Subsurface Structural Framework Of The Tertiary Sedimentary Rocks East Of Jeddah, Saudi Arabia

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Idris, El Tahir Osman
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The study area lies in proximity of the Arabian coast of the Red Sea at about 45 km northeast of Jeddah city. In this study, lithostratigraphic interpretation of optimally processed Landsat TM data at reconnaissance mapping has been carried out as a precursor to geophysical interpretation, providing the basic ‘framework’ in which to view the imaged geophysical data. The Landsat-derived framework shows the correct positions and vergences of major structures, which characterize the study area. Within the structural framework derived from satellite imagery, the locations of major shallow-source aeromagnetic anomalies related to intermediate/mafic extrusive and sub volcanic rocks and the controlling structures of this economically important basin has been interpreted. Structural mapping aided with the interpretation and forward modeling of aeromagnetic and ground magnetic data form complimentary and powerful tools for regional structural analysis. Both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the subsurface structure of the Tertiary sedimentary rocks in the study area. Using the ground magnetic data and integrating digital aeromagnetic data the structural analysis was substantially enhanced to trace the fault zones in the basement underneath sedimentary cover. Aeromagnetic results indicate four main fault directions NE, N, E-W, and NW trends have implications related to the tectonics of the region. The northeast trending which dominates the magnetic pattern of the Assuqah area in all central and east-central parts is related to Precambrian basement faults. These faults were reactivated during the opening of the Red Sea. The interpreted map represents a significant improvement over previous mapping and a major subsidiary basin, Assuqah Basin, has been identified and mapped. Assuqah basin is a longitudinal graben 30 km long and 12 km wide has peak-trough contrasts of as -400 nT contains Tertiary sedimentary rocks. The low gravity anomaly over Wadi Assuqah is consistent with the results of the aeromagnetics and ground magnetics. The Bouguer map shows a prominent gravity low closure over Wadi Assuqah with a minimum of -45 mgals. The aeromagnetic and ground magnetic profiles showing significant magnetic anomalies were analyzed for target depth estimation. The interpretation of Basement structure of the aeromagnetic data indicates that the maximum depth is 6000 m. The derived depths are used, together with available borehole data, to constrain for forward modeling along selected profiles. Several models can be suggested with regard to the evolution and structural framework of the Assuqah. The tectonic evolution of the basin can be explained in terms of extensional tectonics or rifting or it can be concluded that the basin was a shallow water lake or inland sea during the deposition of the Tertiary sediments. The Tertiary sediments were laid down during a structurally unstable period which followed, with sedimentation taking place in the form of fluvial fans, in the down warps between faulted basement domes. In summary, the results provide clear evidence that reveals the good potential occurrence of deep basin where thick sediments exist buried beneath the Quaternary deposits. These conditions are favourable for a hydrocarbon accumulation. Other faults were identified using aeromagnetic data in combination with the satellite images. By using the aeromagnetic data it is possible to identify Assuqah basin and to clarify the small scale faults covered by sediments. This case study demonstrates a good methodology for obtaining maximum geoscientific value from aeromagnetic, ground magnetic data through integration with high-resolution satellite imagery. Deep resistivity detected two major aquifers for groundwater, the first is a shallow unconfined aquifer at a depth ranging between 10-60 meters associated with alluvium gravelly sand deposits and the second is a deep confined aquifer at a depth ranging between 120-180 meters associated with gravelly sandstone layers within the Upper Cretaceous and Tertiary rocks
Subsurface,Structural,Framework,Tertiary Sedimentary Rocks East,Jeddah