Chronostratigraphy and Biofacies of the Middle Eocene Paguey Formation: An Invaluable Tool for Hydrocarbon Exploration in the Barinas Basin, Western Venezuela: ABSTRACT

Abstract

Paleoenvironmental changes within the Paguey Formation gave rise to foraminiferal assemblages which can be used for reservoir depth prognosis and subsurface correlation. The chronostratigraphic framework is provided by the integration of palynological, foraminiferal and calcareous nannoplankton data. The interval spans approximately from 43.0 Ma to 39.4 Ma. A transgressive-regressive second order cycle can be interpreted, since a very rapid deepening takes place at the base, where paleoenvironments vary from middle neritic in the south, to bathyal in the north. Within the lower half of the unit, a gradual filling of the basin begins, with paleoenvironments becoming progressively shallower toward the top of the unit. This formation is a 660-m-thick stratigraphic unit, composed mainly of dark gray mudstones, with fine grained glauconitic sandstones toward the base. It conformably overlies the middle Eocene Gobemador Formation and unconformably underlies the Parangula Formation. The characteristic biofacies are constituted by: Planktic zone (T robri/Globigerina sp.), Nummulites sp., Eggerella/Textularia, Quinqueloculina seminula, Ammobaculites/pteropods. The Nummulites sp. biofacies constitutes the most useful marker, since it extends across the whole basin. From the top of this zone to the top of the reservoir, a thickness of about 50 m in the south to 170 m in the north ismore » observed. The precise prognosis of the reservoir depth is essential during the course of the drilling phase, in order to get the maximum geological information from cores at the lowest economic cost.« less