CENTRAL SAUDI ARABIAN CRUDE OILS: A GEOCHEMICAL INVESTIGATION Mohammad Farhat Ali, 1, * M. R. Al-Khadhrawi, 2 H. Perzanowski, 1 and H. I. Halpern 2 1 Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia 2 Saudi ARAMCO Research & Development Center, Dhahran 31311, Saudi Arabia ABSTRACT A suite of Paleozoic oils from Central Arabia was studied in order to evaluate conventional hopane and sterane biomar- kers, as well as non-conventional markers such as phenan- threne and its methylated isomers. Because it was uncertain whether conventional biomarkers would be found in these light oils, additional analyses were performed on the oils for correlation purposes. These include light-hydrocarbon and mid-range micro-scale correlation techniques. Paleozoic oils and condensates from several fields from Central Arabia as well as Paleozoic condensate and two Jurassic oils from the Eastern Province (for comparison) were studied. Bulk properties, such as API gravity, percent sulfur, and nitrogen content were determined. The oils and condensates were then fractionated into their major compound classes: saturates, aromatics, polars, and asphaltenes. The full range gas chromatographic 633 Copyright & 2002 by Marcel Dekker, Inc. www.dekker.com PETROLEUM SCIENCE AND TECHNOLOGY, 20(5&6), 633–654 (2002) *Corresponding author. E-mail: [email protected]
Transcript
CENTRAL SAUDI ARABIAN CRUDE OILS:
A GEOCHEMICAL INVESTIGATION
Mohammad Farhat Ali,1,* M. R. Al-Khadhrawi,2
H. Perzanowski,1 and H. I. Halpern2
1Department of Chemistry, King Fahd University ofPetroleum & Minerals, Dhahran 31261, Saudi Arabia2Saudi ARAMCO Research & Development Center,
Dhahran 31311, Saudi Arabia
ABSTRACT
A suite of Paleozoic oils from Central Arabia was studied inorder to evaluate conventional hopane and sterane biomar-kers, as well as non-conventional markers such as phenan-threne and its methylated isomers. Because it was uncertainwhether conventional biomarkers would be found in theselight oils, additional analyses were performed on the oils forcorrelation purposes. These include light-hydrocarbon andmid-range micro-scale correlation techniques. Paleozoic oils andcondensates from several fields from Central Arabia as well asPaleozoic condensate and two Jurassic oils from the EasternProvince (for comparison) were studied. Bulk properties, suchas API gravity, percent sulfur, and nitrogen content weredetermined. The oils and condensates were then fractionatedinto their major compound classes: saturates, aromatics,polars, and asphaltenes. The full range gas chromatographic
633
Copyright & 2002 by Marcel Dekker, Inc. www.dekker.com
PETROLEUM SCIENCE AND TECHNOLOGY, 20(5&6), 633–654 (2002)
analysis of the saturate and aromatic fractions did not permit
meaningful distinctions for bimodal distribution of normal
alkanes or identification of aromatic compounds. Micro-
scale correlation technique was used to correlate mid-range
hydrocarbons and the mid-range star diagrams (MRSD) are
drawn. The MRSD showed that all Paleozoic fluids (Central
Arabian oils) follow patterns that are different from those
followed by the Eastern Province. Bulk properties, pristane/
phytane ratios, methyl penanthrene ratio (MPR), and calcu-
lated reflectance (%Rc) values showed that Paleozoic oils
from Central Arabia differ significantly from Jurassic oils of
the Eastern Province. Central Arabian Paleozoic oils and con-
densates are high-API gravity, exceedingly low sulfur, low
nitrogen and low metals fluids. They have pristane/phytane
ratios that are greater than one indicating dysoxic conditions
at the time of deposition of their source rocks. Their MPR
and %Rc values are indicative of differences in timing of
generation from the source rock as well as in maturity and,
perhaps most significantly, indicative of post-generative
alteration such as water washing. Eastern Province Jurassic
oils are: medium-gravity, high sulfur, high nitrogen crude oils.
They have pristane/phytane ratios that are less than one indi-
cating highly reducing conditions at the time of source rock
deposition. Their MPR and %Rc values are indicative of
generation from a source rock that was at peak oil maturity
(VRe� 0.80–1.0%).
