Geophysical Characterization of Lithology-Application to Subtle Traps
In its first half century, geophysical exploration by the seismic reflection technique has been used mainly to delineate the morphology of subsurface strata. While seeking structural information, the technique has yielded little information on the physical properties of the rocks themselves. If lithologic information from nearby wells or outcrops was absent, the wildcatter had to drill blindly into shapes which might be hydrocarbon traps hoping they contained the right kinds of materials. Traps with no morphologic expression or having ambiguous morphologic information were virtually impossible to identify with the seismic technique.
Today, with advanced technology it is 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 increasing porosity and gas content, it is now possible to define some nonstructural traps and to identify directly some hydrocarbon accumulations.
Computer-plotted cross sections, with seismic velocity measurements shown in color, graphically identify a variety of subtle traps in many parts of the world. New seismic techniques presently being investigated should make possible the simultaneous determination of additional physical properties. In the future, these measurements may reveal traps even more subtle than we suspect exist today.