Hydrocarbon Potential of Early Eocene Rocks offshore Northern Libya

Hassan S. Hassan, Tim J. Green, and Richard Woodfine

Eocene sedimentation along the central North African margin was locally controlled by an upwelling system as indicated by the occurrence of associated phosphate deposits in Tunisia and Egypt. This led to the development of an organic-rich interval within the Eocene which provided an excellent source rock in Tunisia and NW offshore Libya (Bou Dabbous Formation) and in the Gulf of Suez (Thebes Formation).

The Top Early Eocene can be confidently tied from wells in offshore Cyrenaica, where this interval consists mainly of wackestone to mudstone carbonates with planktonic forams that were deposited in an outer-shelf setting. NE-SW seismic lines show that the Eocene shelf margin prograded westward into the Sirt Trough and the southern region of BP’s EPSA area C, and that there was significant deepening and thickening, and a facies change into more argillaceous sediments, in this direction. The Top Early Eocene horizon (Ypresian) represents a flooding surface that appears to have great continuity and relatively high amplitude (bright spots) over all of Area C. It is overlain by a Middle Eocene transparent, low-amplitude interval that gradually thins towards the northwest. The strong negative acoustic impedance contrast and the facies changes from the shelf to basin suggest possible source rock development within the troughs.

The Eocene Nummulites-bearing carbonates are proven reservoir rocks in the Gulf of Gabes, Pelagian Shelf and onshore Sirt Basin. Nummulites are benthic forams, which lived and thrived in the warm shallow waters that occurred around the Mediterranean region in the Eocene. Their hydrodynamically controlled accumulation created low-relief banks that have been documented in many places around the Mediterranean margin (e.g. Cyrenaica, Tunisia and Sicily). In Libya and offshore Tunisia, nummulites distribution appears to have coincided with long-lived paleo-highs that can be interpreted from gravity and seismic data. Similar nummulites accumulations are thought to occur on paleo-highs that were part of tectonically isolated platforms formed on the western flank of the main Sirt graben (northern extension of Ajdabya Trough).

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013