ORIGINAL PAPER

Programed oil generation of the Zubair Formation, SouthernIraq oil fields: results from Petromod software modelingand geochemical analysis

Thamer Khazal Al-Ameri & Janet Pitman &

Madhat E. Naser & John Zumberge &

Hiba Adil Al-Haydari

Received: 31 August 2009 /Accepted: 14 May 2010# Saudi Society for Geosciences 2010

Abstract 1D petroleum system modeling was performedon wells in each of four oil fields in South Iraq, Zubair(well Zb-47), Nahr Umr (well NR-9), West Qurna (wellWQ-15 and 23), and Majnoon (well Mj-8). In each of thesefields, deposition of the Zubair Formation was followed bycontinuous burial, reaching maximum temperatures of100°C (equivalent to 0.70%Ro) at depths of 3,344–3,750 m of well Zb-47 and 3,081.5–3,420 m of well WQ-15, 120°C (equivalent to 0.78%Ro) at depths of 3,353–3,645 m of well NR-9, and 3,391–3,691.5 m of well Mj-8.Generation of petroleum in the Zubair Formation began inthe late Tertiary, 10 million years ago. At present day,modeled transformation ratios (TR) indicate that 65% TR ofits generation potential has been reached in well Zb-47,75% TR in well NR-9 and 55-85% TR in West Qurna oilfield (wells WQ-15 and WQ-23) and up to 95% TR in wellMj-8, In contrast, younger source rocks are immature toearly mature (<20% TR), whereas older source rocks are

mature to overmature (100% TR). Comparison of thesebasin modeling results, in Basrah region, are performedwith Kifle oil field in Hilla region of western EuphratesRiver whereas the Zubair Formation is immature withintemperature range of 65–70°C (0.50%Ro equivalent) withup to 12% (TR=12%) hydrocarbon generation efficiencyand hence poor generation could be assessed in this lastlocation. The Zubair Formation was deposited in a deltaicenvironment and consists of interbedded shales and porousand permeable sandstones. In Basrah region, the shaleshave total organic carbon of 0.5–7.0 wt%, Tmax 430–470°C and hydrogen indices of up to 466 with S2=0.4–9.4of kerogen type II & III and petroleum potential of 0.4–9.98of good hydrocarbon generation, which is consistent with55–95% hydrocarbon efficiency. These generated hydro-carbons had charged (in part) the Cretaceous and Tertiaryreservoirs, especially the Zubair Formation itself, in thetraps formed by Alpine collision that closed the TethysOcean between Arabian and Euracian Plates and developedfolds in Mesopotamian Basin 15–10 million years ago.These traps are mainly stratigraphic facies of sandstoneswith the shale that formed during the deposition of theZubair Formation in transgression and regression phaseswithin the main structural folds of the Zubair, Nahr Umr,West Qurna and Majnoon Oil fields. Oil biomarkers of theZubair Formation Reservoirs are showing source affinitywith mixed oil from the Upper Jurassic and LowerCretaceous strata, including Zubair Formation organicmatters, based on presentation of GC and GC-MS resultson diagrams of global petroleum systems.

T. K. Al-Ameri (*) :M. E. Naser :H. A. Al-HaydariDepartment of Geology, University of Baghdad,Jadiriyah, Iraqe-mail: thamer_alameri@yahoo.com

J. PitmanU.S. Geological Survey,MS 939,Denver, CO 80218, USA

J. ZumbergeGeomark Research Ltd,9748 Whithorn Drive,Houston, TX 77095, USA

Arab J GeosciDOI 10.1007/s12517-010-0160-z

Keywords PetroMod software . Zubair Formation .

South Iraq . Oil generation . Accumulation . Biomarkers

Introduction

Zubair Formation is the most important formation of theLower Cretaceous sequence in Iraq (Al-Sayyab, 1989). Ithas widespread in the Arabian Gulf region, Syria, and Iran.It was introduced to designate the prevalently terrigenousclastics and oil-bearing sequences of the southern Iraqifields (Buday 1980).

This formation consists of alternating course to finegrained sandstone, siltstone, and dark gray shales (BellenR.C. Van et al. 1959) in sequence of repeated units; theyrange in thickness from 301 m in well NR-9 to 381 m inwell Zb-47 of transgression and regression within deltaicenvironment (Ali and Nasser 1989). The formationextends from the Tkrit-East Baghdad field in central Iraqtoward the south near the Iraqi-Kuwait border andcontinued through the Arabian Gulf in Iraq southern partof Mesopotamian zone ; it may reach >500 m in the areasouth of Baghdad and is thinning eastward toward theZagross Fold Belt and westward toward the EuphratesRiver. Age of the formation, as determined on the basis ofboth fossils and regional correlation, is Hauterivian tillEarly Aptian (Bellen R.C. Van et al. 1959), whilepalynomorphs evidences extended this formation up toearliest Albian age (Al-Ameri and Batten 1997) withcontacts of mostly gradational and conformable. Theunderlying formation is the Ratawi Formation whichconsists of dark, slightly pyretic shales interbedded withpseudo-oolitic detrital limestone (Bellen R.C. Van et al.1959); and this is overlain by Shuaiba Formation whichconsists of dolomitic limestone.

The petrophysical characteristic of porosity is rangingfrom 19% to 28% and permeability from 80 to 2,500 md.The Zubair Formation is main reservoir in most south oilfields and produced oil and gas from gross thickness of(150–250)m and a net thickness of (90–170)m. Theformation produced oil from the Abu Khaimah and Kifldiscoveries and the Luhais, Majnoon, West Qurna,Ratawi, Siba, and Tuba fields. Oil and gas are producedin commercial quantities from East Baghdad, Nahr Umer,Wesr Qurna, Luhais, Rumaila North and South, andZubair Oil Fields (Al-Gailani 1996 and Al-Sharhan et al.1997). The hydrocarbon pays of Zubair Formationdisplayed in four pay zones (Fig. 1), their AmericanPetroleum Institute (API) gravity value is ranging from16º to 39º (average 34º) and a sulfur content of 1.5-2.18%(average 1.8). The API varies from an average of around34º in the western part of Mesopotamia to around 16-27ºin the east.

