Geochemical Study of Oils and Source Rocks from an Oil Field in Llanos Basin, Colombia

Summary Llanos Basin is one of the most prolific and largest sedimentary basins in Colombia. This work is focused on a field producing from Carbonera Formation (Tertiary). We have undertaken geochemical studies on oil and rock samples from a well (from Carbonera) for oil-source rock correlations. Fingerprint chromatograms were analyzed for paraffin distribution and to infer maturation and alteration processes. Sterane and triterpane biomarkers and aromatic compounds were analyzed by GC-MS to evaluate the origin, age and lithology of the source rock, maturity, biodegradation, and depositional environment conditions. The study revealed that the oils were similarly mature, and probably originated from marine shales deposited under suboxic-anoxic conditions with terrigenous contribution. The presence of oleanane suggests contributions from Tertiary or younger source rock. Oils are mixtures from two different charges, the older one severely biodegraded. By contrast, many rocks have enhanced terrestrial organic matter input, are less mature and have varying depositional environments. Correlations were hampered by large differences in maturity between the oils and rocks. Higher rock terrigenous-aquatic ratios indicate more terrigenous organic matter contributions for the rocks than for the oils. However, this parameter is affected by maturity.