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Abstract

Accurate prediction of porosity and permeability of reservoir targets ahead of the bit is one of the most important tasks facing the petroleum geologist. This is as true for a wildcat well in an undrilled basin as it is for a production or injection well in a producing field. The porosity prediction influences the perception of likely reserves while that of permeability—the key control on well flow rate—has a major impact on the Net Present Value ascribed to an opportunity and thus the likelihood that it will be drilled. Porosity and permeability are related both to the original fabric and composition of sediments and to diagenetic processes. The latter are the subject of this volume.

The usual method of predicting porosity and permeability involves the use of correlations between porosity and depth (or sometimes a time-depth integral) and between porosity and permeability (Scherer, 1987; Schmoker and Gautier, 1988; Schmoker and Hailey, 1982; Bloch, 1991). This procedure requires an estimate of the depth to the target (which is always available); a set of data relating average porosity to depth for the target formation or another selected on some basis as analogous; and core data relating porosity to permeability. It also requires of course that depth, porosity and permeability should show reasonable correlations. Correlations often work well but there are cases in which they can be very difficult to apply with any confidence. The most obvious is in a frontier exploration setting where local empirical relationships between depth, porosity and permeability are simply not available. One can resort to regional or global data sets but these can mislead as much as they can help. Another instance in which empirical prediction is difficult is when depth, porosity and permeability are poorly correlated. The method has one further important drawback. Regression relationships smooth out anomalies and make it impossible to predict, for example, unusually porous or permeable sands at great depths; yet it is often these that are the most important predictions, particularly in basins which are in a mature phase of exploration.

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