In 1982, Bass Enterprises applied successfully for tight gas designation for the Morrow Formation over an area of approximately 320,000 acres encompassing the Big Eddy and Poker Lake Federal Units in Eddy County, New Mexico. Relating to this application, a petrophysical study was made to determine the pay section in the Morrow and the in-situ permeability of it.

Initially log and core data were quality controlled, porosity logs were calibrated using core data, and Pickett plots were used to determine the formation water resistivity (R_w) and the formation resistivity factor (F). Subsequently, the R_w and F values were used in determining water saturation. The pay section was then identified by determining the porosity and water saturation cut-offs from porosity vs. water saturation crossplots and production tests from zones of varying water saturation.

Standard laboratory-measured core data analyzed at 200 psi provided the porosity-permeability relationships which allowed permeability data to be obtained using the porosity logs. Using additional core data, a relationship was established between this "surface" permeability and a permeability measured at subsurface conditions more analogous to that of the reservoir. This relationship was used to determine the in-situ permeability of the pay section.

This discussion covers several critical aspects of reservoir description, and, although the data involved pertain to the Morrow Formation, it is stressed that these principles can be used for other reservoirs. When possible, such aspects should be investigated more frequently, be it in an exploration or a development program.

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