--> The ups and downs in the life of a rift basin, Western Sirt Basin, Libya

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The ups and downs in the life of a rift basin, Western Sirt Basin, Libya


The evolution of the Western Sirt Basin in the region of the Dahra Platform, the Amin High and the Kotla Graben has been evaluated using integrated 2D/3D seismic and correlated wireline log data from 36 wells. Three tectonic styles can be identified involving regional Pre-rift intra-continental sag, complex multi-stage rifting and modification by wrenching. The earliest subsidence phase recognized is intracontinental sag which resulted in deposition of a thick Palaeozoic sequence, characterized by low tectonic subsidence within extensive low relief basins. Depositional sequences are widespread, and the main reservoir of this interval is the Cambro-Ordovician Gargaf Formation sandstones.

According to previous work, rifting in the Sirt Basin initiated during the Upper Jurassic-Lower Cretaceous (syn-rift I, syn-rift II, III, and post-rift IV). Based upon our analysis of subsidence curves in the study area however, the earliest phase (syn-rift I) cannot be recognized. However, backstripping subsidence curves do reveal that rifting did occur during three distinct phases (syn-rift II, III, and IV), which covers a time interval “Upper Cretaceous-Middle Eocene”. These phases have been divided into ten sub-phases based on age and subsidence rate. Contour maps of total tectonic subsidence rates have been constructed through the interpolation of boreholes to understand the evolution of the basin in terms of time, although the deepest parts of the trough is yet to be drilled. The oldest rift deposits in the Western Sirt Basin are from the Cenomanian Bahi Formation, (dated at 93.9 Ma by previous paleontological studies), which unconformably overlies Cambro-Ordovician Gargaf Formation (Hercynian Unconformity).

The elongated asymmetric NNE-SSW pull-apart basin has been delineated as a southern deepening trough (Kotla Graben), which separates the eastern block (Amin High) from the Dahra Platform in the west, upon the extensional zone of the major faults based on seismic data interpretation (Fig. 1), TWT, depth and velocity anomalies maps. Triggering time of the rifting in relation to the extensional structural style is identified as an Upper Cretaceous, and the Palaeocene represents the strongest registered rifting time. While the rifting quiescence and consequent regional thermal sagging of the area has been documented by the end of the Middle Eocene.

2D/3D seismic and regional cross sections across the major structural features interpretation have allowed identification of fault abundance, and orientation, whilst enabling their planform distributions to be mapped. The recognition and mapping of an en echelon system of NNE-SSW normal faults, which are interpreted as having commenced during Upper Cretaceous to Lower Eocene based on the stratigraphic record of the study area. Moreover, this system of en echelon faults was initiated by right-lateral wrenching movements during Santonian and Landenian respectively, based on vertical throw (Δt-Age) analysis and 3D seismic interpretation along the major normal faults. This wrenching system has been identified by negative and positive flower type structures along the releasing and restraining bends.

In conclusion, the integrated 2D/3D reflection seismic and wireline log data interpretation have revealed that the structural style of the study area identified as horst-graben style caused mainly by tectonic rifting interrupted by strike-slip movement resulted in forming a pull-apart basin (Kotla Graben), which might represent the main oil kitchen in the study area and surroundings.