Stratigraphic Controls of Petroleum Systems in the Sirt Basin, Libya
Some +27 x109 bbls oil reserves have been found in the Sirt Petroleum Province in a wide variety of structural and stratigraphic traps. The spatial distribution of these reserves is controlled by the stratigraphic architecture of pre-rift, syn-rift and post-rift source facies, migration conduits and reservoir rocks.
A regional Gondwana sequence once blanketed the region, but was largely stripped away by ‘Hercynian’ unroofing, leaving significant reservoirs in remnant Palaeozoic sandstones and weathered basement. Rifting commenced in the Triassic with rich source rocks deposited locally in lacustrine basins. Extension became more widespread during the Late Jurassic-Early Cretaceous. Syn-rift continental sands and shales of this age now form important reservoirs and source rocks in southeast Sirt.
Major extensional collapse occurred in late Aptian(?). Several grabens developed across the larger Sirt region, separated by prominent horsts. These were flooded from the north during the later Cretaceous. Restricted marine source facies were deposited locally near the base of the transgressive infill, passing up into the Rachmat-Sirt shales, which now form the most prolific source of the Province.
By end Cretaceous, the horst and graben topography had been largely buried. Carbonate shoals developed on residual highs. These coalesced into more extensive platforms during the Paleocene and by mid-Eocene had evolved into a continuous north facing shelf, with a number of prolific reservoirs.
Subsidence increased in the central part of the Sirt Basin in late Eocene, with uplift and unroofing towards the west. Clastic deposition advanced northwards extending across the central Sirt during the Oligocene, to form reservoirs locally. Mixed clastic-carbonate deposition followed in the Miocene, ending with emergence and exposure during the Plio-Pleistocene.
Hydrocarbon expulsion started during the Eocene, and continued to the present. A number of discrete and sometimes overlapping (hybrid) petroleum systems developed with a variety of migration and entrapment styles, reflecting the controlling stratigraphic architecture. These range from short distance vertical downloading within the grabens to long distance lateral migration onto the adjacent horsts. Locally in the western part of the Sirt, late uplift encouraged vertical leakage and partial dispersal into shallower reservoirs above.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009