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Arab J Geosci
DOI 10.1007/s12517-010-0160-z

ORIGINAL PAPER

Programed oil generation of the Zubair Formation, Southern
Iraq oil fields: results from Petromod software modeling
and 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 performed
on wells in each of four oil fields in South Iraq, Zubair
(well Zb-47), Nahr Umr (well NR-9), West Qurna (well
WQ-15 and 23), and Majnoon (well Mj-8). In each of these
fields, deposition of the Zubair Formation was followed by
continuous burial, reaching maximum temperatures of
100°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 WQ15, 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 in
the late Tertiary, 10 million years ago. At present day,
modeled transformation ratios (TR) indicate that 65% TR of
its generation potential has been reached in well Zb-47,
75% TR in well NR-9 and 55-85% TR in West Qurna oil
field (wells WQ-15 and WQ-23) and up to 95% TR in well
Mj-8, In contrast, younger source rocks are immature to
early mature (<20% TR), whereas older source rocks are
T. K. Al-Ameri (*) : M. E. Naser : H. A. Al-Haydari
Department of Geology, University of Baghdad,
Jadiriyah, Iraq
e-mail: thamer_alameri@yahoo.com
J. Pitman
U.S. Geological Survey,
MS 939,
Denver, CO 80218, USA
J. Zumberge
Geomark Research Ltd,
9748 Whithorn Drive,
Houston, TX 77095, USA

mature to overmature (100% TR). Comparison of these
basin modeling results, in Basrah region, are performed
with Kifle oil field in Hilla region of western Euphrates
River whereas the Zubair Formation is immature within
temperature range of 65–70°C (0.50%Ro equivalent) with
up to 12% (TR=12%) hydrocarbon generation efficiency
and hence poor generation could be assessed in this last
location. The Zubair Formation was deposited in a deltaic
environment and consists of interbedded shales and porous
and permeable sandstones. In Basrah region, the shales
have total organic carbon of 0.5–7.0 wt%, Tmax 430–
470°C and hydrogen indices of up to 466 with S2=0.4–9.4
of kerogen type II & III and petroleum potential of 0.4–9.98
of good hydrocarbon generation, which is consistent with
55–95% hydrocarbon efficiency. These generated hydrocarbons had charged (in part) the Cretaceous and Tertiary
reservoirs, especially the Zubair Formation itself, in the
traps formed by Alpine collision that closed the Tethys
Ocean between Arabian and Euracian Plates and developed
folds in Mesopotamian Basin 15–10 million years ago.
These traps are mainly stratigraphic facies of sandstones
with the shale that formed during the deposition of the
Zubair Formation in transgression and regression phases
within the main structural folds of the Zubair, Nahr Umr,
West Qurna and Majnoon Oil fields. Oil biomarkers of the
Zubair Formation Reservoirs are showing source affinity
with mixed oil from the Upper Jurassic and Lower
Cretaceous strata, including Zubair Formation organic
matters, based on presentation of GC and GC-MS results
on diagrams of global petroleum systems.

Arab J Geosci

Keywords PetroMod software . Zubair Formation .
South Iraq . Oil generation . Accumulation . Biomarkers

Introduction
Zubair Formation is the most important formation of the
Lower Cretaceous sequence in Iraq (Al-Sayyab, 1989). It
has widespread in the Arabian Gulf region, Syria, and Iran.
It was introduced to designate the prevalently terrigenous
clastics and oil-bearing sequences of the southern Iraqi
fields (Buday 1980).
This formation consists of alternating course to fine
grained sandstone, siltstone, and dark gray shales (Bellen
R.C. Van et al. 1959) in sequence of repeated units; they
range in thickness from 301 m in well NR-9 to 381 m in
well Zb-47 of transgression and regression within deltaic
environment (Ali and Nasser 1989). The formation
extends from the Tkrit-East Baghdad field in central Iraq
toward the south near the Iraqi-Kuwait border and
continued through the Arabian Gulf in Iraq southern part
of Mesopotamian zone ; it may reach >500 m in the area
south of Baghdad and is thinning eastward toward the
Zagross Fold Belt and westward toward the Euphrates
River. Age of the formation, as determined on the basis of
both fossils and regional correlation, is Hauterivian till
Early Aptian (Bellen R.C. Van et al. 1959), while
palynomorphs evidences extended this formation up to
earliest Albian age (Al-Ameri and Batten 1997) with
contacts of mostly gradational and conformable. The
underlying formation is the Ratawi Formation which
consists of dark, slightly pyretic shales interbedded with
pseudo-oolitic detrital limestone (Bellen R.C. Van et al.
1959); and this is overlain by Shuaiba Formation which
consists of dolomitic limestone.
The petrophysical characteristic of porosity is ranging
from 19% to 28% and permeability from 80 to 2,500 md.
The Zubair Formation is main reservoir in most south oil
fields and produced oil and gas from gross thickness of
(150–250)m and a net thickness of (90–170)m. The
formation produced oil from the Abu Khaimah and Kifl
discoveries and the Luhais, Majnoon, West Qurna,
Ratawi, Siba, and Tuba fields. Oil and gas are produced
in commercial quantities from East Baghdad, Nahr Umer,
Wesr Qurna, Luhais, Rumaila North and South, and
Zubair Oil Fields (Al-Gailani 1996 and Al-Sharhan et al.
1997). The hydrocarbon pays of Zubair Formation
displayed in four pay zones (Fig. 1), their American
Petroleum Institute (API) gravity value is ranging from
16º to 39º (average 34º) and a sulfur content of 1.5-2.18%
(average 1.8). The API varies from an average of around
34º in the western part of Mesopotamia to around 16-27º
in the east.

