The Correlation of a Multi-Level Porosity System with Depositional Cyclicity and Log Signatures: Deep Cretaceous Reservoirs, Lake Maracaibo, Venezuela

VAHRENKAMP, VOLKER C., and RAYMOND C. M. W. FRANSSEN, Shell Research, Rijswijk, The Netherlands, EVERT W. J. E. VAN DE GRAAFF, Shell Expro, Aberdeen, U.K., and PEDRO J. MUNOZ, Maraven S.A., Caracas, Venezuela

The correlation of a multi-level porosity system with depositional cyclicity in cores from central Lake Maracaibo (SVS-225; SVS-229) allows indirect recognition of productive intervals on logs. Deep Cretaceous reservoirs of Lake Maracaibo include Barremanian to Turonian strata (Rio Negro Sandstone, Cogollo Group, and La Luna Formation). In the shallow-water carbonates of the Cogollo Group, large-scale open-marine and restricted-marine cycles (thickness: 50-200 ft) are recognized in cores as well as on logs. Organic-rich and less organic-rich limestone layers (decimeter scale) of the deep-marine La Luna source rocks probably reflect Milankovitch cyclicity.

Cogollo Group/La Luna reservoirs are characterized by a dual/triple porosity system related to depositional cyclicity (matrix porosity = lower level; stylolite porosity = intermediate level; fracture porosity = upper level).

Open fractures act as highly productive fluid conduits while providing little storage capacity (upper level porosity system). Open fractures occur preferentially in open-marine depositional cycles of the Cogollo Group, and fractured intervals can therefore be recognized indirectly by their log signatures. In the La Luna Formation open fractures are concentrated in less organic-rich layers. The lower porosity level provides mainly storage capacity but little permeability. It consists of stylolite and patchy intervals of matrix porosity (leached limestones in open-marine cycles; sucrosic dolomite in restricted-marine cycles).

A network of closely spaced stylolites not only provides significant storage capacity (lower level system) but also serves as an intermediate permeability system connecting matrix porosity with open fractures.

Comparison with cores and logs from other wells in the Lake Maracaibo region confirms the recognized cyclicity pattern and suggests regional validity of the reservoir model.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)