High Resolution Sequence Stratigraphy of the Devonian Jauf Formation Gas Reservoir. Ghawar Supergiant Field, Saudi Arabia

By

 Riyadh A. Rahmani¹, Ronald J. Steel², Abdulaziz A. Duaiji¹

(1) Saudi Aramco, Dhahran, Saudi Arabia (2) University of Wyoming, Laramie, WY

 The Devonian (Praghian to Frasnian) Jauf Formation in Ghawar Supergiant Field thickens from 650 feet in the west to 1100 ft thick in the east, and comprises a marginal to shallow marine siliciclastic succession. It consists of a lower 3rd order sequence (S55), the Lower Jauf, which is dominated by a sand-prone progradational falling stage systems tract (FSST) and totally lacking an overlying coastal plain facies. Whereas the upper 3rd order sequence (S60), the Upper Jauf, is comprised of a mud-prone, estuarine transgressive systems tract (TST), forming the Jauf Embayment and an overlying highstand systems tract (HST) composed of progradational shoreface and overlying coastal plain deposits. A key aspect of the succession, and a critical point in making a predictive model for a wider area, is that the Jauf 3rd-order sequences can be subdivided into higher frequency 4th order depositional sequences of transgressive-to-regressive character. Sequence S55 consists of 16 fourth order sequences, while sequence S60 consists of 15 fourth order sequences. The D3B biozone, an extensive stratigraphic marker consisting mainly of dark colored shale, occurs at the top of the TST of sequence S60 and marks its maximum flooding surface (MFS). A notable feature of 4th order sequences is that the transgressive tract tends to thicken significantly landwards whereas the highstand tract thickens in the opposite direction. This trend becomes an important tool in prediction into unknown areas. Third order sequences are mappable on the regional scale, and are very useful exploration mapping tool. Fourth order sequences are of more local extent, and are extremely useful stratigraphic tool for reservoir characterization purposes on field scale as they furnish a high resolution “window” on depositional environments. Long distance shoreface progradation was accomplished mainly during falling stage of sea level.