a few pure

dolomite layers in Well A). This zone dominantly consists of partly to non-dolomitized wacke-,

pack-, and grainstones (Figure 3d). Grainstones show low but varying degrees of cementation (Figure

4). Syntaxial calcite overgrowths occur on echinoderm debris and brachiopod shells. Debris of

Cladocoropsis generally shows internal cement linings in the pores of the algae. Dolomite rhombs

where present are fabric destructive. Porosity is low where primary inter-granular pores and molds

are absent. Occasionally, sulfate precipitates occlude pores. Micritic rims and pre-lithification

(mechanical) compaction, indicated by breakage of ooids, elongated bioclasts and micritic rims, and

by pressure indents, represent earliest diagenetic alterations (Figure 4b and 4c). Dedolomitization is

restricted to Well A.

Zone 1

Zone 1 samples consist of low-porosity partly dolomitized mudstone to grainstone as well as pure

dolostones. The only occurrence of fabric-preserving dolomite was observed in a zone 2A sample

from Well C (Figure 5). Anhydrite was observed to replace calcitic matrix. Anhydrite infills a low

portion of the porosity. Well A is distinguished by leaching features such as vugs, molds and

dedolomite.

Differences between the Wells

Besides the differences in lithofacies between the different stratigraphic zones, pronounced lateral

differences in diagenetic style are observed between the three wells.

The upper part of Well-A (Zones 1 and 2) is distinguished by pronounced dissolution features such as

leaching of the calcitic matrix and components, and leaching dedolomitization. The super-K interval

in Well A was not recovered, implying that this interval was extremely porous (possibly vuggy).

Partly to non-cemented pack- to grainstones from the upper portion of Zone 2 which shows moldic to

vuggy porosity additional adding to the preserved primary porosity. Skeletal dedolomitization is

common throughout Zone 1 and 2, creating secondary intra-crystalline porosity by dissolving the

cores of the dolomite rhombs. In several samples, This dedolomitization seems related to dissolution

seams that might have formed contemporaneously. In these such solution seams, hollow dolomite

rhomboids are observed to have collapsed as a response to compaction. Zone 3 of Well A in contrast,

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is characterized by low visible porosity and the absence of dedolomite. Well C differs from the other

two wells in the abundance of polymodal dolomite, that is, significant variations in crystal size or

shape (euhedral and anhedral) of the dolomite crystals occur together in the same sample. Leaching

and dedolomitization are rare to absent. High porosity intervals in this well are caused by

intercrystalline pore space between euhedral dolomite rhombs (super-K interval) and by preserved

primary porosity in grainstones (upper part of Zone 2). In Well-C visible porosity is lowest in the

cores examined. The exception is the super-K interval in Zone 2 and some partly cemented

grainstones just below this interval. Dedolomitization is restricted to the lower part of zone 3, and is

characterized by replacement calcite in the cores of dolomite rhombs.

Porosity

Visible porosity includes primary porosity (interparticle pores), molds and vugs, intercrystalline pores

in sucrosic dolomites, intracrystalline pores in dedolomites, and fractures. The most abundant type of

porosity is intercrystalline porosity in the strongly dolomitized intervals. Dedolomite, that occurs

throughout the studied cores, is porosity enhancing where it is simply dolomite dissolution ("skeletal

dedolomite"), and neutral to destructive towards porosity where calcite replaces the dolomite.

Small isolated molds in mudstones and wackestones are abundant, especially in Zone 3. Generally,

molds do not seem to contribute significantly to porosity and permeability characteristics.

Compaction affected the sediments prior to (mechanical compaction) as well as after lithification

(chemical compaction). Mechanical compaction is most obvious in grainstones where components

are deformed or show pressure indents. This mechanical compaction resulted in decreasing primary

porosity.

Mineralogy: The mineralogy as measured using X-ray diffraction is presented in Figure 6-8. In

addition to low-Mg calcite and dolomite, minor amounts of quartz and anhydrite were also detected.

