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A GEOPHYSICAL STUDY OF SOME GEOLOGICAL STRUCTURES IN THE LOW FOLDED ZONE, NORTH IRAQ

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ABSTRUCT The study area, which is characterized by numerous surface and subsurface structures of NW–SE dominating trend, is located in the Low Folded Zone. The gravity responses of these structures are studied. Moreover, the study is further extended to include the vertical and horizontal variations in density derived from seismic reflection data of two seismic profiles a cross Jambur structure. Such variations may be helpful in clarifying the sources of some gravity anomalies and also in theoretical modeling. There are three aims of the present study; the first is to consider these responses, which reflect the effect on the gravity field due to the continuous compression of the Alpine stresses, taking into account that these stresses also have regional effects on the gravity field. Then, the total effects may be removed to obtain a new “corrected field”. The second is to estimate the maximum thickness of the sedimentary cover affected by the Alpine stresses, which might probably be observed in the gravity field “affected depth”, and the third aim is to calculate the gravity response of Jambur anticline from the available seismic data. The results show that the gravity field responses are represented on the Bouguer map as gravity highs, lows and high gradients. The gravity highs coincide perfectly with the surface anticlines; however, some gravity highs show subsurface extensions. The gravity highs are elongate and narrow ellipsoidal or nose-shaped, whereas their magnitudes range from 1.2 to 8.0 mGal. The gravity lows can be divided into two groups according to their “origin”; the first has good matching with the synclines, while the second may be related to depressions. The shapes of these lows are either elongated and ellipsoidal running parallel to the gravity highs or broad circular shaped. The gravity value of those lows is close to -2.0 mGal. Some gravity highs and lows are not reflected in the geologic map. The results also show that the maximum thickness of the sedimentary cover where the Alpine stresses may be observed in the gravity field is about seven kilometers. In addition, two dimensional models of Jambur anticline show that the structure has a residual positive gravity value ranging between 8.0 and 9.0 mGal.
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A GEOPHYSICAL STUDY OF SOME GEOLOGICAL STRUCTURES
IN THE LOW FOLDED ZONE, NORTH IRAQ
Hayder A. Al-Bahadily1
Received: 20/ 06/ 2013, Accepted: 09/ 07/ 2014
Key words: Gravity, Seismic reflection, Structures, Low Folded Zone, Iraq
ABSTRACT
The study area, which is characterized by numerous surface and subsurface structures of
NW SE dominating trend, is located in the Low Folded Zone of Iraq. The gravity responses
of these structures are studied. The study is further extended to include the vertical and
horizontal variations in density derived from seismic reflection data of two seismic profiles
across Jambur structure. Such variations may be helpful in clarifying the sources of some
gravity anomalies and also in theoretical modeling.
There are three aims of the present study; the first is to consider these responses, which
reflect the effect on the gravity field due to the continuous compression of the Alpine stresses,
taking into account that these stresses also have regional effects on the gravity field. Then, the
total effects may be removed to obtain a new “corrected field”. The second is to estimate the
maximum thickness of the sedimentary cover affected by the Alpine stresses, which might
probably be observed in the gravity field “affected depth”, and the third aim is to calculate the
gravity response of Jambur anticline from the available seismic data.
The results show that the gravity field responses are observed on the Bouguer map as
gravity highs, lows and high gradients. The gravity highs coincide perfectly with the surface
anticlines; however, some gravity highs show subsurface extensions. The gravity highs are
elongate and narrow ellipsoidal or nose-shaped, whereas their magnitudes range from 1.2 to
8.0 mGal. The gravity lows can be divided into two groups according to their “origin”; the
first has good matching with the synclines, while the second may be related to depressions.
The shapes of these lows are either elongated and ellipsoidal running parallel to the gravity
highs or broad circular shaped. The gravity value of those lows is close to 2.0 mGal. Some
gravity highs and lows are not reflected in the geologic map.
The results also show that the maximum thickness of the sedimentary cover where the
Alpine stresses may be observed in the gravity field is about seven kilometers. In addition,
two dimensional models of Jambur anticline show that the structure has a residual positive
gravity value ranging between 8.0 and 9.0 mGal.
