Petroleum System Analysis of the Mishrif Reservoir in the Ratawi, Zubair, North and South Rumaila Oil Fields, Southern Iraq

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GeoArabia, v. 14, no. 4, 2009, p. 91-108 Petroleum system analysis of the Mishrif reservoir, southern Iraq Gulf PetroLink, Bahrain

Petroleum system analysis of the Mishrif reservoir in the Ratawi, Zubair, North and South Rumaila oil fields, southern Iraq

Thamer K. Al-Ameri, Amer Jassim Al-Khafaji and John Zumberge

ABSTRACT

Five oil samples reservoired in the Cretaceous Mishrif Formation from the Ratawi, Zubair, Rumaila North and Rumaila South fields have been analysed using Gas Chromatography – Mass Spectroscopy (GC-MS). In addition, fifteen core samples from the Mishrif Formation and 81 core samples from the Lower Cretaceous and Upper Jurassic have been subjected to source rock analysis and palynological and petrographic description. These observations have been integrated with electric wireline log response. The reservoirs of the Mishrif Formation show measured porosities up to 28% and the oils are interpreted as being sourced from: (1) Type II carbonate rocks interbedded with shales and deposited in a reducing marine environment with low salinity based on biomarkers and isotopic analysis; (2) Upper Jurassic to Lower Cretaceous age based on sterane ratios, analysis of isoprenoids and isotopes, and biomarkers, and (3) Thermally mature source rocks, based on the biomarker analysis.

The geochemical analysis suggests that the Mishrif oils may have been sourced from the Upper Jurassic Najma or Sargelu formations or the Lower Cretaceous Sulaiy Formation. Visual kerogen assessment and source rock analysis show the Sulaiy Formation to be a good quality source rock with high total organic carbon (up to 8 wt% TOC) and rich in amorphogen. The Lower Cretaceous source rocks were deposited in a suboxic-anoxic basin and show good hydrogen indices. They are buried at depths in excess of 5,000 m and are likely to have charged Mishrif reservoirs during the Miocene. The migration from the source rock is likely to be largely vertical and possibly along faults before reaching the vuggy, highly permeable reservoirs of the Mishrif Formation. Structural traps in the Mishrif Formation reservoir are likely to have formed in the Late Cretaceous.

INTRODUCTION

The Mishrif Formation is of Late Cenomanian age (Figure 1; van Bellen et al., 1959) and is the most important oil reservoir in the Mesopotamian Basin (Figure 2), southern Iraq (Al-Khaersan, 1975; Reulet, 1982; Alsharhan and Nairn, 1997; Aqrawi et al., 1998), containing some 30% of Iraq’s total oil reserves (Al-Sakini, 1992). It extends throughout the Mesopotamian Basin reaching thicknesses of 100–200 m in the Basrah District at subsurface depths around 2,100–2,400 m. The Mishrif Formation is dominated by a shallow-water, shelf carbonate sequence composed of bioclastic-detrital limestones, including (in places) algal, coral and rudist bioherms with reservoir porosities exceeding 20% and permeabilities of 100 mD to one Darcy. This formation is widespread throughout the Arabian Peninsula.

The underlying Rumaila Formation consists predominantly of chalky and marly limestones. A conformable and gradational junction with the Mishrif Formation has been documented in the Basrah District. Overlying the Mishrif Formation are dark grey and greenish shales, alternating with grey, fine-grained marly limestones of the Khasib Formation (van Bellen et al., 1959). This unit lies unconformably above limonitic, partly argillaceous limestones forming the upper part of Mishrif Formation (Aqrawi et al., 1998).

Due to a lack of published studies describing the petroleum system in this area, this work was undertaken to correlate oils of the Mishrif Formation with Jurassic and Cretaceous source rocks.

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Lithology Age Formation Depth TOC Maturation Tectonics (m) Reservoir % Pliocene Dibdibba

Fatha Regional Zagros Orogeny Miocene Seal Ghar

Y 500 Dammam Eocene Rus TERTIAR

Umm Er Paleocene 1,000 Radhuma

Neo-Tethys Ocean Closing Maastrichtian Tayarat Immature

Shiranish 1,500 Campanian Hartha

Santonian Sa'adi Tanuma Turonian- 2,000 Tethys Obduction Coniacian Khasib Mishrif Rumaila Cenomanian Ahmadi

Mauddud 2,500 ACEOUS

Nahr Umr

CRET Albian

Aptian Shu'aiba 3,000 Neo-Tethys Ocean Opening 1–4 Barremian Zubair

Hauterivian Ratawi 3,500 Valanginian Yamama Berriasian

Sulaiy Mature Tithonian 1–7 Regional Gotnia 4,000 Kimmeridgian Seal Oxfordian Najma 1–6 Callovian

JURASSIC Naokelekan Bathonian 4,500 1–17 Bajocian Sargelu

Anhydrite Limestone Conglomerate Gas reservoir Unconformity Dolomite Shale Sandstone Oil reservoir Source rock

Figure 1: Stratigraphic section and the major tectonic phases relevant to Jurassic through Tertiary Total Petroleum System.