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Stable Isotopes

Oxygen: The stable oxygen isotopic composition (δ18O) of the bulk samples varies between

approximately -0.6 to -6.5‰ (Figure 6-8). The range of C and O isotopic compositions presented in

this paper are similar to those previously reported for dolomites from the Arab-D of the entire

Ghawar field (Cantrell et al., submitted). There appears to be little correlation between the oxygen

isotopic composition of the bulk sample and the percentage dolomite (Figure 9a), although the

samples with the highest oxygen isotopic compositions contain nearly 100% dolomite. If the

relationship between percentage dolomite and the oxygen isotopic composition is examined relative

to the different zones, it is clear that in Zone 1 and 2 there is a positive correlation between

percentage dolomite and δ18O, while in Zone 3, this relationship is reversed (Figure 10). A 10 sample

windowed regression coefficient in shown in figure 7 and 8 for Wells B and C. This type of

correlation examines the association between dolomite and δ18O for successive intervals containing

10 samples.

A large number of dolomite rich samples from both Wells B and C were leached in order to

remove all traces of calcite and then reanalyzed for their carbon and oxygen isotopic compositions.

The resultant analyses of the dolomite separates frequently showed little difference from the bulk

samples analyzed from the same interval. This suggests that the calcite removed from the samples

possesses a δ18O similar to that of the dolomite in the sample.

Carbon: The carbon isotopic composition of the bulk samples all showed relatively positive carbon

isotopic compositions (Figure 6-8). In contrast to δ18O, there is a positive correlation between the

percentage dolomite and the carbon isotopic composition (Figure 9b). Dolomite separates show

slightly more positive carbon isotopic compositions than the bulk dolomites. Overall there appears to

be a positive correlation between the carbon and oxygen isotopic composition (Figure 12) although

this appears to be mainly a result of the presence of a number of dolomites with isotopically heavy

carbon and oxygen isotopic compositions.

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Trace Elements

Strontium: The concentration of strontium was measured in two Wells, B and C. The mean Sr

concentration in Well B was 101 ppm and in Well C 129 ppm. There was no statistically significant

difference in concentration of Sr between the two wells. The results from these analyses (Figure 13a

& d) show that while the concentrations of Sr are generally low, there are occasional values as high

as 1000 ppm in some samples. In both Wells B and C, samples taken from Zone 1 showed high

values. Zones 3b also showed slightly higher Sr concentrations compared to Zones 2A, 2B, and 3A.

Iron: The concentrations of iron average approximately 400 ppm in both wells with values ranging in

Well B from 290 to 1800 ppm and in Well C from 200 to 1400 ppm (Figure 13b & e). No

significant trends were observed relative to depth in the well. The concentration of Fe did not show

any statistically significant correlation with either the percentage of dolomite, the concentration of

Sr, or the oxygen or carbon isotopic composition.

Manganese: The concentration of Mn averaged 27 ppm for both wells and showed a statistically

positive correlation with Fe (Figure 13c & f).

Discussion

Stable Isotopic Data: Although the δ13C and δ18O data measured in this study are similar to that

reported previously (Cantrell et al., In Press) (Fig. 12), we are able to show significant differences

within the δ18O in one core equivalent to the entire range of δ18O values previously reported in the

Arab-D of the Ghawar field. These difference exist despite apparent similarities in the petrographic

types of dolomite. For example, in Well B the interval between 6560 and 6600' possesses non-fabric

preserving sucrosic dolomites, all of which possess relatively isotopically positive (-1 to -2‰) δ18O

values. In contrast, the dolomites below this interval, which are petrographically similar, contain

much more negative δ18O values (-4 to -6‰). Hence dolomites which are separated by only 50' in

depth have a range of δ18O values as wide as 6‰. In order to form dolomites at the same time a

difference in temperature or fluid composition of 20oC and 5‰ would be needed. Using modern

dolomite occurrences from the Bahamas as an analogy (Supko, 1977; Dawans and Swart, 1986;

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Swart and Melim, 1999), it would be extremely unusual for dolomites which have a common origin

to have such wide range of isotopic compositions. Therefore either the δ18O values of one or both of

the dolomites have been altered, or the dolomites formed at different times from different fluids. In

comparison with the δ18O values of modern analogues, even the dolomites with the highest δ18O

values, are approximately 5‰ more negative than modern equivalents. At least part of this difference

(1‰) can be accounted for by an ocean composition which was more isotopically negative than at the

present time as a result of the higher sea level during Jurassic times. The remaining 4‰ either

represents a temperature some 16oC warmer than at present time, or the recrystallization of the

dolomite in the presence of isotopically more negative fluids or at a higher temperature. The

relatively high carbon isotopic composition of all the dolomites suggests that the rocks experienced

alteration in a relatively closed system and were probably not significantly influenced by meteoric

fluids.