1. INTRODUCTION
High quality (sulfur� 10%, API gravity� 40�) petroleum was dis-covered in Central Saudi Arabia in the mid-1980s. The oil in CentralArabian fields is believed to be sourced from the Paleozoic sequence ofArabia, Silurian Qusaiba member of the Qalibah Formation (1,3). Studiesrelated to Central Arabian oils that have been undertaken to data haveshown that these oils are very lean in conventional biomarkers (such ashopanes and steranes) and that the biomarker compounds, where identi-fied, are non-specific (e.g., diasteranes and tricyclics). Furthermore, recentstudies have shown that biomarkers in these Paleozoic fluids are differentfrom biomarkers found in Paleozoic fluids of neighboring regions such asOman (1–4).
634 ALI ET AL.
The objective of this paper is to examine a suite of Central Arabianoils so as to classify the types of biomarkers present. These include bothconventional biomarkers (e.g., steranes and hopanes) and non-conventionalbiomarkers (e.g., bicyclics and aromatized compounds). The paper alsodetails development of ratios of compounds useful for maturity assessment,as well as finding individual compounds potentially useful for correlation.
Because of their high API gravity values (over 40�), the Central Arabianoils are very lean in biomarkers. It was possible that meaningful biomarkers(i.e., non-routine) would not be found in these oils. In this case, detailed oil–oil correlation and data synthesis are accomplished using correlationtechniques employing both the light and mid-range hydrocarbons (5–7).Gas chromatography–mass spectrometry (GC–MS) is used to determinethe distribution of several biomarker classes in oils as well as sourcerocks (8–12).
2. EXPERIMENTAL
2.1. Crude Oil Samples
Thirteen crude oil and condensate samples were collected from sevendifferent fields (A to E and G) in Central Saudi Arabia to be used in thisstudy. Two more samples from an Eastern Province field (H) and one con-densate sample (F) were used for comparison. The samples are divided intothe following sub-groups:
The exact locations of the fields studied here and well identities werenot disclosed due to reasons of confidentiality on the part of Saudi Aramco.However, a general location map is presented in Figure 1.
Area Field I.D. No. of Samples
Central Arabia Field A 3Central Arabia Field B 3
Central Arabia Field C 3Central Arabia Field D 1Central Arabia Field E 1Eastern Province Field F 1
Central Arabia Field G 1Eastern Province Field H 2
CENTRAL SAUDI ARABIAN CRUDE OILS 635
2.2. Crude Oil Characterization
Each crude oil and condensate sample was analyzed for its APIgravity, sulfur, and nitrogen contents. ASTM methods were used for thesemeasurements (13–15).
2.3. Crude Oil Fractionation
Column chromatography was used to separate the topped crude oilinto saturates, aromatics, polars, and asphaltene fractions (16). HPLC gradesolvents (dichloromethane, n-pentane, n-hexane, toluene and methanol)were used.
2.4. Gas Chromatography (GC)
A Hewlett Packard Model 5790 gas chromatograph equipped with aflame ionization detector (FID) and a J & W Scientific DB-1 capillarycolumn (30m� 0.25mm) were used. The injector temperature was set at320�C, FID temperature at 350�C. Helium was used as a carrier gas with a
Figure 1. General location map of fields studied.
636 ALI ET AL.
column pressure of 15 psig. The split ratio was 50 : 1. The amount injectedwas 1 ml. The temperature program for saturates was 80–320�C, rampedat 4�C/min; and for aromatic fraction/whole crude oil was 35–315�C,ramped at 3�C/min. Hold time for all samples was 20min.
3. RESULTS AND DISCUSSION
3.1. Crude Oils: Bulk Properties and Classification
The properties of the composite samples of Arab crude oils are shownin Table 1. The super light Arabian crude (ASL) has a high API gravity andvery low sulfur and exceedingly low nitrogen contents. Table 2 shows the cutyield and quantity data for different cuts from ASL crude oil. The resultsshow that the super light Arabian crude oil has a much higher percentage(81.9%) of low boiling materials (<343�C) and very low percentage (18.1%)of high boiling materials (heavy ends) in the crude oil.
