Geochemical evaluation of East Sirte Basin (Libya) petroleum systems and oil provenance

Abstract

With cumulative reserves exceeding 23 gigabarrels oil recoverable (GBOR), the East Sirte Basin is a prolific oil province hosting supergiants such as the Amal, Augila-Nafoora and Sarir fields. Production from Precambrian-Oligocene reservoirs yields low sulphur and often highly waxy oils.The Late Mesozoic-Cenozoic Agedabia and older Hameimat, Maragh and Sarir troughs provide the main structural features of the habitat and control hydrocarbon prospectivity. Paleogene subsidence has facilitated the generative process with Mesozoic basin-fill sediments hosting source rocks for productive petroleum system(s). Traditionally the marine Upper Cretaceous Sirte Shale Formation source was thought to provide the dominant charge. Application of geochemical inversion procedures to oil data, however, indicates a greater diversity in oil provenance. Delineation of eight end-member generic oil families indicates a number of complex contributory petroleum systems, mixed-system hybrid oils also being evident. Non-marine (lacustrine) source inputs are also in evidence, enhanced waxiness differentiating petroleums of such provenance. Systematic screening of the stratigraphic section has additionally identified source potential in Nubian (Triassic and Lower Cretaceous), Rachmat-Tagrifet (Upper Cretaceous), Harash (Paleocene) and Eocene formations.Assignment of oil provenance has been achieved via multivariate oil data analysis and application of a carbon isotope-based source kerogen-oil correlation procedure. End-member petroleum systems have been definitively identified involving the Sirte Shale Formation, Rachmat-Tagrifet Formations and Nubian (Triassic) as the contributory sources. The remaining major systems rely upon Pre-Upper Cretaceous lacustrine sediments specific to the Hameimat and Sarir troughs. Whereas numerous archetypal Sirte Shale Formation oils were recognized (e.g. Messla, Hamid, Sarir-L etc.), reserves for many of the giant fields, including Amal, Augila-Nafoora and Sarir-C, rely on hybrid system charging.These results confirm that the prospectivity of the Sirte Basin is not exclusively dependent upon the Sirte Shale Formation, with other petroleum systems in operation, often involving hybrid-sourcing.

Map compilations and synthesis of Africa’s petroleum basins and systems

“…The majority of reserves lie in fluvial sands below Triassic salt and are Palaeozoic-sourced. There are also major reserves at this level in the Sirt Basin, sourced stratigraphically downward from Cretaceous source rocks (Burwood et al 2003). Imaging of undrilled traps below shifting sand dunes and salt is a critical-technology technique that has led to considerable recent success (Drummond et al 2003).…”

“…In the Sirt Basin (Burwood et al 2003), which is the predominant hydrocarbon province within the class, the productive stratigraphic interval extends downward stratigraphically to include the important Albian sandstone play (Gras & Thusu 1998). Other basins in this class include the Gabes Basin of Tunisia and the interior rift basins of Sudan and Niger.…”

“…Other basins in this class include the Gabes Basin of Tunisia and the interior rift basins of Sudan and Niger. With the exception of Sudan, where an Early Cretaceous lacustrine section is the source rock (Schull 1988), the main source level within these basins is the Late Cretaceous marine section, which feeds reservoirs ranging in age from Cambro-Ordovician and Triassic to Neogene (Burwood et al 2003). Excluding the Palaeozoic reservoirs discussed earlier, this class of petroleum provinces contains around 40 BBOE of reserves, or about 17% of Africa's petroleum.…”

The Hydrocarbon Prospectivity of Lower Oligocene Deposits in the Maragh Trough, Se Sirt Basin, Libya

“…The main source kitchen supplying the Concession 96 fields is located to the NW of As Sarah in the deepest portion of the Maragh Trough. Burwood (1996) showed that oil generation may have started as early as the Late Cretaceous. Significant expulsion of hydrocarbons however did not occur until Oligocene to Early Miocene times (Fig.…”

“…The Upper Cretaceous -Lower Tertiary succession above the principal source rocksthe Lower Sirt or (Rakb Gamma) shales, which consist of more than 6,000 ft of tight lime mudstones, claystones and marls overlain by evaporiteshas prevented major vertical migration of hydrocarbons in the Maragh Trough. As the Lower Triassic lacustrine bituminous shale (PUC-A: Arratia, 2000;Schrank, 2000) and the Cretaceous Rakb Gamma shales are still within the oil window and are expelling hydrocarbons (Burwood et al, 1996), migration continues to takes place along faults or carrier beds filling structures or traps with good reservoir properties (Fig. 12).…”