The studied area composes geographically the oil fields
of Zubair, Nahr Umr, Majnoon, and West Qurna. It extends
between 30.00–31.10 latitude and 47.10–47.55 longitudes
as shown in Fig. 2. The Zubair Formation in this area was
deposited in Mesopotamian basin of geographically facies
distribution of southern part in Iraq. It is part of the major
Total Petroleum System of Cretaceous-Tertiary in South
Iraq (Pitman et al. 2004 and Al-Ameri et al. 2009) that
extend between the evaporates Upper Jurassic Gotnia
Anhydrite Formation as the lower major seal and the
evaporates Lower Fars Formation as the upper major seal
(Fig. 3).
A source for the oil accumulated in the Zubair Formation
is not definitely assigned, especially those in the stratigraphic traps, and hence this study is aimed to find the oilsource relation and their system in Zubair Formation.
Generation and their accumulation by PetroMod software
basin modeling, organic geochemical analysis and rock thin
sections will be the tools for this study.

Materials and methodology
Cores and cuttings samples were collected from Zubair
Formation and its suggested hard data source rocks from
eight wells (Fig. 2) of five oil fields in South Iraq (Basrah
region): they are six samples from Zb-40, ten samples from
Zb-43, and five samples from Zb-47 of Zubair Oil Field, six
saples from NR-9 of Nahr Umer Oil Field, seven samples
from W.Q-15, eight samples of WQ-1 of West Qurna Oil
Field, four samples from Mj-8 of Majnoon oil field, 29
samples from R-167, six samples from R-26 and four samples
from R-172 of Rumaila North Oil Field, four samples from
Ru-1 of Rumaila South Oil Field as well as four core samples
from the same formation in well AA-1 of Abo Amood oil field
in central Mesopotamian Region and five samples each from
wells Kf-1 in Kifle oil field and Ns-1 in Nasiriyah oil field
along Euphrates River region. Two oil samples are collected
from Zubair Formation of wells Zb-5 depth (2,200 m) and Zb10 depth (2,200.5 m) of the Zubair oil field.
These samples are subjected for organic geochemical
analysis with pyrplysis for source rocks evaluations and
with gas chromatography/mass spectrometry for oil analysis and biomarkers in Geomark Research Ltd in Houston,
Texas. Microscopy is aided by palynological slides that are
prepared by palynological preparations of these rock
samples 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 programmed
by 1D PetroMod software basin modeling for the above
mentioned wells in the US Geological Survey in Denver,
Colorado.

Arab J Geosci

Fig. 1 Major oil pays of the Lower Cretaceous Zubair Reservoir of South Iraq in depth successions with logs of lithologgy, resistivity (converted to
permeability) 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

1D petromod software basin modeling
PetroMod software technology is a finite-element basin
simulator that describes thermal histories, source rock
maturity, and petroleum generation and migration. The
interpreted well logs served as input to the models.
They could be processed in one, two, and three
dimensions (1D, 2D, 3D, respectively). It is integrated
exploration systems (IES 2007) for evaluating petroleum
systems.
The model is a numerical formulation of the region
history based on interpretation of combined geologic and
geochemical data in a temporal framework, and it includes
the principal elements (source rock, reservoir rocks, and
seals) of the total petroleum system.
PetroMod 1D is a newly developed and fully
integrated component of 'IES' PetroMod modeling
system. 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 or
to be extracted directly from PetroMod 2D and 3D
models. Calibration results such as heat flow histories
can then be directly used by the 2D and 3D simulators,
which enables calibration work in all packages to be
performed much faster. Data for 1D PetroMod for this
study, taken well Zb-47 as example, are displayed in
Table 1.

