Thermal history and source rock characterization of a Paleozoic section in the Awbari Trough, Murzuq Basin, SW Libya

摘要

In some areas of the Murzuq Basin, SW Libya, the lower Silurian member comprises highly radioactive shales (Hot Shale), which are regarded as the most significant factor controlling petroleum generation in the basin. In this context, it was the goal of our project to study the distribution and maturity of the Hot Shale in the framework of basin evolution. Organic geochemical, organic petrological and basin modelling methods were used to obtain a more thorough understanding. Four wells from the northern and central part of the Awbari Trough have been selected for this study. The Hot Shale contains mainly Type II kerogen with high TOC values ranging from 3 to 23%, which is considered as an indication for excellent source rock potential. A good petroleum potential is further supported by Hydrogen Indices varying between 220 and 380 (mg HC/g TOC). Numerical 1D basin modelling was performed for four wells in order to assess the thermal maturity of the Tanezzuft Hot Shale and its evolution through time. Mean random equivalent vitrinite reflectance results, using vitrinite-like particles for lower Silurian Hot Shale, pyrolysis T_(max) values and fluorescence colours indicate that the Hot Shale is immature to early mature in the southern part and clearly more mature in the northern part of the basin. Other potential source rocks from Silurian Tanezzuft, middle-upper Devonian Awaynat Wanin and lower Carboniferous Marar formations were analysed. Thermal history modelling indicates that the present-day geothermal gradients and heat flows vary at different locations, but that paleotemperature evolution is mostly related to deepest burial. Peak temperatures were reached either in the Late Carboniferous before Hercynian uplift occurred or in the Jurassic/ Cretaceous, the latter scenario being more probable, because much oil has been preserved.