Abstract
In the central-eastern Sirt Basin, enigmatic Intisar domal structures host significant hydrocarbon accumulations. These structures have been commonly interpreted as pinnacle reefs/bioherms occurring in the open-marine basinal environment. Generally, pinnacle reefs/bioherms are mainly characterized by in situ carbonates. The current study challenges the Intisar pinnacle reef/bioherm model by examining one of the domal structures in terms of biostratigraphy, microfacies and depositional environment. These structures were dated using larger benthic foraminifera, which yielded a Middle to Late Paleocene age (Selandian–Early Thanetian). Thirteen microfacies types representing different carbonate ramp environments ranging from outer ramp to inner ramp, were defined. Outer ramp deposits have been observed adjacent to the domal structure, represented mainly by wackestone with small benthic and planktonic foraminifera. The outer ramp deposits are most likely isochronous to the domal structures. The lower part of the domal structures is composed mainly of foraminiferal–algal–echinodermal packstones. The upper part is characterized by foraminiferal–algal–echinodermal packstones with intercalated microbialite–coral boundstones. The euphotic inner ramp deposits are preserved on the crest of the domal structure, consisting of grainstone and packstone rich in Glomalveolina. As a result of this study, the Intisar domal structures are seen as erosional relics of a carbonate ramp and no evidence for pinnacle reef/bioherm model was found.
Highlights
The Sirt Basin (Fig. 1a, b) is one of the most prolific hydrocarbon provinces globally, and the important reservoir rocks are Middle to Late Paleocene shallow-marine carbonates (Ahlbrandt 2001)
The Paleocene shallow-marine carbonates of the Upper Sabil Formation are dated using larger benthic foraminifera
The Middle to Late Paleocene shallow-marine carbonate deposits of the Sirt Basin are dominated by Ranikothalia, miscellaneids and rotaliids, they can be successfully dated using Shallow Benthic Zonation
Summary
The Sirt Basin (Fig. 1a, b) is one of the most prolific hydrocarbon provinces globally, and the important reservoir rocks are Middle to Late Paleocene shallow-marine carbonates (Ahlbrandt 2001) This time interval has been studied intensively by various authors concerning the Sciences, Jagiellonian University, Gronostajowa 3a, 30‐387 Kraków, Poland 4 Engineering Planning Department, Fourth Floor, Zueitina Oil Company, Sidi Issa Street, P.O. Box 2134, Tripoli, Libya architecture of the carbonate reservoirs (Terry and Williams 1969; Bebout and Pendexter 1975; Brady et al 1980; Gumati 1992; Mresah 1993, 1998; Spring and Hansen 1998), little information has been given regarding the detailed biostratigraphy, microfacies, depositional environment and geometry. According to its original definition (Cumings and Shrock 1928, p. 599), bioherm is “a dome-like, lens-like or other circumscribed mass built exclusively or mainly by sedentary organisms and enclosed in normal rock of different
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