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40°E 44° 48° TURKEY

Zagros Thrust Zone N Fold Belt 0 100

km Khlesia Kirkuk (Baba) Field Uplift SYRIA

g Eup ris Rive hra Zagros Mountains tes Ri Khashim Ahmar Field 34°N r Western Desert ver 34° IRAQ

Baghdad

IRAN Rutbah - Ga'ara Uplift JORDAN Mesopotamian Basin

Study Area SAUDI ARABIA

40° Southwestern Desert

30° 47°E 47°30' 48°

Tigris River Mesopotamian Basin N KUWAIT Majnoon 0 25 Arabian Gulf West Qurna 31°N km Euphrates40° River 44° 48° Qurna Nahr Umr Legend North Rumaila R-36 IRAN Oil field Subba R-167 Gas field R-29 Tertiary outcrop Rt-5 Basrah Ratawi Rt-3 Cretaceous outcrop 30°30' Rt-2 Jurassic outcrop Thrust fault Ru-105 Siba Fault Shat City A Luhais Ru-96 Ru-72 l-Arab Zubair Well

Rachi South Rumaila Jerishan 30° KUWAIT Arabian Gulf Figure 2: Location map of the studied wells along with oil fields.

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MATERIALS AND ANALYTICAL SCHEME

The Mishrif-reservoired oil samples are from wells Zubair 4, Zubair 8, Rumaila North 81, Rumaila North 170 and Rumaila South 131. Source rock analysis was conducted on 40 samples taken from cores in wells in the Ratawi, Rumaila South, and Rumaila North fields. Techniques employed in this study include biomarker fingerprints, palynomorphs, palynofacies, and pyrolysis. The analysis of samples by gas chromatography-mass spectrometry (Zumberge et al., 2005) and pyrolysis were performed by Geomark Research Ltd. in Houston (Texas, USA) in 2005 and other TOC and pyrolysis were performed in the laboratories of Iraqi Exploration Oil Company during the year 1987 and 1989, while palynomorphs and organic matter extracts were analyzed in the Department of Geology, University of Baghdad. Sample and data are shown in Tables 1 and 2 and Figure 3.

Table 1 Rock-Eval Pyrolysis Data for Lower Cretaceous Formations in Basrah, Southern Iraq

Sample Fm Well Depth TOC S1 S2 S3 Tmax HI OI PI (m) Type wt% mg/g mg/g mg/g (°C) S2/TOC S3/TOC S1/(S1+S2) Mishrif Rt-2 2,685 Core 0.47 0.25 3.31 0.19 427 704 40 0.27 Mauddud Ru-1 2,658 Core 0.02 0.02 0.10 0.00 Mauddud Ru-1 2,670 Core 0.36 0.03 0.31 0.35 485 86 97 0.09 Nahr Umr Ru-1 2,791 Core 0.86 0.23 1.08 0.12 425 126 14 0.18 Nahr Umr Ru-1 2,774 Core 3.77 0.50 11.48 2.38 433 305 63 0.04 Zubair Ru-1 3,010 Core 0.58 0.77 2.41 0.00 436 416 0 0.24 Zubair Ru-1 3,230 Core 0.19 0.07 0.27 0.02 Zubair R167 3,388 Cuttings 0.56 0.15 0.57 0.00 432 102 0 0.21 Zubair R167 3,425 Cuttings 0.89 0.07 0.80 0.00 435 90 0 0.08 Zubair R167 3,407 Cuttings 0.78 0.14 1.55 0.00 435 199 0 0.08 Zubair R167 3,507 Cuttings 1.09 0.05 0.73 0.00 435 67 0 0.06 Zubair R167 3,555 Cuttings 0.47 0.04 0.31 0.00 434 66 0 0.11 Zubair R167 3,655 Cuttings 1.30 0.11 1.50 0.00 436 115 0 0.07 Zubair R167 3,730 Cuttings 1.49 0.07 0.86 0.00 437 38 0 0.08 Zubair R/172 3,100 Cuttings 0.32 0.10 0.49 1.10 Zubair R/172 3,220 Cuttings 0.33 0.17 0.33 1.80 Zubair R/172 3,301 Cuttings 2.58 0.58 9.40 1.61 446 364 62 0.60 Ratawi Rt-5 3,728 Core 0.06 0.05 0.07 0.00 Ratawi Rt-5 3,668.5 Core 0.10 0.09 0.35 0.00 Ratawi R167 3,730 Cuttings 1.19 0.09 0.66 0.00 438 55 0 0.12 Ratawi R/172 3,400 Cuttings 1.85 6.00 9.60 2.95 445 519 159 0.38 Ratawi R/172 3,450 Cuttings 0.25 0.11 0.28 1.32 Yamama Rt-13 3,678 Core 0.09 0.25 0.25 0.00 Yamama R167 3,896 Core 0.46 0.06 0.41 0.00 436 89 0 0.13 Yamama R167 4,094 Cuttings 0.56 0.78 1.70 0.00 437 304 0 0.31 Yamama R167 4,129 Cuttings 0.34 0.13 1.04 0.00 437 306 0 0.11 Yamama R167 4,141 Cuttings 0.18 0.02 0.58 0.00 Yamama R167 4,210 Cuttings 4.07 0.75 1.78 0.00 444 44 0 0.30 Yamama R/172 3,700 Cuttings 1.00 1.63 4.00 1.77 437 400 177 0.29 Sulaiy R167 4,456 Core 2.05 0.84 1.00 0.42 464 49 20 0.46 Sulaiy R167 4,375 Core 5.90 1.82 4.15 0.00 470 70 0 0.30 Sulaiy R167 4,220 Cuttings 0.21 0.26 0.49 0.00 Sulaiy R167 4,322 Cuttings 0.10 0.11 0.27 0.00 Sulaiy R167 4,379 Cuttings 0.16 0.17 0.26 0.00 Sulaiy R167 4,421 Cuttings 0.25 0.40 0.27 0.00 Sulaiy R167 4,482 Cuttings 3.26 0.66 1.35 0.00 468 41 0 0.33 Sulaiy R167 4,484 Cuttings 3.82 0.72 4.95 0.00 475 130 0 0.13 Sulaiy R167 4,487 Core 4.47 0.76 2.80 0.00 471 63 0 0.21 Sulaiy R167 4,491 Core 3.28 0.33 4.27 0.00 470 130 0 0.29 Sulaiy R167 4,510 Core 7.33 1.21 3.40 0.00 467 46 0 0.28

