TY - BOOK

T1 - Sedimentology and stratigraphy of the Ajdabiya Trough (Sirt Basin, Libya)

AU - Vrsic, Ales

N1 - no embargo

PY - 2022

Y1 - 2022

N2 - The Sirt Basin is renowned for hosting significant hydrocarbon accumulations and has been the site of extensive hydrocarbon exploration since the 1960s. Paleocene sedimentary rocks account for a large portion of the stratigraphic record, and Paleocene shallow-marine carbonate reservoirs contain about 30% of the oil reserves in the Sirt Basin. These reservoirs were the subject of extensive studies regarding reservoir architecture and storage capacity. Most of the studies were carried out before modern microfacies analysis and biostratigraphic zonations were established. Since most interpretations/models are outdated and numerous questions remain unanswered, a modern revision is necessary. The enigmatic Intisar domal structures (Upper Sabil Formation) of the Ajdabiya trough were up until recently interpreted as pinnacle reefs/bioherms of the Late Thanetian, surrounded by contemporaneous deep-marine sedimentary rocks. These structures were examined in terms of biostratigraphy, microfacies, and depositional environment and a new depositional model was created based on the results. Highly diverse larger foraminiferal assemblages from the shallow-marine carbonates yielded a Selandian to Early Thanetian age. The age was further refined to Selandian by calibrating with planktonic foraminifera and calcareous nannoplankton zones where shallow-marine carbonates are intercalated between deep-marine sedimentary rocks. Microfacies analysis demonstrated that the described microfacies types (MFT) are characteristic for a carbonate ramp geometry and not for reef structures. Thirteen MFT were defined starting with outer ramp deposits characterized mainly by wackestones with planktonic and small benthic foraminifera. These deposits were encountered in the distal area adjacent to the domal structures. The core of the domal structure consists of packstones, rudstones and floatstones. The diverse fauna and flora composed of larger benthic foraminifera, corals, red algae and bryozoans are characteristic for a mid-ramp setting. The crest of the domal structure is composed of Glomalveolina-rich grainstones, packstones and wackestones and are assigned to the inner ramp. The biostratigraphic resolution of larger benthic foraminifera is insufficient to unravel the dynamic depositional history (e.g., timing of the erosional event) of the domal structures. Therefore, the underlying and overlying strata were dated using planktonic foraminifera and calcareous nannoplankton to document the depositional history of the Paleocene to Early Eocene succession. The underlying Sheterat Formation was constrained to the Early Selandian (planktonic zone P3). The Upper Sabil carbonate ramp was formed during the Middle Selandian. The Late Selandian is characterized by a major sea-level fall of 130m, resulting in subaerial exposure and erosion of the carbonate ramp. This erosional event generated hilly morphology, i.e. ¿domal structures¿. During the lowstand and early transgression, an alternation of claystone and limestone was deposited, defined here as the newly introduced Intisar Formation. The stratigraphic range of the Intisar Formation is Globanomalina pseudomenardii zone (planktonic zone P4). The Paleocene/Eocene boundary is characterized by a thermal maximum, faunal turnover and eustatic sea-level rise. The Harash Formation represents a shallow-marine limestone succession deposited during the Early Ilerdian. The Harash Formation as a highstand succession infilled the erosional topography of the older carbonate ramp (Upper Sabil Formation) after a stratigraphic gap spanning approximately 4 million years. The overlying Kheir Formation represents a further deepening of the depositional environment and can be correlated with the Eocene Thermal Maximum 2 (Upper Ilerdian - Planktonic Zone E4). The morphologically highest area of the domal structures was thus covered by dee

AB - The Sirt Basin is renowned for hosting significant hydrocarbon accumulations and has been the site of extensive hydrocarbon exploration since the 1960s. Paleocene sedimentary rocks account for a large portion of the stratigraphic record, and Paleocene shallow-marine carbonate reservoirs contain about 30% of the oil reserves in the Sirt Basin. These reservoirs were the subject of extensive studies regarding reservoir architecture and storage capacity. Most of the studies were carried out before modern microfacies analysis and biostratigraphic zonations were established. Since most interpretations/models are outdated and numerous questions remain unanswered, a modern revision is necessary. The enigmatic Intisar domal structures (Upper Sabil Formation) of the Ajdabiya trough were up until recently interpreted as pinnacle reefs/bioherms of the Late Thanetian, surrounded by contemporaneous deep-marine sedimentary rocks. These structures were examined in terms of biostratigraphy, microfacies, and depositional environment and a new depositional model was created based on the results. Highly diverse larger foraminiferal assemblages from the shallow-marine carbonates yielded a Selandian to Early Thanetian age. The age was further refined to Selandian by calibrating with planktonic foraminifera and calcareous nannoplankton zones where shallow-marine carbonates are intercalated between deep-marine sedimentary rocks. Microfacies analysis demonstrated that the described microfacies types (MFT) are characteristic for a carbonate ramp geometry and not for reef structures. Thirteen MFT were defined starting with outer ramp deposits characterized mainly by wackestones with planktonic and small benthic foraminifera. These deposits were encountered in the distal area adjacent to the domal structures. The core of the domal structure consists of packstones, rudstones and floatstones. The diverse fauna and flora composed of larger benthic foraminifera, corals, red algae and bryozoans are characteristic for a mid-ramp setting. The crest of the domal structure is composed of Glomalveolina-rich grainstones, packstones and wackestones and are assigned to the inner ramp. The biostratigraphic resolution of larger benthic foraminifera is insufficient to unravel the dynamic depositional history (e.g., timing of the erosional event) of the domal structures. Therefore, the underlying and overlying strata were dated using planktonic foraminifera and calcareous nannoplankton to document the depositional history of the Paleocene to Early Eocene succession. The underlying Sheterat Formation was constrained to the Early Selandian (planktonic zone P3). The Upper Sabil carbonate ramp was formed during the Middle Selandian. The Late Selandian is characterized by a major sea-level fall of 130m, resulting in subaerial exposure and erosion of the carbonate ramp. This erosional event generated hilly morphology, i.e. ¿domal structures¿. During the lowstand and early transgression, an alternation of claystone and limestone was deposited, defined here as the newly introduced Intisar Formation. The stratigraphic range of the Intisar Formation is Globanomalina pseudomenardii zone (planktonic zone P4). The Paleocene/Eocene boundary is characterized by a thermal maximum, faunal turnover and eustatic sea-level rise. The Harash Formation represents a shallow-marine limestone succession deposited during the Early Ilerdian. The Harash Formation as a highstand succession infilled the erosional topography of the older carbonate ramp (Upper Sabil Formation) after a stratigraphic gap spanning approximately 4 million years. The overlying Kheir Formation represents a further deepening of the depositional environment and can be correlated with the Eocene Thermal Maximum 2 (Upper Ilerdian - Planktonic Zone E4). The morphologically highest area of the domal structures was thus covered by dee

KW - Paleogene

KW - biostratigraphy

KW - microfacies

KW - shallow-marine carbonates

KW - depositional history

KW - Paleogen

KW - Biostratigraphie

KW - Mikrofazies

KW - Flachmarine Karbonate

KW - Ablagerungsgeschichte

M3 - Doctoral Thesis

ER -