The results of the determinations of API gravity, sulfur, and nitrogencontents for the fifteen oil samples are presented in Table 3. The data showthat oils from Central Arabia, fields A, B, C, and E, have high API gravityvalues that qualify them for the super light crude oil classification. The other
Table 1. Arabian Crude Oils—Whole Crude Properties
Total sulfur, wt.% 0.069Pour point, �F 65Viscosity, SSU @ 122�F 57.7
800�F (427�C), plus residuumYield, vol.% 8.4
Gravity, �API 22.4Total sulfur, wt.% 0.12Pour point, �F 105
Viscosity, SSU @ 210�F 106.5Flash point, COC, �F 530
638 ALI ET AL.
two oils from Central Arabia, fields D and G may be classified as light crudeoils on the basis of their API gravity values. Sample F1 is a Paleozoiccondensate from Eastern Province. The other two oils (Sample H1 andH2), which are from the Eastern Province (Jurassic), may be classified asmedium gravity oils, based on their API gravity values. Higher sulfur con-tents are related to the source rock from which these oils were generated.Carbonate source rocks generate oils with higher levels of heavy ends andsulfur (17). Central Arabian oils have been shown to be generated from theQusaiba Member (shale) of the Silurian (440-410MYA) Qalibah Formationand not from a carbonate source (3,7).
The Central Arabian Paleozoic oils have sulfur contents that are, onaverage, 30–80 times less than the Eastern Province Jurassic oils. These verylow sulfur contents warrant the classification of these Central Arabian crudeoils as very sweet oils. In most oil fields around the world, sulfur is the mostabundant heteroatom (N,S,O) in crude oils. Sulfur is thought to occur natu-rally in early diagenesis in the form of H2S, resulting in part, from the actionof sulfate-reducing anaerobic bacteria. Sulfur can be found in low, medium,and high molecular weight fractions of crude oils (7,17).
Nitrogen contents of the Central Arabian Paleozoic oils are alsoexceedingly low compared to the Eastern Province Jurassic oils. Averagesfor A, B, and C fields are 20, 81, and 85 ppm, respectively. Oils from otherCentral Arabian fields have nitrogen levels ranging between 8 and 350 ppm.
Table 3. Bulk Properties of All Samples Used in the Study
Sample
Code Area
Sample
Type
Sample
Age
Gravity
(�API)
Sulfur
(Wt.%)
Nitrogen
(ppm)
A1 Central Oil Paleozoic 52.60 0.01 22A2 Central Oil Paleozoic 51.00 0.01 21
A3 Central Oil Paleozoic 51.90 0.01 16B1 Central Oil Paleozoic 46.80 0.02 80B2 Central Oil Paleozoic 46.40 0.02 82
B3 Central Oil Paleozoic 46.20 0.02 81C1 Central Oil Paleozoic 49.40 0.03 82C2 Central Oil Paleozoic 46.10 0.03 87
C3 Central Oil Paleozoic 47.70 0.05 87D1 Central Oil Paleozoic 37.10 0.07 350E1 Central Oil Paleozoic 48.40 0.08 100F1 Eastern Condensate Paleozoic 45.80 0.01 8
Nitrogen levels of the Eastern Province Jurassic oils are noticeably higher:1130 and 1110 ppm. The average value in Central Arabian oils is betweenthree and 140 times less than the values of the oils from the EasternProvince.
Correlation studies were made of percent sulfur versus API gravity,percent nitrogen versus API gravity and percent sulfur versus percent nitro-gen for all oils included in this study. The study indicated that, with theexception of the Jurassic fluids from the Eastern province (low API, high%S and %N), and the oils from fields D and G, which have relatively lowerAPI gravity, all Paleozoic fluids group together very tightly. The oils fromfields D and G appear either to have a closer genetic relationship, or pos-sibly a different source rock organic facies than other Central Arabian oils.They may have undergone migration pathways that are similar to eachother, but different from the other hydrocarbons.
3.2. Fractionation of Crude Oils
Fourteen oils (topped) and one condensate were fractionated into theircompound class fractions (saturates, aromatics, polar, and asphaltenes)using silica–alumina column chromatography. Table 4 shows the fractiona-tion data of all fluids used in the study. Samples from field ‘‘A’’ (Paleozoicfrom Central Arabia) contain the highest average percentage (79.6%) ofsaturate compounds amongst all samples. Samples from fields ‘‘B’’ and‘‘C’’ have average percentages of saturate compounds of 71.8 and 70.4%,respectively. Field ‘‘E’’ also has a high saturate content. The API gravity ofthese Paleozoic oils follows the same trend as their saturate content whilethe sulfur and nitrogen contents follow a trend opposite to the saturatetrend (c.f. Table 3) i.e., oils with highest API gravity have the lowestsulfur and nitrogen contents.