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MISHRIF OIL GEOCHEMISTRY Table 2 Analysis Chart of GC and GC-MS Crude oils produced from the Cenomanian Oil Biomarkers Data for Lower Cretaceous Mishrif Formation in the Basrah District of Formations in Basrah, southern Iraq southern Iraq were geochemically assessed in OilMod Ru/81 Ru/131 R/170 Zb/4 Zb/8 order to determine source rock type, geologic Ratio age, level of thermal maturity, and degree C19/C23 0.08 0.08 0.07 0.07 0.09 of biodegradation. All of the oil samples are C22/C21 1.21 1.15 1.18 1.15 1.14 remarkably similar, both in bulk and molecular C /C 0.27 0.27 0.28 0.28 0.29 properties. For example, API gravities average 24 23 C /C 0.79 0.79 0.79 0.76 0.76 24.8° ± 1.5° with weight percent sulfur 4.2% ± 26 25 Tet/C 0.3. A comparison with oils from 150 different 23 1.31 1.33 1.30 1.33 1.30 C T/C petroleum systems (based on GeoMark 27 27 0.01 0.00 0.00 0.00 0.00 C /H Research’s global crude oil geochemical data- 28 0.01 0.01 0.01 0.01 0.04 C /H base; www.RFDbase.com) is shown in Figures 29 1.49 1.52 1.44 1.48 1.49 C X/H 4 and 5. Low to medium gravity, high sulfur 30 0.00 0.00 0.00 0.00 0.00 crudes are usually derived from carbonate OL/H 0.01 0.00 0.00 0.01 0.01

source rocks (containing type IIS kerogen) C31R/H 0.38 0.39 0.40 0.40 0.40

since, during diagenesis, sulfur can become GA/C31R 0.23 0.24 0.22 0.22 0.21

preferentially incorporated in organic matter C35S/C24S 1.12 1.13 1.16 1.14 1.09 (kerogen) that eventually generates high sulfur Ster/Terp 0.16 0.16 0.16 0.16 0.15

oils due to insufficient iron (Fe) to scavenge 2H S Rearr/Reg 0.11 0.10 0.10 0.10 0.13 as pyrite or marcasite. C27% 33.4 33.3 33.7 33.7 33.1

C28% 25.4 25.5 26.3 25.4 26.2 The stable carbon isotopic compositions of the C % 41.1 41.2 40.1 40.8 40.7 saturate and aromatic hydrocarbons are all 29 C 20S/R 0.67 0.68 0.66 0.67 0.65 within analytical error, averaging -27.3 ‰ and 29 C Ts/Tm -27.6 ‰, respectively (relative to PDB; ± 0.1 ‰). 27 0.18 0.17 0.18 0.18 0.18 C Ts/Tm In contrast to oils from marine shales, the aro- 29 0.08 0.07 0.08 0.08 0.07 matic isotopic composition is often isotopically DM/H 0.01 0.01 0.01 0.01 0.01 lighter than the saturate hydrocarbons in low TAS3(CR) 0.22 0.20 0.22 0.22 0.21 maturity, high sulfur oils. This pattern occurs Pr/Ph 0.73 0.73 0.74 0.70 o.72 because high sulfur oils from carbonate source Pr/nC17 0.20 0.20 0.20 0.21 0.21

rocks typically have the most negative CV values Ph/nC18 0.32 0.33 0.33 0.34 0.34

(-1.65 average for carbonates and -0.25 average nC27/nC17 0.19 0.19 0.20 0.22 0.21 for shales; Sofer, 1984) with C15+ aromatic 13C CPI 1.076 1.042 1.051 1.036 1.053 compositions almost as, or even more negative Ru = South Rumaila; R = North Rumaila; Zb = Zubair than the C15+ saturate 13C compositions.