_____________________________________________________________________________
1Assistant Chief Geophysicist, Iraq Geological Survey, P.O. Box: 986 Alwiya, Baghdad, Iraq
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
70
1,28,0 2,0
5,257,5
13
89
INTRODUCTION
The Low Folded Zone (LFZ) (or Foothill Zone) is located between the Mesopotamia
Foredeep from SW and the High Folded Zone from the NE. The southern boundary of the
zone runs parallel to the first topographic break of slope (step) made by first front of the
Zagros mountain range, whereas the northeastern boundary is taken along the second
topographic step made by series of relatively high shortening anticlines (Fouad, 2012) (Fig.1).
It has a complete sedimentary pile due to strongly subsiding since the opening of the southern
Neo-Tethys in the late Jurassic.
Consequently, the area contains three major oil bearing structures; Kirkuk, Bai Hassan
and Jambur anticlines, which awarded the area an economic importance. Gravity survey has
covered many parts of the study area since the earlier half of the last century. The gravity
survey was followed by aeromagnetic survey during the 1970s. Unfortunately, the northern
and northeastern parts of the study area were not covered neither by aeromagnetic nor gravity
surveys. However, the present study is an attempt to understand the influence of such
structures, which are built up through the of Alpine Orogeny, on the gravitational field.
Moreover, the information gained from seismic reflection data on Jambur area in 1976 by the
Companie General De Geophysique (CGG), which was not previously covered by a gravity
survey, may be useful in studying the gravity response of this structure and also contribute to
the enrichment of the geological knowledge of the area. The area of study is located between
43° 30' to 45° easting and 35° to 35° 45' northing (Figs.1 and 2).
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Fig.1: Map of northern Iraq, including the study area, where Nappe, Folded and Unfolded
Zones are recognized. The southern boundary of the LFZ is marked by series of Folds
(after Dunnigton, 1958 in Aqrawi et al., 2010)
Geological Setting
Tectonically, the study area lies in the northeastern part of the Arabian Platform. Fouad
(2010) divided the Platform into Inner and Outer Platforms, and accordingly, the LFZ is
located in the Outer Platform. He mentioned that the LFZ is an integral part of the Western
Zagros Fold Thrust belt. The Zone is bounded from the NE by the prominent mountain front
of the High Folded Zone, and from the SW by relatively flat terrains of Mesopotamian
Foredeep.
Structurally, the LFZ is characterized by structural trends and facies changes that are
parallel to the Zagros Taurus belts (dominantly NW SE or E W) (Jassim and Goff,
2006). The folds generally flatten towards the Mesopotamian Basin where relatively narrow
anticlines are separated by wide synclines (Aqrawi et al., 2010) filled by thick Pliocene and
different types of Quaternary sediments (Sissakian, 1993 and Jassim and Goff, 2006).
Moreover, the Zone has very thick Miocene Pliocene molasse sediments (~ 3000 m thick),
and the Hemrin Makhul block contains relatively narrow long anticlines (100 to 200 Km
long) often associated with reverse and normal faults (Jassim and Goff, 2006). The anticlines
present within the study area include Taq Taq, Cham Chamal N, Cham Chamal S, Bai
Hassan, Jambur, Hemrin N and Makhmur and the synclines are Khal-Khalan and Qara Hanjir.
Kirkuk structure is the longest and the largest structure within the study area extending from
the SE to the NW for a distance of about 114 Km (Fig.2), and consists of three domes; Baba,
Avanah and Khurmala. It is thrusted along its axis. Hemrin, Jambur and Kirkuk anticlines
have extensions outside the study area. Generally, the anticlines are long and trend with
decollement thrust faults originated at detachments surfaces at the base of the saliferous beds
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
72
of Fatha Formation often producing “gamma-structures” and they are also segmented into
doubly plunging domes; the segmentation usually occurs at intersections with transversal
faults where the axis of the anticline are bent (Jassim and Goff, 2006). Different types of
faults exist within the study area (CGG, 1976); all are associated with anticlines. The thrust
faults are believed to be developed during the late stages of fold development to
accommodate shortening as the folds tightened (Fouad, 2012).
