Role of Parasequence-Scale Sequence Stratigraphic Analysis in Integrated Reservoir Characterization and Modeling of Shallow-Water Carbonate Strata

KERANS, CHARLES, F. J. LUCIA, D. G. BEBOUT, R. P. MAJOR, and S. C. RUPPEL, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The predictable stacking patterns of parasequences within different systems tracts of a sequence and of facies and rock-fabric successions within parasequences provide a powerful conceptual framework for integrated analysis of carbonate reservoirs. Stratigraphic analysis using a parasequence approach emphasizes correlation through comparative mapping of genetically related facies successions rather than the classic marker bed approach. The basic shallow-water carbonate parasequence is the upward-shallowing cycle (approximately fifth-order cycle, average period -20,000 yr) that records a single progradational event represented most often by an upward-coarsening facies/rock-fabric succession. Interpretation of depositional environment is best accomplished at the parasequence scale becau e lateral facies relationships within these units best approximate time-equivalent genetically related deposits.

The upward-shallowing facies succession that forms a parasequence provides a natural framework for quantification of geologic descriptions through rock-fabric-oriented petrophysical analysis. Permeability and saturation are directly related to sediment grain size, sorting, and interparticle porosity. Grain-dominated rocks have higher permeability and oil-saturation values than do mud-dominated rocks with the same interparticle porosity. Grain size and sorting occur systematically and predictably in the upward-shallowing parasequences. Porosity, grain size, and sorting can be significantly altered by diagenetic processes such as dolomitization, cementation, and dissolution. Diagenetic overprints are, however, commonly controlled by the depositional textures and thus also are predictabl .

Laterally continuous low-permeability strata at either the base (flooding-surface mudstones) or the top (anhydrite-cemented tidal-flat caps) of parasequences define key components of the flow-unit framework and exert a primary control on fluid flow and vertical sweep efficiency. Data on external dimensions of depositional facies for use in stochastic models of interwell heterogeneity are best derived from maps of individual parasequences that approximate true, rather than time-averaged, dimensions.

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