Both tricyclic and pentacyclic (hopanes) terpane biomarker ratios confirm the carbonate origin of the Mishrif-reservoired oils, again by a comparison with global analogs (Figures 5 and 6). The standard errors of the terpane ratios are about 5% of the mean values for the Mishrif oils. Low pristane/phytane ratios (0.76 ± 0.04; from gas chromatography) are also suggestive of carbonate source rocks. The pentacyclic terpane gammacerane, a biomarker for hypersaline/restricted source environments, is

uniformly low (GA/C30 hopane < 0.1) in all samples. The relatively high C24 tetracyclic to C23 tricyclic terpane ratio of 1.3 suggests shallow, warm water, intra-shelf depositional source rock depositional environments (Zumberge and Summons, 2004).

In petroleum systems with carbonate/marl source units, the C28/C29 regular sterane ratio (αββ 20R isomers based on peak heights from the m/z = 218 mass chromatogram), along with the corresponding

triaromatic steranes (C27R/C28R; peak areas from the m/z = 231 mass chromatograms), is useful for predicting source rock ages (Zumberge and Summons, 2004). As illustrated in Figure 7, the calculated

C28/C29 ratio of 0.92, employing both regular and triaromatic steranes, and weighted using a saturate to aromatic hydrocarbon ratio of about 0.5, suggests that the Cretaceous Mishrif-reservoired oils were generated from Upper Jurassic to Lower Cretaceous carbonate source rocks.

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100.0 Group 1 Ru/131 Z/5 2 R/170 Zb/10 Type III 3 Zb/8 4 Zb/4 Terrigenous Ru/81

10.0 Type II/III Mixed

pe II O Ty Biodegradation xidation Algal Marine Pristane-n C17 R eduction 1.0

0.5

0.2 Maturation 0.1 0.1 0.2 0.5 1.0 10.0 Phytane-n C18 Figure 3: Pristane-n C17 versus Phytane-n C18 plot diagram (following Peters et al., 2005) showing marine algal organic matters of mainly kerogen type 2, reducing paleoenvironments and mature source of the Mishrif oil for the studied wells in southern Iraq.

45 GeoMark OILSTM Database 40 Carbonate SR Marl SR Distal Shale SR 35 Lacustrine SR

Southern Iraq

API Gravity (60°F) 20 Mishrif-reservoired oils

5 0 0.5 1.0 1.5 2.0 2.0 3.0 3.5 4.0 4.5 Weight Percent (Wt%) Sulfur Figure 4: Average API Gravity and weight % sulfur of Mishrif-reservoired oils from southern Iraq. Other data points represent average oil values from 150 global petroleum systems from marine carbonate, distal marine shale, marine marl, and lacustrine shale source rocks from GeoMark Research OILS™ database.

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1.6 GeoMark OILSTM Database 1.4 Carbonate SR Southern Iraq Marl SR Mishrif-reservoired oils Distal Shale SR 1.2 Lacustrine SR

1.0

0.8 erpane C22/C21 T

0.6 ricyclic T

0.4

0.2

0 0.1 0.3 0.5 0.7 0.9 1.1 1.3 Tricyclic Terpane C24/C23 Figure 5. Average tricyclic terpane ratios of Mishrif-reservoired oils from southern Iraq suggesting a carbonate source rock. Other data points represent average oil values from 150 global petroleum systems from marine carbonate, distal marine shale, marine marl, and lacustrine shale source rocks from GeoMark Research OILS™ database.

0.60

0.55

0.50

0.45 Southern Iraq Mishrif-reservoired oils 0.40

0.35

0.30 Hopane C31R/C30

0.25 GeoMark OILSTM 0.20 Database Carbonate SR Marl SR 0.15 Distal Shale SR Lacustrine SR 0.10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Hopane C29/C30 Figure 6: Average hopane ratios of Mishrif-reservoired oils from southern Iraq suggesting a carbonate source rock (SR). Other data points represent average oil values from 150 global petroleum systems from marine carbonate, distal marine shale, marine marl, and lacustrine shale source rocks from GeoMark Research OILS™ database.

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1.6 Cretaceous Jurassic Triassic

1.4

1.2

1.0

Southern Iraq 0.8 Mishrif-reservoired oils

0.6 Calculated C28/C29 Sterane Ratio

GeoMark OILSTM 0.4 Database Carbonate SR Marl SR 0.2 60 110 160 210 260 Source Rock Age (Ma) Figure 7: Calculated average C28/C29 sterane ratio (0.92; based on both regular steranes and triaromatic steranes) of Mishrif-reservoired oils from southern Iraq suggesting a source rock of Upper Jurassic to Lower Cretaceous age. Other data points represent average oil values from global petroleum systems from marine carbonate and marine marl source rocks from GeoMark Research OILS™ database.

