Megaseismic section across the northeastern slope of the Arabian Plate, Iraq Open Access

In 1990 a project was initiated to construct 15 megaseismic lines across Iraq using existing seismic data. The purpose of this project is to provide a regional framework of uniformly presented geological and geophysical data from which the tectono-stratigraphic evolution of Iraq can be studied. The megaseismic lines are positioned in such a manner so as to form a rectilinear grid that consists of 11 SW-NE-oriented lines and four NW-SE-oriented lines (Figure 1). This extensive grid is complimented by seven previously constructed seismogeological depth sections running at various directions (Figure 2). The seismogeological depth lines are shorter in extent and tie key exploration and stratigraphic wells. The megaseismic lines are each accompanied by strips of surface geology, the depiction of the limits of the major tectonic units, as well as profiles showing the Bouguer gravity and total magnetic field (Figure 3).

This paper presents Megaseismic Line 7, the first of the 15 megaseismic traverses to be constructed. Megaseismic Line 7 extends in a SW-NE direction, nearly 500 km from near the Iraq-Saudi Arabia border, to near the Iraq-Iran border (Figures 1–3). This line cuts through the middle of Iraq and crosses different tectonic provinces and geological structures. It passes through the giant East Baghdad oil field, and the smaller Tel Ghazal and Nahrawan oil fields (Figure 3). The paper discusses the data used for Megaseismic Line 7, as well as how it was processed and interpreted.

The geographic position and construction of Megaseismic Line 7 was based on both geophysical and geological considerations, both at the surface and in the subsurface. In this section, the different types of data and the criteria for selecting this data are briefly discussed.

A total of 16 seismic lines were selected for the construction of Megaseismic Line 7 (Figures 1 to 4 and Table 1). The lines were selected such that the gaps between them were minimal in extent, while simultaneously insuring that the cross-section does not repeat itself over the same structural dip. Not all the initially considered lines had magnetic tapes and supporting field reports; accordingly the selection was limited to those lines that could be digitally reprocessed from original field tapes. This reprocessing approach ensured unified digital processing of the constituent lines, and the best quality for the final stacked section.

The seismic lines were selected from various 2-D surveys acquired by the Iraq National Oil Company (INOC) and several foreign seismic crews (Table 1 and Figure 4). The data consists of different vintages (1975 to 1983) and was acquired using either dynamite or vibroseis sources. The recording systems were 48 channels resulting in 24-fold coverage. The configuration of the recording cable is generally symmetric-split with maximum offsets of 2,500–3,200 m (Table 1). Three lines were recorded with asymmetric cable geometry with the maximum offset of 3,200 m. One line was acquired off-end with a maximum offset of 5,000 m.

Various geological information was used in the study. The Surface Geological Map of Iraq (General Directorate of Geological Survey and Mineral Investigation, 1984) was used to determine the outcrop rock unit and its influence on seismic data quality (Figure 3). The seismic reflections were correlated to formations, unconformities or/and disconformities of Iraq (van Bellen et al., 1959-2005; Buday and Tyracek, 1980), the Arabian maximum flooding surfaces (Sharland et al., 2001, 2004), and the Chronostratigraphic Chart of the Gulf (M.I. Al-Husseini and N. Stewart, 2000). Also geological reports and correlation diagrams from various wells were used to identify the reflections. The lithofacies distribution maps of the depositional cycles of Iraq were also used (e.g. van Bellen et al., 1959-2005; Dunnington, 1958-2005; Al-Naqib, 1967; Buday and Tyracek, 1980).

Geophysical borehole information that was used in the study included velocity surveys and synthetic seismograms from five wells to tie the reflections. In particular, the traverse intersects the deep West Kifil-1 well (32^o21’12”N, 43^o43’16”E, total depth of 5,842 meters; Al-Hadidy, 2007 in preparation) that penetrated the Permian Chia Zairi and Ga’ara formations (Figure 2) and was used to identify the oldest reflections. Seismic interpretation reports for nearby survey areas were helpful in identifying key reflections (Figure 4). The geophysical potential information used here included the Bouguer gravity anomaly map (Iraq Petroleum Company - IPC, 1960) and aeromagnetic total field intensity and interpretation map (CGG, unpublished report, 1974). The depth of the Proterozoic Basement was based on modeling by CGG (unpublished report, 1974).