The studied area composes geographically the oil fieldsof Zubair, Nahr Umr, Majnoon, and West Qurna. It extendsbetween 30.00–31.10 latitude and 47.10–47.55 longitudesas shown in Fig. 2. The Zubair Formation in this area wasdeposited in Mesopotamian basin of geographically faciesdistribution of southern part in Iraq. It is part of the majorTotal Petroleum System of Cretaceous-Tertiary in SouthIraq (Pitman et al. 2004 and Al-Ameri et al. 2009) thatextend between the evaporates Upper Jurassic GotniaAnhydrite Formation as the lower major seal and theevaporates Lower Fars Formation as the upper major seal(Fig. 3).

A source for the oil accumulated in the Zubair Formationis not definitely assigned, especially those in the strati-graphic traps, and hence this study is aimed to find the oil-source relation and their system in Zubair Formation.Generation and their accumulation by PetroMod softwarebasin modeling, organic geochemical analysis and rock thinsections will be the tools for this study.

Materials and methodology

Cores and cuttings samples were collected from ZubairFormation and its suggested hard data source rocks fromeight wells (Fig. 2) of five oil fields in South Iraq (Basrahregion): they are six samples from Zb-40, ten samples fromZb-43, and five samples from Zb-47 of Zubair Oil Field, sixsaples from NR-9 of Nahr Umer Oil Field, seven samplesfrom W.Q-15, eight samples of WQ-1 of West Qurna OilField, four samples from Mj-8 of Majnoon oil field, 29samples from R-167, six samples from R-26 and four samplesfrom R-172 of Rumaila North Oil Field, four samples fromRu-1 of Rumaila South Oil Field as well as four core samplesfrom the same formation in well AA-1 of AboAmood oil fieldin central Mesopotamian Region and five samples each fromwells Kf-1 in Kifle oil field and Ns-1 in Nasiriyah oil fieldalong Euphrates River region. Two oil samples are collectedfrom Zubair Formation of wells Zb-5 depth (2,200 m) and Zb-10 depth (2,200.5 m) of the Zubair oil field.

These samples are subjected for organic geochemicalanalysis with pyrplysis for source rocks evaluations andwith gas chromatography/mass spectrometry for oil analy-sis and biomarkers in Geomark Research Ltd in Houston,Texas. Microscopy is aided by palynological slides that areprepared by palynological preparations of these rocksamples in the laboratories of the department of geology,College of Science, University of Baghdad.

Burial depths, erosion, well temperatures, lithologies,organic matter richness, and their kinetics are programmedby 1D PetroMod software basin modeling for the abovementioned wells in the US Geological Survey in Denver,Colorado.

Arab J Geosci

1D petromod software basin modeling

PetroMod software technology is a finite-element basinsimulator that describes thermal histories, source rockmaturity, and petroleum generation and migration. Theinterpreted well logs served as input to the models.They could be processed in one, two, and threedimensions (1D, 2D, 3D, respectively). It is integratedexploration systems (IES 2007) for evaluating petroleumsystems.

The model is a numerical formulation of the regionhistory based on interpretation of combined geologic andgeochemical data in a temporal framework, and it includesthe principal elements (source rock, reservoir rocks, andseals) of the total petroleum system.

PetroMod 1D is a newly developed and fullyintegrated component of 'IES' PetroMod modelingsystem. It has the same user interface and 'look and feel'as all other IES packages and enables single-point data(well and pseudo-well) to be constructed from scratch orto be extracted directly from PetroMod 2D and 3Dmodels. Calibration results such as heat flow historiescan then be directly used by the 2D and 3D simulators,which enables calibration work in all packages to beperformed much faster. Data for 1D PetroMod for thisstudy, taken well Zb-47 as example, are displayed inTable 1.

Model development The chronostratigraphic units in the 1Dmodel were assigned absolute ages of deposition anderosion. The ages of depositional and erosional events weredesignated based on the geologic time scale of Sharland etal. (2001). Lithologies represented as end-member rocktypes or as compositional mixtures of rock types wereassigned to each unit using software default parameters(Pitman et al. 2004).

Determining the timing of petroleum generation andexpulsion required calibration of the thermal regime at eachmodel location. Parameters used in the calibration includedheat flow , thermal conductivity of the rock matrix , surfacetemperature, and sediment thickness (present and past) ofthese factors, heat flow is the least constrained parameter(Pitman et al. 2004). Type IIS kerogen kinetics are used forthe Jurassic and Cretaceous Formations because extracts ofthe kerogen of the Jurassic and Lower Cretaceous sourcerocks have considerable amount of Sulfur (nitrogen,sulphur, and oxygen; NSO=1-34%).

Reservoir and bottom-hole temperature data plus theother parameters mentioned earlier in this paragraph forfour wells (Zb-47, W.Q-15 NR-9 and Mj-8) are used in thesoftware data to estimate the present heat flow in the studyarea as shown in Fig. 4. This integrates the effects oftemperature over time and is used to evaluate thepaleothermal history of the four well locations (Zb-47, W.Q-15 NR-9 and Mj-8). Well Kifle-3 is taken for comparison

Fig. 1 Major oil pays of the Lower Cretaceous Zubair Reservoir of South Iraq in depth successions with logs of lithologgy, resistivity (converted topermeability) and SP (converted to porosity). a The main pay in Rumaila Field, b the fourth pay in Zubair Field, and c the third pay in Zubair Field

Arab J Geosci

with western location in the west of Euphrates River, HillaRegion.