The two Jurassic oils from the Eastern Province have low saturatecontent and low API gravities. This trend is opposite in direction to thatof the Paleozoic oils which have high saturate content and high API gravity.The sulfur and nitrogen contents of the Jurassic oils are very high whencompared to Paleozoic oils.
3.3. Gas Chromatography
The saturate fraction Cþ12 gas chromatography showed that all
Paleozoic fluids have similar GC profiles which do not permit meaningful
640 ALI ET AL.
distinctions between them. No distinctive features, such as bimodally orodd–even predominance were noticed in these chromatograms.
Similarly the aromatic fraction gas chromatography did not allowidentification of aromatic compounds such as naphthalenes, phenanthrenes,or benzothiophenes. Attempts to identify these compounds by concentratingon the middle range of the chromatogram were not successful.
Failure to come up with any useful correlation between the fluidsunder investigation by using gas chromatographic analyses of either thesaturate or aromatic fraction led to the use of a technique known asmicro-scale correlation using mid-range hydrocarbons. Micro-scale correla-tion techniques depend on gas chromatographic analysis of whole oils,rather than compound-class fractions of the crude such as saturates andaromatics. The mid-range hydrocarbons used for micro-scale correlationpurposes represent a random selection of compounds that elute betweennormal alkanes n-C15 and n-C22.
A whole-oil gas chromatogram of oil sample A1 is shown in Figure 2.The middle range used for correlation is expanded in Figure 3. Accuratemeasurement of peak heights is of major importance to the accuracy ofmid-range hydrocarbons micro-scale correlation technique. The basisfor correlation of compounds for this purpose is that these compoundsshould exhibit sample-to-sample variations that permit correlation
Table 4. Fractionation Data of All Samples (Topped, >343�C) Used in the Study
Sample
Code Area
Sample
Type
Sample
Age
%
SATs
%
AROs
%
POLARs
%
Asphaltenes
A1 Central Oil Paleozoic 79.89 15.66 3.73 1.05A2 Central Oil Paleozoic 79.22 15.33 4.21 1.10
A3 Central Oil Paleozoic 79.82 16.51 2.14 0.95B1 Central Oil Paleozoic 71.66 22.96 3.99 1.30B2 Central Oil Paleozoic 72.50 20.86 4.84 1.15
B3 Central Oil Paleozoic 71.46 22.22 4.10 1.20C1 Central Oil Paleozoic 70.55 22.60 4.45 1.25C2 Central Oil Paleozoic 71.25 23.10 4.55 1.00
C3 Central Oil Paleozoic 69.50 23.95 5.18 0.85D1 Central Oil Paleozoic 62.02 31.35 3.87 2.15E1 Central Oil Paleozoic 73.53 20.59 4.12 1.05F1 Eastern Condensate Paleozoic 77.69 17.76 3.53 0.85
Figure 3. Whole-oil gas chromatogram of the middle range (38–67min.) of oil A1.
642 ALI ET AL.
and/or differentiation between fluids. All compounds selected betweenn-C15 and n-C22 elute within the time window between 38 and 67min(Figure 3).
These compounds are usually unknown minor components whichmeet the criteria for selection as cited above. Among the compoundsselected are some known peak such as, n-C17, pristane (i-C19 with formulaC19H40) n-C18, and phytane (i-C20 with formula C20H42).
The main objective here is to correlate the fluids under investigation bycomparing ratios of compounds. To put these comparison in a pictorialform, multivariate plots (star diagrams) are used. These star diagramsmake correlating and/or differentiating fluids easier. Star diagrams are mul-tivariate plots in polar coordinates. Each star diagram can have as manyaxes as one needs to use. However, the more axes a star diagram has, themore complicated the picture becomes. Thus, for clarity, star diagrams thatwill be used in this study will have eight axes per plot, i.e., the 24 sequentialratios will be plotted on three star diagrams. These star diagrams will bereferred to hereafter as mid-range star diagrams (MRSD).
Star diagrams have been used to represent chemical compositions ofwater and oil samples. Studies of oils that employ star diagrams include:evaluation of reservoir lateral and vertical continuity, assigning relativecontributions in multizone producing reservoirs and monitoring contribu-tion changes with time, and identification of production problems such ascasing leakage (5).
