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Most geologists regard resistivity curves simply as sources of information concerning lithology and degree of water saturation of subsurface strata; however, other important geologic information can also be obtained. For example, water saturation is related both to pore characteristics and to the buoyancy pressures that lead to water expulsion from those pores by a hydrocarbon column. Thus, resistivity response is indirectly modified by a complex of factors including pore size, pore geometry, and grain-surface characteristics (including presence of clays). The combined effect of these pore characteristics is revealed most directly by capillary pressure curves. Because these same factors also determine permeability, resistivity itself can be regarded as responding to perme bility.
This approach to resistivity interpretation has several important consequences. For example, true oil-water contacts encountered in the well bore display transitional resistivity values as upwardly increasing buoyancy pressures approach those necessary to produce irreducible water saturation. Complex reservoirs with inclined permeability barriers (such as shale drapes along lateral accretion cross-bedding in point-bar sands or shale interbeds in tilted turbidite sand sequences) may include false water levels where vertically adjacent but separated beds may have different fluid contents and may lack transition zones. Recognition of the distinction between these two types of oil-water contact may profoundly affect reserve calculations and help avoid passing over of viable reservoirs.Correctly interpreted resistivity curves may also permit recognition of water production resulting from leading permeability barriers, may aid in distinguishing different types of oil show (leading, trailing, and residual) and may aid in explaining and predicting early water production, etc. Proper use of this readily available tool can have an important impact on successful exploration and production.
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