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Biomarker Characteristics of Crude Oils from the Murzuq Basin, S.W. Libya

Hodairi, Tarek *1; Philp, Paul 1
(1) School of Geology and Geophysics, University of Oklahoma, Norman, OK.

Twenty crude oil samples from the Murzuq Basin (NC-115 and NC-186 Blocks) were used for this study. Complete removal of asphaltenes from oils was assured by using n-pentane, and the maltenes were subsequently fractionated into saturate hydrocarbons, aromatic hydrocarbons, and polar compounds using high-performance liquid chromatography (HPLC). The saturate fractions were sieved for n-alkanes removal prior GC-MS analyses using high sieve (HI-SIV 3000). Analytical techniques such as gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS) were used to characterize the saturate fractions. The distributions of n-alkanes, isoprenoids, terpanes, and steranes were used to determine organic matter (OM) composition, depositional environment conditions, thermal maturity and extent of biodegradation. Based on biomarker distributions (e.g. n-alkanes, isoprenoids, and steranes), source of the organic matter for the oils is characterized by mixed marine/terrigenous organic matter with a slightly higher contribution of terrestrial organic matter. Oils were sourced from clay-rich sediments deposited in mixed oxic to sub-oxic depositional environments as indicated by the high values of Pr/Ph, C30-diahopane/C29Ts, and diasterane/sterane ratios together with the low values of C29/C30-hopane ratio in most of samples. Assessment of thermal maturity based on phenanthrene and aromatic steroids (e.g. monoaromatic (MA) and triaromatic (TA) steroid hydrocarbons), terpanes (e.g. tricyclic/hopane ratios), diasterane/sterane ratios, as well as the high concentrations of pregnane and homopregnane, indicate that crude oils from A-Field are at high levels of thermal maturity, while oils from R- and I-Fields are characterized by intermediate levels of thermal maturity. Based on the distributions of n-alkanes and the absence of 25-norhopanes in all of crude oils, oils do not appear to have been biodegraded. Correlation of crude oils permits the establishment of one genetic family supported by the stable carbon isotope values. These oils can be divided into two subfamilies based on the differences in maturities and Pr/nC17 vs. Ph/nC18 crossplot. Subfamily A is represented by all oils from A-Field-NC115, which is characterized by high levels of maturity. Subfamily B is described by oils from R- and I-Fields that are at intermediate levels of maturity. The two subfamilies may represent two different phases of hydrocarbon generation that took place in the Murzuq Basin.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California