Generation and migration of oil in the Maturin Subbasin, Eastern Venezuelan basin

Abstract

Regional geochemical evaluation of the possible Cretaceous and Tertiary source rocks and oil-oil and oil-source rock correlation studies, have identified the calcareous shales and limestones of the Upper Cretaceous, Querecual and San Antonio Formations of the Guayuta Group, with amorphous marine type II organic matter, as the source rocks for oil in the Maturin Subbasin. The deposition of the source rocks took place on a passive continental margin whilst the generation and migration of oil occurred in a foreland basin during Miocene-Recent times. The oils found in the northern part of the subbasin in different reservoirs (Cretaceous, Eocene, Oligocene, Miocene and Pliocene formations) are of mature, marine type with or without bacterial alteration. Geochemical data indicate that some of these oils were mixed with the highly altered residue of a previously accumulated oil. Based on maturity and migration modelling, it appears that the oils essentially migrated from active, Upper Miocene-Recent, kitchens in the Cretaceous source rocks below the main thrusts. The highly altered residue was formed during the Upper Miocene by the biodegradation of oil migrating during the Middle and Upper Miocene. Large areas of mature to overmature Cretaceous source rocks (now exposed in the Interior Mountain Front) were in active oil kitchens during the Lower and Middle Miocene before being involved in thrusting around 12 Myr BP. Long distance (150–300 km) southward lateral migration of oils during the Middle Miocene gave rise to the altered moderately mature marine oils in the shallow Miocene stratigraphic reservoirs of Cerro Negro (Orinoco Oil Belt) in the southern fringe of the subbasin.