In the present study, 24 compounds were selected for mid-range micro-scale correlation. These compounds are labeled ‘‘C’’ through ‘‘Z’’ as can beseen in Figure 3. Sequential ratios of these twenty-four compounds are usedto construct mid-range star diagrams (MRSD).
The 24 sequential ratios are calculated based on peak heights of thecompounds selected. The first ratio is calculated by dividing height of peakC by height of peak D; the second ratio is calculated by dividing height ofpeak D by height of peak E; the third ratio is calculated by dividing of peakE by that of peak F, and so on until the last ratio which is the only non-sequential one. This ratio is calculated by dividing height of peak C byheight of peak Z. The 24 ratios are normalized and then represented by24 axes that are divided into three groups each of which consists of eightaxes. The first eight ratios, ‘‘C/D’’ through ‘‘J/K’’, constitute the eight axesfor the first mid-range-star-diagram (MRSD-1). The second eight ratios,‘‘K/L’’ through ‘‘R/S’’, constitute the eight axes for the second mid-range-star-diagram (MRSD-2). The third eight ratios, ‘‘S/T’’ through ‘‘C/Z’’, con-stitute the eight axes for third mid-range-star-diagram (MRSD-3). Themaxima of the axes of the MRSDs vary and are printed in parenthesesnext to the axis endpoints.
CENTRAL SAUDI ARABIAN CRUDE OILS 643
Figures 4–6 depict MRSD-1, MRSD-2, and MRSD-3, respectively,for all oils. Since it is somewhat hard to distinguish individual oils fromthese figures, the star diagrams were further subdivided.
Figure 7 depicts MRSD-1 of field A oils and shows the close groupingamong field A oils, and their extreme differences from field H oils. Thisdifferentiation is in line with the differences in source rock between thetwo fields (source rock of field ‘‘A’’ is Paleozoic shale while that of field‘‘H’’ is Jurassic carbonate).
Similar MRSDs were plotted for all Paleozoic fluids and oils from theEastern Province. The star diagram showed that all Paleozoic fluidsfollow patterns that are different from those followed by oils fromEastern Province. Field B and C follow fairly similar pattern on theMRSDs.
Gross differences were observed among the remaining oils fromfield D, E, F, and G. However, oils from field D and G do follow similarpaths on the three MRSDs suggesting a strong similarity between thesetwo oils.
3.4. Gas Chromatograph–Mass Spectrometry (GC/MS)
GC–MS fragmentograms of saturate fractions showed that CentralArabian oils are exceedingly lean in conventional biomarkers (e.g., hopanes
Figure 4. MRSD-1 of all oils included in the study.
644 ALI ET AL.
and steranes). These biomarkers were not present in high enough concen-trations and the compounds that are present were not diagnostic enough topermit using them for correlation purposes.
Due to the lack of the conventionally applied biomarkers (hopanesand steranes) in the Central Arabian oils and condensate, it was decided to
Figure 5. MRSD-2 of all oils included in the study.
Figure 6. MRSD-3 of all oils included in the study.
CENTRAL SAUDI ARABIAN CRUDE OILS 645
look for unconventional markers in the aromatic fractions. Phenanthreneand its four methylated isomers (MP) are amongst the promisingcompounds that one may find in the aromatic fraction of crude oils. Thediagnostic ion for phenanthrene is m/z 178 and that for the methylatedisomers is m/z 192. GC–MS fragmentogram for the aromatic fraction ofoil A-1 for m/z 178 shows a single peak (Figure 8) which is that of phenan-threne. The fragmentogram for m/z 192 (Figure 9) shows four major peakswhich are, in order of increasing elution time, those of 3MP, 2MP, 9MPand 1MP.
Table 5 lists the abundances of phenanthrene and its methylatedisomers in all oils calculated from their GC–MS chromatograms. Thevalues in the table are based on area integration of each peak. Theseabundances are then used to calculate various parameters that are reportedin the literature, such as the maturity-sensitive Methylphenanthrene Index(MPI-1), which is used in the calculation of the ‘‘calculated reflectance’’,%Rc. The Methylphenanthrene Ratio (MPR) is calculated from the ratio ofthe abundances of 2-MP to 1-MP and provides the criterion as to whetherEq. I or II should be used in calculating %Rc. The criterion is that Eq. Ishould be used for samples with MPR values of less than 2.24 andEq. II should be used for samples with MPR values of higher than2.24 (18–22).
