PAPER KK

EXTRAPOLATING LOGS RUN IN EXPLORATION OR DEVELOPMENT WELLS USING SEISMIC DATA

Dale G. Stone and Hilton B. Evans

Seismograph Service Corporation

Tulsa, Oklahoma

ABSTRACT

Well logs yield a high resolution to vertical earth layering, but are sparsely spaced horizontally. Seismic data have a relatively dense horizontal sampling, but are limited in vertical resolution. Seismic responses are sensitive to lateral lithologic (bed thickness or properties) changes, although the vertical resolution may not be adequate to define the nature of the change. Statistically, seismic data can be used to detect the amount of lateral variation based on the seismic trace at the well site. Thus, the high resolution data of the well log can be extrapolated using the seismic data as a transfer function. Vertical seismic profile data are used to calibrate both the seismic and the sonic log data. The vertical seismic profile also yields below-the-bit extrapolation of seismic/log data to aid in target definition.

The well logs are aligned and statistically evaluated with the corresponding seismic traces. Similarly, the traces in the target horizon are aligned with the reference traces. A time variant coherency function examines the similarity of each trace with the reference traces. The log data are combined with seismic data through

G_t = C_t * R_t + (l-C_t)  * S_t,

where C_t is the coherence, R_t is the well log and S_t is the seismic trace. The composite signal is the extrapolation result. This procedure uses the similarity of seismic traces as a criteria for the applicability of the well log extrapolation; seismic data are used where the seismic trace has no similarity to any of the well logs on the seismic line.

The extrapolation procedure has potential to aid the interpreter on subtle stratigraphic problems in several ways. Well site conditions, defined by the logs, can be laterally delimited. Similar well site conditions can be detected along the horizontal line. The amplitude envelope of the section can be determined, which improves the amplitude ratio of reflections and increases the lateral coverage defined by high resolution well log data.