In Well B, the super-K zone which accounts for over 70% of the production of the well

(Figure 6), is a 4 ft interval between 6581 and 6585', in the middle of Zone 2B, and corresponds with

the zone of more positive oxygen isotopic compositions. This is similar to Well C where the super-K

zone is slightly thicker (Figure 7), but is situated at the base of Zone 2B. The super-K is also

associated with a bore hole caliper log in Wells A and B which indicates a larger than normal bore

hole diameter, a sonic and neutron density log which indicates high porosity, and core samples with

high porosity and permeability. Petrographic examination of this interval reveals a small amount of

quartz and anhydrite, a finding confirmed by XRD analysis. The strontium concentration of the

dolomites throughout this interval appears to be similar to normal values with the exception of one

sample which contained an unusually high Sr concentration for this core. Within the super-K of

Wells B and C (no core was recovered in Well A in the super-K zone and its presence is inferred

from the caliper log and production data), the δ18O of the bulk rock was positively correlated with the

percentage of dolomite (Figure 7 and 8). This is normally to be expected as a result of the

approximate 3‰ equilibrium enrichment of dolomite relative to LMC (Land, 1980). However if this

relationship is examined in all the dolomites from the three wells, then it is evident that there is no

general correlation between the concentration of dolomite and δ18O, and as may be observed in

figures 7, 8 and 10, there are some intervals in which there is an inverse correlation between these

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two parameters. The dolomite component in these samples is therefore isotopically negative relative

to the LMC component. These intervals occur in Zones 2A, 3A, and 3B in all wells. Based on the

correlation between the percent dolomite and δ18O, the isotopic composition of the bulk samples, the

isotopic composition of dolomite in these samples would be approximately -7‰, similar to that

measured in baroque dolomite by Cantrell et al. (Submitted). However despite the similarity in the

δ18O of the baroque dolomite and the dolomites measured in this study, they may not be related. This

conclusion is based on the dissimilarity of the petrography and the absence of any high

concentrations in Fe which appear to characterized the baroque dolomite (Cantrell et al., submitted).

Strontium: The concentration of Sr was analyzed in order to ascertain the nature of the fluids

responsible for the dolomitization. Based on previous work (Vahrenkamp and Swart, 1994; Swart

and Melim, 2001), it is known that stoichiometric dolomites formed from fluids with Sr/Ca ratios

close to seawater have a Sr concentration of approximately 100 ppm. Dolomites formed from

solutions with more evolved Sr/Ca ratios can have Sr concentrations of 2000 ppm and higher. The

results from the analyses of the cores are shown in Figure 11. Both these wells possessed dolomites

with Sr concentrations close to that which might be expected as a result of formation from a open and

rapidly circulating seawater system. In the upper portion of Wells-B and C, Sr concentrations were

elevated suggesting some association with the overlying evaporites which may have contained some

Sr bearing mineral such as celestite. Concentrations of Sr were slightly elevated in Zone 3B.

Although the Sr was only measured on two cores in this study, the previous work (Cantrell et al.,

submitted) measured the Sr concentration of numerous dolomites throughout the Ghawar. Many of

these dolomites contained Sr concentrations similar in concentration to that measured in this study.

The concentration of Sr however, did not appear to be correlatable to properties such as permeability,

porosity or other geochemical properties such as stable isotopes or the concentration of calcite or

dolomite. The Sr content therefore appears to be an indicator of the types of fluids responsible for

dolomitization and not of the processes responsible for the development of the high porosity and

permeability.

Iron and Manganese: Concentrations of these elements were correlated with each other in both wells,

but showed no correlation with oxygen isotopic composition. The absence of correlation with the

isotopically negative dolomites seem to suggest that the dolomites in Zone 3 have a different origin

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than the isotopically similar baroque dolomites described by Cantrell et al. (Submitted) which have

iron concentrations in excess of 10,000 ppm.