Figure 7. MRSD-1 of field A oils contrasted with field H oils.
646 ALI ET AL.
Figure 8. GC–MS chromatogram (m/z 178) of the aromatic fraction of oil A1.
Figure 9. GC–MS chromatogram (m/z 192) of the aromatic fraction of oil A1.
Oils from field A (Central Arabian) have low gas/oil ratios (GOR) andMPR values that are greater than 2.24 (average¼ 5.46), thus Eq. II was usedto determine their calculated vitrinite reflectance values (%Rc). The %Rc
values (average¼ 1.35%) of the three oils from field A indicate the %Rm
value of the source rock at the time of generation, i.e., the source rock was atthe ‘‘late-post mature’’ stage of oil generation. Low GOR for such highgravity oils may be taken as an indication of water-washing since water-washing results in depleting the gas in the oil phase and, in turn, depletingthe gas cap present over the oil via a dissolution phenomenon (5).
The %Rc of these oils are on the high ends of oil generation; this maybe an artificial elevation resulting from water-washing, not true maturity.Water-washing may deplete phenanthrene concentration to a greater extentthan it depletes the concentration of methylphenanthrenes, thus altering theoutcome of the equation (18).
Since aqueous solubility data for phenanthrene and methylphenan-threnes are not available, the above conclusions (about increased solubilityof phenanthrene relative to methylphenanthrenes) were reached from solu-bility trends of benzene, toluene, and dimethylbenzene, which decrease byabout an order of magnitude with each addition of a methyl group (17).
Oils from field B (Central Arabia) have low GOR and MPR valuesthat are less than 2.24 (average¼ 1.99) meaning that equation I should beused for the calculations of %Rc. The calculated %Rc values have anaverage of 1.22%. Taking these values to correspond to the % Rm valuesof the source rock at the time of generation, means that the source stock wasin the ‘‘late mature’’ stage of oil generation. Since that is also a low GORfield, the %Rc values obtained may again be an artifact of water-washingrather than real maturity.
Field C (Central Arabia) oils have MPR values that are less than 2.24(average¼ 1.83), thus equation I was used in calculating the %Rc values ofthese oils. The calculated %Rc show an average of 0.99%. These %Rc
values suggest that the source rock was at the ‘‘late mature’’ stage of oilgeneration. Field C has a slightly higher GOR than fields A and B. Thus, itmay be as heavily water-washed as those two fields. However, the %Rc ismore reasonable than that for fields A and B.
Oils D1 and G1 (Central Arabia) have GOR values that are consider-ably higher than all the above field, and MPR values that are less than 2.24.
CENTRAL SAUDI ARABIAN CRUDE OILS 649
The %Rc values (using Eq. I) are 0.79 and 0.90%, respectively, suggestingthat they were generated from source rocks that were at the ‘‘peak time’’stage of oil generation, with oil G1 generated at a stage bordering on the‘‘late mature’’ stage. Maturity seen here may be closer to reality than atfields, A, B, or C.
The oil from field E (Central Arabia) has a low GOR and MPR valuethat is greater than 2.24 and corresponding %Rc value (using Eq. II) of1.45%. The same reasoning as given for field A oils is also applicable here.
F1 (Paleozoic condensate from the Eastern Province) has the highestGOR of all the Paleozoic fluids considered in this study. It has an MPRvalue that is greater than 2.24 and corresponding %Rc value (using Eq. II)of 1.35%. This suggests that it was generated from a source rock at the‘‘post mature’’ stage of oil generation and is in agreement with its believedtrue maturity.
Field H oils (Jurassic, Eastern Province) have MPR values that are lessthan 2.24 and %Rc values (using Eq. II) of 0.68% (Oil-H1) and 0.67% (Oil-H2). These %Rc values indicate that these oils were generated from a sourcerock that was at the early ‘‘peak mature’’ stage of oil generation.
Calculated reflectance values (%Rc) for Paleozoic oils and condensateshow that oils from fields A, B, and E (low GOR, high API) were generatedwhen the source rock was at the ‘‘late-post’’ mature stage. These %Rc
values, however, are unlikely in view of the fact that at such values of%Rc, only gas should be produced by the source rock. However, we findblack oils. Because of the low GOR of the field, water-washing may becausing preferential depletion of phenanthrene relative to methylphenan-threnes, thus affecting (raising) the MPI-1 values and resulting in an artifi-cially high %Rc.