Calibration of the paleothermal regime required match-ing measure Ro values for each well with values of Rocalculated using the easy Ro of (Sweenay and Burnham,1990) for wells (Zb-47, W.Q-15 NR-9, Mj-8 and Kifle-3,

respectively) with depths as illustrated in Fig. 4 for Zb-47as an example.

Modeled Ro depth trends constructed for the paleother-mal analysis were used to estimate the amount of strataeroded in the model area. In each of the studied wells, theRo trend that best fit the data was extrapolated at an Ro

Fig. 2 Location map of Iraq showing northeast Arabian Peninsula of the region Iraq with locations of basins, oil fields, and wells on which thisstudy is based

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value of 0.25%, which was assumed to approximate thevitrinite reflectance at surface conditions (Fig. 4) for Zb-47 as an example. Computed Ro trends for the studiedwell in South Iraq intersect the present-day surface at0.25% Ro , indicating that erosion during late Cenozoictime generally was minimal (less than 500 m) in thisregion.

The amount of stratigraphic section removed duringMesozoic erosional episodes has not been reported;however, sensitivity tests indicate that pre-tertiary erosion,

although locally intense, had little impact on the Jurassicsource rock maturation history.

Model analysis One dimensional burial-thermal profiles asshown in Fig. 4 for well (Zb-47, as example) demonstratethe effects of continuous burial had been affected onsource rock of the Zubair Formation. Zubair Formationsource rocks are presently at maximum burial tempera-ture of 120°C at depths of 3,344 m for well Zb-47,3081.5 m for well W.Q-15, 3,353 m for well NR-9 and

Fig. 3 Stratigraphic column ofSouth Iraq, Basrah region withhydrocarbon generation param-eters and seal within a totalpetroleum system of this field

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Tab

le1

Exampleof

ExcellDataInpu

tforwellZb-47

inZub

airoilfieldin

Sou

thIraq

Formation

Formation

Formation

Formationage

Erosion

age

Formation

Petroleum

Total

Sou

rcerock

Initial

Nam

eTop

Base

From

ToFrom

To

Lith

olog

ySystem

Organic

carbon

Kinetics

Hyd

rogenindex

(m)

(m)

(Ma)

(Ma)

(Ma)

(Ma)

Element

Con

tents(%

)mgg

TOC

0.0

292.0

10.2

5.0

5.0

0.0

SANDcong

lDibdd

iba

292.0

472.0

17.0

10.2

LIM

E&EVAP

Fatha_L

.Fars

472.0

624.0

25.2

17.0

SANDcong

lGhar

624.0

849.0

49.0

34.0

34.0

25.2

LIM

Edo

lom

Dam

mam

849.0

990.0

52.0

49.0

LIM

E&EVAP

Rus

990.0

1,44

0.0

58.0

52.0

LIM

E&EVAP

Umm

Er.Rad.

1,44

0.0

1,65

5.0

66.0

63.0

63.0

58.0

LIM

E&EVAP

Tayarat

1,65

5.0

1,79

4.0

69.0

66.0

LIM

Emarly

Shiranish

1,79

4.0

1,95

9.0

82.0

69.0

LIM

Emarly

Hartha

1,959.0

2,284.4

84.0

82.0

LIM

ESTONE

Sadi

2,28

4.4

2,31

7.0

89.0

84.0

SHALE

Sou

rce

5Pho

spho

ria_HP

600

Tanum

a

2,31

7.0

2,38

1.5

94.0

89.0

LIM

Emarly

Khasib

2,381.5

2,560.0

95.0

94.0

LIM

ESTONE

Mishrif

2,56

0.0

2,64

8.0

97.0

95.0

LIM

EShaly

Rum

aila

2,64

8.0

2,81

8.0

99.0

97.0

SHALE

Sou

rce

5Pho

spho

ria_HP

600

Ahm

adi

2,818.0

2,959.0

112.0

99.0

LIM

ESTONE

Maudd

ud

2,95

9.0

3,27

0.7

117.0

112.0

SAND&SHAL

Sou

rce

2Pho

spho

ria_HP

600

HahrUmr

3,270.7

3,344.0

121.0

117.0

LIM

ESTONE

Shu

aiba

3,34

4.0

3,72

5.0

127.0

121.0

SAND&SHAL

Sou

rce

2Pho

spho

ria_HP

600

Zub

air

3,725.0

3,872.0

137.0

127.0

LIM

ESTONE

Source

2Phosphoria_HP

600

Rataw

i

3,87

2.0

4,25

8.0

141.0

137.0

LIM

Edo

lom

Yam

ama

4,25

8.0

4,51

8.0

144.0

141.0

LIM

Eshaly

Sou

rce

2Pho

spho

ria_HP

600

Sulaiy

4,51

8.0

Wellname

Depth

(m)

Tem

perature

value

Unit(°C)

Min

(°C)

Max

(°C)

ZB-47

3,34

4.0

96.9

C86

.910

6.9

Surface

temp∼2

0°C

Arab J Geosci

3,391 m for well Mj-8 with equivalent vitrinite reflec-tance of 0.8-0.9 %Ro that lie in the oil window forZubair Formation according presentation on van Krevlendiagram of Tissot and Welte, (1984). On the other hand,the burial temperature of well Kifle-3 is 60-70°C at depthof 1,969 m with equivalent vitrinite reflectance of 0.3%Rothat lie in the immature zone.

Transformation ratio (TR) could be defined (Follow-ing Lewan and Ruble 2002 and Peter et al. 2005) as thedifference between the original hydrocarbon potential of asample before maturation and the measured hydrocarbonpotential divided by the original hydrocarbon potential orcould be measured by Bitumen/total organic carbon(TOC), the value could range from 0 to 1.0 or cold beconverted to percentages. Accordingly, timing of oilgeneration for each of the proposed source rocks unit inthe studied wells of the studied oil fields could beestimated from the transformation ratio diagram and henceclarified the following generations.

