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Compressional and horizontal shear wave data recorded at the surface are used to detect lateral changes in the physical properties of a clastic unit. Shear and compressional wave transit times across the formation were measured from CDP-stacked sections derived from data collected along collocated shear and compressional seismic lines. At each surface position, the ratio of the shear to compressional transit times is formed from common CDP traces. It is shown how lateral changes in the transit time ratio primarily correlate with variations in the sand-shale ratio in the zone of interest.
The horizon selected for this case study was the Morrow Formation, which produces gas from sand-channel bodies in the Empire-Abo field, New Mexico. As this field has been extensively drilled, a detailed cross section of the producing horizon was mapped along a seismic line which crossed two wells. In well "A," the nonproductive Morrow cycles 1, 2, and 3 are principally shale and nonpermeable shaly sand facies. In well "B," this same interval contains as much as 160 ft (49 m) of permeable gas-productive sand which has a calculated absolute open flow of 61 MMCFGD. Shear wave and compressional wave Vibroseis surveys were conducted along this seismic profile using data acquisition parameters designed to produce comparable signal-to-noise ratios and resolution in the field data. Similar ca e was taken during data processing to insure that the differences and similarities observed in the final CDP sections were due to variations in geology and not simply artifacts of the particular set of processing parameters that were employed.
Along the seismic profile both compressional and shear wave interval transit times across the Morrow Formation showed a statistically significant decrease in going from the nonproductive to productive thicknesses of sand. There is, however, a proportionately greater decrease in the shear wave transit time than in the compressional transit time which results in an overall decrease in the shear-to-compressional transit time ratio. There are two changes in the physical properties of the Morrow which could account for the observed transit time ratio variations. First, the replacement of pore fluid in the sand by a small amount of gas would cause a decrease in the transit time ratio in going from well "A" to well "B." However, the compressional wave transit time should increase drastically in this case, while the shear wave transit time would decrease slightly. This behavior was not observed. A second possible explanation is that the decrease in transit time ratio was due to an increase in the sand-shale ratio between the two wells. Because of the marked differences in the shear-to-compressional transit times for pure shale (2.4) and pure sandstone (1.7), any increase in the sand-shale ratio should be accompanied by a decrease in the formation transit time ratio. Furthermore, it would be anticipated that a change in the sand-shale ratio would influence the shear wave transit time more significantly
than the compressional transit time--a fact which is consistent with the observations. We thus conclude that shear-to-compressional transit time ratio measurements provide a method for estimating variations in the sand-shale ratio of a formation.
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