Abstract: Hydrocarbon Systems of the Frontal Fold Structure, East Venezuela

Daly, M. C.; Dailey, D. V. and Jones, N. E. - BP Exploracion de Venezuela; Coward, M. - Ries-Coward Associates Ltd.; Shann, M. V. - BP Exploracion de Venezuela

The Eastern Venezuela Foreland Basin is traversed by a major fold structure that extends some 200 km. from east to west (Fig. 1). The structure contains at least three petroleum systems, one of which has proven commercial volumes of hydrocarbons. This huge structure has been explored at its east and west extremities which led to the discovery of the Pedernales Field by Creole in 1933. However, 90 km. of the central portion of the structure remains unexplored and has significant petroleum potential.

The known reservoirs lie within the Mio-Pliocene La Pica Formation, deposited as delta front and delta top sandstones. Only the Pliocene Pedernales Member is commercially significant to date. Oil migration into the reservoirs is occurring present day, post-dating structural growth which took place during the early Pleistocene. The trapping mechanism is defined laterally by major cross faults, whilst the up-dip seal is poorly understood. The Pedernales Field lies along the northern flank of the fold structure, the southern flank being largely unexplored.

From an exploration perspective, 3D seismic is being used to predict over pressure along the fold structure, which has proven to be a good indicator of seal effectiveness. Additionally, 3D data in conjunction with the well data is being used to calibrate new and different stratigraphic and structural models to better predict reservoir distribution.

Several models have been proposed to account for the development of the fold structure, which include, 1) mud diapirism, 2) a thin skinned thrust detaching in the Early to Mid Miocene Carapita and fishtailing to surface, and 3) a basement involved thick skinned thrust. Interpretation of over 500 sq. km. of 3D seismic has highlighted another possible interpretation, that of a fold produced by Pleistocene inversion of late Miocene extensional faults.

The fold post-dates the Caribbean thrust tectonics, which ceased in the Late Miocene. The thrust tectonics were followed by subsidence, in part interpreted as Late Miocene extension in a narrow rift formed within the foreland basin. The proto-Orinoco delta built out into this evolving depocenter apparently from both the west and south. This rift later inverted to form the major fold structure mapped today (Fig. 2).

Although interpretation of this alternative model is still in the early stage, it has a major impact on the perceived prospectivity of the fold structure. The inversion model implies a simple anticlinal structure with good stratigraphic continuity from north flank to south. Recent exploration results have proven the continuation of the Pedernales North Flank Play into the Guanipa area, 20 km. west of Pedernales. This opens the way to a much greater reserves base being developed along the northern flank of the fold and heightens the anticipation for an exploration test of the little explored southern flank.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil