Organic geochemical characterization of source rocks (Sirt Shale) and crude oils from Central Sirt Basin, Libya.

摘要

The Sirt Basin is located in the north of Libya, bordered on the north by the Gulf of Sirt in the Mediterranean Sea. Sirt Shale is dark-brown to black laminated organic rich shale containing abundant foraminifera, deposited during Maastrichtian to lowermost Campanian. The samples analyzed in this study were collected from Sirt Basin, concession 6 and 47.;This organic geochemical study was performed on eighty-six samples taken from the Sirt Shale. The total organic carbon content is a poor to excellent source rock for oil generation and the kerogen types are type II and II/III. The samples from greater depths (more than 12000 ft) have kerogen type III with low HI and relatively high OI. Tmax from Rock-Eval pyrolysis was found to be unreliable at the higher depths, and was in the mature range in Well BBB1-6 between depths of 10460 to 11425 ft. Excellent source rock quality was observed in the well BBB1-6. In this well the samples have the highest TOC value, some greater even than 5%. This interval (10460-11425 ft) of well BBB1-6 is divided into two parts based on the Gamma Ray profile and Rock Eval data parameters. The lower strata were deposited during sea level rise within transgressive systems tracts and the upper strata were deposited within highstand systems tracts. The total organic carbon content increases with depth and supports sea level changes.;Organic geochemical analysis shows a relatively high abundance of low molecular weight n-alkanes compared to high molecular weight n-alkanes suggesting an absence, or low contribution of terrigenous organic matter. Pristane/phytane and isoprenoid/n-alkane ratios, the presence of gammacerane, regular sterane distributions and the occurrence of C30 steranes suggest marine organic matter deposited under oxic to suboxic saline conditions. These rocks are mature and the main phase of oil generation has been reached.;Organic geochemical analysis of the eighteen crude oil samples shows pristane/phytane ratios between 1 and 3, which may suggest a marine depositional environment for the organic matter. The crude oils also show a high abundance of low molecular weight n-alkanes which suggests a high contribution of marine organic matter. A ternary diagram of regular steranes and the occurrence of C30 steranes also strongly suggest marine organic matter contributions. Geochemical parameters suggest that the lithology of the source rocks of these oils are shale deposited under oxic to suboxic marine conditions. According to the age parameters, the source rock of these crude oils is most likely Upper Cretaceous in age. The oils were divided into four groups (A, B, D and C) according to thermal maturity parameters and carbon stable isotope and hydrogen stable isotope analysis. Two of the shallowest samples from the Gialo reservoir show very slight biodegradation. These two samples show depletion of normal alkane while the isoprenoids, hopane, and steranes were unaffected which has been observed to be associated with light biodegradation.;The group A and B oils have a positive correlation with the source rock in well BBB1-6 according to the biomarker parameters. The absolute concentrations of biomarkers support this correlation where both oils (group A and B) and source rock extracts have relatively close biomarker concentrations; therefore, the crude oils in these groups could be sourced from similar source rocks.