Recent advances in seismic acquisition and processing techniques have greatly improved the usable bandwidth and signal-to-noise ratio of seismic data collected in the Paradox basin. Historically, the irregular terrain coupled with shallow velocity gradients limited the usable stratigraphic thin-bed resolution to near 60 Hz. Analysis of filtered sonic logs of the upper Ismay formation suggests that even with 100-Hz seismic data, carbonate porosity will not be represented seismically as an individual low-velocity trough on wavelet-processed data. Conventional seismic stratigraphy attempted to interpret these zones based on a qualitative study of synthetic seismograms.

The purpose of this paper is to evaluate an inverse method of calibrating seismic data by the integration of quantitative analysis of filtered sonic logs from bore-hole data with seismically derived sonic logs (Seislogs). The Seislog display has a horizontal scale in transit time and a vertical scale in depth. In this format it is possible to compare quantitative variations observed on the Seislogs with changes derived in model studies that relate filtered velocity on sonics to lithologic variation.

This paper discusses the application of this method in an upper Ismay field (Patterson Canyon), located in San Juan County, Utah. The field is currently producing oil from a porous carbonate algal-mound facies, which in filtered-sonic logs is typified by a decrease in both velocity and thickness when compared to the anhydrite facies. When the carbonate facies becomes tight, there is a transit-time decrease and an apparent increase in thickness. The anhydrite facies sonically has the lowest transit time and also is the thickest facies. It is important to note that a limestone with intercalated clastics may have exactly the same sonic response as the porous carbonate facies. The distribution of these facies in map view provides the key to differentiation of the two types of low velocity response in this trend. The porous carbonate facies is typically narrow and lenticular in contrast to the more sheetlike clastic carbonate facies.

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