Previous Studies
The following geophysical works were made on the study area:
The gravity survey that covered most of the Iraqi territory except the northern and
northeastern parts. Many parts of the study area are covered by this survey. The results are
compiled and unified as Bouguer Gravity maps scale 1: 250 000 by Al-Kadhimi et al.
(1984).
The seismic reflection survey was executed by the CGG in (1976) in Jambur area. Seismic
time sections showed four reflectors (H1 to H4) represented by:
- H1 represents the top of Fatha Fn.
- H2 could be the base of Jeribe Fn.
- H3 is Kometan Fn.; it represents the base of this formation (very close to the top of
Qamchuqa Fn.).
- H4 could be the top of the Jurassic.
Time sections 10.2 Km long which indicate more than 2.5 Km of depth show the actual
size of Jambur anticline (Figs.3a and 3b). Again, no gravity measurements cover this area.
Such measurements would be useful in quantitative evaluation when compared with
seismic and boreholes information. Nevertheless, the gravitational effect of this structure
could be calculated using the available seismic velocities (Gardner et al., 1974 and
Sharma, 1976).
Al-Bahadily (1997) studied different types of empirical relationships derived from velocity
analysis of seismic reflection data and sonic logs. The relationships relate the travel time,
average and interval velocities with depth. He interpreted the parameters of these relations
in terms of variations in porosity and compaction throughout the study area. The average
velocity and the travel time may be used to study the relationship between density and
depth as it will be explained subsequently in a paragraph of density-depth relationship
derived from seismic reflection data.
The aim of the present study is to interpret the gravity responses of some Alpine
structures of the LFZ by isolating their effects from the background with special reference to
the Jambur structure. It also aimed to estimate the maximum thickness of the sedimentary
cover affected by the Alpine stresses “effective depth” by studying the density-depth
relationship.
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Fig.2: Geologic map of the study area (after Sissakian, 1993)
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
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Fig.3a: Time section across Jambur anticline (seismic line J1) shows the decollement thrust
fault (Fa) originated in a detachment surface at the base of the saliferous beds of Fatha Fn.;
VA.1 represents the location and the number of the velocity analysis. H1, H2 and H3
represent the tops of Fatha, Jeribe and Kometan Formations, respectively,
(CGG, 1976). For location refer to Fig.4
Fig.3b: Time section across Jambur anticline (seismic line J2)
(CGG, 1976). For the location refer to Fig.4
NE
SW
NE
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GEOPHYSICAL DATA
Description and Some Possible Interpretations of the Gravity Anomalies
Bouguer gravity and residual maps of Al-Kadhimi et al., (1984) scale 1: 250 000 are used
to delineate the gravity anomalies especially that of regional nature; the studied anomalies are
more than 10 Km long (Fig.4). They mentioned that the regional field is calculated by using
Griffin's ring (eight points method) in which the circle radius (R) = 2 √5 Km. The rings of this
radius may cover the studied anomalies.
The most prominent features in Fig. (4) are the elongated gravity highs (often ellipsoidal
and nose-shaped anomalies), which geologically reflect the narrow anticlines of NW SE
trends. These highs are numbered with the letter G from the SW to the NE. Table (1) gives the
gravity value of each high and its corresponding anticline. The gravity values are extracted
from the residual map.
Table 1: The magnitudes (∆g) of the gravity highs
Gravity high
Structure
∆g (mGal)
G1
Hemrin anticline
7.0
G2
Southwestern part of Makhmur anticline
2.8
G3
Southeastern part of Bai Hassan anticline
1.2
G5
Southeastern part of Jambur anticline
7.0
G6
Kirkuk anticline
8.0
G9
Chamchamal N anticline
4.0
G10
Southern part of Taq Taq anticline
1.0
The geological map (Fig.2) shows that G3 (the gravity effect of Bai-Hassan anticline)
disappears gradually towards the SE while the Bouguer map (Fig.4) shows a distinctive
subsurface continuation of this high. Also, some gravity highs are not reflected in the
geological map. These are G4, G12, G13 and G14. G4 has an extension towards the southeast
outside the study area with maximum amplitude of 3.0 mGal. G12, G13 and G14 trend
NE SW, and are relatively short (11, 8 and 17 Km, respectively) with ellipsoidal shapes and
∆g of about 2.2, 1.6 and 1.4 mGal, respectively. They may be attributed to subsurface
antiforms or facies changes. In addition, the gravity lows are shown in Table (2) as follows.
