Controls on Hydrocarbon Generation from Eocene Source Rocks in the Ash Shulaydimah Trough, NE Libya

In north-eastern Libya, the Eocene succession within the Ash Shulaydimah Trough is not well understood with respect to its potential as a source rock for petroleum, despite the presence of organic-rich shale layers. The geological and geochemical characteristics of the source rocks of the Eocene period in the Ash Shulaydimah trough have been studied, with particular attention to the Lower shale and the Eocene Antelat formation. The assessment is conducted by re-evaluating existing pyrolysis and total organic carbon (TOC) Rock-Eval data from numerous exploration wells. The data is combined with information from burial history analysis, stable isotope data, observations of the rock's texture, information about the environment in which the rocks were formed and a stratigraphic analysis. These source rocks are of Eocene age and contain a generally high organic content, ranging from 1.8 to 8.7 percent by weight. In some regions of the basin, organic content was very high, as reflected by high hydrogen index values; this high hydrogen index is characteristic of oil-generating kerogen. It appears that the oxygen levels in the area were low, and the organic matter found is primarily marine life, with low carbon-13 levels and deposited in low-energy environments where oxygen was blocked mainly out, especially in the clayey limestone formations. The quality of the source rock does not appear to limit hydrocarbon production in the Eocene succession. The majority of source rocks in the Eocene are immature; this is shown by the thermal maturity data which indicate that in many cases, the samples have a Tmax value that is too low to have generated oil. The history of sediment burial suggests hydrocarbons began forming during the Eocene and increased until the sedimentary rocks were exposed in the late Pliocene. In the Ash Shulaydimah trough, hydrocarbon generation seems to be limited to specific areas where expulsion occurs. In the D1-NC129 well, the presence of oil indicates that the Antelat Formation can generate oil if burial is deep enough. The main risk during the exploration of the Eocene petroleum system in western Cyrenaica comes from the effectiveness of the charge rather than from the source rock's quality.