Model development The chronostratigraphic units in the 1D
model were assigned absolute ages of deposition and
erosion. The ages of depositional and erosional events were
designated based on the geologic time scale of Sharland et
al. (2001). Lithologies represented as end-member rock
types or as compositional mixtures of rock types were
assigned to each unit using software default parameters
(Pitman et al. 2004).
Determining the timing of petroleum generation and
expulsion required calibration of the thermal regime at each
model location. Parameters used in the calibration included
heat flow , thermal conductivity of the rock matrix , surface
temperature, and sediment thickness (present and past) of
these factors, heat flow is the least constrained parameter
(Pitman et al. 2004). Type IIS kerogen kinetics are used for
the Jurassic and Cretaceous Formations because extracts of
the kerogen of the Jurassic and Lower Cretaceous source
rocks have considerable amount of Sulfur (nitrogen,
sulphur, and oxygen; NSO=1-34%).
Reservoir and bottom-hole temperature data plus the
other parameters mentioned earlier in this paragraph for
four wells (Zb-47, W.Q-15 NR-9 and Mj-8) are used in the
software data to estimate the present heat flow in the study
area as shown in Fig. 4. This integrates the effects of
temperature over time and is used to evaluate the
paleothermal history of the four well locations (Zb-47, W.
Q-15 NR-9 and Mj-8). Well Kifle-3 is taken for comparison

Arab J Geosci

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

with western location in the west of Euphrates River, Hilla
Region.
Calibration of the paleothermal regime required matching measure Ro values for each well with values of Ro
calculated 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-47
as an example.
Modeled Ro depth trends constructed for the paleothermal analysis were used to estimate the amount of strata
eroded in the model area. In each of the studied wells, the
Ro trend that best fit the data was extrapolated at an Ro

Arab J Geosci
Fig. 3 Stratigraphic column of
South Iraq, Basrah region with
hydrocarbon generation parameters and seal within a total
petroleum system of this field

value of 0.25%, which was assumed to approximate the
vitrinite reflectance at surface conditions (Fig. 4) for Zb47 as an example. Computed Ro trends for the studied
well in South Iraq intersect the present-day surface at
0.25% Ro , indicating that erosion during late Cenozoic
time generally was minimal (less than 500 m) in this
region.
The amount of stratigraphic section removed during
Mesozoic erosional episodes has not been reported;
however, sensitivity tests indicate that pre-tertiary erosion,

although locally intense, had little impact on the Jurassic
source rock maturation history.
Model analysis One dimensional burial-thermal profiles as
shown in Fig. 4 for well (Zb-47, as example) demonstrate
the effects of continuous burial had been affected on
source rock of the Zubair Formation. Zubair Formation
source rocks are presently at maximum burial temperature 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

2,317.0
2,381.5
2,560.0
2,648.0
2,818.0
2,959.0
3,270.7
3,344.0
3,725.0
3,872.0
4,258.0
4,518.0

2,284.4
2,317.0
2,381.5
2,560.0
2,648.0
2,818.0
2,959.0
3,270.7
3,344.0
3,725.0
3,872.0
4,258.0
4,518.0
Depth (m)
3,344.0

Well name
ZB-47
Surface temp ∼20°C

Sulaiy
Temperature value
96.9

292.0
472.0
624.0
849.0
990.0
1,440.0
1,655.0
1,794.0
1,959.0
2,284.4

0.0
292.0
472.0
624.0
849.0
990.0
1,440.0
1,655.0
1,794.0
1,959.0

Dibddiba
Fatha_L. Fars
Ghar

Dammam
Rus
Umm Er. Rad.
Tayarat
Shiranish
Hartha
Sadi
Tanuma
Khasib
Mishrif
Rumaila
Ahmadi
Mauddud
Hahr Umr
Shuaiba
Zubair
Ratawi
Yamama

Base
(m)

Top
(m)

Name

Formation

Formation

Formation

Unit (°C)
C

89.0
94.0
95.0
97.0
99.0
112.0
117.0
121.0
127.0
137.0
141.0
144.0

10.2
17.0
25.2
49.0
52.0
58.0
66.0
69.0
82.0
84.0

From
(Ma)

Min (°C)
86.9

84.0
89.0
94.0
95.0
97.0
99.0
112.0
117.0
121.0
127.0
137.0
141.0

5.0
10.2
17.0
34.0
49.0
52.0
63.0
66.0
69.0
82.0

To
(Ma)

Formation age

25.2

58.0

34.0

63.0

Max (°C)
106.9

0.0

To
(Ma)

5.0

From
(Ma)

Erosion age

Table 1 Example of Excell Data Input for well Zb-47 in Zubair oil field in South Iraq

SHALE
LIMEmarly
LIMESTONE
LIMEShaly
SHALE
LIMESTONE
SAND&SHAL
LIMESTONE
SAND&SHAL
LIMESTONE
LIMEdolom
LIMEshaly

SANDcongl
LIME&EVAP
SANDcongl
LIMEdolom
LIME&EVAP
LIME&EVAP
LIME&EVAP
LIMEmarly
LIMEmarly
LIMESTONE

Lithology

Formation

5

5
2
2
2
2

Source
Source
Source
Source
Source

Organic carbon
Contents (%)

