Geochemical analysis of oils from Barbados and basin modeling of Paleozoic units in the southern Llanos basin, Colombia

Abstract

This study evaluates two separate areas using different approaches that help improve knowledge regarding their hydrocarbon potential: 1 - A frontier region lying between the junction of the Barbados accretionary prism and the Tobago basin; 2. - A mature basin such as the southern Llanos basin in Colombia. Both areas are located within the hydrocarbon-rich southern Caribbean margin. Commercial oil production from The Woodbourne oil field and the presence of migrated petroleum in outcropping rocks onshore Barbados prove the existence of a working petroleum system in the region. Barbados petroleum is suggested (Lawrence et al., 2002; Burggraf et al., 2002; Leahy et al., 2004; Hill and Schenk, 2005) to have been generated by facies similar to the Upper Cretaceous carbonate rich La Luna Formation onshore South America, but it has so far not been proved. This study presents detailed organic geochemical observations on six crude oils, one seepage, and bitumen from seven outcrop samples, in order to investigate heterogeneities in petroleum composition, thermal maturity, and biodegradation, and to investigate the filling history of the Woodbourne field. The results were also compared with published geochemical data for other northern South American and Caribbean oils/source rocks. In addition, the hydrocarbon potential of four Paleogene source rocks was evaluated. The geochemical data suggest that the petroleum present in Barbados can be divided into two compositional and maturity groups (group A and B). The petroleum in both groups was derived from Cretaceous shaly source rocks deposited in oxic-to-dysoxic marine environments with varying contribution of marine and land plants-derived organic matter. Group A petroleum was generated and expelled at low maturity levels (0.72-0.77%Ro), and was derived from predominantly marine organic matter. By contrast, petroleum in group B was generated at higher maturity levels (0.87-0.94%Ro), and was derived from a more proximal source rock influenced by terrestrial organic matter. These observations indicate the existence of two separate kitchens sourcing the Barbados petroleum. Organic geochemical data also suggest that reservoirs at the Woodbourne field has received two pulses of oil. The first oil pulse represents a filling event believed to have charged the reservoirs after the Mid-Miocene uplift of the Barbados ridge. This oil was biodegraded in the reservoirs above 1000 meter depth. The second more recent pulse consists of very light hydrocarbons (n-C3 to n-C9). This oil probably got separated from the parental oil and escaped through faults and/or failing seals during/after the last tectonic event in the Pliocene. Both oils seem to be compositionally similar and to have the same maturity level. v Geochemical comparison of the Barbados sample set with Upper Cretaceous oils and source rocks from several basins in northern South America and Caribbean region indicates that Barbados petroleum was not derived from carbonate facies typical of La Luna Formation or its equivalent in eastern Venezuela The Querecual Formation. Finally, Rock-Eval pyrolysis data indicate that the studied Paleogene source rocks have poor generation potential. On the other hand, hydrocarbons exploration in the Llanos basin began in the 1980’s. Exploration trends have focused exclusively on the Cretaceous-Cenozoic foreland plays, and no hydrocarbons sourced by Paleozoic rocks have yet been discovered. Forward 2D basin modeling was performed along a profile to evaluate timing of hydrocarbon generation and expulsion and hydrocarbon phase from a potential Lower Ordovician source rock interval. The 2D model of the subsurface was constrained using a published interpretation of an E-W regional 2D seismic line, in which the basin sedimentary infill is divided into five tectono-stratigraphic sequences, and data from three exploration wells. Calibration data was available for the three wells, and included vitrinite reflectance and temperature data. Modeling results show that by Late Ordovician-Early Silurian a first major phase of transformation (average 60%) occurred in the deepest places of the basin. Later, Permian uplift and denudation most probably destroyed any hydrocarbon accumulation existing in the western and central parts of the basin. A second phase of generation begins in the Eocene-Paleogene and continues up to present day within the easternmost extension of the basin. This potential petroleum system has not undergone the degree of uplift, erosion, and destruction of reservoirs in the eastern part of the basin, making preservation of any petroleum accumulation much more feasible. Thus, potential for finding an alternative source of hydrocarbons in a mature basin exists in the eastern depocenters, where newly generated hydrocarbons (mainly gas) could coexist with older petroleum preserved from the first generation phase.