Abstract
The maturity parameters obtained from the analysis of saturated and aromatic hydrocarbons and petroporphyrins in marine rock extracts (La Luna source rock) and crude oils from the Maracaibo basin were examined by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). The results obtained indicate that in very mature samples there is a preferential struction of C 29 steranes over C 27 and C 28 as well as an increase in the C 21 and C 22 regular steranes. These changes seem to indicate that thermal maturity affects the correlation parameter normally used in the characterisation of oils and rock extracts. The demetallated petroporphyrins show a decrease of DPEP series and increase in ETIO components with increasing maturity. In very mature samples the ETIO porphyrin ratio, C 28E′/C 27E′ and C 29E/C 28E, together with the methylphenanthrenes parameter (PP-1), are very useful for classifying both rock extracts and crude oils in terms of maturity. Of the parameters presented here, the methylphenanthrene parameter (PP-1) and the ETIO petroporphyrin series are the most promising source of geochemical information for better assessment of maturity of oil and source rock extracts beyond the main oil generation zone ( R 0 ⩾ 0.9%), after which other molecular maturity parameters have reached equilibrium.
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