Total

Source

System
Element

Petroleum

Phosphoria_HP

Phosphoria_HP
Phosphoria_HP

Phosphoria_HP

Phosphoria_HP

Phosphoria_HP

Kinetics

Source rock

600

600
600

600

600

600

Hydrogen index
mgg TOC

Initial

Arab J Geosci

Arab J Geosci

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

3,391 m for well Mj-8 with equivalent vitrinite reflectance of 0.8-0.9 %Ro that lie in the oil window for
Zubair Formation according presentation on van Krevlen
diagram of Tissot and Welte, (1984). On the other hand,
the burial temperature of well Kifle-3 is 60-70°C at depth
of 1,969 m with equivalent vitrinite reflectance of 0.3%Ro
that lie in the immature zone.
Transformation ratio (TR) could be defined (Following Lewan and Ruble 2002 and Peter et al. 2005) as the
difference between the original hydrocarbon potential of a
sample before maturation and the measured hydrocarbon
potential divided by the original hydrocarbon potential or
could be measured by Bitumen/total organic carbon
(TOC), the value could range from 0 to 1.0 or cold be
converted to percentages. Accordingly, timing of oil
generation for each of the proposed source rocks unit in
the studied wells of the studied oil fields could be
estimated from the transformation ratio diagram and hence
clarified the following generations.
1. Zubair Oil Field, well (Zb-47): at Zubair Formation, TR
curve 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 till
present time at temperature ∼100°C as illustrated in
(Fig. 5) which could reflect 65% of its oil had been
generated.
The mature formations bellow the Zubair Formation (the
Lower Cretaceous Ratawi Formation) showed that (Fig. 5)
the oil generation had increased up to 100% in the Late
Neogene to present time at temperature ∼100-110°C
(equivalent to 0.70%Ro); while the mature formations
under the Ratawi Formation (Upper Jurassic Sulaiy Formation) have oil generation from TR >1% in the Cretaceous at
temperature ∼80-90°C (equivalent to 0.60%Ro) and
reached TR >100% in the Late Neogene at temperature
110-120°C (equivalent to 0.75%Ro). Accordingly, the
generated hydrocarbons (till present) from these formations
are 95% in Ratawi Formation, 100% in Sulaiy Formation
accomplished within 20 million years ago. The overlying
formations (Nahr Umr, Maudud, Ahmadi) are immature as
shown in Fig. 5 of bellow 60°C (equivalent to 0.40%Ro)
thermal burial with very few hydrocarbon (less than 20%

Arab J Geosci

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

TR) generation and hence not considered for the whole
generation.
2. West Qurna Oil Field, well (W.Q-15): at Zubair
Formation, the oil generation start with TR >1% in
the Middle Paleogene at temperature ∼70-90°C and
reached TR >50% at temperature ∼90-100°C in the
Late 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 oil
generation ∼10%; therefore it is neglected, while the mature
formations under the Zubair Formation like the Lower
Cretaceous (Ratawi Formation) had oil generation TR >
80% at temperature ∼100-120°C in the Late Neogene to
present time. The Jurassic Formations of Sulaiy, Najmah,
Naokelekan, and Sargelu have reached the oil generation of
TR >100% at temperature ∼100-120°C in the Late
Paleogene.

3. Nahr Umr Oil Field, well (NR-9): at Zubair Formation,
the oil generation start with TR >1% in the Early
Palaeogene 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 (the
Middle Cretaceous Ahmadi and Nahr Umr Formations) had
very few oil generation vary between 4% and 25%; therefore,
its generation consider negligible while the mature formations
bellow Zubair Formation (the Lower Cretaceous Ratawi
Formation) had oil generation reached TR >100% in the Late
Neogene at temperature ∼100-120°C.
4. Majnoon Oil Field, well (Mj-8): at Zubair Formation,
the oil generation start with TR >10% in the Early
Paleogene at temperature 70-80°C, accelerated its
generation 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.

Arab J Geosci

The overlying formations are transitional immature and
hence generate less oil while the underlying formations of
the Jurassic and Lower Cretaceous Formations are mature
and hence have generated oil of TR >100.
5. Kifle Oil Field, well Kifle-3: at Zubair Formation, no
oil 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 of
well Kifle-3 in shallow depths (1,968-2,407 m) with
burial thermal temperature of 60-70°C
Source rock maturation and petroleum generation Oil TR
and the temperatures of petroleum generation were modeled
for Zubair, Nahr Umr, West Qurna, and Majnoon Oil Fields by
(Zb-47, W.Q-15, MJ-8, and NR-9) wells to simulate the
temperature and timing of major petroleum events (generation
onset and completion), and the extent of generation at these
locations as shown in (Fig. 6) for well Zb-47 as an example.
Vitrinite reflectance curves depicting thermal stress and
shown in Fig. 7 for comparison. TR curves represent the
fraction of petroleum that was generated at a given moment
in geologic time. Temperatures during petroleum generation
were modeled with integrated heat flow, matrix conductivity, and decompacted thickness calculations.
Accordingly and on the basis of these results, the
generated 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
Fig. 6 Hydrocarbon transformation ratios of Zubair Oil
Field, well Zb-47

