Diagenetic and Depositional Constraints in Reservoir Rock Quality: Examples from North Monagas Oil Fields of Eastern Venezuela

Jose Ignacio Guzman, Ana Maria Hernandez

Porosity and permeability distributions in Cretaceous and Tertiary clastic reservoirs of the North Monagas area of eastern Venezuela, are affected by differences in diagenetic effects and by the environment of deposition. Diagenesis in turn is controlled by the original sand composition, facies distribution and structural depth.

A total of 1700 feet of cores recovered from 4 wells, were described and sampled for detailed petrographical, biostratigraphical, x-ray diffraction and SEM analyses The cored sections cover a stratigraphic interval ranging from late Campanian to early Miocene. Four major sandy lithofacies were recognized on the basis of texture and sedimentary structures, providing adequate geological constraints for the distribution of petrophysical properties into discrete categories.

Secondary porosity in the subarkosic Cretaceous reservoirs is enhanced due to leaching of detrital potassium feldspars, whereas in structurally deeper quartzitic Tertiary reservoirs, the dissolution of early carbonate and siliceous cements and of infiltrated clay matrix are the major porosity-generating processes. Sandstone texture differences due to varying depositional facies is locally an important control on reservoir rock quality. Primary and secondary porosity is destroyed in the deeper reservoirs due to cementation by late quartz and carbonate cements. The proximity of some wells to younger fault surfaces, results in precipitation of pyrite cements that also inhibit pore volume. The presence of ilite and interstratified clays reduces permeability in the Cretaceous reservoirs.

The representation of porosity-permeability distributions in standard scatter plots, helps to visualize the striking differences in reservoir rock quality due to varying diagenetic and depositional conditions.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995