Table 2: The magnitudes (∆g) of the gravity lows
Gravity Lows
Structure or other possible source
∆g (mGal)
G17
A structural low trending NW SE
2.2
G18
A narrow and elongated depression runs
along the southwestern flank of Kirkuk
anticline
6.0
G20
Syncline
2.4
G21
E W ellipsoidal depression
2.0
G22
Qara Hanjir syncline
2.0
G24
E W ellipsoidal depression
2.0
The gravity lows G17, G18 and G21 have almost the same direction as that of the above
mentioned gravity highs. Furthermore, two high gradients are seen on the Bouguer map; the
first is accompanied with Hemrin gravity high (G1), with an average of 5.25 mGal/Km. This
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
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high gradient may be related to subsurface thrust faults which is not reflected on geological
map. The second one is accompanied with Kirkuk gravity high (G6), with an average of
7.5 mGal/Km. This high gradient represents the gravity response of the thrust fault. The
magnitudes of these high gradients depend mainly on the intensity of folding and the depth of
the structure. The Himreen north gravity field over the S of Makhmur to Himreen north
anticlines exhibits a uniform gradient (0.5 mGal/Km) in nearly E W direction and it shows
different gravity response to that in the area north of Makhmur anticline.
Fig.4: Bouguer gravity map of the study area (after Al-Kadhimi et al., 1984)
Seismic Data
The available seismic reflection data of Jambur structure are two migrated time sections
(seismic lines J1and J2) and the velocity analyses of eleven locations distributed on these
lines. Velocity analyses from VA.1 to VA.8 lie on line J1 and the others (from VA.9 to
VA.11) lie on line J2 (CGG, 1976).
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Density Depth Relations Derived from Seismic Reflection Data: There is no gravity
measurement covering Jambur area (Fig.4). However, any information related to distributions
of density with depth may be useful for gravity interpretations. Therefore, the velocity
analyses are used to calculate the density using Nafe and Drake's relationship (Nafe and
Drake, 1957), which is commonly used to estimate density from Vp:
ρ= 0.23Vp^0.25
Where: ρ = bulk density in g/cm3; Vp (Vave.) = P-wave velocity in ft/sec
Figure (5) shows the distribution of densities along the entire sedimentary column. Wide
range of densities from 2.04 to 2.38) gm/cm3 is observed near the surface of the earth while
a relatively narrow range at the deeper part from 2.53 to 2.62) gm/cm3. Based on these
results, the following conclusions are revealed:
Fig.5: Density depth curves derived from seismic reflection velocities
of eight locations
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
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1- The density changes remarkably within the first 7 Km of the sedimentary cover; beyond
that it tends to be constant. Therefore, it is expected that most of the gravity anomalies are
related to the upper seven kilometers of the sedimentary cover.
2- The density is relatively higher at the core of the anticline (2.4 gm/cm3), which
corresponds to the massive anhydrite of Fatha Fn., and it gradually decreases reaching the
lower limit at the flanks (2.04) gm/cm3corresponding to a relatively light clastic content of
Mukdadiyah Fnormation.
3- Any intrasedimentary granitic intrusion (ρ ~ 2.64 gm/cm3) or structural uplift, occurring at
deep levels (≥ 7 Km) within sedimentary cover, is not easy to be detected by gravity
measurements, because of the low density contrasts with the surroundings. However, basic
or ultrabasic intrusions (ρ 3.0 gm/cm3) may have a measurable density contrast.