Qurna Oil Field of well W.Q.-15. The Zubair Formation in
Majnoon oil field is showing higher maturation than other
mentioned fields and has oil generation of 90% in well MJ8 and 100% in well MJ-19 (Fig. 7). This could indicate
rising maturation of the Zubair Formation up in the
Majnoon oil field to the 100% transformation ratios
(Fig. 8) compared to 60% transformation ratios in Zubair
oil field (Fig. 6).
The comparison indicates that the Zubair formation in
the Kifle Oil Field in Hilla region in Western Euphrates
River is of shallower depths (1,968-2,407 m) in well Kile-3
than it is in the Basrah Region. It has less burial thermal
temperatures (60-70°C) of Ro 0.45-0.52 equivalent and
much less generated hydrocarbons of 0.1-12% TR.
Modelled hydrocarbon generations These modeling informed us that Zubair Formation have started its main oil
generation 10 million years ago with 50% hydrocarbon
transformation in Zubair oil field while deeper formations
of Sargelu, Naokelekan, Najmah, and Sulaiy have generated their oil during the paleogene time (65-25 million years
ago). The overlying strata such as Nahr Umr and Ahmedi
Formations have little or no generation in Zubair oil field.
Accordingly, and as illustrated in Figs. 5 and 6, the
hydrocarbon generations in this total petroleum system
could be modeled within middle area of Mesopotamian
Basin into three phases of the following:The first phase was 100% transformation ratios to hydrocarbons obtained from Jurassic Formations of Sargelu,

Arab J Geosci

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

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

Arab J Geosci

Najmah, and Sulaiy within temperature effect of 120140°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-anoxic
environment of marine carbonate that has total organic
carbon of 0.5-8.0 wt% (Al-Ameri et al. 1999 and 2009)
which confirm the hydrocarbon generation. The trap

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

formation is taken from the abrupt subsidence line in
burial thermal history during the Upper Cetaceous time at
65-60 million years ago (Fig. 4), which conform to the
Global Cretaceous/Tertiary Event of its indirect effect on
the Arabian Plate. The mechanism of its development is
basement movements in pre-existed trends as response to
thick sediment accumulations in the Mesopotamian Basin

Arab J Geosci

Fig. 10 Dinoflagellate cysts recovered from the Zubair Formation

Arab J Geosci

and the global cratonic reaction to plate collision (AlSharhan et al. 1997 and Sharland et al. 2001)
Accordingly, these traps are mainly structural with
some syndepositional traps of stratigraphic nature. These
generated hydrocarbons have charged the already formed
Cretaceous structural traps such as Mishrif Formation
Reservoir by the event discussed above and the stratigraphic traps such as the Sulaiy, Ratawi, and Zubair
Formations that formed by trasgressions and regressions
during their deposition time of Lower Cretaceous (AlAmeri and Batten 1997).
The second phase was the petroleum generation from
Lower Cretaceous Ratawi and Zubair Formations. At
present day, modeled transformation ratios indicate that
53-90% for Zubair Formation and 85-100% for Ratawi
Formation had occurred within temperature effect of 100120°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 be
based on abrupt change in the thermal subsidence line
(Fig. 4) during the Miocene time at 20-10 million years ago
and which conform with the closing phase of the Late
Alpine Orogeny effect on northeast Arabian Plate (AlSharhan et al. 1997 and Sharland et al. 2001). Accordingly,
the oil of the second phase of generation has been
distributed within the whole petroleum system of the
Cretaceous and Tertiary between the two major seals of
Upper Jurassic Kimmeridgian Gotnia Anhydrites Formation
and the Middle Miocene Lower Fars Anhydrites Formation.
This late dynamicity of the generated hydrocarbons might
had charged the already formed stratigraphic traps of
sandstone facies embedded in shales that formed during
the deposition of mainly Zubair Formation in transgression
and regression phases .
The third phase: younger non-efficient source rocks of
Nahr Umr, Maudud, and Ahmadi Formations are immature
to early mature (0.7%Ro) and hence less than 10%
hydrocarbons were generated (Figs. 5 and 6) while
overlying formations up to the present sediments are
immature with no hydrocarbon generation. Accordingly,
negligible consideration is given to their quantity generated
with respect to the formations bellow them.

Table 2 List of spores and pollen grains, and dinoflagellate cysts
recorded 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

Hydrocarbon potential

Circulodinium distinctum (Deflandre and Cookson) Jansonius 1986
Cleistosphaeridium? aciculare Davey 1969

Kerogen analysis for the hydrocarbon generation and oil
analysis for their biomarkers to find oil-source correlation
could form the base for confirming the PetroMod software
Basin modeling of the Zubair Formation in South Iraq.
The generation Assessment of hydrocarbon generation and
timing of the Zubair Formation could be confirmed by

Cribroperidinium auctificum (Brideaux) Stover and Evitt 1978
Cribroperidinium orthoceras (Eisenack) Davey 1969
Oligosphaeridium pulcherrimum (Deflandre and Cookson) Davey and Williams 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