1. Zubair Oil Field, well (Zb-47): at Zubair Formation, TRcurve indicate that oil generation had commenced with

(TR >1%) in the Middle Paleogene time at temperature∼70-80°C and reached TR >60% in the Neogene tillpresent time at temperature ∼100°C as illustrated in(Fig. 5) which could reflect 65% of its oil had beengenerated.

The mature formations bellow the Zubair Formation (theLower Cretaceous Ratawi Formation) showed that (Fig. 5)the oil generation had increased up to 100% in the LateNeogene to present time at temperature ∼100-110°C(equivalent to 0.70%Ro); while the mature formationsunder the Ratawi Formation (Upper Jurassic Sulaiy Forma-tion) have oil generation from TR >1% in the Cretaceous attemperature ∼80-90°C (equivalent to 0.60%Ro) andreached TR >100% in the Late Neogene at temperature110-120°C (equivalent to 0.75%Ro). Accordingly, thegenerated hydrocarbons (till present) from these formationsare 95% in Ratawi Formation, 100% in Sulaiy Formationaccomplished within 20 million years ago. The overlyingformations (Nahr Umr, Maudud, Ahmadi) are immature asshown in Fig. 5 of bellow 60°C (equivalent to 0.40%Ro)thermal burial with very few hydrocarbon (less than 20%

Fig. 4 Thermal history diagram of Zubair Oil Field, well Zb-47

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TR) generation and hence not considered for the wholegeneration.

2. West Qurna Oil Field, well (W.Q-15): at ZubairFormation, the oil generation start with TR >1% inthe Middle Paleogene at temperature ∼70-90°C andreached TR >50% at temperature ∼90-100°C in theLate Neogene (present time) which could reflect 53%of its oil generation.

The immature formation above the Zubair Formation(Middle Cretaceous Nahr Umr Formation) had very little oilgeneration ∼10%; therefore it is neglected, while the matureformations under the Zubair Formation like the LowerCretaceous (Ratawi Formation) had oil generation TR >80% at temperature ∼100-120°C in the Late Neogene topresent time. The Jurassic Formations of Sulaiy, Najmah,Naokelekan, and Sargelu have reached the oil generation ofTR >100% at temperature ∼100-120°C in the LatePaleogene.

3. Nahr Umr Oil Field, well (NR-9): at Zubair Formation,the oil generation start with TR >1% in the EarlyPalaeogene at temperature ∼70-80°C until reaching TR>70% in the Late Neogene (present time) at temperature∼100-120°C.

The immature formations above the Zubair Formation (theMiddle Cretaceous Ahmadi and Nahr Umr Formations) hadvery few oil generation vary between 4% and 25%; therefore,its generation consider negligible while the mature formationsbellow Zubair Formation (the Lower Cretaceous RatawiFormation) had oil generation reached TR >100% in the LateNeogene at temperature ∼100-120°C.

4. Majnoon Oil Field, well (Mj-8): at Zubair Formation,the oil generation start with TR >10% in the EarlyPaleogene at temperature 70-80°C, accelerated itsgeneration in the Late Neogene (10 million years ago)from TR=20% to reach TR=90% at present time(Fig. 7) with temperature of 120°C.

Fig. 5 Upper Jurassic and Lower Cretaceous hydrocarbon generation and extent of Sulaiy, Zubair and Nahr Umr Formations in well Zb-47

Arab J Geosci

The overlying formations are transitional immature andhence generate less oil while the underlying formations ofthe Jurassic and Lower Cretaceous Formations are matureand hence have generated oil of TR >100.

5. Kifle Oil Field, well Kifle-3: at Zubair Formation, nooil or little (up to TR=12%) oil could be generated(Fig. 7) because of transitional immature level (%Ro=0.45-0.52) of this formation in the Kifle Oil Field ofwell Kifle-3 in shallow depths (1,968-2,407 m) withburial thermal temperature of 60-70°C

Source rock maturation and petroleum generation Oil TRand the temperatures of petroleum generation were modeledfor Zubair, Nahr Umr,West Qurna, andMajnoon Oil Fields by(Zb-47, W.Q-15, MJ-8, and NR-9) wells to simulate thetemperature and timing of major petroleum events (generationonset and completion), and the extent of generation at theselocations as shown in (Fig. 6) for well Zb-47 as an example.

Vitrinite reflectance curves depicting thermal stress andshown in Fig. 7 for comparison. TR curves represent thefraction of petroleum that was generated at a given momentin geologic time. Temperatures during petroleum generationwere modeled with integrated heat flow, matrix conductiv-ity, and decompacted thickness calculations.

Accordingly and on the basis of these results, thegenerated hydrocarbon from Zubair Formation were; 65%of its efficiency from Zubair Oil Field of well Zb–47, 75%from Nahr Umr Oil Field of well NR-9, 53% from West

Qurna Oil Field of well W.Q.-15. The Zubair Formation inMajnoon oil field is showing higher maturation than othermentioned fields and has oil generation of 90% in well MJ-8 and 100% in well MJ-19 (Fig. 7). This could indicaterising maturation of the Zubair Formation up in theMajnoon oil field to the 100% transformation ratios(Fig. 8) compared to 60% transformation ratios in Zubairoil field (Fig. 6).

The comparison indicates that the Zubair formation inthe Kifle Oil Field in Hilla region in Western EuphratesRiver is of shallower depths (1,968-2,407 m) in well Kile-3than it is in the Basrah Region. It has less burial thermaltemperatures (60-70°C) of Ro 0.45-0.52 equivalent andmuch less generated hydrocarbons of 0.1-12% TR.