Gravitational Model of Jambur Anticline: The sedimentary column can be divided into
five intervals; H0 H1, H1 H2, H2 H3, H3 H4 and H4 H5. H0, H1, H2, H3, H4 and
H5 are the ground surface, top of Fatha Fn., base of Jeribe Fn., top of Qamchuqa Fn., top of
Jurassic and top of the basement, respectively. The average velocities under each location of
velocity analysis are given in the Table (3). However, the corresponding average density
(arithmetic mean) for each interval is given in Table (4).
Table 3: The average velocities at each reflector
Depth(Z)
in (m)
VA.1
VA.2
VA.3
VA.4
VA.5
VA.6
VA.7
VA.8
VA.9
VA.10
VA.11
H1
2319
2149
1887
2149
700
1304
2357
2249
2607
499
2632
H2
3022
2857
2749
2857
1920
2351
3579
3081
3074
2135
2995
H3
4157
3895
3813
3895
2933
3177
4338
3863
3872
3003
4228
H4
6500
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
H5
12500
12500
12500
12500
12500
12500
12500
12500
12500
12500
12500
Table 4: The average density for each interval
Ave.
Density in
(gm/cm3)
VA.2
VA.3
VA.4
VA.6
VA.7
VA.8
VA.9
VA.10
VA.11
H0 H1
2.177
2.408
2.368
2.208
2.306
2.196
2.252
2.295
2.230
H1 H2
2.298
2.436
2.478
2.325
2.429
2.306
2.375
2.389
2.378
H2 H3
2.356
2.465
2.559
2.393
2.472
2.382
2.405
2.453
2.418
H3 H4
2.510
2.555
2.618
2.585
2.535
2.491
2.460
2.510
2.464
H4 H5
2.650
2.650
2.650
2.650
2.650
2.650
2.650
2.650
2.650
H5
2.935
2.935
2.935
2.935
2.935
2.935
2.935
2.935
2.935
The gravitational effect of Jambur anticline can be calculated according to the data given
in Tables (3 and 4) (Sharma, 1986 and El-Kelani, 2005). For this purpose, 2-D modeling
along seismic lines J1 and J2 using Geosoft version 7.3 are shown in Figs. (6a and 6b,
respectively).
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Fig.6a: Gravitational model of Jambur anticline along seismic line J1. The model is built up according to information
gained from eight velocity analyses (from VA.1 toVA.9)
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
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Fig.6b: Gravitational model of Jambur anticline along seismic line J2
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The gravity anomaly is about 9 mGal in Fig. (6a) whereas it is about 8 mGal in Fig. (6b).
However, Fig. (6a) reflects a more detailed picture of the gravity field across the anticline due
to using eight velocity analyses in the model compared with three velocity analyses used in
the model shown in Fig. (6b).
It is important to mention that the values of depth and average density used in the models
are 12.5 Km and ~ 2.935gm/cm3 for the Precambrian basement as extracted from the
basement depth map of CGG (1974) and the postulated basement composition given by
Jassim and Goff (2006). However, the average density of the rocks below depth of 6500 m is
supposed to be constant at its average value of 2.65 gm/cm3 according to density depth
curves discussed in the Section of Density Depth Relations.
Gravity Field
Gravity field is much affected by the upper part of the sedimentary cover than the deepest
part or basement. The Alpine stresses have generated numerous surface and subsurface
geological structures that modified this field. The responses are generally represented as
gravity highs, lows and sharp gradients. Gravity highs are narrow and elongated in shapes and
closely coincide with the surface anticlines on the geological map. The amplitudes of the
gravity highs, however, depend upon the depth and amplitudes of these structures. Some
gravity lows are matched with the synclines. However, thrust faults along anticlines are
reflected as sharp gradients, and these faults play the main role in increasing the gravity
response of these anticlines.
DISCUSSION AND CONCLUSIONS
Bouguer map (Fig.4) shows that the area between the southwestern part of Taq Taq
anticline and the southwestern part of Makhmur anticline appears much more affected by
Alpine stresses than that occupying the area to the S of Makhmur to Hemrin north anticline,
which, in turn, exhibits uniform gradient, of about 0.5 mGal/Km in nearly E W direction.
This area has gently steepening gravity field that indicates a different response to the Alpine
stresses.
