Application of new diterpane biomarkers to source, biodegradation and mixing effects on Central Llanos Basin oils, Colombia

Llanos Basin oils have been commonly attributed to the prolific Upper Cretaceous age source formations outcropping in the Eastern Cordillera. Recently, however, variable oleanane indices have been reported for Llanos oils, suggesting a contribution from Tertiary source sequences largely on the basis of the presence of high relative abundance of oleanane, a Tertiary age-diagnostic biomarker. A homologous series of 25-norhopanes in many central Llanos Basin oils indicates that heavy biodegradation is particularly common to the initial oil charging reservoirs from Upper Cretaceous marine sources. One homologue, 20S-25-norhomohopane [C30 25-norhopane], co-elutes with oleanane and may thus contribute to the peak attributed to oleanane. Also, confusing the source identification is the possibility that some Cretaceous facies also contain oleanane. Three plant diterpanes, which may be isomers of fichtelite and are common to the Central Llanos oils, have been observed in an Eocene source rock sample, but not in Cretaceous rocks or oils, providing additional strong evidence for a Tertiary contribution to some Llanos Basin oils. In mixed oils with low to moderate oleanane contents and with a strong, co-eluting C30 25-norhopane, the fichtelite isomers are the more reliable indicators of Tertiary source input.