Figure 8 shows an example geochemical summary sheet for a Mishrif oil from the Zubair Field. The whole crude gas chromatogram indicates a lack of biodegradation due to the presence of abundant n- paraffins. The low API gravity and high sulfur and aromatic content of these oils, as well as very low Ts/Tm terpane ratios, suggest that these oils were generated from a source rock of low to moderate thermal maturity within the oil window.

SOURCE ROCK EVALUATION

Forty samples were selected from the Mishrif, Mauddud, Zubair, Ratawi, Yamama, and Sulaiy formations (Figure 1) for source rock analysis and palynofacies studies.

Source Rock Potential and Palynofacies Assessment

The rock samples were analysed for their total organic carbon by Leco. The samples were also subjected to standard palynological analysis involving dissolving the carbonates with HCl and silicates with HF. Slides were then prepared by strewing and sticking the organic matter for examination by light refracted microscopy. The microscopic study ascertained the proportional occurrences of the different organic matter types (Bujack et al., 1977; Batten, 1996a), palynomorph colour (Staplin, 1969; Batten, 1996b), reworked organic matter (Al-Ameri, 1983 and Tyson, 1995), and foraminiferal test linings (FTL, according to Al-Ameri et al., 1996; Tyson, 1995). The analyses are drafted in a model of Organolog palynofacies for each well to evaluate the paleoenvironment of the organic matter and its capability to generate hydrocarbons from each palynofacies type. Figure 9 shows one example for Rumaila South 172 (Ru-172) well to illustrate the hydrocarbon generation potential. The names for each palynofacies type in Figure 9 are based on the above criteria and are equivalent in correlation to palynofacies types of the Ternary Kerogen Plot suggested by Tyson (1993).

The position of Palynofacies Type IX (Figure 9) in the Ternary Kerogen Plot (Figure 10) corresponds to phytoclast-AOM-palynomorphs. This palynofacies shows peak oil generation and the main phase of oil expulsion for the Upper Jurassic – Lower Cretaceous Sulaiy and Yamama formations (Al-Ameri

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Bulk Properties

%Asph API Gravity: 26.7 ppm V: 50 Tricyclic Terpanes (m/z =191) %NSO %

C15+ Aromatic: -27.48 n-Paraffin/ Abundace C26tri Canonical Variable: -3.84 Naphthene = 0.64 600 C24tri 400 C22tri C25tri Whole Oil Gas Chromatogram C20tri C21tri C19tri 200 Whole Crude Gas 24 28 32 36 40 40 48 52 nC10 Chromatography Time (min) nC5 Pr/Ph = 0.78 Pr/n-C17 = 0.22 Triaromatic Steranes (m/z = 231) nC15 Ph/n-C18 = 0.33 1,400 n-C27/n-C17 = 0.21 C20 C26R&C27S CPI = 1.046 1,200 Abundace Pr Ph nC20 1,000 C28S nC25 C21 C27R 800 C28R

10 20 30 Abundace Time (min) 600 C26S Biomarkers (ppm C30 Hopane: 246) 400

Terpanes 12,000 200 C29H 51 54 57 60 63 66 69 Time (min) 9,000 C30H Steranes (m/z = 218) 1,000 ßßC29S 6,000 900 C31H ßßC27S R&S 800

Abundance Tm 700 3,000 ßßC28S 600 C23tri C24Tet Ts 500 0 30 40 50 60 70 80 Abundace 400 Time (min) 300

Steranes 200 700 100 C29 C27 600 60 62 64 66 68 70 Time (min)

480 C28

360 OilMod Ratios

Abundance C19/C23 = 0.07 C30X/H = %C28 = 25.3 C22/C21 = 1.29 OL/H = 0.01 %C29 = 40.9 240 C24/C23 = 0.28 C31R/H = 0.41 C29 20S/R = 0.76 C26/C25 = 0.72 GA/C31R = 0.22 C27 TsTm = 0.18 120 Tet/C23 = 1.29 C35S/C34S = 1.15 C29 Ts/Tm = 0.08 C27T/C27 = Ster/Terp = 0.15 DM/H = 0.01 C28/H = 0.00 Rearr/Reg = 0.12 TAS(CR) = 0.25 35 40 45 50 55 60 65 70 C29/H = 1.50 %C27 = 33.7 Time (min)

Figure 8: Example Geochemical Summary Sheet of a Mishrif-reservoired oil from southern Iraq Zubair field showing bulk properties, whole oil gas chromatogram as well as terpane and sterane biomarker mass chromatograms.