The seismic lines that were used to construct Megaseismic Line 7 are of various vintages (Table 1) and required reprocessing so that they could be presented as one uniform and continuous image. The data was reprocessed from the original digital field tapes. The first step was to apply a bulk static time shift to each line so as to have a single datum. The datum starts at 300 meters above sea level to the far southwest, rises to 350 m at Qurnain, and then descends gradually in steps to sea level from Najaf-Kerbala to Khanakin (Figure 4 and Table 2). The reference datum chosen for the Megaseismic section is fixed at 300 m above sea level. The static bulk shifts that that were applied ranged from 0.0 to 225 milliseconds (Table 2).

The seismic data quality is strongly related to the surface geology (Figure 3). Data quality is generally better in areas covered by the Miocene-Pliocene-Pleistocene Dibdibba Formation, the Pleistocene Habbariya Formation and the Holocene flood plain deposits. These rock units are extensive in outcrop in Iraq, and consist mainly of sand and gravel. In fact these outcrops are ideal surfaces for reflection seismic because the unconsolidated flood-plain deposits provide better geophone coupling to the ground and do not propagate high-amplitude ground roll. In contrast, regions covered by the upper Miocene Upper Fars Formation, Miocene Euphrates Limestone and the Eocene Jil and Dammam formations are characterized by poor seismic data. These formations consist of highvelocity carbonates and evaporites, which tend to have karsts in the near-surface zone. The karsts and dissolution features cause the seismic energy to be scattered and dispersed in the near-surface zone.

The quality of the deeper data may also be related to the energy source. The energy source for the southwestern part of the traverse is vibroseis, while the majority of the traverse was acquired with a dynamite source. The data quality is significantly better in the eastern part of the traverse that was acquired with a dynamite source (Figure 7).

The data was processed using the conventional Cogniseis VAX 11-780 software packages. Residual statics and dip filters were applied to all lines forming the southwestern half of the megaseismic section under consideration. For the seismic lines in the Khanakin survey area, to the far east, the data was migrated.

The final stacked section was displayed at different scales to determine the optimal perspective for interpretation (Figure 7). The line was also displayed in terms of the Hilbert attributes: (1) reflection amplitude (Figure 5), (2) instantaneous phase (Figure 6) and (3) instantaneous frequency. The Hilbert reflection amplitude, in particular (Figure 5), illustrates the relative strength of the different reflections. In terms of amplitude some of the most continuous reflections are due to thick evaporites (with salt) such as the Lower Fars and Gotnia reflections. The strength of the Top Ratawi reflection is probably due to the major acoustic contrast between the Ratawi carbonates and overlying clastics of the Zubair Formation.

The instantaneous phase illustrates how certain stratigraphic geometries are enhanced (Figure 6). For example the continuity and appearance of the seismic package between the tops of the Gotnia and Ratawi reflections is much clearer in the instantaneous phase section relative to the stacked section (Figure 7) and amplitude section (Figure 5).

A total of 14 horizons were identified and picked along the constructed Megaseismic Line 7 (Figures 7 and 8). The synthetic seismograms from the following five wells were used to identify the reflections: Ekhaider-1, West Kifil-1, Kifil-3, East Baghdad-2 and Tel Ghazal-1. The picked horizons were selected on the basis of reflection quality and continuity, and range in age from Miocene to Permian. Some of the identified horizons correspond to important seals, sources and reservoir. Where possible the reflections are tied to regional Arabian Plate Megasequence boundaries, major unconformities or near to maximum flooding surfaces (MFS, Sharland et al., 2001, 2004).

The youngest horizon to be picked is the reflection from the top of the middle Miocene Lower Fars Formation (near MFS Ng30). Other Cenozoic reflections are from the top of the lower-middle Miocene Jeribe Formation (near MFS Ng10), and the top of the Eocene Jaddala Formation (top of AP10 Megasequence).

