Abstract: Upper Cretaceous Source Rocks of Northern South America

Suhas C. Talukdar, B. DeToni, F. Marcano, J. Sweeney, A. Rangel

Upper Cretaceous source rocks sourced most of the oil accumulations in Trinidad, Venezuela and Colombia. Extensive areas of mature to overmature organic-rich, calcareous shales and limestones are present in basins throughout the area. Paleogeographic reconstruction shows deposition of the source rocks in open marine, outer shelf-inner slope environments on the passive margin of the Cretaceous South American continent. Deposition is primarily related to marine transgressions caused by eustatic sea level changes and to Upper Cretaceous oceanic anoxic events.

Sedimentological and geochemical data show regional variations in organic facies across the huge Upper Cretaceous shelf margin. Source rocks deposited in the pelagic domain in outer shelf slope towards north and northwest are commonly organic-rich and contain oil prone, type II kerogen with essentially marine amorphous organic matter. In this organic facies, variations in organic content were primary controlled by changes in marine organic productivity and sedimentation rate: while the kerogen quality was influenced by short-term fluctuations in the thickness of oxygen-minimum layer. Source rocks deposited in outer neritic environment, towards south and southeast are usually less organic-rich and include oil and gas prone kerogen between types II and III with both marine amorphous and terrestrial organic matter. Temporal changes in the influx of terrestrial organic matter and fluctuations in relative anoxicity controlled the kerogen quality.

Hydrocarbon generation from these source rocks occurred during the Tertiary.

Basin modeling quantifies oil and gas generation and expulsion from the various organic facies. Oil characteristics can be predicted by the organic facies in the effective source area.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994