Geochemical Evidence of Fracturing

The development of the high permeability zones (super-K) within the Arab-D is of obvious interest in

that it is a major control on the oil production. It has been recognized from previous work that the

super-K is mainly associated with dolomites in Zone 2A or 2B of the Arab-D, although there are rare

instances of its association with the top of Zone 3A or to non-dolomitized sections. In this study we

were able to examine the geochemistry of two wells in which the super-K occurred in a dolomitized

interval (the super-K from well A was not recovered). Petrographically dolomites in the super-K

appeared to be similar to those above and below the interval, with the exception of course that there is

a greater porosity associated with the super-K. The super-K also has a tendency to occur in intervals

dominated by the stromatoporoid, Cladocoropsis. This species was presumed to be originally

composed of LMC, but is now frequently completely leached, leaving molds and cavernous porosity.

In term of the stable isotopic geochemistry of the super-K zones, in the two examples which we

examined in this study, they both occur in intervals which possess isotopically enriched oxygen

isotopic compositions. This enrichment is attributed to the presence of isotopically positive

dolomites, hence within the broad interval there is a positive correlation between the δ18O and the

percentage of dolomite. In contrast above and below the zone of isotopically positive dolomite, there

is an inverse relationship between the δ18O and dolomite. This implies that regions surrounding the

interval containing the super-K intervals were recrystallized by later higher temperature fluids, while

the super-K zone was not. In order to examine the association between the percentage of dolomite

and δ18O in the super-K in more detail we carried out a moving correlation between these two

variables. The calculated regression coefficients are shown in figures 7 and 8 and indicate that in the

center of the isotopically positive zone in which the super-K occurs, the positive relationship between

these parameters disappears. Hence there is a suggestion that the super-K-k intervals within these

overall isotopically positive units was also altered by a later stage higher temperature fluid. Although

high temperature baroque dolomites have been reported in other cores from the Arab-D (Cantrell,

Submitted), these are associated with significantly higher concentrations of iron than measured in the

16

dolomites isolated from the super-K samples and therefore the fluids responsible for the super-K may

be different to those which formed the baroque dolomites.

Paragenetic Sequence

Based on the petrographic and geochemical data presented above we propose the following

paragenetic sequence.

Early Dolomite

-A- Dolomitization associated with Zone 3 of the Arab-D: A portion of the dolomite in Zone 3 is

considered to have formed in situ and is associated with the presence of numerous firmground

surfaces throughout the Zone. Typically, concentrations of dolomite are high immediately below

these surfaces and decrease with depth indicating an association with the surfaces or the overlying

depositional unit. The dolomites in Zone 3 are sucrosic, non-fabric preserving, and approximately

100 µm non-stoichiometric dolomite rhombs. It is postulated that these dolomites formed in

association with the non-depositional surfaces, that contain abundant burrows. These burrows in

many cases are preferentially dolomitized, first as small non-stoichiometric rhombs (10-50 µm) that

subsequently coarsened to their present size. This is consistent with the observation that in Zone 3

dedolomitization is restricted to the cores of dolomite rhombs that correspond to the earliest, calcian

dolomite growth. The mechanism of formation is postulated to be similar to that documented by

Swart and Melim (2000) for dolomites forming off the margin of Great Bahama Bank ( Figure 13).

Originally these dolomites would have had δ18O and δ13C values reflecting deposition from normal

marine fluids δ18O = +2 to +4 ‰ and δ13C = +2 to +4‰). The fact that they now have quite negative

oxygen isotopic compositions (-6 to -7 ‰), suggests later alteration by warm fluids during burial or

recrystallization in a closed system in a normal geothermal gradient during burial. This process

would have reset the oxygen isotopic systematics, but not the carbon. Hence the dolomites still

possess δ13C values typical of modern marine sediments. A closed system dolomitization might also

be expected to lead to a higher Sr concentration (Swart and Melim, 2001). However, if

17

dolomitization occurred close to the non-depositional surface it might be expected that the Sr would

still be close to that expected from formation from normal seawater.

-B- Dolomitization in Zone 2: A second phase of dolomitization affected the generally more grain-

dominated sediments characteristic of Zone 2B. Although original depositional textures are often

difficult to discern as they have been masked by dolomitization, these sediments probably represent a

slightly shallower water facies compared to the sediments of Zone 3. The dolomites formed in these

sediments are principally non-fabric preserving in nature, but the crystals are larger in size (200 to

500 µm) compared to those found in Zone 3 and they posses significantly more positive oxygen

isotopic compositions δ18O = -2 to -1‰). Some of these dolomites possess extremely high

permeability and porosity and are responsible for some of the high flow zones (super-K) encountered

in this field, while others possess low permeability and porosity. We interpret that the dolomites in

Zone 2 formed through the reflux of higher salinity fluids down into these sediments, as evidenced by

the presence of dolomites with relatively positive δ18O values in this zone.

