This study aims to evaluate geologic and structural influences on groundwater in El Obeid area as an essential resource in outcrop area, and to evaluating the structures of the site because of local field conditions within the site, such as rock type and complex structure. The geology of the area is range in age from Precambrian for the old rocks of granite, gneisses, schist and quartzite rocks, to recent age overlain by thick layers of superficial and alluvial deposits. The older rocks comprise the major hydrogeologic units of the deep groundwater system in the study area. The tectonics was important in establishing the distribution of the rocks that control the present flow patterns of groundwater. The study area is affected by tectonic that was caused faulting, folding, and fracturing. Four main types of structures that are investigated which including faults, fractures, folds and veins. These structures were influence of groundwater occurrence and flow. Two type of folds (anticline and syncline) act as accumulation and drainage channels of groundwater flow and also as aquifer in the area. The structures in the target area they are responsible for the dryness of wells in parts of the El Bangadeid, El Molbas, and Keryako hand dug wells. The relationships between surface and subsurface structural geology observed in the study of fractures in drill holes at the target area, the subsurface structures above about 100 m depth at boreholes are similar to surface structure, which have been helpful in assessing the surface structure. Surface structural geology fractures at the site consist of widely spaced, and the distribution of fractures relative to rock type at the surface; more fractures occur in brittle especially in schist rocks. The geological contacts between hydraulically different lithologies, they have virtually no primary porosity and they have secondary structures developed from the tectonic events. These include porosity, lithological boundaries, large faults, fracturing and weathering, which permits the flow and storage of groundwater. In the study area there are two main groundwater aquifers namely; Quaternary, and fractured basement. It’s recharged directly from the rainfall and indirect from the basement rocks through fracture systems. Quaternary aquifer is restricted and recorded especially in the El Bangadeid, El Molbas, and Keryako hand dug wells, where the fractured basement aquifer is widely distribution.

Keywords: Basement, geological, structures, groundwater occurrence, El Obeid

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