Cretaceous Source Rocks Enigma: Northern Libya Case Study
Hassan, Hassan S.; Kendall, Christopher G.
Geology, University of Benghazi, Benghazi, Libyan Arab Jamahiriya.
The Cretaceous period contains the best source rock horizons of the geologic record. Sourcing some 29.5% of the world's oil reserves, they are especially important in the Mediterranean realm forming the source for 85% of Libya's 45 billion of oil reserves.
Critical factors combine to enhance the Cretaceous as time for the accumulation of organic shales with tectonic configuration and depositional history playing major roles. Together the break-up of Pangea, extensive volcanic and plume activity, globally warm climates, and an eustatic sea level high all favor high rates of organic carbon accumulation.
The organic rich layers of this time coincided with periods of warm climate. At this time the atmosphere contained an appreciable increase in quantities of carbon dioxide while there was a depletion of the oxygen content of ocean water. Major global marine transgressions occurred in the Cretaceous and vast quantities of terrestrial plant material were transported into the oceans.
Controls on the spatial and temporal distribution of Cretaceous organic shales in northern Libya include; formation of the Sirt Basin as a rifted cratonic structure, high concentrations of algae type organic matter deposited during this syn-rift stage, limited water circulation and development of local anoxic conditions in deep grabens.
The vertical and horizontal variations in organic matter richness within the shales of the Sirt Basin suggest that the accumulations of these shales were controlled by the local paleogeography that included the horst and graben (paleo-depression) topography.
Though locally the Late Cretaceous section of Libya contains important oil source rocks, the Cyrenaica section does not match the paradigm of the Sirt Basin. Here, located on the adjacent eastern side of the Sirt Basin and separated by Antelat fault, this region had a different tectonic and depositional history that did not favor the accumulation or preservation of organic shales as was the case of the Sirt Basin. The section is comprised of two major facies; a restricted shallow (proximal) platform facies and open (distal) platform facies that accumulated within a low angle platform depositional system. Probably the local oxygenated water and exposure to wave action on the basin floor destroyed the organic potential. In contrast to Sirt Basin the upper Cretaceous sedimentary section of this part of NE Libya had a total organic content with less than 0.4% organic matter and their source rock maturity is very low.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012