Controls on Permeability from Conventional Core Data in Unconsolidated Sands, Offshore Gulf of Mexico

Michael Marzano

One of the major shortcomings of many geologic models in attempting to characterize reservoirs is the uncertainty of permeability estimates. This uncertainty has been 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 11 wells, taken in various parts of the Gulf of Mexico and from different depositional environments, were examined to develop a better correlation of porosity to permeability with rocks of different grain size and shaliness. Results from this study would allow an analyst without conventional core data to stimate permeability using porosity and grain size from sidewall core data.

The data set includes hundreds of conventional core samples with porosities that range from 12 to 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 more analytical examination of controls on permeability. Z-plots were constructed, which allowed porosity-permeability crossplots and a third variable to be displayed. For a constant porosity, an increase in grain size and a decrease in shaliness were observed in a higher permeability, and by using particle size, one can determine at what percentage an increase in grain size or a change in shaliness becomes significant. Though the data showed some scatter, a better correlation between porosity and permeability is possible if the grain size of the rock is known. A good correlation appears between the volume of matrix material and grain size in this data set.

Large declines in permeability observed at increased confining pressures confirms the need to correct core data to reservoir conditions and to properly plan core analysis.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.