BIOMARKER CHARACTERISTICS OF CRUDE OILS FROM THE MURZUQ BASIN, SW LIBYA

T.  A.  Hodairi*+ and R. P.  Philp*

* School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, USA.

+ corresponding author, email: hoderi75@ou.edu

Twenty crude oil samples from the Murzuq Basin, SW Libya (A-, R- and I-Fields in Blocks NC115 and NC186) have been investigated by a variety of organic geochemical methods. Based on biomarker distributions (e.g. n-alkanes, isoprenoids, terpanes and steranes), the source of the oils is interpreted to be composed of mixed marine/terrigenous organic matter. The values of the Pr/Ph ratio (1.36-2.1), C₃₀-diahopane / C₂₉Ts ratio and diasterane / sterane ratio, together with the low values of the C₂₉ / C₃₀-hopane ratio and the cross-plot of the dibenzothiophene/phenanthrene ratio (DBT/P) versus Pr/Ph ratio in most of oil samples, suggest that the oils were sourced from marine clay-rich sediments deposited in mild anoxic depositional environments. Assessment of thermal maturity based on phenanthrenes, aromatic steroids (e.g. monoaromatic (MA) and triaromatic (TA) steroid hydrocarbons), together with terpanes, and diasterane/sterane ratios, indicates that crude oils from A-Field are at high levels of thermal maturity, while oils from R- and I-Fields are at intermediate levels of thermal maturity. Based on the distributions of n-alkanes and the absence of 25-norhopanes in all of the crude oils analysed, none of the oils appear to have been biodegraded. Correlation of the crude oils points to a single genetic family and this is supported by the stable carbon isotope values. The oils can be divided into two sub-families based on the differences in maturities, as shown in a Pr/nC₁₇ versus Ph/nC₁₈ cross-plot. Sub-family-A is represented by the highly mature oils from A-Field. Sub-family-B comprises the less mature oils from R- and I-Fields. The two sub-families may represent different source kitchens of different thermal maturity or different migration pathways.

In summary, the geochemical characteristics of oil samples from A-, R-, and I-Fields suggest that all the crude oils were generated from similar source rocks. Depositional environment conditions and advanced thermal maturities of these oils are consistent with previously published geochemical interpretations of the Rhuddanian “hot shale” in the Tanezzuft Formation, which is thought to be the main source rock in the Murzuq Basin.

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