Impact of Paleokarst on the Pinnacle Reef Reservoirs in the Sirt Basin, Libya

More than twenty pinnacle reefs have been discovered in the southeast of Ajdabiya Trough within the Paleocene carbonate sediments, most of which are oil-bearing. However, detailed reservoir characterization and conditions governing oil fill-up in this reef remained unresolved. The major faults provide paths for significant vertical movement of fluids at the edges of the Intisar reef reservoirs. At the same time, the ongoing karst-solution collapse also creates vertical zones for fluids encroachment both outside of and within the productive area of the Intisar reef reservoirs. The seismic data shows numerous karst-collapse features up to 300 m in diameter which developed shortly after the final drowning of Intisar ‘B’ and ‘C’ reefs. These karst-collapse features may be the main contributing factor to the escape of hydrocarbons within these reefs, which could explain the high-water cuts in the Intisar ‘B’ and ‘C’ reefs. On the other hand, the porosity of the southeastern part of the Intisar ‘A’ reef was significantly improved by fracturing and dissolution, where faults associated with fractures are most common in this part of this reef.