Arab J Geosci

fossil palynomorphs (spores, pollen, and dinoflagellate)
identifications for dating the strata, palynofacies concept for
optical views of the total organic matters for finding
environment of the organic matters accumulations and
pyrolysis technique for chemical analysis for finding the
hydrocarbon generation.
Palynomorphs assemblages of mainly the spores,
pollen, and dinoflagellate cysts but no bisacate (Figs. 9
and 10) are recorded from the Zubair Formation and are
listed in Table 2 along with Botryococcus, foraminiferal
test lining, and fungal remains that are potential for
generating hydrocarbons.
The relative abundance of triradiate spores is consistent
with a warm climate, as implied by the latitude of southern
Iraq during the Early Cretaceous (within 5º either side of
the equator). It is not possible, however, to conclude from
the assemblages whether most of the plants from which
they were derived lived in generally damp conditions on,
and in the vicinity of, a delta because many fern taxa
occupy dry habitats (see, e.g., Tryon and Tryon 1982). On
the other hand, representative of plants that are widely
accepted as indicating dry conditions (e.g., Ephedripites)

are less common. In general, the dinoflagellate cyst
associations compare well with other described from the
Tethyan province; in particular from Libya (Uwins and
Batten 1988) and offshore Morocco (Ogg 1994).
For purpose of age determination, the palynomorphs
recovered are considered to comprise a single assemblage because no significance difference in composition
were recorded through the section examined (Al-Ameri
and Batten 1997). Comparison were made between
occurrences of the taxa encountered and those recorded
in the literature from widely scattered elsewhere so that
the dating of the assemblages could be put into a global
perspective. In addition to the publications already
mentioned, this literature include papers by Batten and
Li Wenben (1987), Brideux and McIntyre (1975), Dettman
(1986), Helby et al. (1987), Herngreen and Chlonova
(1982), Kotova (1978), Milliod et al. (1974) and Williams
and Brideaux (1975).
The published ranges of all the taxa listed on Table 2
impinge on Barremian-Aptian or up to Albian stage.
Bearing in mind that the first and last occurrences of the
majority 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

Arab J Geosci

geographical region, on the evidence currently erected, the
Zubair Formation cold be most appropriately dated as
Barremian-Early Albian. The lack of elaters-bearing palynomorphs, and tricolpate and tricolporate pollen, is taken to
imply that the assemblage is older than late Albian,
although it is recognized that the absence of elaterate forms
could be for other reasons, such as a climate that was
unsuitable for the parent plants.
Palynofacies could be based on Al-Ameri and Batten
(1997) who have recorded, by palynological studies for
Rumaila North Oil Field (well R-26) Zubair Oil Field (well
Zb-43) and West Qurna Oil Field (WQ-1), four palynofacies types that may infer to indicate swamp and marsh
environments on delta top for PF1, and platform conditions
of delta front for PF2, prodelta for PF3, and open marine
for PF4, respectively (Fig. 11). Other wells of this study are
recorded by palynological analysis to have been undergoing
the same environmental laws within similar palynofacies
types. Swamps and marsh as near to shore line of early
Cretaceous prevailed in the western Iraqi Desert and Saudi
Arabian region and open marine conditions dominated in
easternmost Iraq and Iran, with more varied deltaic

environments in between, within Iraq and Kuwait. The
repetition of the palynofacies types reflect transgressive and
regressive phases during the deposition of the detrital
sediments of the Zubair Formation and hence formed
Barrier Island which lead to reducing circulation. Accordingly, anoxic conditions leading to the preservation of
abundant organic matters along with the palynomorphs
were created.
Rocks of Zubair Formation are deposited in subsiding
deltaic deposits of favorable conditions for the preservation
of the accumulated organic matter, albeit mostly in a
biodegraded state (Al-Ameri and Batten 1997). The TOC
content is between 0.5% and 6% for most of the samples
analyzed. Biodegradation and thermal alteration of the
organic matter led in particular to the abundant amorphous
organic matters in PF2 and PF3 (Fig. 12) with abundant
dinoflagellate cysts (15-30% of the total palynomorphs) and
fluorescent alginate material derived from algae of the
genus Botryococcus as well as foraminiferal linings, fungal
remains, and resin that indicate highly oil prone (following
Tyson 1995). In the upper part of the formation, these two
palynofacies 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 palynofacies
demonstrating lower part of the formation (1, 3, 5, and 7) and upper part of the formation (2, 4, 6, and 8)

