FORMATION VELOCITY AND DENSITY—THE DIAGNOSTIC BASICS FOR STRATIGRAPHIC TRAPS

Abstract

A multiplicity of factors influence seismic reflection coefficients and the observed gravity of typical sedimentary rocks. Rock velocity and density depend upon the mineral composition and the granular nature of the rock matrix, cementation, porosity, fluid content, and environmental pressure. Depth of burial and geologic age also have an effect. Lithology and porosity can be related empirically to velocity by the time‐average equation. This equation is most reliable when the rock is under substantial pressure, is saturated with brine, and contains well‐cemented grains. For very low porosity rocks under large pressures, the mineral composition can be related to velocity by the theories of Voigt and Reuss. One effect of pressure variation on velocity results from the opening or closing of microcracks. For porous sedimentary rocks, only the difference between overburden and fluid pressure affects the microcrack system. Existing theory does not take into account the effect of microcrack closure on the elastic behavior of rocks under pressure or the chemical interaction between water and clay particles. The theory of Gassmann can be used to calculate the effect of different saturating fluids on the P-wave velocity of porous rocks. The effect may be large enough in shallow, recent sediments to permit gas sands to be distinguished from water sands on seismic records. At depths greater than about 6000 ft, however, the reflection coefficient becomes essentially independent of the nature of the fluid. Data show the systematic relationship between velocity and density in sedimentary rocks. As a result, reflection coefficients can often be estimated satisfactorily from velocity information alone.