The quality of carbonate petroleum reservoirs is a product of not only the overall porosity and permeability of the reservoir, but also the variation in these attributes within the reservoir lithology. Variation in porosity and permeability within a reservoir or reservoir compartment is a reflection of heterogeneity in the reservoir lithology. Heterogeneity can occur at megascopic to microscopic scales. Megascopic and macroscopic heterogeneities influence produceability by defining individual reservoirs or reservoir compartments, microscopic heterogeneity controls the produceability within these reservoirs or compartments.

Microscopic heterogeneities can be classified as depositional, diagenetic, and/or structural. Depositional heterogeneities include laminae, grain size, sorting, and infiltered micrite. Diagenetic heterogeneities are the product of cementation, dissolution, dolomitization, and compaction. Structural heterogeneities include microfractures, microfaults, and stylolites.

The level of microscopic heterogeneity can vary dramatically within individual reservoirs or reservoir compartments. While accurate evaluation of microscopic heterogeneity requires comprehensive megascopic and microscopic examination of reservoir lithologies, estimates of heterogeneity can be developed based on an understanding of the depositional setting and diagenetic history of the reservoir unit.

Variation in microscopic heterogeneity can influence the overall produceability of a reservoir and can affect the accuracy of recovery estimates based on average porosity and permeability values. The produceability of a microscopically homogeneous. reservoir can be adequately predicted by evaluation of porosity and permeability data derived from core analysis or petrophysical data. These data, however, are much less efficient in predicting reservoir performance for reservoirs with high degrees of microscopic heterogeneity.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas