Thick Paleozoic succession of siliciclastics and carbonates is well exposed and penetrated in the greater Arabian Basin. The succession included source, reservoir, and seal rocks. Though, oil and gas have been discovered in sandstone and limestone reservoirs in these rocks in several oilfields in the basin, the tectonic history of this succession has not been fully understood yet.The Paleozoic succession of the basin is severely affected by series of major climatic and tectonic events which have caused facies change and major stratigraphic breaks. The major tectonic movements have resulted in dividing the Paleozoic succession into mega-depositional cycles. The succession is subdivided into pre-and syn-climatic and tectonic events representing well defined depositional cycles separated by regional unconformities. These mega-depositional cycles are well preserved in basinal or less tectonically affected areas where boundaries between the respective cycles are marked by possible hiatus. In more tectonically active areas, the Paleozoic succession is less preserved and boundaries between the cycles are more complicated due to longer periods of erosion by later movements and/or non-deposition. Minor stratigraphic breaks within the mega cycles subdivide them into smaller sub-cycles.Following the deposition of the Late Caradocian Quwarah member of the Qasim Formation the area went through a gentle uplift and tilt probably related to the Taconic tectonic movements, and a drop of sea level due to glaciation. Thick succession of the Quwarah, Ra'an, Kahfah, and Hanadir members of the Qasim Formation and the Risha and Sajir members of the Saq Formation were eroded and deep paleo-valleys incised in outcrops of the two formations. Glacial and periglacial deposits of the Zarqa and/or Sarah Formations were unconformably rest on older units from Ordovician to Precambrian.
The Midyan Basin is characterized by arid climate with rare surface water flow. Groundwater is the main water resources in this region. In order to evaluate their suitability for drinking, domestic and irrigation purposes, chemical characteristics of groundwater in the Midyan Basin (northwestern Saudi Arabia) have been investigated and evaluated. A total of 72 water samples were collected from different wells and analyzed for hydrogen ion concentration, total dissolved solids, electrical conductivity, total hardness, Ca2+, Na+, Mg2+, K+, Cl-, HCO3-, NO3- and SO42-. To understand quality of groundwater and their suitability, chemical indices like sodium percentage, sodium adsorption ratio and salinity values have been calculated using analytical techniques. From traditional Piper diagram for water classification, the water quality is placed at Na+-Ca2+-SO42--Cl- type. According to the results of electrical conductivity and sodium adsorption ratio, 37% of the studied samples are regarded as highly saline, while 63% of them are classified as very highly saline. As per the Arab Gulf and international standards, such waters are not suitable for irrigation under normal condition and further action is required to remediate such problem by salinity control. Sodium content in 24% of the studied samples is regarded as high that can’t be used for irrigation purposes for any soils. Thus high level of salinity, sodium adsorption ratio and sodium percentage in most water samples have made them unsuitable.
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