Sagasti, Guillermina¹, Langhorne B. “Taury” Smith², Alejandra Rinaldi³, Gregor P. Eberli¹ 
(1) University of Miami, Miami, FL 
(2) New York State Museum, Albany, NY 
(3) Shell Venezuela S.A, Maracaibo, Venezuela

ABSTRACT: Characteristics of an Unusual Reservoir Facies in the Lower Cretaceous Cogollo Group, Maracaibo Platform, Venezuela

The Lower Cretaceous Cogollo Group in the Maracaibo Platform consists mostly of shallow-water deposits with co-existing carbonates and siliciclastics. Although abundant high-energy grainstone shoals are present in the stratigraphic succession, the main reservoir is confined to two intervals of leached oyster/mollusc float/rudstone. They occur in transgressive portions of third-order sequences and are, thus, predictable in their stratigraphic position. Porosity in these beds is enhanced by early (fabric selective) and late (non-fabric selective) leaching. 
The approximately 300 m thick Cogollo Group consists of six and a half third-order sequences. The transgressive system tracts are dominated by lagoonal carbonates, shales and oyster/mollusc float/rudstones, with lesser amounts of sandstones and grainstones. Highstand system tracts primarily comprise grain-dominated carbonates and tide-influenced quartzitic sandstones, with lesser amounts of lagoonal facies and shales. Facies with reservoir quality are restricted to two intervals of oyster/mollusc float/rudstones that occur in two transgressive intervals. The first interval (sequence three) is only 3 – 6 m thick and has limited lateral continuity and average reservoir quality. The second interval (sequence seven) is approximately 10 m thick and laterally continuous throughout the developed area. Two leaching events make this interval a high porosity unit. Early leaching occurred in the shallow burial environment, while warm fluids that rose along faults and invaded the formation laterally produced the late leaching. The leaching is variable along strike, resulting in lateral reservoir heterogeneity. Consequently, although the stratigraphic position of the reservoir can be predicted, lateral changes in the reservoir quality are hard to assess.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.