Modelled hydrocarbon generations These modeling in-formed us that Zubair Formation have started its main oilgeneration 10 million years ago with 50% hydrocarbontransformation in Zubair oil field while deeper formationsof Sargelu, Naokelekan, Najmah, and Sulaiy have generat-ed their oil during the paleogene time (65-25 million yearsago). The overlying strata such as Nahr Umr and AhmediFormations have little or no generation in Zubair oil field.Accordingly, and as illustrated in Figs. 5 and 6, thehydrocarbon generations in this total petroleum systemcould be modeled within middle area of MesopotamianBasin into three phases of the following:-

The first phase was 100% transformation ratios to hydro-carbons obtained from Jurassic Formations of Sargelu,

Fig. 6 Hydrocarbon transfor-mation ratios of Zubair OilField, well Zb-47

Arab J Geosci

Fig. 8 Hydrocarbon transformation ratios Majnoon oil field, well MJ-8

Fig. 7 Hydrocarbon generation and extent of the Zubair Formation in West Qurna, Nahr Umr, Majnoon, and Kifle oil fields for the wellsindicated inside the figures

Arab J Geosci

Najmah, and Sulaiy within temperature effect of 120-140°C and calculated vitrinite reflectance of 0.9-1.0%Ro(Fig. 4), during times of 80-30 million years ago (Fig. 5).These formations were deposited in dysoxic-anoxicenvironment of marine carbonate that has total organiccarbon of 0.5-8.0 wt% (Al-Ameri et al. 1999 and 2009)which confirm the hydrocarbon generation. The trap

formation is taken from the abrupt subsidence line inburial thermal history during the Upper Cetaceous time at65-60 million years ago (Fig. 4), which conform to theGlobal Cretaceous/Tertiary Event of its indirect effect onthe Arabian Plate. The mechanism of its development isbasement movements in pre-existed trends as response tothick sediment accumulations in the Mesopotamian Basin

Fig. 9 Spores and pollen grains recovered from the Zubair Formation

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Fig. 10 Dinoflagellate cysts recovered from the Zubair Formation

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and the global cratonic reaction to plate collision (Al-Sharhan et al. 1997 and Sharland et al. 2001)

Accordingly, these traps are mainly structural withsome syndepositional traps of stratigraphic nature. Thesegenerated hydrocarbons have charged the already formedCretaceous structural traps such as Mishrif FormationReservoir by the event discussed above and the strati-graphic traps such as the Sulaiy, Ratawi, and ZubairFormations that formed by trasgressions and regressionsduring their deposition time of Lower Cretaceous (Al-Ameri and Batten 1997).

The second phase was the petroleum generation fromLower Cretaceous Ratawi and Zubair Formations. Atpresent day, modeled transformation ratios indicate that53-90% for Zubair Formation and 85-100% for RatawiFormation had occurred within temperature effect of 100-120°C and calculated vitrinite reflectance of 0.8%Ro(Fig. 4), during times of 50 million years ago to present.Traps formation of structural types of this phase could bebased on abrupt change in the thermal subsidence line(Fig. 4) during the Miocene time at 20-10 million years agoand which conform with the closing phase of the LateAlpine Orogeny effect on northeast Arabian Plate (Al-Sharhan et al. 1997 and Sharland et al. 2001). Accordingly,the oil of the second phase of generation has beendistributed within the whole petroleum system of theCretaceous and Tertiary between the two major seals ofUpper Jurassic Kimmeridgian Gotnia Anhydrites Formationand the Middle Miocene Lower Fars Anhydrites Formation.This late dynamicity of the generated hydrocarbons mighthad charged the already formed stratigraphic traps ofsandstone facies embedded in shales that formed duringthe deposition of mainly Zubair Formation in transgressionand regression phases .

The third phase: younger non-efficient source rocks ofNahr Umr, Maudud, and Ahmadi Formations are immatureto early mature (0.7%Ro) and hence less than 10%hydrocarbons were generated (Figs. 5 and 6) whileoverlying formations up to the present sediments areimmature with no hydrocarbon generation. Accordingly,negligible consideration is given to their quantity generatedwith respect to the formations bellow them.

Hydrocarbon potential

Kerogen analysis for the hydrocarbon generation and oilanalysis for their biomarkers to find oil-source correlationcould form the base for confirming the PetroMod softwareBasin modeling of the Zubair Formation in South Iraq.

The generation Assessment of hydrocarbon generation andtiming of the Zubair Formation could be confirmed by

Table 2 List of spores and pollen grains, and dinoflagellate cystsrecorded from the Zubair Formation in South Iraq

Spores and pollen grains

Afropollis operculatus (Brenner) Doyle, Jardine and Doecrenkamp 1982

Appendicisporites cristatus (Markova) Pocock 1965

Appendicisporites potomacensis Brenner 1963

Asbekiasporites borysphenicus (Voronova) Fedorova (Theodorova)-Shakhmundes 1976

Asbekiasporites hoennensis von der Brelie 1964

Balmeisporites holodictyus Cookson and Dettmann 1958

Brenneripollis peroreticulatus (Brenner) Juhász and Gόczán 1985

Callialasporites dampieri (Balme) Sukh Dev 1961

Cicatricosis potomacensis Brenner 1963

Clavatipollenites hushesii Couper 1958

Concavisporites Obtusangulus (Potonié) Krutzsch 1954

Concavissimisporites variverrucatus (Couper) Brenner 1963

Costatoperforosporites fistulosus Deák 1962

Cyathidites australis Couper 1953

Cyclosporites hughesii (Cookson & Dettmann) Cookson & Dettmann 1959

Densoisporites microrugulatus Brenner 1963

Deltoidospora spp.

Ephedripites jansonii (Pocock) Muller 1968

Ephedripites multicostatus Brenner 1963

Eucommiidites sp.

Exesipollenites tumulus Balme 1957

Gleicheniidites senonicus Ross 1949

Interulobites triangularis (Brenner) Paden Phillips and Felix 1971

Microfoveolatosporites canaliculatus Dettman 1963

Monosulcites sp.