The gravity responses of these structures display the effects on the local field only.
However, the regional component is also affected by the Alpine stresses that should be
considered for any attempt of removing the total effects of these stresses from the observed
gravity field to obtain a new “corrected field”.
Some anticlines have subsurface plunging extensions that are easily detected by the
gravity measurements and others are not reflected on the geological map and only can be seen
on the gravity map. However, some gravity lows may not be synclines according to their
orientations and shapes. They may be depressions caused by solution.
Density depth relationship shows that the upper 7 Km of the sedimentary cover is the
"effective depth" for any source of gravity anomaly within the sedimentary cover. However,
the deep-source anomalies (≥ 7 Km) should be related to dense intrusions due to an
unexpected high density contrast. Therefore, the effects of the Alpine stresses on the
sedimentary cover that may be reflected on gravity field are restricted to the upper seven
kilometers only. However, the sedimentary cover of the study area reaches 13 Km, thus, there
is an ambiguity associated with the gravity field about the sedimentary structures deeper than
7 Km.
A Geophysical Study of Some Geological Structures in the Low Folded Zone ,
Hayder A. Al-Bahadily
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Two dimensional modeling of Jambur anticline using the information deduced from
seismic reflection data shows that the structure has a residual gravity value ranging from
8 to 9 mGal.
REFERENCES
Al-Bahadily, H.A., 1997. The use of seismic velocity data for the study of stratigraphic variations at Jambur
area. Unpub. M.Sc. Thesis, College of Science, University of Baghdad, 136pp. (in Arabic with English
abstract)
Al-Kadhimi, J.A., Abbas, M.J., and Fattah, A.S., 1984. Unified Bouguer Gravity Maps of Iraq, scale 1: 250 000
GEOSURV, Baghdad, Iraq.
Aqrawi, A.A.M., Goff, J.C., Horbury, A.D. and Sadooni, F.N., 2010. The petroleum geology of Iraq. Scientific
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Nafe, J.E. and Drake, C.E., 1957. Variation with depth in shallow and deep water marine sediments of porosity,
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Fouad, S.F., 2010. Tectonic and structural evolution of the Mesopotamia Foredeep, Iraq. Iraqi Bull. Geol. Min.,
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Jassim, S.Z. and Goff, J.C., 2006. Geology of Iraq. Dolin, Prague, and Moravian Museum, Brno, 341pp.
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About the Author
Mr. Hayder A. Al-Bahadily, graduated from University of Baghdad in 1994.
He got his M.Sc. from the same university in 1997 in geophysics and joined
GEOSURV in 1999. Currently, he is working as Assistant Chief Geophysicist
in the Geophysics Department, GEOSURV. His main field of interest is
gravity and magnetic prospecting. He has 29 documented reports and
published papers.
e-mail: hayder.adnan@geosurviraq.com
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The Low Folded Zone is an integral part of the Western Zagros Fold - Thrust Belt of the Iraqi territory. Field, seismic and well data have been integrated to interpret the structural styles and geometries of the folds and the associated structures and their variations across and along the zone. It is concluded that the mechanical properties of the folded sedimentary pile as well as the presence or absence of early formed structural lines of weakness have exerted first order impact on the nature of the folding and faulting processes and their subsequent evolution in this structural domain. Moreover, the classical structural - mechanical grouping of the Zagros stratigraphy in southwest Iran appears not uniform and may show significant variation in the mechanical properties. Therefore, it is found that this grouping might be partially applied in Kirkuk region of the Low Folded Zone, whereas it is totally inapplicable in Mosul region of the zone.
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The Petroleum Geology of Iraq by A. A. M. Aqrawi, J. C. Goff, A. D. Horbury and F. N. Sadooni ISBN: 978-0-901360-36-8 424 pages +xvi Format: hardback Publisher: Scientific Press Ltd., PO Box 21, Beaconsfield, Bucks, HP9 1NS, UK This book presents a comprehensive, up-to-date appraisal of the reservoir rocks, source rocks, seals and traps that control Iraq’s petroleum resources. Early chapters review the history of the oil industry in Iraq and outline Iraq’s tectonic setting and evolution. A five-chapter section on stratigraphic elements, arranged by megasequence, is followed by an assessment of Iraq’s petroleum systems. The book provides an invaluable source of information for petroleum geologists and other researchers.