Two possible time periods for this reflux are possible. The first is immediately following the

deposition of Zone 2B, when shallow seas could have been conducive to the formation of

hypersaline brines, perhaps in a situation analogous to the Bahamas. A schematic representation of

this is shown in figure 14. The second period of possible reflux is during or shortly after the

deposition of the evaporites overlying Zone 1. Although there is some evidence to support the first

time period of dolomite formation in Zone 2B (specifically the paucity of dolomite in Zone 2A), it is

not certain that the fluids present on the platform at that time would have had sufficient

thermodynamic drive to produce the amount of dolomite observed. In contrast, fluids originating

from the overlying evaporites (the CD Evaporite) would have been powerful dolomitizing agents. In

order to deliver the fluids to Zone 2B without influencing Zone 2A we suggest that a series of

fractures and joints developed in Zone 1 and 2A coincident with deposition of the CD evaporite (at

the boundary between Arab D and Arab C members).

As a result of non-fabric-preserving dolomitization of Zone 2, the sedimentology is not

precisely known. We suggest, however, that the grainstone units were perhaps more pervasively

dolomitized at this time, while the muddier units may have been only partially dolomitized. The

18

Cladocoropsis-rich intervals were probably not dolomitized early and were preserved as low-Mg

calcite.

C- Dolomites associated with Zone 1 are petrologically distinctive (fabric preserving) and are

intimately associated with the evaporites in this zone. These dolomites possess relatively positive

δ18O values (-1‰) and are interpreted as being formed by hypersaline fluids at or very shortly after

the time of deposition.

D- During increasing burial the sediments became compacted with the formation of stylolites in the

muddier portions of the section (Zone 3) and fractured in the portions of the section which were

dolomitized early (Zone 2B). These fractures served as conduits for alteration by later fluids. Fluids

altered the sediments during late stage migration. The influence of more than one dolomitization

event is evidenced by the polymodal dolomites present in Well B. These fluids were elevated in

temperature and moved preferentially through the fracture system (which probably developed fairly

early, at or shortly after the time of deposition of the evaporite) and dissolved a portion of the

sediment which was not dolomitized. Hence within Zone 2B, parts of the sediments, which were

only partially dolomitized, were leached and the matrix removed leaving behind sucrosic dolomite

possessing very high porosity and permeability. Cladocoropsis fragments could have been leached at

this time leaving molds, which are typical of this zone. These zones, which eventually formed the

super-K intervals, occurred preferentially in the muddier portions of the sediments. In contrast the

more grain-dominated sediments may have been completely dolomitized early and were therefore

less affected by this late stage alteration event. Evidence for the association between the super-K

zones and the late-stage dolomitization event is evident in the δ18O composition of the dolomites

associated with the super-K zone in Well C. This zone always shows an enrichment in the δ18O

values, but is surrounded by dolomites depleted in δ18O. This depletion is consistent with alteration

by late-stage hydrothermally derived fluids. It is also possible that these late stage fluids also altered

the dolomites in Zone 3 which have δ18O values as negative as -7‰.

An alternative explanation is that these dolomites simply recrystallized in the presence of a

geothermal gradient associated with increased burial. Assuming an original oxygen isotopic

19

composition for these dolomites of approximately +3‰, the present δ18O suggests an alteration

temperature of about 50 to 60oC. These late stage alteration features are conspicuous in that they

differ strongly between the individual cores. This supports the notion of fractures being the conduits

for the late fluids, even though such fractures are provable neither in seismic sections nor in the cores

themselves.

Summary

C At least four episodes of dolomite can be identified . The earliest episode occurred near the

time of deposition and affected the sediments that eventually formed the majority of the high

permeability super-K units. Dolomitization at this time was probably not complete and

occurred preferentially in fine grained sediments. The fluids responsible for dolomitization

could have been derived from normal marine waters or could alternatively have been derived

from the overlying evaporitic unit. Dolomitization below this unit also occurred

contemporaneously with deposition.

C Compaction of the sediment occurred, but was less pronounced in the early dolomitized

sediments.