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Reworked

nomorphs 10

Immature Mature Maturation 1.0 trinite Vi

0.5 Reflectance ration Index ration

- Alte-

Thermal Thermal + + +

2 3 2 3 4

Color

Palynomorphs Melanogen

75 Hylogen s 50 Bujack’ Organic Log Amorphogen Phyrogen

25 Palynofacies V V IX IX VII VIa IVb well Ru-172, Rumaila South oil field. (See Figure 1 for legend) (m) 3,000 3,500 4,000 Depth Lithology mama Sulaiy Zubair Gotnia Ratawi Najmah Sargelu Shu'aiba Ya Mauddud Nahr Umr Formation thonian Albian Aptian langinian Bajocian Age Ti Callovian Oxfordian Berriasian Bathonian Barremian

Hauterivian Va

Cenomanian Figure 9: Organlog of palynofacies and assessment of hydrocarbon generation potential for the Upper Jurassic – Lower Cretaceous Figure 9: Organlog of palynofacies and assessment hydrocarbon generation potential for the Upper Jurassic Kimmeridgian

ACEOUS CRET JURASSIC

100

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et al., 1999). The organic matter accumulated PHYTOCLAST in distal suboxic-anoxic basins and extended IV stippled: note over across the Southern Mesopotamian Basin (Al- Nahr Umr 95 with II Shelf - basin Zubair transport path A 10 Ameri et al., 1999). Source rock richness can Ratawi A Yamama B 15 reach up to 7 wt% total organic carbon (TOC) Proximity of fluvial sources Sulaiy of mainly marine algal components and may II contain up to 100% Amorphous Organic Matter

(AOM). They form kerogen type A (Thompson d sources 65 and Dembicki, 1986) or equivalently kerogen IVa III Type II for the pyrolysis result plotted in a van B E B Krevelen diagram. They are highly oil prone. VI C 55 The high percentage occurrence of foraminiferal D IVb plus sortin test linings of about 10% and very low reworked 40 60 palynomorphs of about 1%, may confirm high oil generation potential (Al-Ameri, 1983; Al- Redax plus proximity to flui g Ameri et al., 1996; Tyson, 1993, 1995). A VII V IX D

The post-depositional alteration of the organic E VIII matter is a result of increasing temperature as the formations were buried to depths of around 60 35 3,500 m in this area. This process resulted in AOM Redox plus masking effect Palynomorph the thermal maturation of the organic matter to Figure 10: Organic Matter of the Lower a late oil or early gas maturation stage with a Cretaceous source rock formations of southern brown colour of the macerals and an equivalent Iraq plotted on the ternary kerogen plot of Thermal Alteration Index (TAI) of 3+ (3.75) Tyson (1993). equivalent to a VRo of 1.0 to 1.4% (Staplin, 1969; Batten, 1996a and b).

Source Rock Potential by Rock-Eval Pyrolysis

Rock samples from core and cuttings in the Sulaiy and Yamama formations, as well as other Upper Jurassic – Lower Cretaceous formations, were analysed using Rock-Eval pyrolysis (Hunt, 1996; Tissot and Welte, 1984). In Figure 11, Rock-Eval parameters are plotted to determine the organic matter type, maturation level, and source quality. They illustrate the following results for each source rock unit:

• The Upper Jurassic – Lower Cretaceous Sulaiy and the Lower Cretaceous Yamama formations have the best oil source potential with TOC values up to 7 wt%. They represent mature (Ro 1.0% to 1.4%) kerogen types II and III with Tmax of 437–471oC. Consequently they currently have fair to very good petroleum potential and are probably the Upper Jurassic – Lower Cretaceous source rock responsible for hydrocarbon generation and expulsion.

• The Ratawi Formation has TOC values ranging from 0.06 to 1.9 wt%; kerogen type II and III, mature (Ro = 0.5–1.0%) with Tmax of 430–450 oC. These rocks have poor to fair current petroleum potential and have generated relatively low amounts of hydrocarbons.

• The Zubair Formation has TOC values of 0.2–2.6 wt% and varies from immature to mature (Ro = 0.3–0.8%) with Tmax of 420–440 oC. Hence it is a poor to fair potential source rock and some oil could have been generated from these clastic rocks.

Accordingly, the most likely source rock for the Mishrif reservoired oils are the Sulaiy and Yamama formations. The Jurassic Najmah and Sargelu formations were not subjected to pyrolysis because of their position bellow the good regional seal of Gotnia Anhydrites. For future work these should be analized to find the possible relation with the Cretaceous oil in southern Iraq.

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1,000 ~0.55 Vro Type I a Immature Oil Zone Gas Zone Oil prone 900 (usu. lacustrine) Oil window c Ty 0.5 pe 800 750 I

C) Nahr Umr Nahr Umr 700 Zubair Zubair TO Ratawi Ratawi 600 600 Yamama Yamama Type II Sulaiy Sulaiy oil prone (usu. marine) Vitrinite 500 0.5 450 isoreflectance Increasing 400 Ty evolution pe II

300 Mixed type 300 II/III

Hydrogen Index (mg oil/g oil/gas prone

200 Hydrogen Index (mg HC/g Corg.) ~1.00 Vro ndow Type III 150 1.3 as prone 100 Condensate - wet gas wi ~1.40 Vro I Dry gas window pe II Type IV Ty 0 0 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 400 420 440 460 480 500 Calculated vitrinite reflectance equivalent (Cal. Vreq.) Tmax (°C)

1.0 30 b d Immature Oil zone Dry gas zone Nahr Umr 20 0.9 Zubair Ratawi Good Yamama 0.8 10 Sulaiy Condensate zone

0.7 on Rock) Fair 0.6 Stained or 0.5 contaminated 2 Poor 0.4 1

Production index (PI) Nahr Umr 0.3 Zubair Ratawi Very Poor 0.2 Yamama

Sulaiy Petroleum potential (Kg HC/T 0.1 Low-level High-level conversion conversion - expulsion Poor Fair Good Very Good 0 0.2 380 430 480 530 580 0.15 0.50 1 2 10 Maturity (based on Tmax (°C)) TOC (wt. %) Figure 11: Pyrolysis output plots for source rocks of the Upper Jurassic − Lower Cretaceous Forma- tions in southern Iraq; (a) Kerogen types and maturity, (b) Kerogen conversion and maturity, (c) Van Krevlen diagram, and (d) Petroleum potential diagram.

