Seismic Ties from Well Data
Reflection seismology is essential for modern petroleum exploration. It allows us to estimate the changes in geologic formations between control points of existing wells. In this sense, seismic data allow us to interpolate between wells.
Conversely, this interpolation is possible only if we can identify seismic reflections. This is done by using correlations to well data. Although formation depths, resistivity, SP, and gamma-ray logs are useful, sonic and density logs provide the most useful information for correlating with seismic data. This correlation often is done with synthetic seismograms. Synthetic seismograms of primary reflections usually can be considered as band-limited reflection coefficients. These coefficients are dependent on contrasts in acoustic impedance (product of density, ρ, and P-wave velocity, v), that is, the reflection between two layers (say layer 2 and layer 1) is given by
Therefore, the density and velocity logs of wells are used to calculate a set of reflection coefficients, which is then filtered so that the dominant frequencies match those in the surface seismic data. The set of reflection coefficients is then band limited to the same frequency band as the seismic data. The formations are identified on the synthetic seismogram by using well-log formation information. Correlations are then made between the reflections on the well-log synthetic seismograms and those on the seismic data. Several examples appear in this book, including examples in Chapter 15.
Figure 1 shows an example of such correlations for traces from a Grand Banks seismic line. In the reflection arrivals,
Figures & Tables
Fundamentals of Geophysical Interpretation, SEG Geophysical Monograph Series No. 13, is a practical handbook for the petroleum geophysicist. Fundamental concepts are explained using heuristic descriptions of seismic modeling, deconvolution, depth migration, and tomography. Pitfalls in processing and contouring are described briefly. Applications include petroleum exploration of carbonate reefs, salt intrusions, and overthrust faults. The book includes past, present, and possible future developments in time-lapse seismology, borehole geophysics, multicomponent seismology, and integrated reservoir characterization.