Murospora florida (Balme) Pocock 1961

Patellasporites distaverrucosus (brenner) Kemp 1970

Perotriletes laceratus (Norris) Rumeau 1981

Reticulisporites sp.

Reyrea polymorpha Herngreen 1963

Retitriletes austroclavatidites (Cookcon) Döring, Krutzsch, Mai and Schulz 1963

Retitriletes austroclavatidites (Cookson & Dettmann) Backhouse 1978

Rugulatisporites sp.

Sestrosporites pseudoalveolatus (Couper) Dettman 1963

Steevesispollenites sp.

Trilobosporites ivanovae Batten 1973

Trilobosporites trioreticulosus Cookson and Dettman 1958

Dinoflagellate cysts

Apteodinium micracanthum Cookson and Eisennack 1974

Cerbia tabulata (Davey and Verdier) Below 1981

Circulodinium distinctum (Deflandre and Cookson) Jansonius 1986

Cleistosphaeridium? aciculare Davey 1969

Cribroperidinium auctificum (Brideaux) Stover and Evitt 1978

Cribroperidinium orthoceras (Eisenack) Davey 1969

Oligosphaeridium pulcherrimum (Deflandre and Cookson) Davey andWilliams 1966

Pareodinia psilata Jain and Millipied 1975

Spiniferites twistringiensis (Maier) Fensome et al. 1990

Subtilisphaera perlucida (Alberti) Jain and Millipied 1973

Taleisphaera hydra Duxbury 1979 emend. Harding 1986

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fossil palynomorphs (spores, pollen, and dinoflagellate)identifications for dating the strata, palynofacies concept foroptical views of the total organic matters for findingenvironment of the organic matters accumulations andpyrolysis technique for chemical analysis for finding thehydrocarbon generation.

Palynomorphs assemblages of mainly the spores,pollen, and dinoflagellate cysts but no bisacate (Figs. 9and 10) are recorded from the Zubair Formation and arelisted in Table 2 along with Botryococcus, foraminiferaltest lining, and fungal remains that are potential forgenerating hydrocarbons.

The relative abundance of triradiate spores is consistentwith a warm climate, as implied by the latitude of southernIraq during the Early Cretaceous (within 5º either side ofthe equator). It is not possible, however, to conclude fromthe assemblages whether most of the plants from whichthey were derived lived in generally damp conditions on,and in the vicinity of, a delta because many fern taxaoccupy dry habitats (see, e.g., Tryon and Tryon 1982). Onthe other hand, representative of plants that are widelyaccepted as indicating dry conditions (e.g., Ephedripites)

are less common. In general, the dinoflagellate cystassociations compare well with other described from theTethyan province; in particular from Libya (Uwins andBatten 1988) and offshore Morocco (Ogg 1994).

For purpose of age determination, the palynomorphsrecovered are considered to comprise a single assem-blage because no significance difference in compositionwere recorded through the section examined (Al-Ameriand Batten 1997). Comparison were made betweenoccurrences of the taxa encountered and those recordedin the literature from widely scattered elsewhere so thatthe dating of the assemblages could be put into a globalperspective. In addition to the publications alreadymentioned, this literature include papers by Batten andLi Wenben (1987), Brideux and McIntyre (1975), Dettman(1986), Helby et al. (1987), Herngreen and Chlonova(1982), Kotova (1978), Milliod et al. (1974) and Williamsand Brideaux (1975).

The published ranges of all the taxa listed on Table 2impinge on Barremian-Aptian or up to Albian stage.Bearing in mind that the first and last occurrences of themajority are uncertain, and in any case vary according to

Fig. 11 Palynofacies map (a) and block diagram depicting depositional environments (b) for the Zubair Formation

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geographical region, on the evidence currently erected, theZubair Formation cold be most appropriately dated asBarremian-Early Albian. The lack of elaters-bearing paly-nomorphs, and tricolpate and tricolporate pollen, is taken toimply that the assemblage is older than late Albian,although it is recognized that the absence of elaterate formscould be for other reasons, such as a climate that wasunsuitable for the parent plants.

Palynofacies could be based on Al-Ameri and Batten(1997) who have recorded, by palynological studies forRumaila North Oil Field (well R-26) Zubair Oil Field (wellZb-43) and West Qurna Oil Field (WQ-1), four palynofa-cies types that may infer to indicate swamp and marshenvironments on delta top for PF1, and platform conditionsof delta front for PF2, prodelta for PF3, and open marinefor PF4, respectively (Fig. 11). Other wells of this study arerecorded by palynological analysis to have been undergoingthe same environmental laws within similar palynofaciestypes. Swamps and marsh as near to shore line of earlyCretaceous prevailed in the western Iraqi Desert and SaudiArabian region and open marine conditions dominated ineasternmost Iraq and Iran, with more varied deltaic

environments in between, within Iraq and Kuwait. Therepetition of the palynofacies types reflect transgressive andregressive phases during the deposition of the detritalsediments of the Zubair Formation and hence formedBarrier Island which lead to reducing circulation. Accord-ingly, anoxic conditions leading to the preservation ofabundant organic matters along with the palynomorphswere created.

Rocks of Zubair Formation are deposited in subsidingdeltaic deposits of favorable conditions for the preservationof the accumulated organic matter, albeit mostly in abiodegraded state (Al-Ameri and Batten 1997). The TOCcontent is between 0.5% and 6% for most of the samplesanalyzed. Biodegradation and thermal alteration of theorganic matter led in particular to the abundant amorphousorganic matters in PF2 and PF3 (Fig. 12) with abundantdinoflagellate cysts (15-30% of the total palynomorphs) andfluorescent alginate material derived from algae of thegenus Botryococcus as well as foraminiferal linings, fungalremains, and resin that indicate highly oil prone (followingTyson 1995). In the upper part of the formation, these twopalynofacies had changed their amorphous organic matter

Fig. 12 Palynofacies types viewed under refracted microscope for the Zubair Formation in the studied wells, two photos for each palynofaciesdemonstrating lower part of the formation (1, 3, 5, and 7) and upper part of the formation (2, 4, 6, and 8)

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structures to chunky compact masses and gelatinousappearance with palynomorph preservation into degradedstate. The composition of this material is similar to oilprone kerogen Type A of Thompson and Dembiki (1986)and is considered to be mature, the thermal alteration indexbeing 2.5 (Staplin 1969; see Batten 1996), and hencecapable of generating liquid hydrocarbons.

