SEVERE BIODEGRADATION OF CRUDE OILS FROM THE QUIRIQUIRE FIELD, EASTERN VENEZUELAN BASIN

Five crude oil samples from the Quiriquire field (Maturin sub‐basin, Eastern Venezuelan Basin) were analysed to evaluate their levels of biodegradation. The oils were obtained from coarse sandstones and conglomerates of the Pliocene Quiriquire Formation at depths <1000 m. Analyses of the samples’ bulk physicochemical parameters indicate variations in API gravity and in the content of saturated hydrocarbons and NSO+asphaltenes, and also in the saturate/aromatic ratio which increases in more biodegraded oils. n‐Alkane distributions are characterized by a dominant unresolved complex mixture (UCM) or hump under an envelope of peaks which lack the acyclic isoprenoids pristane and phytane. The alteration of steranes and terpanes together with the presence of 25‐norhopanes and 17‐nor‐tricyclic terpanes, and the alteration of low molecular‐weight (C20‐C21) triaromatic steroids, phenanthrene, methyl‐phenanthrene, dibenzothiophene and methyl‐dibenzothiophene, indicate that the oils have undergone severe biodegradation.