ABSTRACT: Characterization of an Overpressured, Cretaceous Carbonate Reservoir, Lake Maracaibo, Venezuela

E. J. M. Willemse, W. J. E. Van De Graaff, Z. Sancevic

In the Lake Maracaibo basin (Venezuela), the Cretaceous Cogolio Group carbonates form an important light oil reservoir. Production is mostly from fractures because matrix porosity is negligible over most of the sequence. Based on cores from well VLA-711 and VLA-722 in the central part of Lake Maracaibo, the productive fracture systems probably formed by hard overpressures. Overpressuring occurred when the Cogolio Group and the overlying regional source rock, the La Luna Formation, reentered the oil window after a period of uplift. Reopening of fractures preferentially occurred close to source rock intervals with an important productive zone being located at about the contact of the competent, thick-bedded, upper Cogolio Group limestones and the more easily deformed, thinl bedded La Luna source rock. Because core recovery across productive zones is very poor, the orientation and connectedness of the fracture network has been modeled using recovered cemented fractures. Orientation was determined with paleomagnetic techniques.

The validity of this approach was confirmed by the results of the two new wells, SVS-225 and SVS-229, in the Lama area, some 40 km to the south of wells VLA-711 and VLA-722. In cores from these two new wells, open fractures with orientations consistent with the structural model confirm the regional validity of the overpressuring model. Integration of in-situ stress data, rock strength measurements, and fracture

system orientations consistent with the structural model confirm the regional validity of the overpressuring model. Integration of in-situ street data, rock strength measurements, and fracture system orientation enables well tracks to be optimized in terms of accessing the maximum number of open, productive fractures.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990