Biofacies and palaeoenvironments of the Jurassic Shaqra Group of Saudi Arabia

• Autorzy: Hughes G.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland)
• Języki publikacji: EN

Abstrakty

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.

Słowa kluczowe

EN

Saudi Arabia; Jurassic; Shaqra Group; micropalaeontology; biostratigraphy; sequence stratigraph; palaeoenvironments

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 89--90

Twórcy

autor

Hughes G.

• Saudi Aramco, Geological Technical Services Division, Dhahran 31311, Saudi Arabia,