Lineaments Determination of Western Part of Iraqi Western Desert using Aeromagnetic and Gravity Data
Abstract
The aim of this study is to investigate the boundaries of geological structures of the western part of Iraqi Western Desert using aeromagnetic and Bouguer gravity data. To achieve this, The CET (Centre for Exploration Targeting) Grid Analysis extension was applied to aeromagnetic and gravity data, and It is supposed that the provide results are unbiased. Decompensative anomaly map of an isostatic residual gravity anomaly map shows NNW maxima trend in western part of the study area, while the eastern part shows NS minima, where this gravity low is thought to be an important basin. The reduced to the pole magnetic map in general agrees with the gravity interpretation, however there are some differences related to the magnetic susceptibility variations within the basement. The basement topography was furthermore estimated using Oasis Montaj that based on Bill Pearson’s implementation of R. L. Parker’s algorithm (1972). Four sets of lineaments were determined using the CET grid and derivative analysis. These lineaments are thought to be related to the formation of the basins in the Western Desert.
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Introduction
The Western Desert region of Iraq is part of the Arabian plate which created nearly 25 Ma when the Red Sea and Gulf of Aden rifted away from northeast Africa (Stern and Johnson, 2010). The Arabian plate is surrounded by active tectonic margins (Stern and Johnson, 2010); the northern and eastern margins of the plate are composed of the compressional regimes of the Taurus suture zone to the north and Zagros thrust zone in the east (Fox and Ahlbrandt, 2002; Stern and Johnson, 2010). To the west and the south, the margins are extensional defined by the Red Sea and Gulf of Aden rifts.
Gravity and magnetic measurements have a chief role in understanding the structure and evolution of sedimentary basins and basement. The basement structures can propagate into the overlying sedimentary rocks and influence fluid flow and the distribution of hydrocarbon traps. For example, many oil and gas fields occur along fault-controlled linear trends. Therefore, the structural framework of the basement has a significant impact on basin evolution (Ali et. al, 2014).
In 1974, an aeromagnetic survey of Iraq was performed by Compagnie General de Geophysique (CGG). GETECH Group has carried out a detailed reprocessing study to recover all the aeromagnetic and gravity data into a standard Geosoft database format. Gravity and magnetic maps have commonly been used in hydrocarbon exploration to the thickness of sedimentary basins (Hinze, 1985). The Western Desert within which the study area (Fig. 1) is mainly within Iraq but extends to the west and south into Syria, Jordan and Saudi Arabia. Most of the Western Desert has been a region of erosion a land since the Oligocene.
Conclusion
The study area is an important one because of its tectonic structure, with its resources of ground water, mines, quarries and oil and gas. For the first time, several fault sets located in the study area are estimated. Lineaments area detected using CET grid analysis, then THD, AS, and TDR methods under the concern of the DAM and RTP anomaly data of the area. The lineament maps of the study area have been offered in this paper. The results are summarized as follows:
1. 3D model of the basement topography of the study field have been presented.
2. The new lineaments have been discovered using the CET grid analysis and to prove the results three approaches as the maximum of the THD and AS maps and zero contours of the TDR maps.
3. The lineaments have been compared with the 3D inversion map of the gravity field, and four direction lineament sets have been predicted.
4. The minima represents sedimentary basins, this case is dominated in the study area (Afif Terrane). Actually the basement is uplifted in such location. Valleys of the study area may controlled by N-S fault system.
5. Major NW-SE fault system (Najd fault system) interpreted mainly as normal faults that separate horsts and graben structures attributed to the rifting phase that affected the Arabian plate during the Precambrian to Cambrian and that lead to the opening of the Paleo-Tethys Ocean. The system is best compiled from magnetic and gravity interpretation of CET. It is clear that the system is developed in left transpressional shear zone during Nabitah Orogenies (Jassim and Goff, 2007). Many of N-S structures are offset and deformed by Najd shears.
6. 3D inversion map of DAM shows detailed features in the study area, but these maps need constrains which are not available now (seismic reflection controls. However, many oilfields occur at the edge of the basement blocks with low magnetic susceptibility outside study area, It is believed that the tectonic events causing re-activation of Precambrian basement contacts, faulting and folding of the overlying sediments causing the formation of domal oilfield structures So maps show two main trends: NE-SW to E-W trend is represented mainly by strike slips lineaments, they represent the transversal faults. NW-SE to N-S trend is represented mainly by normal faults that separate horsts and graben structures attributed to the rifting phase that affected the Arabian plate during the Precambrian to Cambrian and that lead to the opening of the Paleo-Tethys Ocean, they represent the Najd and Nabitah faults systems
7. In offshore of Abu Dhabi most of the oilfields are located at the edge of the two positive magnetic anomalies and along magnetic linear trends, suggesting that the blocks are possibly bounded by steep faults. It is hypothesized that the faults in the basement may have triggered migration of infra-Cambrian Hormuz salt bodies, folding of the overlying sediments and formation of domal oilfield structures (Salem, 2015) and (Ali et al, 2013), actually this is may be the case in the study area, many expected structures that could relate with oil field lie on gradient zones in DRTP map where there is a faults separate between different basement blocks and after reactivation of those faults many positive structures formed in Paleozoic sediments.
8. The Western Desert affected by different stages of Orogenies as mentioned above, the main effects generated from basement tectonic that are reactivated later for several times among different blocks. The normal faults and strike slip faults are the dominant in the studied area, usually extension forces caused normal faults associated with horse and graben, but it is though that the most important effect is due to transtension (phase of oblique extension) and transpression (the later phase of oblique compression). Focusing should be along the zone that separated between any two faulted adjacent basement blocks as several of structures align there, that later affected by strike slip faults structures may be very good targets. Western Desert shows heavy faulted blocks, determination of them is important for future seismic surveys, where the most important targets are surrounded by those fault systems. Divided the Western Desert at least to three zones for more detailed gravity and magnetic studies.
9. Depending on the results focusing should be on gradient zones for the purpose of oil and gas investigation speciously around anomalies (A, B, C and E), (Fig. 4)