The term "rock pressure" is now commonly used in oil, gas, and underground water technology to refer to the pressure under which fluids are confined in rocks and takes no account of the cause or origin of the pressure, but simply of the fact of its existence. Geophysicists and isostasists are, however, using the term in the primitive and more correct sense of pressure exerted on underlying rocks by superimposed rocks, and it is suggested that in order to avoid confusion it is desirable to substitute for the term "rock pressure" as now used in oil, gas, and underground water technology the more appropriate term "reservoir pressure."

The various methods of determining reservoir pressure are discussed with special attention to the interpretation of the result secured. In the case of a closed-in well in which there is free gas and a column of oil, it is shown that a correct measurement of the pressure of the gas at the casing-head does not give the correct reservoir pressure. In the case where the reservoir pressure in oil and gas pools is due to hydrostatic pressure, it is shown that the reservoir pressures are progressively greater from the lower limit of the oil zone to the top of the oil zone than they would be in the same structure filled with water, and that the reservoir pressure throughout a gas pool is initially the same as the reservoir pressure at the point of contact of the gas with the adjoining liquid, and thus independent of the differences in elevation of the different parts of the gas reservoir.

Accurate and properly interpreted pressure determinations have value in the correlation of sands, in the determination of whether in a given region faults produce a seal or merely open channels of communication, and in the problems related to the depletion of oil pools and the encroachment of water. They are essential to oil

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operators in the management of their properties, so that the maximum production may be obtained from the oil deposits.