Until recently, the relatively monotonous, thick sequence of Wajid Sandstone which crops out in southwestern Saudi Arabia has been mapped as a single lithologic unit with a Cambrian-Ordovician age. Seismic ties to the outcrop, recent fieldwork by Bureau de Recherches Geologiques et Mineres (BRGM) and Saudi Aramco' geologists and stratigraphic information obtained from a shallow 2260-foot test well (V-608) suggest that the Wajid Sandstone (as mapped in the outcrop) may be divided into five units and that the rocks range in age from Silurian and older to Permian. From oldest to youngest, the five units are the Saq and Qasim formations undivided (Cambrian-Ordovician age), the Zarqa and Sarah formations (Late Ordovician to Early Silurian age), the Qalibah Formation of Early Silurian age, rocks of Devonian to Middle Carboniferous age undivided, and the Unayzah Formation (Haushi Group) of late Carboniferous to Early Permian age. Except for the Qalibah Formation which is the source-rock of recently discovered oil in the Hawtah, Dilam, Raqhib, Nuayyim and Hazmiyah fields to the north, all of these Paleozoic sands have excellent reservoir potential in the subsurface. It is proposed that the term Wajid Sandstone be dropped from usage and that the existing Paleozoic nomenclature discussed above and used by Saudi Aramco be applied to the Wajid Sandstone outcrop.

The Middle to Upper Ordovician Qasim Formation is well exposed in the Qasim region of central Saudi Arabia and is recognized from many wells to the north and east. It consists of the Hanadir, Kahfah, Ra’an, and Quwarah members that are arranged in two coarsening-upward progradational sequences that have a total thickness of about 200 m in outcrops. Two sections were measured in each sequence. The lower sequence is a storm-dominated, shallow-marine depositional system composed of the late Llanvirn to middle Caradoc Hanadir and Kahfah. At least five progradational beach parasequences were identified. The Hanadir overlies deltaic deposits of the upper Saq Formation and marks a major marine transgression onto Gondwana. It is composed of laminated fissile shale formed in an offshore marine environment together with siltstone laminae that indicate periodic influxes of low-density turbidity currents. The overlying Kahfah is composed of shale and sandstone. The upper progradational sequence consists of the middle Caradoc to middle Ashgill Ra’an and Quwarah members. The Ra’an is a succession of homogeneous fissile shale and minor siltstone laminae formed in an offshore marine environment following a major transgression onto the Kahfah. The overlying Quwarah consists of sandstone and minor amounts of siltstone and shale in its lower parts, and a thickly bedded sandstone facies above. The diagnostic sedimentary structures in the Quwarah are large-scale lens-shaped tidal sand waves composed of sigmoidal bundles deposited in a mesotidal setting of barrier bars, tidal channels, ebb-dominated deltas and lagoons. In the subsurface, the Formation thickens northeastward (basinward) to over 4,000 ft (1,220 m). In the same direction, the sand-dominated Kahfah and Quwarah gradually become shale-dominated, small-scale bed forms are more abundant, and identification of members based on lithofacies is more difficult. After deposition of the Quwarah, present-day Saudi Arabia was affected by a Late Ordovician (late Ashgill) glaciation. The Qasim and older formations are deeply incised by glacial channels at the base of the Zarqa and Sarah formations. Basinward, the glacial unconformities (or their submarine erosion surface equivalents) become less significant and a thick succession of the Formation is preserved.

Turbidite sand stones de pos ited in rift set tings are cur rently among the main tar gets of hy dro car bon ex plo ra tion. How ever, the im pact of style of sed i men ta tion, cyclicity and diagenesis on res er voir qual ity of such sand stones is rel a tively poorly explored in the lit er a ture. The sedimentology, strati graphic ar chi tec ture, and diagenetic al ter ations of deep ma rine sand stones of the Miocene Nutaysh Mem ber of the Burqan For ma tion in the Midyan area (Saudi Ara bia) are de scribed based on num ber of mea sured sedimentologic sec tions, lithofacies iden ti fi ca tion in the field and lab o ra tory stud ies. Three lithofacies types are here iden ti fied in the mea sured sec tions. These are from bot tom to top: (1) "lithofacies A" con sist ing of mas sive to thickly-bed ded, coarse-to-very coarse-grained sand stone and con glom er ates; (2) "lithofacies B" con sist ing of well-bed ded, coarse-to-me dium-grained, well-sorted sand stone, and (3) "lithofacies C" con sist ing of thin-bed ded, fine to very fine-grained, cur rent-rip pled sand stone, bioturbated shaley siltstone and marl. The main diagenetic pro cesses in the sand stones in clude the for ma tion of grain-coat ing smectite and rhombic do lo mite. Small amounts of ce ments in clude the for ma tion of authigenic kaolinite and cal cite. The res er voir qual ity is an tic i pated to have been pre served un der the trans for ma tion of smectite to deep burial illite, which is be lieved to pre vent for ma tion of quartz ce ments.

The hydrocarbon-producing Midyan Basin is located in northwestern Saudi Arabia and is surrounded by the Proterozoic igneous basement of the Arabian Shield. It includes thick hydrocarbon-producing sedimentary sequences deposited in half-grabens that formed during rifting of the Red Sea and the gulfs of Suez and Aqaba in the Early Miocene (23.3 Ma). The early syn-rift succession consists of arid alluvial fan sediments and playa evaporates, followed by shallow marine carbonates. The late syn-rift sequences consist of progradational deep sea turbidites and Alpine-type glacial deposits indicating strong vertical uplift during the climax of the rifting (19 Ma). The post-rift succession overlies the late syn-rift successions and consist of shallow marine marls and evaporites. The aim of this study is to examine the hydrocarbon potential of the turbidite sandstones and the formation of various types of glacial deposits in the Burqan Formation. This study also encompasses the importance of various geologic processes in order to understand their significant influence on the geometry, continuity and reservoir quality of oil and gas producing genetically different sandstones in the subsurface of the Burqan Formation in the Midyan Basin. The Alpine-type glacial deposits provide an excellent opportunity to study the presence of continuous vertical and lateral facies variations between true glacial, glacio-fluvial and glacio-marine deposits in the direction of sediment transportation. Unsorted moraines deposited in the deep and U-shaped glacial valleys occupy the northwestern part of the basin. They pass gradually into glacio-fluvial sandstones that contain large polished and striated boulders. In the southeastern part of the deep basin, the glacio-marine deposits are associated with deep sea turbidites and pelagic shales. Many stratigraphic and sedimentologic sections were measured from well-exposed outcrops in every part of the basin to establish various depositional environments. A large number of sandstone samples was collected to examine their reservoir quality.