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1

Sedimentological and micropalaeontological evidence to elucidate post-evaporitic carbonate palaeoenvironments of the Saudi Arabian latest Jurassic

Hughes G., Naji N.

Volumina Jurassica

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2008

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Vol. 6, no. 1

61-73

EN

The Hith Formation forms the youngest lithostratigraphic unit of the Jurassic Shaqra Group. It represents the culmination of a succession of hypersaline and euryhaline cycles that characterise the Late Jurassic of Saudi Arabia. The Formation is poorly exposed in central Saudi Arabia, but it has been studied in detail in subsurface eastern Saudi Arabia where the upper carbonate member hosts an important hydrocarbon reservoir called the Manifa Reservoir. Chronostratigraphic control is absent from the formation itself, and the Tithonian age is suggested for the Hith Formation based on its stratigraphic position between the underlying Arab Formation of Late Kimmeridgian age, and the overlying Sulaiy Formation, of Late Tithonian to Berriasian age. The Hith Formation needs redefining in the light of new lithological evidence, and a tripartite member scheme is suggested. This includes the lower anhydrite-dominated member here termed the “anhydrite” member, and considered to represent hypersaline subaqueous deposition within a restricted deep lagoon during the lowstand systems tract of the Manifa sequence. A “transitional“ member consists of interbedded anhydrites and carbonates and approximates with the transgressive zone. The overlying “carbonate” member represents the results of a prograding shallow, normal salinity marine succession related to the highstand systems tract. Interbedded carbonates within the evaporites are interpreted to represent superimposition of a higher frequency, possibly 4th order eustatic cyclicity. The “carbonate“ member hosts the Manifa Reservoir, and here proposed as the Manifa Member, consist of five parasequences, each of which represents a shoaling-upwards cycle with a succession of up to five repeated lithofacies and biofacies that commences with a stromatolitic, microfaunally-barren unit followed by fine-grained grainstones with a monospecific but abundant ostracod biofacies. A succession of coarse pelloidal grainstones with rare foraminifera, including Redmondoides lugeoni, Trocholina alpina with a variety of unidifferentiated valvulinids and miliolids then follows, that passes vertically into coarse ooid grainstones, with rare Redmondoides lugeoni, forming the uppermost part of each parasequence.

2

Oxfordian biofacies and palaeoenvironments of Saudi Arabia

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

Volumina Jurassica

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2008

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Vol. 6, no. 1

47-60

EN

The Hanifa Formation in Saudi Arabia consists of a succession of carbonates, over 100 m thick, that were deposited during the Late Jurassic. It consists of two depositional sequences represented by the lower Hawtah Member and an upper Ulayyah Member, respectively. The Hawtah Member is assigned an Early (?) to Middle Oxfordian age, based on brachiopod, nautiloid and coccolith evidence. The Ulayyah Member is assigned a Late Oxfordian age based on ammonite, nautiloid, coccolith and foraminiferal evidence. Detailed study of the microbiofacies and lithology of the late highstand succession of the Ulayyah sequence in 41 cored wells distributed across the Kingdom was aimed at determining the most suitable locations for porous and permeable grainstone accumulation as lithofacies hosts the Hanifa Reservoir elsewhere in the region. A range of palaeoenvironments has been determined, based on integrated biofacies and lithofacies, that 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 intrashelf basin-flank mudstones and wackestones containing sponge spicules, deep marine foraminifera and coccoliths. This study provided control to delimit an intrashelf basin with an irregular margin situated in the east-central part of the Saudi Arabian portion of the Arabian Plate carbonate platform during 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 dominated basin margin facies 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.

3

Biofacies and palaeoenvironments of the Jurassic Shaqra Group of Saudi Arabia

Hughes G.

Volumina Jurassica

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2008

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Vol. 6, no. 1

33-45

EN

The Jurassic succession in Saudi Arabia consists of seven formations, forming the Shaqra Group, of which most are carbonate and some are partly evaporitic, and is of economic importance because it hosts twelve hydrocarbon reservoirs, including the Arab-D Reservoir within the world’s largest oilfield at Ghawar. The Minjur-Marrat formational boundary marks the Triassic-Jurassic boundary, of which the Marrat is dated as Toarcian. A significant unconformity separates this unit from the overlying Dhruma Formation, of Bajocian to Bathonian ages. The Tuwaiq Mountain Formation, of Callovian age, overlies the Dhruma Formation, with reduced hiatus. The Hanifa Formation, of Oxfordian age, is separated from the Tuwaiq Mountain Formation by a minor hiatus, as are the successive Jubaila, Arab and Hith formations, of Kimmeridgian to Tithonian ages. The Jurassic-Cretaceous boundary is currently placed within the lower part of the overlying Sulaiy Formation. A combination of semi-quantitative micropalaeontological and macropalaeontological analysis of closely-spaced thin sections from these carbonates displays a tiered relay of clearly defined microbiofacies cycles. These cycles reveal palaeoenvironmental trends that subdivide the succession into parasequences, transgressive and highstand systems tracts that are not always evident from the sedimentology alone. The biofacies approach to elucidating palaeoenvironmental variations of the Shaqra Group provides significant insights to the Jurassic history of the Arabian Plate, as well as serving to explain the origin and stratigraphic location of hydrocarbon reservoirs, seals and potential source rocks. The Shaqra Group spans at least 36 Ma, and qualifies as a second order depositional sequence, within which the formations can be considered as third order sequences. Better chronostratigraphic constraint of the depositional sequences reveals elevated subsidence rates for the Dhruma, Tuwaiq Mountain and Hanifa formations that can be related to established episodes of global plate readjustment. Tectonoeustatic and possibly glacioeustatic controls on depositional cyclicity probably exerted an influence on the major unconformities within the Shaqra Group.