Chemistry of the Zubair Formation organic matters inthe Rumaila South, Rumaila North, Abo Amood, Nasiriyahand Kifle oil fields (wells Ru-1, R167, R-172, Ns-1, AA-1,and Kifle-3) is based on the results of pyrolysis analysistechniques (Table 3). Analyzed samples are plotted on vanKrenlen diagram of hydrogen index (S2/TOC=mg HC/gCorg) versus maximum temperature (Tmax °C) for hydro-carbon pulse in the pyrolysis devise (Tmax) accordingEspitalie et al. (1980; Fig. 13). The positions of the intersectpoints in the figure could indicate that Zubair Formation inBasrah Region is a source rocks of kerogen type II and IIIand of early mature organic matter content with 430-440°CTmax and hydrogen index of up to 450 mg HC/gm rocksfor the Rumaila North, Rumaila North, and Zubair oilfields while Abo Amood oil field is transitional imma-ture. The petroleum potential (mgHC/gRock) of ZubairFormation in well Ru-1 vary between 0.34 and 3.18, andreached 0.35–9.78 in well R-167, while it is between 0.5and 9.98 in well R-172 and 0.59-1.53 in well AA-1 thatled to the petroleum potential assessment of fair sourcerock in well Ru-1 and good source rock in wells R-167and R-172 as shown in Fig. 14. Nasiriyah oil field has

Fig. 13 Hydrocarbon generation and kerogen type of the ZubairFormation source rocks based on pyrolysis values plotted on VanKrevlen diagram in wells of selected oil fields

Table 3 Rock-eval pyrolysis data for Lower Cretaceous Zubair Formations in selected wells in South Iraq

PP PI OI HI TMax (°C) S3 S2 S1 TOC Sample type Depth (m) Well name No.

3.18 0.24 0 416 436 0 2.41 0.77 0.58 Core 3,010 Ru-1 1

0.34 0.20 11 142 432 0.02 0.27 0.07 0.19 Core 3,230 Ru-1 2

0.72 0.20 0 101 432 0 0.57 0.15 0.56 Cutting 3,388 R167 3

0.87 0.08 0 89.9 435 0 0.8 0.07 0.89 Cutting 3,425 R167 4

1.69 0.08 0 198.7 435 0 1.55 0.14 0.78 Cutting 3,457 R167 5

9.78 0.92 0 67.8 435 0 0.73 9.05 1.09 Cutting 3,507 R167 6

0.35 0.11 0 66 434 0 0.31 0.04 0.47 Cutting 3,555 R167 7

1.61 0.06 0 115.4 436 0 1.5 0.11 1.3 Cutting 3,655 R167 8

0.93 0.07 0 57.7 437 0 0.86 0.07 1.49 Cutting 3,730 R167 9

0.59 0.17 316 153 429 1.1 0.49 0.1 0.32 Cutting 3,100 R/172 10

0.5 0.34 327 100 440 1.8 0.33 0.17 0.33 Cutting 3,220 R/172 11

9.98 0.05 62 364 442 1.61 9.4 0.58 2.58 Cutting 3,301 R/172 12

1.53 0.06 37 148 434 0.36 1.44 0.09 0,97 Core 3,360 AA-1 13

0.71 0.10 92 121 433 0.49 0.64 0.07 0.53 Core 3,590 AA-1 14

0.59 0.08 79 126 436 0.34 0.54 0.05 0.43 Core 3,634 AA-1 15

0.93 0.06 60 107 430 0.49 0.87 0.06 0.81 Core 3,682 AA-1 16

24.81 0.37 16 321 420 1.05 20.93 3.78 6.52 Core 2,615 Ns-1 17

4.62 0.15 73 174 420 1.05 1.62 3.00 0.93 Core 2,880 Ns-1 18

2.67 0.46 54 162 424 0.48 1.44 1.23 0.89 Core 1,974 Kf-3 19

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high petroleum potential of 4.62-24 mgHC/gRock thatindicate good petroleum potential. Plotting the studiedsamples in production index versus Tmax diagram areshowing locations in the oil zone too.

The Zubair Formation organic matters in the Kifle OilField (Kifle-3) is of immature (Tmax=420-427), lowhydrogen index of 161 mgHC/gRocks (Fig. 13), and of2.7 mgHC/gRocks and hence of poor petroleum potential(Fig. 14). This result show negative correlation with Basrahoil fields and confirming the transformation ratio differenceby the PetroMod basin modelling.

Zubair oil geochemistry Zubar Formation oil of wells Zb-5(depth 2,200.0 m) and Zb-10 (depth 2,200.5 m) is mainlyaromatic hydrocarbons on the basis of C15† with 43-47aromatic, 37-40 saturate, 11-14 NSO and 2-6 asphalt. Plottingthe analysis result from gas chromatography (GC) and GC-mass spectrometry of wells of the Zubair and Luhais oil fields(Table 4) on diagrammatic presentation of global standardparameters suggested by Peter et al. (2005) and Zumberge etal. (2005) and compared with Geomak Oils™ database couldassess the following oil-source relations:

Pristane-nC17 Versus Phytan-nC18 The pristane versusphytane (Pr/Ph) plot can be used to infer oxidisation andorganicmatter type in the source rock depositional environment(Hunt 1997 and Peter et al. 1999). In Fig. 15, the ZubairFormation oils are plotted in the area of algal marine, reducingand type II kerogen of mature and low biodegradation. BothPr-nC17 and Ph-nC18 decreases with thermal maturity of oiland increase with degree of biodegradation. The Pr/Ph ratiosof the Zubair oils vary between 0.72 and 0.73 and hencecould indicates anoxic source rock deposition by comparisonwith the less than 1.0 of Peter et al. (2005), while Pr/Ph >1indicates that the oil is rich in lipids and waxes organic matter.

