origin of dolomite in the arab-d reservoir from the ghawar...
TRANSCRIPT
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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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ABCSan SalvadorClino
Seawater
3. Diffusive seawater dolomitization of burrow fill and mud matrix
Dolomiteabundance
Mineralized R ind
2. Seafloor erosion, deposition of firm- ground gravel lag, and burrow infill
Seawater Intraclast rudstone
Erosionalsurface
1. Burrowing, formation of firmground, pause in sedimentation
SeawaterOpenburrows Firmground
surface
Mudstone-Wackestone
HaradhEvaporation
Arab-D Evaporite MatrixPorosity
Stage 1 - Fracturing
HaradhEvaporation
Arab-D Evaporite
Stage 2 - Reflux Dolomitization
Fluid-TransmissiveFractures
Limestone MarineDolomite
Early Reflux Dolomite
SalinaDolomite
Reflux-Fracture Dolomite