Evidence for mixed and biodegraded crude oils in the Socororo field, Eastern Venezuela Basin

Abstract

This work presents a geochemical study of crude oils from the Socororo field (Socororo Major Area) in the Maturín Sub-basin of the Eastern Venezuelan Basin. The V+Ni total concentrations and V/(V+Ni) ratios suggest the presence of two oil groups. These oils are heavy aromatic–asphaltic and according to their sulfur content, API gravity and hydrocarbon compositions, fall into the same two groups established from the V and Ni proxies. Although the analyzed crude oils show evidence of biodegradation, it was possible to determine, based on various biomarkers parameters, that the oils originated from marine organic matter in a marine carbonate or marine marl source rock deposited under anoxic to suboxic conditions. The oils were generated near the peak of the oil window. Distributions of n-alkanes, isoprenoids, hopanes and steranes combined with a prominent unresolved complex mixture (UCM) indicate that the oils are biodegraded. Moreover, the presence of pristane and phytane, along with alteration of hopanes and steranes and the presence of 25-norhopanes, reveals that these oils are mixtures showing different levels of biodegradation. Sulfur content, V and Ni concentration and saturate, aromatic, resin and asphaltene (SARA) composition provide additional evidence for oil mixing. Such mixed oils originate from several episodes of charging and biodegradation in the reservoirs. The first charge was severely biodegraded and many biomarkers susceptible to degradation (n-alkanes, acyclic isoprenoids, terpanes, methyl hopanes, methyl steranes and steranes) were extensively altered or completely removed. The second oil charge was subjected to a lower degree of biodegradation and only the n-alkanes show significant biodegradation. Both oil charges seem to come from the same source rock, which was deposited under variable redox conditions. Moreover, due to their similar maturity, the generation and migration of these oils to reservoirs likely occurred close together in time.