Re-evaluation of the petroleum potential of the Kufra Basin (SE Libya, NE Chad): does the source rock barrier fall?

The Kufra Basin is a large, underexplored, Palaeozoic intracratonic sag bas in in SE Libya and NE Chad with extensions into NW Sudan and SW Egypt. The basin fill consists of shallow marine to fluvial deposits ranging in age fr om infracambrian to Cretaceous. Geologically, the basin is very similar to the Murzuq Basin in SW Libya which recently presented Libya with its larges t oil discovery for over a decade. Most of the hydrocarbon play elements kn own from the Murzuq Basin also occur in the Kufra Basin: thick, porous Camb ro-Ordovician sandstones are present and would form good reservoirs, lower Silurian shales may act as effective seals, and there are potential structu ral traps in seismically defined fault blocks. However, the source rock ava ilability in the Kufra Basin is currently unclear. One of the two main sour ce rock candidates in the basin is a lower Silurian shale unit (Tanezzuft F ormation). The Tanezzuft shales have been described as being up to 130 m th ick in outcrops at the basin margins, but the shales were found to be repla ced by siltstones and sandstones in two dry exploration wells drilled in th e northern part of the basin by AGIP between 1978 and 1981. Hot shales deve loped at the base of this widespread Silurian shale unit form important sou rce rocks in many areas of North Africa and Arabia. These hot shales are in terpreted to have been deposited in palaeodepressions, such as incised vall eys of the preceding lowstand, or intrashelf basins, during the initial tra nsgression after the melting of the late Ordovician ice cap. The areal dist ribution of the organic-rich unit is, therefore, discontinuous. Fieldwork i n the Kufra Basin has shown that the basal Tanezzuft horizon is not exposed on the northern and eastern margins of the basin. Deep infracambrian rift grabens have been interpreted on seismic lines from the Kufra Basin and, in analogy to Oman and Algeria, could contain organic-rich infracambrian depo sits. The infracambrian succession in the Kufra Basin may contain a second major potential source rock and warrants further investigation. (C) 1999 El sevier Science Ltd. All rights reserved.