THE ROLE OF GEOSTATISTICS IN GEOLOGICAL AND PETROPHYSICAL OIL RESERVOIR MODELING

The oil reservoirs are the result of a complex succession of geological, physical, chemical, and biological process acting during the geological past. The processes have created a unique distribution of rocks with petrophysical properties that conforms the current petroleum reservoirs. Although these processes may be understood, we do not understand completely all the details and their interactions, more over we do not have the complete data set to provide a unique true distribution of rock types and their properties within the reservoir. Nevertheless, we can try to create numerical models that represent the result of all these processes and their features. The main idea is to make numerical models consistent with the conceptual idea of the geological setting and also honor all the available information. So, reservoir modeling is a multidisciplinary task which involves disciplines like geology, geostatistics and stochastic simulation (Cosentino, 2001).

The started point of the work was a sedimentological and stratigraphic interpretation of the reservoir from which a geological conceptual model was established. The definition of facies was based on well log interpretation, core description and seismic interpretation. Other step of the study consisted of a basic statistical analysis in order to explore the information and made a statistical characterization of the facies in combination with their petrophysical properties. More over the modeling task includes the calculation of facies proportion curves and the estimation and modeling of variograms (Deutsch, 2002). Reservoir heterogeneity is defined as a variation in reservoir properties as a function of the space. These properties may include porosity, permeability, thickness, saturation of fluids, faults, fractures, facies and all rock characteristics, for a proper reservoir description is important to predict theirs variations as a function of spatial locations. (Kelkar & Perez, 2002) In the context of reservoir modeling, the geological and petrophysical model obtained can be applied to evaluate different geological hypotheses, to assessment the associated uncertainty and to explain the behavior of the production oil wells.