ORGANIC GEOCHEMISTRY OF CRUDE OILS AND UPPER CRETACEOUS SOURCE ROCKS FROM CONCESSION 11, WEST SIRTE BASIN, LIBYA

W. Sh. El Diasty^1*, S. Y. El Beialy¹, K. E. Peters², H. El Atfy¹, A. M. Gheith¹ and N. N. Agha¹

¹ Mansoura University, Faculty of Science, Geology Department, Mansoura 35516, Egypt.

² Schlumberger, Mill Valley, CA 94941 & Geological and Environmental Sciences Department, Stanford University, Stanford, CA 94305, USA.

⁺ Author for correspondence, awaleed@mans.edu.eg

This paper reports the results of Rock-Eval pyrolysis and total organic carbon analysis of 46 core and cuttings samples from Upper Cretaceous potential source rocks from wells in the West Sirte Basin (Libya), together with stable carbon isotope (δ¹³C) and biomarker analyses of eight oil samples from the Paleocene - Eocene Farrud/Facha Members and of 14 source rock extracts. Oil samples were analysed for bulk (°API gravity and δ¹³C) properties and elemental (sulphur, nickel and vanadium) contents. Molecular compositions were analysed using liquid and gas chromatography, and quantitative biological marker investigations using gas chromatography - mass spectrometry for saturated hydrocarbon fractions, in order to classify the samples and to establish oil-source correlations.

Core and cuttings samples from the Upper Cretaceous Etel, Rachmat, Sirte and Kalash Formations have variable organic content and hydrocarbon generation potential. Based on organofacies variations, samples from the Sirte and Kalash Formations have the potential to generate oil and gas from Type II/III kerogen, whereas samples from the Etel and Rachmat Formations, and some of the Sirte Formation samples, have the potential to generate gas from the abundant Type III kerogen. Carbon isotope compositions for these samples suggest mixed marine and terrigenous organic matter in varying proportions. Consistent with this, the distribution of n-alkanes, terpanes and steranes indicates source rock organofacies variations from Type II/III to III kerogen. The petroleum generation potential of these source rocks was controlled by variations in redox conditions during deposition together with variations in terrigenous organic matter input.

Geochemical analyses suggest that all of the oil samples are of the same genetic type and originated from the same or similar source rock(s). Based on their bulk geochemical characteristics and biomarker compositions, the oil samples are interpreted to be derived from mixed aquatic algal/microbial and terrigenous organic matter. Weak salinity stratification and suboxic bottom-water conditions which favoured the preservation of organic matter in the sediments are indicated by low sulphur contents and by low V/Ni and Pr/Ph ratios. The characteristics of the oils, including low Pr/Ph ratio, CPI ~l, similar ratios of C₂₇:C₂₈: C₂₉ ααα-steranes, medium to high proportions of rearranged steranes, C₂₉<C₃₀-hopane, low Ts/Tm hopanes, low sulphur content and low V/Ni ratio, suggest a reducing depositional environment for the source rock, which was likely a marine shale. All of the oil samples show thermal maturity in the early phase of oil generation.

Based on hierarchical cluster analysis of 16 source-related biomarker and isotope ratios, four genetic groups of extracts and oils were defined. The relative concentrations of marine algal/microbial input and reducing conditions decrease in the order Group 4 > Group 3 > Group 2 > Group1. Oil - source rock correlation studies show that some of the Sirte and Kalash Formations extracts correlate with oils based on specific parameters such as DBT/P versus Pr/Ph, δ¹³C saturates versus δ¹³C aromatics, and gammacerane/hopane versus sterane/hopane.

Keywords:: Sirte Formation, source rocks, Cretaceous, organic geochemistry, carbon isotopes, biomarkers, Concession 11, Sirte Basin, Libya.

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