--> Barbados Petroleum and its role in understanding source rock distribution in the southern Caribbean region

Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins

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Barbados Petroleum and its role in understanding source rock distribution in the southern Caribbean region

Abstract

Oil production from the Woodbourne field and the presence of migrated petroleum in outcropping rocks onshore Barbados prove the existence of an active source rock(s) and a working petroleum system within the Barbados accretionary prism (BAP). This very fact makes the BAP an oddball among accretionary systems as it is the only commercially hydrocarbon producing prism in the world (Mann et al, 2003). The Barbados Island, as the topographically highest and the only subaerially exposed part of the prism, is an open window to a poorly-understood petroleum system within the leading age of the Caribbean plate. In particular, the geochemical nature and distribution of the Barbados source rock relative to the surrounding hydrocarbon rich provinces remain unsolved. Observations This study presents organic geochemical observations that reveal heterogeneities in composition, thermal maturity and biodegradation of petroleum occurrences onshore Barbados (figure 1). Geochemical data suggest that Barbados petroleum can be divided into two compositional and maturity groups (A and B) (figure 1C). The petroleum in both groups was derived from a 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 (Woodbourne field) was generated and expelled at the early oil window (0.72-0.77%Ro) from a source rock containing predominantly marine organic matter. By contrast, petroleum in group B (Shale Quarry) was generated at the peak of the oil window (0.87-0.94%Ro) from a more proximal source rock with higher content of terrestrial organic matter. Petroleum of different maturity found onshore the Barbados Island is circumstantial evidence to support that the source rock is present at different stratigraphic levels. The consequences of this are important because it implies the existence of a multi-stacked source rock system within the leading edge of the Caribbean plate. Organic geochemical data also reveal that reservoirs in the Woodbourne trap system have received two main pulses of oil. A paleo-oil filling event interpreted to have charged the reservoirs with the lower maturity oils after the Mid-Miocene uplift of the Barbados ridge. This oil was then biodegraded in reservoirs located above 1000 meter (figure 1A and B). The second more recent pulse consists of light oils expelled from the source rock at the peak of the oil window. These light hydrocarbons got separated from the parental oils in a gaseous media (Evaporative fractionation?) and migrated upwards through faults and/or partially leaking seals. This difference in maturity between the light and the heavier fractions hints on the presence of deeper accumulations below the presently drilled targets. After determining the Cretaceous age for the Barbados source rock, it is tempting to suggest that the aforementioned source rock may be genetically related to other source rocks of Cretaceous age in northern south America and Caribbean regions. Such statement, however, demands further investigations, particularly when considering that deposition of the source rock facies varied significantly across the passive margin of the Cretaceous South American continent. Such variability, together with the need for contextualizing it in a tectonic framework, has led the Conjugate Basins, Tectonics and Hydrocarbons consortium (CBTH) to start a research project to further investigate this topic. The project aims to acquire organic geochemical data (GC, GC/MS, and isotopes) from several geological provinces that would be the basis for performing a mega-regional comparison. Sampling includes source rocks and related oils from both the northern and the southern boundary of the Caribbean plate (Cuba, Belize, Jamaica, Colombia, Venezuela, Barbados, Trinidad, Suriname). This comparison will contribute to validate/contradict the resemblance of the Cretaceous source rocks within the leading edge of the Caribbean plate with source rocks and related oils of similar age in the circum-Caribbean region. This biomarkers-based comparison, when georeferenced and integrated with the extensive CBTH paleogeographic database, will ultimately shed light on the lateral variability of the Cretaceous source rock facies in Caribbean region. Preliminary results using both in-house and published biomarkers data show significant variations across the Caribbean region (figure 2). In particular, this seems to indicate that Barbados petroleum was not derived from carbonate facies typical of La Luna Formation or its equivalent in eastern Venezuela the Querecual Formation as previously proposed. It hence presumes the existence of a shaly Cretaceous source rock within the leading edge of the Caribbean plate. existence of a shaly Cretaceous source rock within the leading edge of the Caribbean plate. Perspective on the distribution of the Barbados source rock Plate reconstructions by the CBTH show that in Eocene times, the leading edge of the Caribbean plate was located much farther west than it is today, north of the Maracaibo Basin. In such a paleogeographic setting, the early stages of offscraping and accretion of previously deposited sediments, probably including the organic carbon rich sediments of cretaceous age, occurred at the leading edge of the Caribbean plate. This hence points to the organic-carbon rich sediments sourcing the Barbados oils having being deposited somewhere offshore western Venezuela or northern Colombia, and could in this regard be the deep-water, shaly equivalents of La Luna Formation in the vicinity of the Maracaibo basin.