The identification of pay in low porosity shaly gas sands is made difficult by the inaccuracy of calculating and interpreting water saturations. Outlined below are techniques that have been used to calculate water saturations in this type of reservoir.

Laboratory tests have been performed to measure saturation exponent and to define the formation factor-porosity and cation exchange capacity-clay relationships. The tests indicate that saturation exponent, n, is 1.36, and that formation factor, F, is given by .9/øe2.17. Formation factor tests and empirical calculations imply that m may be a function of porosity in porosities less than five percent.

The cation exchange capacity of the rock is due to illite. As the illite contains radioactive potassium, gamma ray activity can be used to calculate cation exchange capacity. Laboratory spectral gamma ray analysis indicates that the potassium component is a good indicator of cation exchange capacity.

Water saturations are calculated using the Waxman-Smits technique. Errors induced by not accounting for clay conductivities are presented as functions of porosity, water saturation, and illite content. The induced errors are greatest in low porosity zones.

Error analysis indicates that the major source of error in calculating water saturations (excluding the effects of exchangeable cations) is the inaccuracy of determining porosity. At porosities less than five percent, the possible error in water saturations approaches one hundred percent.

Data are presented relating the error in water saturations to errors in porosity and resistivity.

The significance of water saturations in low porosity, low permeability sandstones is unclear. Drainage relative permeability data predict that except at extremely high water saturations the flowing phase should be predominantly gas.

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