Reservoir quality of the Unayzah Formation in Waqr, Tinat, and Haradh fields (Saudi Arabia)

The Unayzah Formation is a succession of shallow marine sediments which were deposited on the pre-Unayzah unconformity and unconformably overlain by the Khuff Formation. The Unayzah Formation comprised of sandstones, shale and siltstone. The sandstones are fine to medium-grained and most of the samples are moderately well-sorted quartz arenite. Calcite, clay and quartz overgrowths are the major authigenic cements.; Major diagenetic processes of the Unayzah Formation include compaction, phases of cementation and extensive grain and cement dissolution. Physical compaction of the sediments due to burial diagenesis is followed by cementation processes that include early calcite cement, authigenic clay coatings, quartz overgrowth and late calcite cementation. The development of secondary porosity due to extensive grain and cement dissolution is an important diagenetic event in the Unayzah sandstone. Depositional environment of the Unayzah Formation was interpreted using probability cumulative curves technique, factors analysis and wireline logs. All the studies deduced a shallow or marginal marine depositional environment for the Unayzah Formation in the study area.; Back propagation neural network was applied for lithofacies classification and porosity prediction of the Unayzah Formation using wireline log data. Genetic and nongenetic approaches were applied for the prediction of porosity. The predicted lithofacies profiles were found in agreement with the well logs. Genetically derived porosity however, was in better agreement with the neutron porosity.