Generation and migration of hydrocarbons in the Maracaibo basin, Venezuela: An integrated basin study

Abstract

The history of hydrocarbon generation and migration in the Maracaibo Basin, Venezuela, is closely related to its stratigraphic, structural and tectonic evolution during the Cretaceous and Cenozoic times. Geochemical evaluation of the possible source rocks within the Cretaceous and the Tertiary sequences indicates that the argillaceous limestones and calcareous shales of the La Luna Formation (Cenomanian-Coniacian) are the most important source rocks in the basin. These rocks contain organic carbon ranging from 1.5 to 9.6% ( c. original TOC 2.5–10.8%), have fluorescent amorphous marine, type II organic matter (oil prone) and have in the immature stage a H-index of 700 mg HC/g TOC and a H/C ratio of 1.5. The beginning of the oil window in these rocks occurs at a relatively low thermal maturity (Lopatin TTI about 10) and the peak of oil generation corresponds to a TTI value of 35. Geochemistry of oils from the different reservoirs (Cretaceous, Paleocene, Eocene and Miocene beds), and oil-oil and oil-source rock correlation studies based on biomarkers have identified three genetic types of oil in the basin: a marine type essentially derived from the La Luna source, a terrestrial type possibly originating either from the Cretaceous Lisure shales or the Paleocene shales, and a third type derived from the mixing of the first two types. The marine type oil is distributed extensively throughout the basin whereas the terrestrial as well as the mixed marine and terrestrial types are found only in the southwestern part of the basin. The marine type oils have been further subdivided based on their alteration and maturity. Reconstruction of the hydrocarbon generating areas at different geological times using TTI-maturity diagrams indicates that the generation of oil and gas in the basin took place during two periods; the first phase occured during the Middle to Upper Eocene times in the northeastern part of the basin, while the second phase occured during the Upper Miocene to Recent times in the central, western and southern parts of the basin. Based on organic geochemical and sedimentological characteristics, it is proposed that primary migration of oil from the La Luna source rocks took place by expulsion of hydrocarbons through abundant microfractures developed by the pore fluid overpressure during hydrocarbon generation. Mass balance considerations suggest that the expulsion probably occurred as a hydrocarbon single phase and the expulsion efficiency for oil in La Luna source rocks should have been very high (30–50%). Taking into consideration the development of oil and gas kitchens through time, the paleostructures at the time of hydrocarbon generation, and the characteristics of oils, the migration history of the oil and gas generated from the La Luna source has been outlined. Mixing of oils, derived at different times (Eocene and Miocene-Recent) from the La Luna source of different areas, appeared to have taken place in Block IV, Block 11, Lago-Lamar, Motatan and Bolivar Coastal fields.