The hydrocarbon source potential of the palaeozoic rocks of the Ghadames Basin, NW Libya

Abstract

Lower Silurian and Upper Devonian organic-rich "hot shales" are the source of almost all Palaeozoic oils in North Africa and the Middle East. They are also developed in the Ghadames Basin, and a better understanding of their organic facies character, maturity and their correlation to Palaeozoic oils of the basin is considered crucial for the future oil exploration strategy. Core and ditch cuttings samples from the alternating transgressive and regressive marine sandstone and shales of the Tanezzuft, Acacus, Tadrart, Ouan Kasa, Aouinet Ouenine, and Tahara formations are investigated using a combination of organic geochemistry, palynofacies and organic petrology. The bulk geochemical results demonstrate the presence of various organic-rich horizons within the Silurian and Devonian formations. The total organic carbon (TOC) values are generally between 0.5 and 25.0%. The highest TOC values are measured within the dark-coloured non-bioturbated graptolitic "hot shales" of the Silurian lower Tanezzuft Formation (average 7%), and the Devonian Aouinet Ouenine Formation (average 2%); other formations such as Tahara Sandstone Formation contain some thin black shales interbeds that have an average TOC of 4.5%. Hydrogen indices (HI) are mostly below 450,150-435 in the lower Tanezzuft Formation, and reaching 50-300 in the Aouinet Ouenine Formation. Palynofacies analysis permits the recognition of different organic facies: a terrestrially-dominated oxic facies in the Upper Devonian, oxic nearshore shallow marine to proximal shelf facies enriched in thin-walled prasinophyte algal phycomata (leiospheres) with low AOM and phytoclasts in alternating sandstone and shale of the Upper and Lower Silurian, and dysoxic-anoxic hemipelagic facies dominated by well preserved AOM (genetically oil-prone Type II kerogen) in the Lower Silurian lower Tanezzuft Formation `hot-shale'. Results from 25 boreholes distributed throughout the basin indicate a significant spatial variation in the Silurian hot shales, including significant variation in gamma ray values, hot shale thickness and organic facies quality. Maturity evaluation based on Tmax, SCI, ACI, %VRE, biomarker and aromatic hydrocarbon distributions for Lower Silurian lower Tanezzuft "hot-shale" source rock facies revealed a trend of increasing maturity from middle mature (oil expulsion zone) in the north and south east to very mature (gas generation zone) towards the central and southwestern parts of the Ghadames Basin. The Upper Devonian samples are immature in the northern and south eastern parts of the basin and mature in the southwestern and central parts of the basin. Oil-source correlations revealed that all the analysed Palaeozoic oils of the Ghadames Basin display similar facies features to the Lower Silurian Tanezzuft Formation source rock facies. Therefore there is a high probability that these oils were generated from the Lower Silurian `hot-shale' source rocks. Maturity evaluation of the oil samples based on the biomarker and aromatic hydrocarbon ratios revealed that the oil samples collected from the oil fields located in the south and southwestern parts of the basin are more mature than the oil samples collected from the northern parts of the basin. This is consistent with the maturity trends of the source rocks of the Ghadames Basin. Most of the Upper Devonian source rock samples have unusual proportions of the 20S isomers (relative to 20R isomers) of the C29 steranes, with values of more than 55% 20S. Such high values could be due to factors other than maturity.