Seismic Amplitude Measurement for Primary Lithology Estimation (SAMPLE): Case Histories From Tertiary Western Basins

ABSTRACT Amplitude variations within a common depth point (CDP) gather can be interpreted to yield shear wave velocities or Poisson's ratios. The SAMPLE method does such an interpretation and from a crossplot of pressure wave velocities versus Poisson's ratios, lithologies, and pore fluids are estimated. The SAMPLE method works because reflection and transmission of elastic waves (seismic waves) at the boundary between two media are a function of six elastic parameters. P-wave velocity, Poisson's ratio, and bulk density are the three elastic parameters in each media. Given these six parameters (three in each media) and the angle of incidence, reflection, and transmission amplitudes can be calculated using Zoeppritz's equations (Richter, 1958). SAMPLE is an inversion of this calculation, where;P-wave velocity is determined in a conventional manner, i.e. Dix interval velocity.Bulk density is assumed to be a function of P-wave velocity, (Gardner et al., 1974).Unknown Poisson's ratios can be determined from the reflection amplitude variation versus shot-to-geophone offset within a CDP gather, (Gassaway and Richgels, 1983).From a crossplot of Poisson's ratio versus P-wave velocity, lithologies and pore fluids are estimated, (Gassaway and Richgels, 1983). Live data examples from the Denver-Julesburg Basin in Nebraska and the Sacramento-San Joaquin Basins in California indicate that this technique can identify gross average lithology over zones 50 milliseconds (approx. 250 feet) thick in consolidated rocks. Gaseous hydrocarbons (approx. 30 feet thick) can be recognized from zones greater than ten milliseconds. INTRODUCTION During the last two decades seismic field recording instruments and techniques have made great strides in collection data to unravel subsurface geology and directly detect hydrocarbons. With the advent of CDP stacking, digital recording and processing, and large multiplicity of recording channels at large shot-to-geophone offset, the simplified normal incidence reflection wave theory no longer describes the observed seismic data. In fact, much important and relevant data collected by these new methods is ignored because of the development history of the present seismic technique. At the turn of the century, the elastic wave propagation theory was developed from first principles of physics in order to apply the results of these theories during the first two-thirds of this century (precomputer era), assumptions were made to simplify the mathematics. The location of the geophone and shot at the same position greatly simplified the application of elastic wave theory. This simplification was valid until the late 1950?s when the maximum offsets were 1000 to 2000 feet from shot to geophone. Today, however, offsets of 5000 to 25,000 feet are the norm. It is no longer valid to assume that the shot and geophone are close together when they are three miles apart. Therefore it is necessary to return to the still valid elastic wave theory without these assumptions in order to interpret modern seismic data correctly.