Three horizons were picked in the Upper Cretaceous successions: (1) top of the Shiranish Formation (near top AP9 Megasequence); (2) top of the Hartha Formation (near MFS K80); and (3) top of the Sa’adi Formation (near MFS K160). Another three horizons were also picked in the Lower Cretaceous successions: (1) top of the Mauddud Formation (near Top AP8 Megasequence); (2) top of the Aptian Shu’aiba Formation (near MFS K90); and (3) top of the Ratawi Formation (near MFS K40).

Three horizons were picked in the Jurassic section: (1) top of the Upper Jurassic Gotnia Formation (near top of the AP7 Megasequence); (2) top of the Middle Jurassic Sargelu Formation (near MFS J40); and (3) top of the Lower Jurassic Alan Formation (top AP6 Megasequence).

The oldest two horizons that could be picked are from the top the Triassic Kurra Chine Formation (near MFS Tr40), and top of the Permian Chia Zairi Formation (near MFS P40). Below the oldest Permian reflection, the middle Carboniferous “Hercynian unconformity” is tentatively picked. The relatively short maximum offset of most of the lines and the low fold appear to preclude the imaging of deeper events.

The Paleozoic pre-Permian succession is not adequately imaged in the seismic data. The crystalline basement is not seen; the presence of such rocks is usually expressed by chaotic and strong diffraction patterns. The thickness of the Paleozoic sedimentary column along the southwestern half of the constructed seismic profile seems to be much greater than previously anticipated.

The picked horizons show a general dip and increasing interval times towards the northeast, except at the location of the Tel Ghazal oil field where syndepositional growth of this structure can be inferred (Figures 7 and 8). A wedge-shaped mass of Cenozoic rocks is interpreted within the northeastern half of the constructed section. The thickening of the section towards the northeast is caused by the appearance of new layers in that direction. Two zones of different seismic response can be observed at the top representing the Upper Fars and the Bakhtiari formations. The weak, discontinuous, subparallel and wavy character of the seismic signal of these zones indicate deposition within a rapidly varying environment and basin subsidence.

More than 50 seismic stratigraphic features representing onlap, toplap and downlap geometries were identified. These are especially prevalent in the Cretaceous interval and on top of the inferred “Hercynian unconformity” (Figures 7 and 8). These features were used to define disconformities and angular unconformities. Numerous truncations of seismic events against unconformities (e.g Sa’adi, Mauddud, etc.) were identified in the interpretation. Future work may include flattened sections on key reflections to show stratigraphic and structural relationships more clearly.

Three limited zones of chaotic reflections were also observed representing possible organic buildups. The locations at which the seismic response changes suddenly were also noted. The latter feature is considered to be an expression of changes in sedimentary facies. Seismic response changes within the Cenozoic succession in the central part of the section indicate possible facies changes. Such variations are also observed below the top of the Ratawi, Alan and Kurra Chin formations at various parts of the section.

Another observed regional stratigraphic feature is the large progradational sigmoid body of earliest Cretaceous age, between the Gotnia and Ratawi reflections, in the area southwest of East Baghdad field (Figures 7 and 8). It shows clear topset, foreset and bottomset geometries thus indicating rapid basin filling and a major sea-level rise during the earliest Cretaceous times. Being so large, this feature was not identified by previous studies due to their local extent.

The construction of this regional line has helped identify a few structural features that were not previously recognized. These include a structural feature within the Ma’aniya Depression in the Western Desert, and a second directly to the southwest of Tel Ghazal oil field (asterisks in Figure 8). A positive time anomaly is interpreted to exist to the east of the Ma’ania Depression within the Paleozoic reflection package. This anomaly appears to be cut by a major angular “Hercynian Unconformity” of possible middle Carboniferous age.

A good correlation exists between the Bouguer gravity anomaly profile and the position of the Ma’ania Depression (Figures 3, 7 and 8). The Bouguer gravity anomaly profile also correlates to the main Paleozoic anticline and the greater thickness of the Cenozoic succession to the northeast. Also, a good correlation is observed between the local increment in the Bouguer values and the position of East Baghdad and Nahrawan oil fields (Figure 3).

A satisfactory correlation was obtained between the total magnetic field and the inferred Paleozoic structural image along the distance extending from the Iraq-Saudi Arabia border to the East Baghdad oil field (Figure 8). This is followed by a gradual increase in the field intensity despite the increasing depth of these rocks. This may indicate a possible change in the basement rock lithology from acidic to basic/ultrabasic in composition, or alternatively being shallower in that direction. A good correlation is also observed between the local changes of total magnetic field with the location of faults seen on the section.