Article
A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Dead Sea Transform (DST) is presented. A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the Rift. The negative Bouguer anomalies (-130 mGal) along the axial portion of the Rift floor, as deduced from the modelling results, are mainly caused by deep seated basins of light sediments (≥10 km). The inferred zone of intrusion coincides with the maximum gravity anomaly over the eastern flank of the Rift. The intrusion is displaced at different sectors along the NW-SE direction. The zone of the maximum crustal thinning (≤30 km) is attained in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal thickness in the region (38-42 km). Linked to the left lateral movement of ~ 107 km at the boundary between the African and Arabian plates, and constrained with recent seismic data, a small asymmetric topography of the Moho beneath the DST was modelled. The thickness and densities of the crust ranging from 2650-2900 kg/m suggest that the DST underlain by a continental crust. The deep basins, the relatively large nature of the intrusion and the asymmetric topography of the Moho, lead to the conclusion that a small-scale asthenospheric upwelling(?) might be responsible for the thinning of the crust and subsequent rifting of the DST during the left lateral movement.
Article
A multiplicity of factors influence seismic reflection coefficients and the observed gravity of typical sedimentary rocks. Rock velocity and density depend upon the mineral composition and the granular nature of the rock matrix, cementation, porosity, fluid content, and environmental pressure. Depth of burial and geologic age also have an effect. Lithology and porosity can be related empirically to velocity by the time-average equation. This equation is most reliable when the rock is under substantial pressure, is saturated with brine, and contains well-cemented grains. For very low porosity rocks under large pressures, the mineral composition can be related to velocity by the theories of Voigt and Reuss. One effect of pressure variation on velocity results from the opening or closing of microcracks. For porous sedimentary rocks, only the difference between overburden and fluid pressure affects the microcrack system. Existing theory does not take into account the effect of microcrack closure on the ela
Article
In a study of the dependence of the velocity of compressional waves in marine sediments upon the thickness of overburden, the velocity-depth relationship in shelf sediments is shown to be distinctly different from that in deep basin sediments. The difference between the two cases may be illustrated by comparing the straight lines that best represent the data. These areEquationwhere V is in km/sec and Z is in kilometers. Shallow and deep water are defined arbitrarily to be under 100 fathoms and over 1,500 fathoms respectively.The observed variation of average compressional velocity in the shallow and deep water sediments, taken together with the known limited range of variation of velocity for a given porosity, yields limits in turn upon the porosity-depth dependence in the two environments. It is shown that at the same depth of overburden porosity is much greater in deep water sediments than in shallow.A physical argument is presented to show that there is implicit in the observed narrow range of varia
The use of seismic velocity data for the study of stratigraphic variations at Jambur area
  • H A Al-Bahadily
Al-Bahadily, H.A., 1997. The use of seismic velocity data for the study of stratigraphic variations at Jambur area. Unpub. M.Sc. Thesis, College of Science, University of Baghdad, 136pp. (in Arabic with English abstract)
Interpretation report, Jambur -Jedaida area
  • Compnie General De Geophysique
Compnie General De Geophysique (CGG), 1976. Interpretation report, Jambur -Jedaida area.OEC Library, Baghdad.
  • S Z Jassim
  • J C Goff
Jassim, S.Z. and Goff, J.C., 2006. Geology of Iraq. Dolin, Prague, and Moravian Museum, Brno, 341pp.
Geophysical methods in geology The geology of Kirkuk Quadrangle, Sheet NI-38-2
  • P V Sharma
  • V K Sissakian
Sharma, P.V., 1976. Geophysical methods in geology. Elsevir Scei. Publi. Com. Amsterdam, Netherland, 428pp. Sissakian, V.K., 1993. The geology of Kirkuk Quadrangle, Sheet NI-38-2, Scale 1: 250 000. GEOSURV, int. rep. no. 2229.