Cretaceous Source Rocks Potential of Northern Libya
World wide Cretaceous organic rich horizons are numerous, particularly in North Africa and the Middle East. These organic rich layers produced some 29.5% of the world’s oil reserves, sourcing some 85% of Libya’s. It is estimated that the Upper Cretaceous strata in the Sirt Basin of northern Libya have generated 117 million barrels of in place oil. These Upper Cretaceous strata contain three major stratigraphic source intervals; the upper Cenomanian Etel Formation, upper Santonian to Campanian Rachmat Formation and Campanian Sirt Shale. The later is the most important. To the east the Sirt Shale is a thick, dark black to brown, laminated and bituminous rock. The black color is the result of high percentages of organic matter that accumulated in the deeper troughs. The troughs were favored by restricted water circulation, stagnant water and anoxic conditions. As the Sirt Basin subsided, sediment accumulations filled the deeper parts including the subsiding and generally northeast-southwest trending Maradah, Hameimat and Zallah Troughs. In the upper Cretaceous-Tertiary this resulted in up to 3000 m of thickening sediments. As burial depths increased, this enhanced greater pressures and temperatures, producing high quality oil with an API gravity that ranges between 33°-42° and a sulfur content of between 0.15 and 0.66%. The accumulation of the organic rich Sirt Shale coincided with periods of warm climate in the Campanian. The atmosphere then contained appreciable quantities of carbon dioxide while a depletion of the oxygen content in the Mediterranean water occurred with accumulation of organic matter, derived mainly from planktonic fauna. Jenkyns (1980) suggested that three major oceanic anoxic events occurred within the late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and that of the Coniacian-Santonian.boundary. The organic rich Rachmat shale accumulated in the Sirt Basin during the Coniacian-Santonian at the margin of the anoxic Tethys Ocean. We also contend there was a further major source rock event in the Sirt Basin that produced the organic rich Sirt Shale. This coincided with the major Campanian marine transgression of the southern Mediterranean region. Our geochemical analysis of the Campanian shale from well E-59 indicates a TOC average of 2.48. It has a low hopane/sterane ratio and a hydrogen/oxgene index ratio of the kerogen that indicates that these source rocks were marine and contain mainly type I and II kerogens.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009