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Formulation of depositional models of carbonate reservoirs are now sufficiently sophisticated so that structural and stratigraphic traps of considerable complexity can be identified by use of core data. However, limited core data have, in many cases, made interpretation of the lateral extent and continuity of these beds difficult. As a result, location of wells, determination of zonal continuity, and enhanced recovery operations have presented difficulties to the petroleum geologist and engineer.
In an attempt to determine if well logs, using minimal core data, could be used to define carbonate facies by depositional texture, a study of the Sligo Formation (Cretaceous), south Texas, was recently published. This study of a shelf-type stratigraphic trap concluded that high- and low-energy carbonate facies could be readily identified by use of well-log cross plots, allowing for identification of reservoir parameters of size, geometry, zonal continuity, and distribution of porosity and permeability--parameters traditionally determined by use of cores.
This study of Hanford field represents a more sophisticated approach to definition of facies by use of well-log cross plots. Based on another study of Hanford field, core descriptions were correlated with well logs, and the digitized logs were then subjected to numerous cross plots. It was determined that facies could be readily disaggregated in this manner, and that permeability could be reasonably estimated using the Kozeny equation, where core data were unavailable. The fact that the San Andres Formation (Permian) has been subjected to extreme diagenesis makes this study a significant step beyond the primary textural patterns encountered in earlier work on the Sligo Formation. Its implications are important, since extensive log data, together with minimal core information, will all w the explorationist and engineer to locate and better define individual productive zones and their extent at reduced cost.
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