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1 2006

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Oxfordian palaeoenvironments of Saudi Arabia

Hughes G., Al-Khaled M., Varol O.

Volumina Jurassica

2006

Vol. 4, no. 1

90-91

The Hanifa Formation in Saudi Arabia consists of a succession of carbonates, over 90 m thick that were deposited during the Late Jurassic, Oxfordian. It consists of a lower Hawtah Member and an upper Ulayyah Member. A Late Oxfordian age is based on the first appearance of the benthonic foraminifera Alveosepta jaccardi and, within the upper part of the underlying Hawtah Member, the local extinctions of the calcareous nannofossil species Watznaueria manivitiae and Stephanolithion bigotii together with an influx of Ellipsagelosphaera britannica. A detailed study of the micropalaeontology, nannopalaeontology, biofacies, sedimentology and wireline log character of the uppermost parts of 35 cored wells distributed across the Kingdom has revealed a variety of depositional environments. The late highstand succession of the Formation displays a variety of biofacies and lithotextures, of which the grainiest host the Hanifa Reservoir. These are associated with stromatoporoid banks that developed on the flanks of intrashelf basins, although grain-dominated shoals within the lagoons also present reservoir potential. A range of palaeoenvironments has been determined, based on integrated biofacies and lithofacies that include shallow lagoon packstones and foramininiferal dominated grainstones, deep lagoon wackestones and packstones with Clypeina/Pseudoclypeina dasyclad algae, stromatoporoid back bank packstones and grainstones with the branched stromatoporoid Cladocoropsis mirabilis, bank crest grainstones with encrusting and domed stromatoporoids and intrashelf basin flank mudstones and wackestones containing sponge spicules, deep marine foraminifera and coccoliths. This study has assisted delimitation of an intrashelf basin with an irregular margin situated in the East Central part of the Saudi Arabian portion of the Arabian Plate palaeoenvironments during the Late Oxfordian. The basin is flanked by a belt of stromatoporoid banks that pass laterally into a back-bank facies before developing into a lagoon facies. There is no evidence for the shoreline of this basin, although the presence of rare charophytes in the northwest testifies to possible proximity of fluviatile input. The grainstone margin presents good hydrocarbon reservoir facies and its juxtaposition to intrashelf basinal sediments with potential source rock character provides exciting new prospects in areas hitherto uninvestigated for hydrocarbon reservoirs.