ABSTRACT

One of the major shortcomings of many geologic models in attempting to characterize reservoirs is the uncertainty of permeability estimates. This is especially true in the offshore Gulf of Mexico where the cost of acquiring data is high. Most fields have no conventional core data; more often than not the geologist or petrophysicist has only wireline logs, occasionally supplemented by sidewall core data, to use in describing the reservoir. Conventional core data from eleven wells, taken in various parts of the Gulf of Mexico and from different depositional environments, were examined in an attempt to develop a better correlation of porosity and permeability with rocks of different grain size and shaliness. This study will demonstrate how an analyst without conventional core data can estimate permeability using wireline log porosity and grain size from sidewall core data.

The data set is based on hundreds of conventional core plug samples that have measured porosities which range from 12-43% with a mean of 33% and permeabilities of 1 to near 10,000 md with a mean of 261 md. The data were divided into two groups, one with only qualitative information on rock fabric from core descriptions and another group with particle-size data, which allowed a quantitative examination of controls on permeability. It was observed that for a given porosity, an increase in grain size and a decrease in shaliness resulted in a higher permeability. By using particle size, it was possible to determine a threshold percentage for which an increase in grain size or a change in shaliness becomes significant. A better correlation between porosity and permeability is possible if the dominant grain size of the rock is known.

Large declines in permeability observed at increasing confining pressures confirms the importance of correcting core data to reservoir conditions.