Subsurface Stratigraphy, Depositional History, and Reservoir Development of the Early-to-Late Permian Unayzah Formation in Central Saudi Arabia

Abstract

ABSTRACT Six oil and gas fields have been discovered in Permian strata in central Arabia since June of 1989. The majority of reserves are contained within the Unayzah Formation, a siliciclastic succession of fluvial, coastal plain, deltaic, and shallow marine facies, ranging in age from Early to Late Permian. Distribution of Unayzah facies is controlled by paleotopography on the Hercynian Unconformity, sea-level fluxuations, and both local and regional structure. The Unayzah C member records the first post-Hercynian marine incursion. The Unayzah A and B members record a second transgressive-regressive cycle. A final Permian transgression caps the Unayzah Formation. A complex interplay of structurally and sea-level controlled facies trends, coupled with erosion during sea-level lowstands, has resulted in the development of a variety of reservoir types. Six different reservoir facies have been identified: Unayzah B member braided river sandstones; shoreface/foreshore, delta channel, and coastal plain channel sandstones of the Unayzah A; and valley fill channel and transgressive lag sandstones deposited during the Khuff transgression. Each reservoir type is characterized by different reservoir attributes. In addition to proven reservoir types, significant potential exists for stratigraphic trap development along depositional boundaries of the Unayzah A and B members against the Central Arabian Arch.