Abstract
The Early Silurian “hot” shales and Late Devonian black shales are major regional oil and gas source rocks in North Africa. Their deposition probably played a major role in global carbon cycling in general because of the large areas of the ocean affected. Comparing the Libyan δ13C record with records
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from other areas enables unraveling local effects from larger scale processes during these intervals. Silurian and Devonian formations in the western part of Libya are generally poorly studied and their basin-scale facies distribution is largely unknown. The main goal is to reconstruct the depositional environment on the North Africa margin during the deposition of the Silurian and Devonian black shales in the western part of Libya and to assess their maturity. The Tanezzuft and Awaynat Wanin formations were investigated using a combined palynological, carbon isotopic and organic geochemical approach. The Silurian carbon isotopic signal (δ13CTOC) from the west of Libya shows several events correlating with events observed earlier in isotopic curves from other areas. Positive 13CTOC excursions occur close to important bioevents and three of these events are correlated to the global carbon isotopic curve. During the Devonian Frasnian organic-rich shales were deposited across much of the North African margin, forming a potential secondary hydrocarbon source rock in the western part of Libya. The abundant and high diversity of prasinophytes, high TOC content and well preserved amorphous organic matter deposited during the Early Frasnian strongly supports enhanced sea surface productivity and oxygen depleted bottom water conditions. The high abundance of the flash pyrolysis products 1,2,3,4- and 1,2,3,5-tetramethylbenzenes together with appreciable amounts of 1-ethyl-3,4,5-trimethylbenzene and 1-ethyl-2,3,6-trimethylbenzene argue for the presence of macromolecularly-bound diaromatic carotenoids derived fromphototrophic anaerobes. Continued photic zone euxinia into the lean shale suggests that other factors than enhanced preservation controlled TOC contents in this shale, such as changes in admixing of lithogenic material. The maturity evaluations, based on equivalent vitrinite reflectance and Rock Eval pyrolysis, indicates that the Silurian source rock in Murzuq Basin is thermally more mature than that in the Ghadamis Basin. Significant differences in alkylbenzene distribution patterns were observed between Ghadamis and Murzuq Basins kerogen pyrolysates Two-dimensional gas chromatography, coupled to time of flight mass spectrometry was applied to identify diamondoids in source rock extracts and crude oils from the western part of Libya. This approach revealed different levels of thermal maturity within the basin. Also significant differences were noted between source rock extracts and the crude oils. Whereas the source rock extracts contain both adamantanes and diamantanes, the latter group is absent from the crude oils. The lack of diamantanes in the crude oils probably indicates that crude oils were expelled before diamantanes formed in the source rock.
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