Numerous normal faults were identified in the Western Desert bordering the Ma’ania Depression and the Abu Jir fault zone (Figures 7 and 8). Some of these faults were not previously identified seismically and additional data is required to map their extent and directions. Normal faults are also prevalent in the central part of the section near the East Baghdad and Nahrawan oil fields. Reverse faults are evident in the northeastern part of the line and affect the Tel Ghazal structure.

The depth of the basement rocks, and hence the thickness of the sedimentary cover as obtained by the interpretation of the aeromagnetic survey data, cannot be confirmed by the present work. The seismic section does not clearly show such a rock complex even in the southwestern part where these rocks are expected to be at their shallowest level.

This study has tentatively specified the location of the Paleozoic platform edge along the eastern flank of the Ma’ania Depression, which marks the start of the Mesozoic near-platform flank of the Mesopotamian Foredeep. The section also defines the spatial migration during the Mesozoic and Cenozoic times of the hinge line separating the Arabian Platform from the Mesopotamian Basin. It corresponds to the Lower Cretaceous progradational pattern seen as clinoforms in Figures 6 and 8.

This study presents the construction and interpretation of the first regional seismic cross-section across Iraq, which forms part of the northeastern slope of the Arabian Plate. The present work required the integration and uniform presentation of numerous data including 16 seismic lines of various vintages, five wells with synthetic seismograms, geophysical potential data (Bouguer gravity and aeromagnetic data) and surface geology. The constructed seismic section is presented in two-way seismic time and will be converted to depth in the future. This conversion can be done with regional interval velocity maps that are derived from wells. The velocity intervals should account for the different lithologies (e.g. clastics, carbonates, evaporites) and their associated average velocities. Accordingly a step-wise conversion of time to depth (“layer-cake”) approach would appear to be appropriate for regional mapping.

The resulting interpretation of the Megaseismic Line 7 provides new insights into the regional geometry and tectono-stratigraphic architecture of Iraq across a 500-km long SW-NE-oriented crosssection. In particular, the regional relationship between the relatively stable shelf in western Iraq and the tectonically active zone of eastern Iraq is seen in one image. This traverse is the first of 15 traverses that will form a grid across all of the country. It provides a base line for the other traverses to tie into. The traverse shows that the tectonic provinces of Iraq require reinterpretation to encompass all tectono-stratigraphic and structural features revealed in this regional grid.

The regional Megaseismic Project highlights many tectono-stratigraphic and chronostratigraphic aspects of Iraq. It presents numerous insights for future exploration opportunities. For example, the sigmoidal body of sediments located southwest of Baghdad and other structural features defined in this study should be considered in terms of exploration potential. Use can be made of this section to accurately define the limits at which various kinds of source rocks enter maturity for oil and gas generations. All previous studies that deal with this aspect relied on inaccurate depth models.

The author thanks the Ministry of Oil of Iraq for permission to publish this paper. The assistance that was provided by many processing and interpretation staff at the Oil Exploration Company of Iraq is highly appreciated. The author also thanks Chevron for the support that made this publication possible. The comments provided by two anonymous reviewers proved helpful in preparing the final manuscript. GeoArabia is thanked for assisting with the editing and design of the manuscript.

Sabah A. G. Mohammed is a Geophysicist at the Oil Exploration Company, the exploration arm of the Ministry of Oil, Iraq. He has over thirty years of professional experience in acquisition, processing and interpretation of seismic data. Sabah holds a BSc (Honors) in Geology from University College London, and MSc and PhD degrees in Geophysics from Leeds University, England. Throughout his career, he conducted, supervised and quality-controlled 100s of geological and geophysical studies that helped in the assessment and evaluation of the hydrocarbon potential of Iraq. He also participated in various integrated geological, geophysical and reservoir studies with international oil companies for Iraq. Sabah has published several scientific papers on various topics in geophysics including resistivity, reflection and refraction seismics. He supervised a number of MSc and PhD research projects, and conducted many training and development programs for the Oil Ministry technical staff. Sabah has also co-chaired many sessions in local, regional and international conferences.

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