Arab J Geosci
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

3.18
0.34
0.72
0.87
1.69
9.78
0.35
1.61

0.24
0.20
0.20
0.08
0.08
0.92
0.11
0.06

0
11
0
0
0
0
0
0

416
142
101
89.9
198.7
67.8
66
115.4

436
432
432
435
435
435
434
436

0
0.02
0
0
0
0
0
0

2.41
0.27
0.57
0.8
1.55
0.73
0.31
1.5

0.77
0.07
0.15
0.07
0.14
9.05
0.04
0.11

0.93
0.59
0.5
9.98
1.53
0.71
0.59
0.93
24.81
4.62
2.67

0.07
0.17
0.34
0.05
0.06
0.10
0.08
0.06
0.37
0.15
0.46

0
316
327
62
37
92
79
60
16
73
54

57.7
153
100
364
148
121
126
107
321
174
162

437
429
440
442
434
433
436
430
420
420
424

0
1.1
1.8
1.61
0.36
0.49
0.34
0.49
1.05
1.05
0.48

0.86
0.49
0.33
9.4
1.44
0.64
0.54
0.87
20.93
1.62
1.44

0.07
0.1
0.17
0.58
0.09
0.07
0.05
0.06
3.78
3.00
1.23

structures to chunky compact masses and gelatinous
appearance with palynomorph preservation into degraded
state. The composition of this material is similar to oil
prone kerogen Type A of Thompson and Dembiki (1986)
and is considered to be mature, the thermal alteration index
being 2.5 (Staplin 1969; see Batten 1996), and hence
capable of generating liquid hydrocarbons.
Chemistry of the Zubair Formation organic matters in
the Rumaila South, Rumaila North, Abo Amood, Nasiriyah
and Kifle oil fields (wells Ru-1, R167, R-172, Ns-1, AA-1,
and Kifle-3) is based on the results of pyrolysis analysis
techniques (Table 3). Analyzed samples are plotted on van
Krenlen diagram of hydrogen index (S2/TOC=mg HC/g
Corg) versus maximum temperature (Tmax °C) for hydrocarbon pulse in the pyrolysis devise (Tmax) according
Espitalie et al. (1980; Fig. 13). The positions of the intersect
points in the figure could indicate that Zubair Formation in
Basrah Region is a source rocks of kerogen type II and III
and of early mature organic matter content with 430-440°C
Tmax and hydrogen index of up to 450 mg HC/gm rocks
for the Rumaila North, Rumaila North, and Zubair oil
fields while Abo Amood oil field is transitional immature. The petroleum potential (mgHC/gRock) of Zubair
Formation in well Ru-1 vary between 0.34 and 3.18, and
reached 0.35–9.78 in well R-167, while it is between 0.5
and 9.98 in well R-172 and 0.59-1.53 in well AA-1 that
led to the petroleum potential assessment of fair source
rock in well Ru-1 and good source rock in wells R-167
and R-172 as shown in Fig. 14. Nasiriyah oil field has

S1

TOC

Sample type

Depth (m)

Well name

No.

0.58
0.19
0.56
0.89
0.78
1.09
0.47
1.3

Core
Core
Cutting
Cutting
Cutting
Cutting
Cutting
Cutting

3,010
3,230
3,388
3,425
3,457
3,507
3,555
3,655

Ru-1
Ru-1
R167
R167
R167
R167
R167
R167

1
2
3
4
5
6
7
8

1.49
0.32
0.33
2.58
0,97
0.53
0.43
0.81
6.52
0.93
0.89

Cutting
Cutting
Cutting
Cutting
Core
Core
Core
Core
Core
Core
Core

3,730
3,100
3,220
3,301
3,360
3,590
3,634
3,682
2,615
2,880
1,974

R167
R/172
R/172
R/172
AA-1
AA-1
AA-1
AA-1
Ns-1
Ns-1
Kf-3

9
10
11
12
13
14
15
16
17
18
19

Fig. 13 Hydrocarbon generation and kerogen type of the Zubair
Formation source rocks based on pyrolysis values plotted on Van
Krevlen diagram in wells of selected oil fields

Arab J Geosci

The Zubair Formation organic matters in the Kifle Oil
Field (Kifle-3) is of immature (Tmax=420-427), low
hydrogen index of 161 mgHC/gRocks (Fig. 13), and of
2.7 mgHC/gRocks and hence of poor petroleum potential
(Fig. 14). This result show negative correlation with Basrah
oil fields and confirming the transformation ratio difference
by 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 mainly
aromatic hydrocarbons on the basis of C15† with 43-47
aromatic, 37-40 saturate, 11-14 NSO and 2-6 asphalt. Plotting
the analysis result from gas chromatography (GC) and GCmass spectrometry of wells of the Zubair and Luhais oil fields
(Table 4) on diagrammatic presentation of global standard
parameters suggested by Peter et al. (2005) and Zumberge et
al. (2005) and compared with Geomak Oils™ database could
assess the following oil-source relations:
Pristane-nC17 Versus Phytan-nC18 The pristane versus
phytane (Pr/Ph) plot can be used to infer oxidisation and
organic matter type in the source rock depositional environment
(Hunt 1997 and Peter et al. 1999). In Fig. 15, the Zubair
Formation oils are plotted in the area of algal marine, reducing
and type II kerogen of mature and low biodegradation. Both
Pr-nC17 and Ph-nC18 decreases with thermal maturity of oil
and increase with degree of biodegradation. The Pr/Ph ratios
of the Zubair oils vary between 0.72 and 0.73 and hence
could indicates anoxic source rock deposition by comparison
with the less than 1.0 of Peter et al. (2005), while Pr/Ph >1
indicates that the oil is rich in lipids and waxes organic matter.

