Geophysical Characterization of Lithology--Application to Subtle Traps: ABSTRACT

Abstract

Traps for oil or gas consist of a geologic material whose physical properties may differ from those above and around it. In its first half century, geophysical exploration by the reflection seismic technique was used to delineate the boundaries between subsurface materials of different properties. The technique could not discriminate between the properties on either side of those boundaries. If geologic information from nearby wells or from outcrop was absent, the wildcatter drilled blindly through trap boundaries in the hope of finding the right kind of material. Technology has advanced sufficiently that it is now possible to determine from seismic data at least one physical property of subsurface materials. That property is seismic velocity. Because seismic velocity generally decreases with increased porosity and because gas under pressure further decreases that velocity, it is now possible to define some non-structural traps and to identify some hydrocarbon accumulations. Seismic velocity measurements displayed on computer-plotted cross sections in color, graphically identify a variety of subtle traps in many parts of the world. When applied to 3-D survey data the velocity methods can yield enough information to define all the boundaries of a hydrocarbon accumulation. The characteristic seismic velocites in a 3-dimensional subsurface block can most graphically be displayed as a motion picture depicting passage through the earth in selected directions. New seismic techniques under investigation should make possible the simultaneous determination of additional physical properties so that even more subtle traps can be found. End_of_Article - Last_Page 986------------