4

Post-evaporitic carbonates of the Saudi Arabian Late Jurassic

Hughes G., Naji N., Al-Khaled M., Harland T., Bacchus D.

Volumina Jurassica

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2006

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Vol. 4, no. 1

91-91

EN

The Hith Formation forms the latest lithostratigraphic unit of the Jurassic Shaqra Group, and was deposited during the Tithonian. The Formation outcrops in central Saudi Arabia, but has been studied in detail in subsurface eastern Saudi Arabia where the upper carbonate member hosts an important hydrocarbon reservoir called the Manifa reservoir. Chronostratigraphic control is absent, and the Tithonian age is based on stratigraphic position between the underlying Kimmeridgian Arab Formation, and the overlying Sulaiy Formation, of Berriasian age. The lower anhydrite-dominated member is un-named, and considered to represent subaqueous deposition representing the transgressive systems tract of the Manifa sequence. A transitional unit, consisting of interbedded anhydrites and carbonates, approximates with the maximum flooding zone, and the overlying carbonates are considered to represent the results of a prograding shallow marine succession related to the highstand systems tract. The carbonates of the Manifa reservoir consist of five parasequences, each of which represents a shoaling-upwards cycle that commences with a stromatolitic, microfaunally-barren unit that is followed by fine-grained grainstones with a monospecific but abundant ostracod biofacies. A succession of coarse pelloidal grainstones with rare Foraminifera, including Redmondoides lugeoni, Trocholina alpina and miliolids, then follows, that passes vertically into coarse ooid grainstones that form the uppermost part of each parasequence. The Hith Formation represents the culmination of a succession of hypersaline and euryhaline cycles that characterise the Late Jurassic of Saudi Arabia, and provides an insight to the palaeoenvironmental conditions that existed across the Arabian Plate at the end of the Jurassic.

5

Oxfordian palaeoenvironments of Saudi Arabia

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

Volumina Jurassica

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2006

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Vol. 4, no. 1

90-91

EN

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.

6

Biofacies and palaeoenvironments of the Jurassic Shaqra Group of Saudi Arabia

Hughes G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

89-90

EN

The Jurassic succession in Saudi Arabia consist of eight formations, of which most are carbonate and some are partly evaporitic, and is of economic importance because it hosts twelve hydrocarbon reservoirs, including the Arab-D reservoir within the world’s largest oilfield at Ghawar. The Minjur-Marrat formational boundary marks the Triassic/Jurassic boundary, of which the Marrat is dated as Toarcian. A significant unconformity separates the overlying Dhruma Formation, of Bajocian to Bathonian age. The Tuwaiq Mountain Formation, of Callovian age, overlies the Dhruma Formation, with reduced unconformity duration. The Hanifa Formation, of Oxfordian age, is separated from the Tuwaiq Mountain Formation by a minor unconformity, as are the successive Jubaila, Arab and Hith formations, of Kimmeridgian to Tithonian age. The Jurassic/Cretaceous boundary is currently placed at the Hith-Sulaiy formational contact. Intensive analyses of the carbonates reveals variably rich micropalaeontological biofacies that contain foraminiferal species of potential palaeoenvironmental significance, especially when applied to cored mudstone to grainstone repeated successions of which the hierarchy is often difficult to elucidate. A combination of semi-quantitative micropalaeontological and macropalaeontological analysis of closely-spaced thin sections from these carbonates reveals clearly defined microbiofacies cycles. Their stacking order provides clearly defined palaeoenvironmental trends that subdivide the succession into potential parasequences, transgressive and highstand systems tracts. The Shaqra Group spans at least 38 Ma, and qualifies as a second order depositional sequence, within which the lithostratigraphic units of formation identity fall into third order sequences. The extensive duration of unconformities spanning the Hettangian-Sinemurian and the Aalenian-Bajocian need explaining, especially when compared to the relatively minimal interformational unconformities that characterize the Bajocian-Tithonian succession. Eustatic sea-level data indicates that the Hettangian to Early Pliensbachian is characterized by a relatively insignificant sea level variation when compared with the gradual rise from the Late Bajocian to the Kimmeridgian. The mid Bajocian event approximates with the global increase in percent dolomite in abiotic muds, and may be associated with a transitional icehouse-greenhouse phase, for which post Bajocian eustatic falls would be expected to display increasing greenhouse affinity. Preservation of calcitic ooids within the Middle and Late Jurassic carbonate reservoirs within Saudi Arabia, together with excellent intergranular porosity testify to the calcitic oceanic conditions associated with greenhouse times. The biofacies approach to elucidating palaeoenvironmental variations of the Shaqra Group provides significant insights to the Jurassic history of the Arabian Plate, as well as serving to explain the origin and stratigraphic location of hydrocarbon reservoirs.