PETROLEUM SYSTEM OF THE MISHRIF OIL

Petroleum system modeling in one dimension (1-D) was performed using the PetroModTM Basin Modeling software (PetroMod developed by Integrated Exploration Systems - IES) to demonstrate hydrocarbon generation and expulsion for the Sulaiy and Yamama formations. The model was applied to well Zubair 47 (Zb-47) following Pitman et al. (2004) using well information for depths, thickness and temperatures.

The results for this well (Figure 12) demonstrate that the timing of petroleum generation is consistent with the source rocks of the Sulaiy and Yamama formations expelling their oil during the Early Cenozoic (Paleogene). No oil should have passed from below the Gotnia Anhydrite regional seal. This probability is due to the perfect effect of the Gotnia Seal due to its thick and impermeable rocks and the lack of tectonic activity (Alsharhan and Nairn, 1997).

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(a) Sulaiy Source Rock (Well ZB-47) Mesozoic Cenozoic Cretaceous Paleogene Neogene 200 2.0 100

90 TR Lewan (2002) 180 1.8 Sweeny & Burnham (1990) EASY%Ro [%Ro]

80 160 1.6

TR Lewan 70 1.4

140 TIIS (Phosphoria) P64-HP 60 1.2 120 50 1.0 Temperature 100 40 emperature (Celcius) T 0.8 80 30 [percent] 0.6 Sweeney & Burnham (1990) 20 60

0.4 10 40 30 0.2 0 141 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma)

(b) Timing of Petroleum Generation (Well ZB-47) Mesozoic Cenozoic Formations Cretaceous Paleogene Neogene 0 Dibdibba Fatha L. Fars Ghar Dammam Rus 1,000 Umm Er Radhuma Tayarat Shiranish Hartha 2,000 Sa'adi

Mishrif

Depth (meters) Ahmadi Mauddud 3,000 Nahr Umr Shu'aiba

Source rock Zubair Start oil generation : TR=0.01 Ratawi 4,000 Peak oil generation : TR=0.50 Yamama End oil generation : TR>0.95 Sulaiy 4,518 141 130 120 110 100 90 80 70 60 50 40 30 20 10 0 Time (Ma) Figure 12: Timing and extent of Petroleum Generation from Sulaiy-Yamama Source Rocks, Well Zubair 47, southern Iraq.

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1 Vugg 20 µm 2 Intra-particle 20 µm

3 Growth Framework 20 µm 4 Intercrystal 20 µm

5 Channel/Oil Flow 20 µm 6 Hydrocarbon Accumulation 20 µm Figure 13: Porosities of the Mishrif Formation Reservoir as site for hydrocarbon accumulation.

Traps in the Mishrif Formation Reservoir formed mainly during the Late Cretaceous. These traps are mainly structural anticlines although stratigraphic traps below the Mishrif Formation might also be developed, for example, in the Yamama and Ratawi formations, as structural fault traps.

After expulsion, the oil migrated from the mainly impermeable strata of the Upper Jurassic – Lower Cretaceous Sulaiy and Lower Cretaceous Yamama source rock formations to the high porosity and permeability reservoirs of the early Upper Cretaceous (Cenomanian) Mishrif Formation. These units are algal and detrital limestones that contain vuggy, intra-particle growth frameworks with intercrystallar and moldic porosities (Figure 13) and form the main sites for hydrocarbon accumulation. The migration path was primarily vertical with relatively short horizontal movement to faults that are shown in the seismic section of the area (Figure 14), as well as dissipation along joints

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West East 0.0 400 Source Oil reservoir m 0 2 Seal Gas reservoir 0 km Migration path

Rus 0.5 Umm Er Radhuma Tayarat Shiranish F2 F1 Hartha 1.0 Sa'adi Tw o-way Tanuma Khasaib T

Mishrif ime (second) Rumaila Ahmadi Mauddud 1.5 Nahr Umr Shu'aiba

Zubair

Ratawi Yamama 2.0 Sulaiy Gotnia Najma Sargelu Mus

2.5

Figure 14: Seismic section along Rachi-Rumaila-Ratawi oil fields showing migration path to assess hydrocarbon accumulation sites.

and permeabilities in a lateral migration through some of the Cretaceous strata. This hydrocarbon system (Figure 1) has as a lower regional seal the Upper Jurassic Gotnia anhydrites, while the Mishrif Formation reservoir is sealed locally by a clay layer along its upper unconformable boundary with the Khasib Formation. But the main regional upper seal for the Mesopotamian Basin in general is the highest regional seal of the Middle Miocene Lower Fars anhydrites on top of Tertiary strata. The sequence of events in this total petroleum system is tabulated in Figure 15.