C The second episode of dolomitization occurred from evolved fluids circulating through the

platform. This episode affected all the sediments and partially leached and further

dolomitized the early formed dolomites. These fluids were slightly elevated in temperature

compared to the fluids responsible for the early dolomitization.

C During burial dolomites formed as a result of recrystallization in the presence of the normal

geothermal gradient.

C During burial and tectonic movements, fracturing of the reservoir occurred. This fracturing

preferentially affected cemented units, such as the early formed dolomites. Sediments which

20

were not dolomitized were subjected to formation of stylolites and loss of porosity. Fluids

passing along these fractures influenced the early dolomitized units.

Acknowledgments

The authors would like to thank Saudi Aramco for permission to publish this paper.

21

References

Cantrell, D., Swart, P.K. and R. Hagerty (In Press). Geology and production significance of dolomite

trends, Arab-D reservoir, Ghawar and Ab Quaiq, Saudi Arabia, Geo-Arabia.

Dawans, J., and Swart, P.K. 1988. Textural and geochemical alternations in late Cenozoic Bahamian

dolomites: Sedimentology, v. 35, p. 385-403.

Handford, C. R., Cantrell, D.L., and Keith, T.H., 2002 (in press), Regional facies relationships and

sequence stratigraphy of a supergiant reservoir (Arab-D member), Saudi Arabia, in Armentrout, J.M.,

and Pacht, J.A. (eds.), Sequence Stratigraphic Models for Exploration and Production: 22nd Bob F.

Perkins Research Conference.

Handin, J., Hager, R.V., Friedman, M., and Feather, J.N., 1963, Experimental Deformation of

Sedimentary Rocks under Confining Pressure: Pore Pressure Tests: Amer. Assoc. Petroleum

Geologists Bull., v.47, p.717-755.

Hugman, R.H.H. and Friedman, M., 1979, Effects of Texture and Composition on Mechanical

Behavior of Experimentally Deformed Carbonate Rocks: Amer. Assoc. Petroleum Geologists Bull., v.

63, p.1478-1489.

Koepnick, R.B. and Waite, L.E., 1991. Integrated description of the Hadriya and Hanifa Reservoirs;

Berri Field, Saudi Arabia: Mobil Report, Chapter 2, 38 p.

Land, L. S. 1980. The isotopic and trace element geochemistry of dolomite: the state of the art:

SEPM Special publication v. 28, p. 87-110.

Meyer, F., Price, R., and Al-Raimi, S.M., (2000) Stratigraphic and Petrophysical Characteristics of

CoredArab-D Super-k Intervals, Hawiyah Area, Ghawar Field, Saudi Arabia, Geo-Arabia, v. 5, p.

355-384.

22

Mitchell, J.C., P.J. Lehmann, D.L. Cantrell, I.A. Al-Jallal and M.A.R. Al-Thagafy , 1988. Lithofacies,

diagenesis, and depositional sequence; Arab-D Member, Ghawar field, Saudi Arabia. In, J.A.

Lamondo and P.M. Harris (Eds.) Giant Oil and Gas Fields: A Core Workshop. Society of Economic

Paleontologists and Mineralogists, Tulsa, p. 459–514.

Murris, R.J., 1980, Middle East: Stratigraphic evolution and oil habitat: American Association of

Petroleum Geologists Bulletin, v. 64, p. 597-618.

Stearns, D.W., 1967, Certain Aspects of Fractures in Naturally Deformed Rocks: In NSF Advanced

Science Seminar in Rock Mechanics (R.R. Riecker, ed.): Bedford, Massachusetts, Air Force

Cambridge Research Lab. Spec. Rept., p 97-118.

Supko, P.K., 1977. Subsurface dolomites, San Salvador Bahamas, Journal of Sedimentary Petrology,

v. 47, p. 1063-1077.

Swart P. K. and Melim L. 2000 The origin of dolomites in Tertiary sediments from the margin of

Great Bahama Bank. Journal Sedimentary Research., v. 70, p. 738-748.

Vahrenkamp V. C., and Swart, P. K., 1990. New distribution coefficient for the incorporation of

strontium into dolomite and its implication for the formation of ancient dolomites, Geology, v. 18, p.

387-391.

