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 Creta ceous age source formations outcropping in the Eastern Cordillera. Recently , however, variable oleanane indices have been reported for Llanos oils, su ggesting a contribution from Tertiary source sequences largely on the basis of the presence of high relative abundance of oleanane, a Tertiary age-dia gnostic biomarker. A homologous series of 25-norhopanes in many central Lla nos Basin oils indicates that heavy biodegradation is particularly common t o the initial oil charging reservoirs from Upper Cretaceous marine sources. One homologue, 20S-25-norhomohopane [C-30 25-norhopane], coelutes with ole anane and may thus contribute to the peak attributed to oleanane. Also, con fusing the source identification is the possibility that some Cretaceous fa cies also contain oleanane. Three plant diterpanes, which may be isomers of fichtelite and are common to the Central Llanos oils, have been observed i n an Eocene source rock sample, but not in Cretaceous rocks or oils, provid ing additional strong evidence for a Tertiary contribution to some Llanos B asin oils. In mixed oils with low to moderate oleanane contents and with a strong, co-eluting C-30 25-norhopane, the fichtelite isomers are the more r eliable indicators of Tertiary source input. (C) 1999 Elsevier Science Ltd. All rights reserved.