CONCLUSIONS

The biomarkers of Mishrif Formation oil indicate source rocks of mainly Upper Jurassic and Lower Cretaceous age, thermally mature, and of algal marine carbonate origin. The palynofacies and pyrolysis results indicate hydrocarbon generation and expulsion occurred from the Upper Jurassic – Lower Cretaceous Sulaiy and Lower Cretaceous Yamama formations. Oil migration is mainly from impermeable strata of the Sulaiy Formation into the Mishrif Formation Reservoir. The entire Petroleum System was controlled in this region by migration paths, which were vertical as well as lateral to form the regional Petroleum System. Its main upper seal is the Middle Miocene anhydrites of the Lower Fars Formation and its lower regional seal is Upper Jurassic Gotnia anhydrites.

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Generation,

Age Stage Formation ap Migration and

(Ma) Tr Rocks Rocks Accumulation Source System Reservoir Formation Seal Rocks Overburden Preservation

Pliocene Dibdibba Zubair 5 Fm Fatha Miocene Ratawi Fm

NEOGENE Ghar 23 Oligocene 36

Dammam Eocene ALEOGENE

P Rus 57 Umm Er Paleocene Radhuma Sargelu - 65 Sulaiy Fm Maastrichtian Tayarat 70 Shiranish

Campanian Hartha

Santonian Sa'adi Coniacian Tanuma Khasib 91 Turonian Cenomanian Mishrif 95 Rumaila Ahmadi Mauddud

ACEOUS Albian Nahr Umr 112 CRET Aptian Shu'aiba ? 125 Barremian Zubair

Hauterivian Ratawi Valanginian Yamama 145 Berriasian Sulaiy Tithonian 151 Gotnia Kimmeridgian Oxfordian Najma 161 ? Callovian Naokelekan Bathonian Sargelu Bajocian 172 JURASSIC Aalenian Alan Anhydrite Mus Limestone Toarcian Adaiyah Anhydrite Figure 15: Events chart showing total petroleum system of the Mesopotamian Basin in southern Iraq.

ACKNOWLEDGMENTS

Sincere acknowledgments are due to Geomark Research Limited of Houston-Texas for analyzing the oil and source rock samples as well as the laboratories of Iraqi Oil Exploration Company for using their data on TOC and pyrolysis, and to the U.S. Geological Survey, in particular to Dr. Janet Pitman for PetroMod basin modeling to make the Petroleum System clearer. The GeoArabia anonymous reviewers and, in particular, Henry Halpern of Saudi Aramco in Saudi Arabia are greatly acknowledged for their editing of this paper. The final design by GeoArabia’s Arnold Egdane is much appreciated.

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ABOUT THE AUTHORS

Thamer K. Al-Ameri is Professor of Petroleum Geology at the University of Baghdad, Iraq, and a Consultant for the Iraqi Oil Exploration Company. He holds a BSc in Geology from the same university (1969) and a Higher Diploma in Engineering Geology and Sulfur Exploitation from the Polish Academy of Mines in Krarov (1971). Between 1971 and 1975, he was involved in mining sulfur in the Mishraq Sulfur Mine in Iraq. In 1975, Thamer continued his higher education at the University of London, UK, where he received MSc and PhD (1975-1980) in Palynology and Petroleum Geology. He also obtained a certificate in organic geochemistry from the Russian Geochemical Institute in Moscow (1985). Before taking up his present position in 1992, Thamer was Assistant Professor in the University of Salahuddin, Iraq (1980-1991). During his academic career, Thamer has supervised 18 MSc and 20 PhD theses, published five books in Arabic and written more than 100 scientific and general interest articles in international and Iraqi journals. [email protected]

Amer Jasim Al-Khafaji teaches geology to the chemistry and biology undergraduate students at Babylon University, Iraq. He holds an MSc in Petroleum Geology from the Baghdad University, Iraq. Amer is currently enrolled in the PhD program at Baghdad University and preparing his thesis entitled “Petroleum system and organic geochemistry of the Euphrates River Sub zone, Mesopotamian Basin, Iraq”. He is also involved in the planning of the Petroleum Research Center at Babylon University. [email protected]

John Zumberge has been President and cofounder of Geomark Research in Houston, USA, since 1991. He was Manager of geochemical and geological research for cities service-Occidental, General Manager for Ruska, and Director of geochemical services for Core Laboratories. He has global experience in petroleum geochemistry, focusing on crude oil biomarkers. He obtained a BSc in Chemistry from the University of Michigan, USA, and a PhD in Organic Geochemistry from the University of Arizona, USA. [email protected]

Manuscript received May 18, 2008 Revised April 27, 2009 Accepted May 15, 2009 Press version proofread by authors June 10, 2009

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