oa Sedimentology and petrographic study of the Upper Carboniferous-Lower Permian Unayzah Formation, Block C, Rub’ Al-Khali, Saudi Arabia

The lower boundary of the Khuff Formation has not been consistently established, since the basal siliciclastic unit is included by some authors in the Unayzah Formation and by others in the Khuff Formation. A regionally correlatable unconformity marked by thick caliche, soil horizons and colour changes is considered to be representative of fundamental changes in the depositional environment that marked the onset of the marine conditions of the Mid to Late Permian. Four siliciclastic depositional units were recognized below the pre-Khuff unconformity: basal Khuff clastics, Unayzah A, B and C depositional units. These units were related to regional tectonic and climatic events that prevailed during the Late Carboniferous to Early Permian times. The regional<br>interpretations suggest the occurrence of the Unayzah C unit throughout Block C, Rub’ Al-Khali Desert, Saudi Arabia. The presence of Unayzah B sandstone is likely in the southern part of the block, however the northern part is characterised by relatively less sandstone presence. A similar distribution with relatively good sandstone reservoir quality in the south and mixed reservoir/ non-reservoir facies to the north is expected in the Unayzah A unit. The basal Khuff clastic unit is probably characterised by relatively thin sandy reservoir facies throughout the block. The petrographic results indicate that the basal Khuff clastics and Unayzah A units may retain intermediate reservoir quality if the grains are sheltered from quartz cementation by illite coatings. The Unayzah B and C units do not show illite coatings and need to be sheltered from quartz cementation by other porosity preserving mechanisms, such as early hydrocarbon migration.