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

high petroleum potential of 4.62-24 mgHC/gRock that
indicate good petroleum potential. Plotting the studied
samples in production index versus Tmax diagram are
showing locations in the oil zone too.

Sterane triangle Plots of gas chromatography/mass spectrometry of the analyzed oil on triangle of C27-C28-C29 ratios
are used to interpret depositional environment and type of
source rock and to classify crude oil into groups (Peter et al.
2005). Accordingly, the resultant diagram (Fig. 16) is
showing that the Zubair Formation oil samples are in the
area 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

Table 4 Analysis chart of GC and GC-MS oil biomarkers data for Lower Cretaceous Zubair Formations in the studied oil producing wells in
Basrah, South Iraq
C29%
24.8
27.0
28.0
41.8
40.4

C28% Sterane

C27%

Ph/nC18

Pr/nC17

C24/C23 Terpane

C22/C21 Terpane

34.1
34.0
34.0
24.7
25.8

42.1
39.0
38.0
33.5
33.9

0.35
0.31
0.33
0.35
0.36

0.21
0.21
0.21
0.23
0.23

0.28
0.24
0.23
0.31
0.39

1.0
1.1
1.1
1.09
0.97

Biomarker
Lu-1
Zb-16
Zb-163
Zb-10
Zb-5

Arab J Geosci
Fig. 15 Pristane-n C17 versus
Phytane-n C18 plot diagram
(following Peter et al. 2005)
showing marine algal organic
matters of mainly kerogen type
2, reducing paleoenvironments
and mature source of the Zubair
oil for the studied wells in South
Iraq

Zumberge et al. (2005) could assess source lithology by
comparison with global oil fields data. Accordingly, the plots
of the analyzed samples of Zubair Formation (Fig. 17) are
lying in carbonate source rocks area with some shale and marl.
Fig. 16 Sterane triangleof C27,
C28, and C29 (following Peter
et al. 2005) showing marine oil
from carbonate with some shales
for Zubair Formation oil of the
studied wells in South Iraq

In the same scenario, plotting analyzed samples from
Zubair Formation on diagram of hopane (C31R/H) versus
tricyclic terpanes (C26/C25) could assess carbonate and
shale source rocks too.

Arab J Geosci
Fig. 17 Average tricyclic
terpane ratios of Zubairreservoired 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

Oleanane/hopane and source rock age plot The oleanane
is terpanes derivatives with origin from angiosperm
plants (Grantham et al. 1983 in Peter et al. 2005) and
hence could be used to interpret the geologic age of source
rock by the phenomena of increasing angiosperm diversity
from the Upper Cretaceous to present. Its absence does not
mean the crude oil generate in Lower Cretaceous and older
source rock. The hyper saline water could helps to save and
protect the oleanane. The Ol/H ratio vary from 0 to 0.01 in
Zubair Formation crude oil samples (Fig. 18) and hence could
Fig. 18 Calculated average
C28/C29 sterane ratio (based on
both regular steranes and
triaromatic steranes) of Mishrifreservoired oils from Southern
Iraq suggesting a source rock of
Jurassic age. 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

assess Upper Jurassic-Lower Cretaceous (including Zubair
Formation 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 to
indicate Lower Cretaceous age influence from the source
rocks for the oil accumulated in the Zubair Formation
Reservoir by comparison with global Geomark Oils™
database. This could confirm source for the oil accumulated
in Zubair Formation is to be from Lower Cretaceous (mainly
Zubair Formation itself) as well as Upper Jurassic carbonates.

Arab J Geosci

Conclusions
Zubair Formation is the boundary layer of transitional
immature organic matters. Of its oil, 60% have been
generated in the Zubair Oil Field while little oils were
generated from younger strata and 100% oil generation
possessed from older strata. Oil generated from Zubair
Formation is mixed with the Upper Jurassic oil and both
have charged the sand facies stratigraphic traps as well as
fold structure traps of the Zubair Formation itself. In the
northwest of Zubair Oil Field, toward Mesopotamian Basin
boundary with the Zagross Fold Belt, Zubair Formation
have higher maturation in Nahr Umr oil field and hence
75% hydrocarbon generation of its efficiency had been
possessed while farther toward northeast 100% of its
efficiency could be generated in Majnoon oil field which
could be due to increased thermal maturity nearer to the
suture of Zagross Fold Belt. On the other hand, in the west
of Zubair Oil Field toward the western side of the
Euphrates River in the Kifle Oil Field, the Zubair
Formation become immature with very low hydrocarbon
generation (TR=0.01-12).

Acknowledgments PetroMod software basin modelings are performed in the USGS and the geochemical analysis of the crude oil
and source rocks are analyzed in Geomark Research Ltd of
Houston, Texas. To both we give our sincere acknowledgements
for their help.

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