BEYOND THE BOSTON BASIN: PREDICTION OF CARBONATE DEPOSITIONAL SYSTEMS, OFFSHORE SIRT BASIN, LIBYA USING REGIONAL ANALOGS

The onshore Sirt Basin, Libya, is one of the most prolific rift basins in the world, with reserves of approximately 54 bboe. More than half of this hydrocarbon is reservoired in three main carbonate intervals – Late Cretaceous rudist buildups, Paleocene coralgal reefal buildups and Eocene nummulitic banks. Significant exploration activity has recently accelerated in the deep offshore Sirt Basin; however exploration wells are limited to the shelf margins. Because of the limited seismic and well data, the Pelagian basin, Cyrenaica Platform and onshore Sirt basin are the best analogs available for understanding carbonate depositional systems of the offshore Sirt Basin.

Paleogeographic maps of the Cyrenaica Platform, Pelagian basin and onshore Sirt Basin were constructed for Early Cretaceous through Middle Eocene times. These maps are based on well data, industry reports, and, where possible, seismic data. They suggest that at least since Cenomanian time, the offshore Sirt basin has experienced deeper water sedimentation than onshore Sirt basin. This transition from onshore to offshore can be defined as a ramp that dips to the NNW, and reflects the onset of the transition from continental crust to oceanic crust. This ramp geometry was an important control on facies distribution from Early Cretaceous through present.

Intracontinental rifting in the onshore and offshore Sirt began in the Early Cretaceous, and influenced the overall ramp geometry. Cyrenaica, in particular, was cut by this rifting, which increased the degree of topographic complexity and potentially provided additional high blocks for carbonate development. When much of the onshore Sirt Basin was experiencing shallow marine deposition of carbonate facies, the offshore Sirt is predicted to have been too deep for the development of reservoir quality shallow marine, high energy carbonates. However, during the Cenomanian-Turonian, while onshore Sirt was non-marine to very shallow marine, the offshore Sirt may have been at the appropriate depth for the growth of shallow marine rudistid carbonate reservoirs. Similiarly, in the Early Eocene, dolomites and evaporites were deposited onshore Sirt, while offshore Sirt may have had growth of nummulitic banks.