Online Journal for E&P Geoscientists

Soliman, Osama M.^*1; Shammari, Salem H.¹; Dwaihy, Ahmed S.¹
(1) Exploration, Saudi Aramco, Dhahran, Saudi Arabia.

The Lower Permian of central Saudi Arabia represents a continental succession that is bounded by major unconformities. It spans ~25 Ma and includes the informal Unayzah A and B reservoir units, which are largely barren of palynomorphs. This study attempts to define high-resolution sequences to help predict reservoir facies distribution within this succession.

Core logging allowed the recognition of fluvial, eolian dune, sand sheet, playa and floodplain facies. These are stacked into fining-coarsening or top-truncated fining upward sequences that are bounded by unconformities and correlative conformities. A sequence reflects a cycle of base level rise and fall. It may consist of stacked eolian dune sandstones changing upsection, across a flooding super surface, into sand sheet; playa; sand sheet and then eolian dune facies. Alternatively, it may be comprised of amalgamated fluvial channel sandstones changing upward into thick playa siltstones. These fluvial sandstones may change distally into eolian dune and sand sheet facies. Alternatively, they may change; toward high-subsidence areas; into floodplain and playa facies. Mass transport deposits and seismite facies commonly occur adjacent to faults in these low areas.

Three third order depositional sequences have been recognized in the Lower Permian succession. They reflect distinct phases of basin rifting and changes in climate. The lower sequence records fluvial to lacustrine sedimentation in large underfilled half-graben basins during a wet climatic period. The middle sequence was deposited in increasingly segmented half-graben sub-basins and is characterized by significant lateral thickness and facies changes. It is dominated by eolian deposits, which suggests a shift towards an arid climate. The upper sequence is marked by large valleys that are incised into older eolian facies. It records terminal fluvial sedimentation during a semiarid climatic period, progressively diminishing accommodation, overfilled basin conditions and pervasive paleosol formation due to repeated uplifts, which heralded the development of the top-bounding Basal Khuff Unconformity.