Sterane triangle Plots of gas chromatography/mass spec-trometry of the analyzed oil on triangle of C27-C28-C29 ratiosare used to interpret depositional environment and type ofsource rock and to classify crude oil into groups (Peter et al.2005). Accordingly, the resultant diagram (Fig. 16) isshowing that the Zubair Formation oil samples are in thearea of marine shale and marine carbonate source rock.

Terpane biomarker ratios Diagram of C22/C21 versus C24/C23 Tricyclic Terpane ratios of Peter et al. (2005) and

Fig. 14 Hydrocarbon generation potential of the Zubair Formationsource rocks based on pyrolysis values plotted on petroleum potentialversus TOC in wells of selected oil fields

Table 4 Analysis chart of GC and GC-MS oil biomarkers data for Lower Cretaceous Zubair Formations in the studied oil producing wells inBasrah, South Iraq

C29% C28% Sterane C27% Ph/nC18 Pr/nC17 C24/C23 Terpane C22/C21 Terpane Biomarker

24.8 34.1 42.1 0.35 0.21 0.28 1.0 Lu-1

27.0 34.0 39.0 0.31 0.21 0.24 1.1 Zb-16

28.0 34.0 38.0 0.33 0.21 0.23 1.1 Zb-163

41.8 24.7 33.5 0.35 0.23 0.31 1.09 Zb-10

40.4 25.8 33.9 0.36 0.23 0.39 0.97 Zb-5

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Zumberge et al. (2005) could assess source lithology bycomparison with global oil fields data. Accordingly, the plotsof the analyzed samples of Zubair Formation (Fig. 17) arelying in carbonate source rocks area with some shale and marl.

In the same scenario, plotting analyzed samples fromZubair Formation on diagram of hopane (C31R/H) versustricyclic terpanes (C26/C25) could assess carbonate andshale source rocks too.

Fig. 15 Pristane-n C17 versusPhytane-n C18 plot diagram(following Peter et al. 2005)showing marine algal organicmatters of mainly kerogen type2, reducing paleoenvironmentsand mature source of the Zubairoil for the studied wells in SouthIraq

Fig. 16 Sterane triangleof C27,C28, and C29 (following Peteret al. 2005) showing marine oilfrom carbonate with some shalesfor Zubair Formation oil of thestudied wells in South Iraq

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Oleanane/hopane and source rock age plot The oleananeis terpanes derivatives with origin from angiospermplants (Grantham et al. 1983 in Peter et al. 2005) andhence could be used to interpret the geologic age of sourcerock by the phenomena of increasing angiosperm diversityfrom the Upper Cretaceous to present. Its absence does notmean the crude oil generate in Lower Cretaceous and oldersource rock. The hyper saline water could helps to save andprotect the oleanane. The Ol/H ratio vary from 0 to 0.01 inZubair Formation crude oil samples (Fig. 18) and hence could

assess Upper Jurassic-Lower Cretaceous (including ZubairFormation itself) age equivalent for their source rocks.

Calculated C28/C29 sterane ratio within the diagram(Fig. 18) showed 1.18 for the Zubair Formation oil toindicate Lower Cretaceous age influence from the sourcerocks for the oil accumulated in the Zubair FormationReservoir by comparison with global Geomark Oils™database. This could confirm source for the oil accumulatedin Zubair Formation is to be from Lower Cretaceous (mainlyZubair Formation itself) as well as Upper Jurassic carbonates.

Fig. 18 Calculated averageC28/C29 sterane ratio (based onboth regular steranes andtriaromatic steranes) of Mishrif-reservoired oils from SouthernIraq suggesting a source rock ofJurassic age. Other data pointsrepresent average oil valuesfrom 150 global petroleumsystems from marine carbonate,distal marine shale, marine marl,and lacustrine shale source rocksfrom GeoMark ResearchOILS™ database

Fig. 17 Average tricyclicterpane ratios of Zubair-reservoired oils from SouthernIraq suggesting a carbonatesource rock. Other data pointsrepresent average oil valuesfrom 150 global petroleumsystems from marine carbonate,distal marine shale, marine marl,and lacustrine shale source rocksfrom GeoMark ResearchOILS™ database

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Conclusions

Zubair Formation is the boundary layer of transitionalimmature organic matters. Of its oil, 60% have beengenerated in the Zubair Oil Field while little oils weregenerated from younger strata and 100% oil generationpossessed from older strata. Oil generated from ZubairFormation is mixed with the Upper Jurassic oil and bothhave charged the sand facies stratigraphic traps as well asfold structure traps of the Zubair Formation itself. In thenorthwest of Zubair Oil Field, toward Mesopotamian Basinboundary with the Zagross Fold Belt, Zubair Formationhave higher maturation in Nahr Umr oil field and hence75% hydrocarbon generation of its efficiency had beenpossessed while farther toward northeast 100% of itsefficiency could be generated in Majnoon oil field whichcould be due to increased thermal maturity nearer to thesuture of Zagross Fold Belt. On the other hand, in the westof Zubair Oil Field toward the western side of theEuphrates River in the Kifle Oil Field, the ZubairFormation become immature with very low hydrocarbongeneration (TR=0.01-12).

Acknowledgments PetroMod software basin modelings are per-formed in the USGS and the geochemical analysis of the crude oiland source rocks are analyzed in Geomark Research Ltd ofHouston, Texas. To both we give our sincere acknowledgementsfor their help.

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