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Abstract: Geostatistical Reservoir Characterization of Complex Lateral and Vertical Sequences in a Mixed Carbonate Platform

R. J. Norris, F. G. Alabert, G. J. Massonnat

In recent years reservoir characterization through the use of geostatistics has become an almost routine part of production geology. Many techniques are available within the broad title of geostatistics, having been developed in response to many types of problem. One characteristic feature of almost all techniques (Stochastic Indicator Simulation, Boolean "object" Modeling, Gaussian [and Truncated Gaussian] methods and Optimized Markov-fields) is their reliance on the concept of quantifiable correlations, which reflect some aspect of the shape of "objects." For example, almost any of the above noted techniques, and their variants, could be used to model fluvial, deltaic, or turbiditic reservoirs because in each case facies can be described in terms of geometries (channels, lobes, etc. .

This study considers the complex lateral and vertical variations of a mixed carbonate platform environment, where facies can not be easily characterized by simple geometries. The complex heterogeneities are a function of changes in sea level, representing fluctuations over several orders of cyclicity. Given facies have no characteristic form, being the product of the interplay between sediment supply and sea level change. This type of environment is, therefore, characterized by a good deal of information concerning trends in the data, while correlations and "geometries" are almost meaningless. Associated with the concepts of cyclicity, "rules" concerning the reappearance of facies, or otherwise, were developed. For example, minor recurrences of maximum flooding surfaces could be toler ted within individual units but other specified recurrence need to be excluded.

Classical geostatistical approaches are hard pressed to model a reservoir in the presence of such constraints due to the importance of facies relationships in three dimensions. A modified version of the Truncated Gaussian Fields approach, wherein the truncations for each facies are based on kriged cutoff maps, has been developed. These maps represent the spatial variability of the probability of finding a given facies. When these truncation maps are applied to probability fields, facies maps are generated that represent the specified sequence logic of the sedimentology.

The reservoir characterization is continued with the replacement of facies by porosity classes, on which a trend is imposed and finally an assignment of porosity values to the porosity classes.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994