English abstract
The Late Cretaceous (Maastrichtian) Ufuq carbonate reservoir was discovered by accident in 2015 during drilling of Ufuq-XX exploration well in Ufuq Field, western Oman. Late Cretaceous (Maastrichtian) Ufuq reservoir is completely different (based on lithological and faunal assemblage) from the Late Cretaceous (Maastrichtian) carbonate which belongs to the Simsima Formation, across Oman. Therefore, according to stratigraphic nomenclature and rules, the Late Cretaceous (Maastrichtian) reservoir of the Ufuq Field cannot be assigned to the known Maastrichtian Simsima Formation. This research aims to characterize the Late Cretaceous (Maastrichtian) Ufuq carbonate reservoir by an outcrop potential analogue study. The probable outcrop analogue for the Ufuq subsurface reservoir, which is selected based on chronological similarities and lithology is, the Late Cretaceous (Maastrichtian) Upper Thaqab Member, which is exposed in Jabal Ruwaydah in Saham, north Oman.Jabal Ruwaydah is mapped in Saham map as Late Cretaceous deposits which belongs to the Upper Thaqab Member. However, Jabal Ruwaydah is mapped in Buraymi map with a scale of 1: 250,000 as belonging to both Late Cretaceous (Thaqab Formation) on the northwestern edge and Tertiary rock in the central and northeastern edge. Biostratigraphic result of this study is agreed with what has been published on the Buraymi map. The northeastern and central Jabal Ruwaydah contains faunal assemblage indicating Tertiary in age (late Paleocene to middle Eocene) which belongs to Ruwaydah Formation. However, the northwestern edge of Jabal Ruwaydah contains a diverse fauna of both large benthic foraminifera and planktonic foraminifera indicating Late Cretaceous age specifically latest Maastrichtian which represent the Upper Thaqab Member. Ecological distribution of Tertiary planktonic fossils is outer neritic zone environment while the ecological distribution planktonic foraminifera of Late Cretaceous age (Upper Thaqab Member) interpreted as outer neritic zone, relatively deeper (bathyal) than the Tertiary fossils.The Ruwaydah Formation is composed of four fining-upward sequences. The base of each sequence consists of a matrix-supported breccia passing up to intercalation of calcarenite and calcirudite and calcareous claystone. Limestone microfacies are dominated by hybrid composition of allocthonous (grain-supported texture) which in turn prevailed by different typified peloids and autochthons pelagic sediments. However, Upper Thaqab Member is prevailed by calcareous claystone interbedded by tabular bedded of polymictic breccia passing-upward to limestone. The depositional model of Ruwaydah Formation has been constructed based on deposit type, platform morphology and biota content, considered as slope carbonate apron model (grain-dominated) whereas, the Upper Thaqab Member is lope carbonate apron model (mud-dominated).Late Cretaceous (Maastrichtian) carbonate of Ufuq reservoir is characterized by the dominance of grain-supported texture (packstone and grainstone). The interpreted depositional system is a carbonate ramp (gentle slope) dominated by, inner ramp, shoal depositional system.The results of this study will shed light to further understand the Ufuq subsurface reservoir in the future such as diagenetic alteration and reservoir quality. Moreover, further detailed study for Jabal Ruwaydah is recommended for regional scale, basin analyses understanding.