Vahrenkamp, V.C. and Swart P.K., 1994. Late Cenozoic dolomites of the Bahamas: metastable

analogues for the genesis of ancient platform dolomites, Spec. Publs. Int. Ass. Sediment, v. 21, p.

133-153.

Wilson, J.L., 1975, Carbonate Facies in Geologic History: Springer-Verlag, New York, 471 p.

Figure 1: Map showing location of Wells A, B, and C used in this study. The wells are located in the

Hardh region of the Ghawar field.

23

Figure 2: a) Partly dolomitized grainstone (7318.7 ft) and (b) mudstone (6571.6 ft) from Well C. (c)

Dissolution dedolomite in entirely dolomitized sample. Cores of dolomite rhombs are dissolved.

(Well A, 6335.0 ft). (d) Dissolution dedolomite in partly dolomitized mudstone. Dedolomitization is

associated with solution seam. Note breakage of partly dissolved rhombs. (Well A, 6331.5 ft). Scale

bar =500 µm except Figure 2b in which it equals 1 mm.

Figure 3: a) Sucrosic dolomite from Well B (6571.6 ft); (b) Dense dolomite from Well B (6576.7 ft);

(c) chert with dolomite rhombs. (Well C, 7252.2 ft) (d) partially dolomitized packstone/wackestone.

(Well B, 6555.2 ft). Scale bar =500 µm.

Figure 4: a) Partly cemented grainstone (Well C 7204.8 ft); (b) Cemented grainstone (Well B, 6536.6

ft); (c) Pressure indents in uncemented grainstone (Well B, 7211.3 ft; (d) Broken micritic envelopes

and grains (arrows). (Well B, 6535.1 ft). Scale bar =500 µm.

Figure 5: Moldic dolo grainstone from zone 1 (Well C, 7194.8 ft). Scale bar =500 µm.

Figure 6: Geochemical parameters from Well A; a) Mineralogy, b) porosity and permeability, c)

oxygen and carbon isotopes, d) well caliper. Zonation of reservoir is shown on the right hand side of

the figure and is based on log correlations. Position of the super-K zone is inferred from caliper log

and well production data and is shown as horizontal shaded box. In the case of well A this zone was

not recovered.

Figure 7: Geochemical parameters from Well B; a) Mineralogy, b) porosity and permeability, c)

oxygen and carbon isotopes, d) well caliper, e) correlation between the percent dolomite and the

oxygen isotopic composition (see text for details). Zonation of reservoir is shown on the right hand

side of the figure and is based on log correlations. Position of the super-K zone is inferred from

caliper log and well production data and is shown as horizontal shaded box.

24

Figure 8: Geochemical parameters from Well A; a) Mineralogy, b) porosity and permeability, c)

oxygen and carbon isotopes, d) well caliper, e) correlation between the percent dolomite and the

oxygen isotopic composition (see text for details). Zonation of reservoir is shown on the right hand

side of the figure and is based on log correlations. Position of the super-K zone is inferred from

caliper log and well production data and is shown as horizontal shaded box.

Figure 9: a) Percentage of dolomite vs δ13C , and b) percentage of dolomite vs δ18O in bulk samples.

The carbon isotopic and concentration of dolomite is reasonably well correlated, while there appears

to be a wide range of oxygen isotopic compositions of dolomite, but a rather narrow range of

isotopic composition for LMC. Consequently these are not well correlated.

Figure 10: Correlation between dolomite vs δ18O in zones 1, 2 and 3 in the bulk samples. Dolomite in

zones 1 and 2 are generally positively correlated, while dolomite within zone 3 is inversely

correlated.

Figure 11: Concentration of a) Sr, b) Fe, and c) Mn in Well B and d) Sr, e) Fe, and f)Mn in Well C.

Data are from the chemically isolated dolomites only.

Figure 12: δ13C vs δ18O crossplot of the chemically isolate dolomites measured in this study

compared to dolomites from San Salvador in the Bahamas (Supko, 1977; Dawans and Swart, 1988)

and from Hole Clino in the Great Bahama Bank (Swart and Melim, 2000). Dolomites from the Arab-

D have similar ranges of δ13C values to these more modern examples, but a more negative and wider

range of δ18O values.

Figure 13: Schematic of dolomitization in zone 3 (see text for details).

Figure 14: Schematic of dolomitization in zone 2 (see text for details).

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Arab-D Evaporite MatrixPorosity

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Early Reflux Dolomite

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