ABSTRACT: El Furrial Oil Field, a New Giant in an Old Basin

Rodulfo Prieto, Gustavo Valdes

In 1978, Lagoven S.A., affiliate of Petroleos rted an exploration program for deeper tracts in the northern flank of the Eastern Venezuelan basin, where the shallow upper Tertiary section had been explored for more than eight decades. The quality of the seismic data collected was sufficient to indicate thrust faults with associated structures. From 1978 to 1985, surface geology surveying, structural modeling, and interpretation of seismic data were combined to define the first prospect, 1 El Furrial.

The discovery well, 1 FUL, penetrated 276 m (905 ft) of net oil sand in an 80-km² (31-mi²) structure with a closure of 915 m (3000 ft). 1 FUL tested up to 12,500 bbl/day of 29° API oil. This is one of the largest single discoveries of medium-gravity oil in the last 25 yr in South America.

Folding and thrusting of the northern flank of the Eastern Venezuela basin occurred during the collision of the Caribbean and the South American plates. The evolution began at least by the early Paleocene. Folds associated with thrusts tend to be cylindrical and are aligned in series up to 70 km (43 mi) long. There are three tectonic domains within the foreland basin: the northern edge, constituted by the Pirital allochthonous block; the thrusted and folded middle area where the field is located; and the extensional province toward the south.

The reservoir rock is a shallow-marine sandstone deposited during the late Oligocene. Gross thickness ranges from 457 to 518 m (1500 to 1700 ft) and porosities range from 11 to 16%; the oil is considered of marine origin, mature, and from a Cretaceous source (Cenomanian-Turonian).

The El Furrial discovery represents an excellent example of the prospectivity of a foreland overthrust area and an example of continued successful exploration in an area already considered mature in the pursuit of deeper objectives.

AAPG Search and Discovery Article #91000©1990 AAPG Conference-Giant Oil and Gas Fields of the Decade 1978-1988 Conference, Stavanger, Norway, September 9-12, 1990