The %Rc values for oils from field C (higher GOR than A and B, highAPI) show that they were generated when the source rock was at the ‘‘late-mature’’ stage or that they have been affected by water-washing though notas heavily as field A and B. %Rc values for this field are more reliable thanthose of field A and B.
The %Rc values for oils from fields D and G indicate that these oilswere generated when the source rock was at the ‘‘peak mature’’ and border-ing on the ‘‘late mature’’ stage. These two fields have considerably higherGORs. Thus, maturity at these fields assessed on the basis of %Rc is closerto reality than at fields A and B.
These %Rc values show that the condensate from field F was gener-ated when the source rock was at the ‘‘post mature’’ stage.
Calculated %Rc values for the Eastern Province Jurassic oils suggestthat they were generated from source rock which was at the ‘‘peak mature’’stage.
650 ALI ET AL.
CONCLUSIONS
The following conclusions are drawn from the previously describedresults and discussions:
(1) Bulk properties: API gravity values of Paleozoic Central Arabian oils(Fields A to G) are much higher than Jurassic oils from the EasternProvince (Field H). Percent sulfur and nitrogen contents of Paleozoicoils are exceedingly low, and especially so when compared withJurassic oils from the Eastern Province. Bulk properties reveal similar-ities between Fields B and C, D and G, as well as E and F.
(2) Pristane/phytane ratios indicate that source rocks of the CentralArabian oils were deposited under dysoxic conditions and that sourcerocks of the Eastern Province oils were deposited under highly reduc-ing conditions.
(3) The mid-range star diagrams (MRSD) show that all Paleozoic fluids(Central Arabian oils and the condensate from Eastern Province) fol-low patterns that are different from those followed by oils from theEastern Province. Fields B and C follow fairly similar patterns on theMRSDs. MRSDs also show that the two oils from field D and G have,for the most part, similar patterns, which is in agreement with theirsimilar bulk properties.
(4) Central Arabian oils are exceedingly lean in conventional biomarkers(e.g., hopanes and steranes). These biomarkers are not present in highenough concentrations and the compounds that are present are notdiagnostic enough to permit using them for correlation purposes.
(5) The methylphenanthrene ratio (MPR) values of the oils from CentralArabia are all higher than those of Jurassic oils from the EasternProvince, with the exception of the oils from fields D and G. Theseexceptions may result from the effects of water-washing on all CentralArabian fields except D and G. The latter two fields exhibit MPRvalues that are closer to the Eastern Province Jurassic oils than toCentral Arabian oils.
(6) Calculated reflectance values (%Rc) for Paleozoic oils and condensateindicate the following:
(a) Oils from fields A, B and E [low gas/oil ratio (GOR), high APIgravity] were generated when the source rock was at the ‘‘late-post’’ mature stage. Their %Rc values, however, are unrealistic inview of the fact that at such values of %Rc, only gas should beproduced by the source rock. However, we find black oils.
(b) Because of low GOR of the field, water-washing may becausing preferential depletion of phenanthrene relative to
CENTRAL SAUDI ARABIAN CRUDE OILS 651
methylphenanthrenes, thus affecting (raising) the MPI-1 values
and resulting in an artificially high %Rc.
(c) Oils from field C (higher GOR than A and B, high API gravity)
were generated when the source rock was at the ‘‘late mature’’
stage or they have been affected by water-washing though not as
greatly as fields A and B. Calculated reflectance (%Rc) values for
this field are more reliable than those of field A and B.
(d) Oils from field D and G were generated when the source rock was
at the ‘‘peak mature’’ and bordering on the ‘‘late mature’’ stage.
These two fields have considerably higher GORs. Thus, maturity
at these fields assessed on the basis of %Rc is closer to reality
than at fields A and B.
(e) Condensate from field F was generated when the source rock was
at the ‘‘post mature’’ stage.
(7) Calculated %Rc values for the Eastern Province jurassic oils suggest
that they were generated from source rock which was at the ‘‘peak’’
mature stage.
ACKNOWLEDGMENT
The facilities and support provided by King Fahd University of
Petroleum & Minerals and Saudi ARAMCO are gratefully acknowledged.
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