Basin Evolution and Geochemical Modeling
Apatite fission track analysis; a regional study of North Africa
Postdoctoral research by Dr Frank Lisker, Dr Jonathan Redfern, and Dr Stefan Lubeseder
Objective:
A study covering the offshore basins of North Africa was undertaken. The objective of this project was to review the regional geologic evolution of offshore north Africa and to study in detail some selected study areas characterised by different feeder systems, variations in shelf width, differences in slope morphology and tectonics, in order to determine the evolution and variability of the shelf margin and slope systems, reservoir and source potential and trapping mechanisms.
Data Availability:
Data availability controlled the scope of the study. Three key areas were identified for this study:
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The Pelagian Basin, Offshore eastern Tunisia, whose structural and stratigraphic evolution can be linked to the offshore Sirte Basin of Libya;
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Offshore northern Tunisia, characterised by the presence of the Maghrebian Thrust Belt, a largely unexplored area whose petroleum potential has recently received new interest;
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The offshore Nile Delta, currently one of the most important deepwater provinces of the world.
Offshore eastern Tunisia ( Gulf of Gabes ) was the object of an MSc research project at Oxford Brookes University. Data was provided by Burlington Resources.
The Gulf of Gabes is a prolific hydrocarbon province with multiple targets and numerous producing oil and gas fields. The basin was affected by multiple tectonic episodes, including Early Mesozoic rifting and Mid-Cretaceous to Tertiary alpine compressional events. These resulted in a series of northwest-southeast trending highs and lows that had a significant effect on deposition and facies distribution. Stratigraphically, Mesozoic and Cenozoic reserves are spread over a range of reservoirs, predominantly carbonate. Fields produce oil and gas from a wide range of siliciclastic and carbonate reservoirs associated with structural and stratigraphic traps.
The project aimed to evaluate 2D Seismic data integrated with available analogue well data and published literature and allowed to:
- Critically assess the prerequisites necessary for the accumulation of hydrocarbon reserves.
- Map regional structural trends.
- Reconstruct the timing of deformation and its effects on trap formation.
- Undertake seismic facies and geometry analyses.
- Combine wireline log and seismic data to identify, describe and map source horizons and potential reservoirs.
- Assess the petroleum potential of the Block.
Offshore Northern Tunisia:
The project on “ Regional Structural and Stratigraphic Evolution of the Oligo-Miocene Numidian Flysch, Offshore Northern Tunisia” aimed to:
- Review the structural and stratigraphic evolution and the hydrocarbon habitat of offshore northern Tunisia.
- Reconstruct the palaeogeography and depositional environments of the Numidian Flysch.
- Discuss the provenance of the Numidian Flysch.
- Review the sedimentology of the Numidian Flysch on the base of outcrop data from onshore Tunisia and Sicily.
- Recommend future potential research.
The Numidian Flysch is a thick Oligo-Miocene syntectonic turbidite unit mostly consisting of shales, siltstones, quartz-rich coarse sandstones and subordinate conglomerates. It can be traced discontinuously along the North African margin from the Straits of Gibraltar to Tunisia , and from there eastward to Sicily and Southern Italy (Benomran et al., 1987; Bouillin et al., 1986; Guerrera et al., 1986, 1987, 1992; Hoyez, 1989; Torricelli, 2000). In Tunisia the Numidian Flysch is formed by several stacked turbiditic systems deposited during the Late Oligocene-Early Miocene ( El Maherssi et al., 2001 ) in a tectonic mobile foredeep basin parallel to the developing Maghrebide Belt. It was then thrusted southward over the Tellian units as part of the Maghrebian Chain during Late Serravallian to Early Tortonian (Rouvier, 1977; El Maherssi, 1992; El Euchi et al., 1998).
Regional seismic data was provided by BG, but, due to limited good quality seismic coverage, the study was mainly based on literature available in the public domain. A complete literature review of the Mediterranean Region has been undertaken to address the available information from Tunisia , and also possible analogue data from the wider Mediterranean area. This included a review of the stratigraphy and structural evolution of the area and regional palaeogeographic reconstructions. A regional review of the Numidian Flysch has also been carried out. This summarises the structural and stratigraphic evolution of the area, discuss the provenance and the sedimentology of the Numidian Flysch, and compare the offshore Numidian Flysch with that outcropping in onshore northern Tunisia and offshore Sicily .
Interpretation of available unprocessed seismic lines provided by BG Tunisia has been attempted. The quality of seismic and the limited coverage allowed identification of only a limited number of major structures on some lines; it has not been possible to follow structures and reflectors across lines or to map these features with confidence. No stratigraphic interpretation has been possible to-date.
The review of available geological data across the area revealed favourable source and reservoir rocks for the generation and entrapment of hydrocarbons. Similarities with the Numidian Flysch outcropping in Sicily (where the Numidian Flysch is an exploration target) and onshore Tunisia have been identified. Issues remain on the scarcity of available data, the intense structural deformation and the internal stratigraphic complexity of the Numidian Flysch.