Late Oxfordian micropalaeontology, nannopalaeontology and palaeoenvironments of Saudi Arabia
Authors: Hughes Geraint Wyn ap Gwilym , Varol Osman , Al-Khalid Mokhtar 
Published: October 2021

ABSTRACT
The Hanifa Formation in Saudi Arabia consists of a succession of carbonates, over 100 m thick, that were deposited during the Late Jurassic in an equatorial position on the west flank of the Neo-Tethys Ocean. It consists of the Hawtah and overlying Ulayyah members, each of which is considered as a third-order depositional sequence. The Hawtah Member is assigned an ?Early to Mid-Oxfordian age, based on brachiopod, nautiloid and coccolith evidence; ammonite, nautiloid, coccolith and foraminiferal evidence indicate a Late Oxfordian age for the Ulayyah Member.
A detailed study of the microbiofacies and lithology of the late highstand succession of the Ulayyah member sequence was conducted in 41 cored wells distributed across Saudi Arabia. The aim of the study was to determine the most likely locations for porous and permeable grainstone lithofacies that host the Hanifa Reservoir in the region. A range of palaeoenvironments has been determined which include shallow-lagoon packstones and foraminiferal-dominated grainstones and deep-lagoon wackestones and packstones with Clypeina/Pseudoclypeina dasyclad algae. In addition, a series of basin-margin, shoal-associated biofacies are present that include stromatoporoid back-bank packstones and grainstones with the branched stromatoporoid Cladocoropsis mirabilis, bank-crest grainstones with encrusting and domed stromatoporoids. A few wells also proved the presence of intra-shelf, basin-flank mudstones and wackestones containing tetraxon sponge spicules, deep-marine foraminifera and coccoliths. The Hanifa Formation demonstrates the high environmental sensitivity of the Oxfordian biocomponents in Saudi Arabia. The study has exploited this feature to interpret the regional Late Oxfordian palaeoenvironmental variations, together with inferred hydrocarbon implications, with a moderately high degree of certainty.
This essentially micropalaeontologically based study has revealed the approximate limit of an intra-shelf basin, with an irregular margin, located in the east-central part of the Saudi Arabian portion of the Late Oxfordian Arabian Plate carbonate platform. The basin is flanked by a belt of stromatoporoid banks that pass laterally into a back-bank facies before developing into a lagoonal facies. There is no evidence for shoreline of this basin, although the presence of rare charophytes, wood fragments and quartz grains in the northwest testifies to possible proximity of fluviatile input. The grainstone-dominated basin-margin facies presents good hydrocarbon reservoir facies and its juxtaposition to intra-shelf, potential source-rock basinal sediments provides important new exploration prospects in areas hitherto uninvestigated for hydrocarbon reservoirs, for which the overlying Jubaila Formation provides an efficient regional seal.
The study provides a template for low-cost, high-value guidance for the selection of seismic survey sites in remote, under-explored areas where only a few wildcat well samples are available. The study could also be performed using cuttings samples where cores are not available. The varied biofacies within the Hanifa Formation could be applied for biosteering applications should this tool become necessary in coiled-tube, underbalanced horizontal development wells.