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This paper summarizes some relations between rock physical properties and the influence of subsurface environmental conditions that commonly are encountered in seismic stratigraphy problems. Many of these relations are empirical correlations based on laboratory and field data, but theory also provides useful guidelines for explaining observed relations. Correlations between velocity, porosity, density, mineral composition, and geologic age, and their dependence on pressure and temperature are documented. Permeability remains an elusive parameter that is not directly measurable by geophysical methods.
Techniques for measuring velocity and attenuation of rock samples in a laboratory environment are discussed to show the capabilities and limitations of these methods. Predictions based on the theory of Biot indicate that viscous losses caused by fluid motion in rocks are of minor importance at low frequencies compared with losses caused by solid friction. Evaluation of the elastic constants of grossly anisotropic rocks requires specialized laboratory techniques. Differences between elastic moduli derived from static and dynamic measurements appear to be related to the presence of microcracks in rocks at low pressures.
The effect of temperature on elastic properties is too large to be ignored in many reservoirs and especially in those located in geothermal zones. The theory of Gassmann is used to show that velocities and reflection coefficients are relatively independent of the type of pore fluid at depths greater than about 6,000 ft (1,830 m) in Miocene sediments in a Gulf Coast area. Generally, when both overburden pressure and formation fluid pressure are varied, only the difference between the two (the effective overburden pressure) has a significant influence on velocity.
Results of laboratory studies show that fluid saturation effects on compressional wave velocity are much larger in low porosity than in high porosity rocks. Shear-wave velocities of sedimentary rocks fully saturated with gas or water do not always agree with the Biot theory; agreement is dependent on pressure, porosity, fluid-mineral chemical interactions, and presence of microcracks in the cementing material. The presence of gas in sedimentary rocks reduces the elastic moduli, and the effect is greatest at low pressures. Elastic moduli and ratios of compressional and shear-wave velocities have significant diagnostic value for differentiating between gas and liquids in sedimentary rocks.
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