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Two high-resolution seismic reflection surveys were conducted to identify shallow natural gas that had caused explosions in Hutchinson, Kansas, in January 2001. Gas presence is associated with both a bright spot and a dim-out on the seismic reflection profiles. Core and log data from wells drilled to vent the gas indicate that the gas-bearing interval corresponds to thin dolomite layers, which have higher P-wave velocities than the surrounding shales. Gas in fractures can reduce the velocity of the dolomite interval to that of the shales (or lower). Depending on the magnitude of the velocity change, either a dim-out or bright spot is produced. Sonic logs from gas-bearing vent wells, recorded after venting of gas, show no anomalous velocity, indicating that as gas dissipates, any associated seismic anomaly will be reduced.
Lateral variations in the seismic properties of the gas-bearing interval and adjacent strata (namely, variations in dolomite and shale content) also have a significant effect on the seismic signature of the interval, mimicking the effect of a small amount of gas. Only where the gas zone is relatively thick (2–3 m; 7–10 ft), creating a high-amplitude negative seismic reflection, is the seismic signature diagnostic of gas. Therefore, whereas the dim-outs observed on the seismic reflection profiles may be the result of gas presence, they are equally well explained by lateral variations in lithology. Dim-outs should not be used in the Hutchinson area as an indicator of gas. The observed bright spot, however, is most likely a unique gas response.
Susan E. Nissen is a geophysicist at the Kansas Geological Survey (KGS). She received a B.S. degree (1983) from the University of Delaware and a Ph.D. (1992) from Columbia University. Prior to joining the KGS in 2000, she spent 8 years as a research scientist with Amoco. Her main interest is integrating 3-D seismic attributes with well data to improve geological interpretations. W. Lynn Watney is a senior scientist at Kansas Geological Survey and executive director at the University of Kansas Energy Research Center. He joined KGS in 1976 and is an alumnus of Chevron. He received a Ph.D. (1985) from the University of Kansas and B.S. (1970) and M.S. (1972) degrees from Iowa State University. His current interests are late Paleozoic stratigraphy, multidisciplinary reservoir modeling and play characterization, and developing collaborative web-based, integrated petroleum reservoir modeling tools.
Jianghai Xia received a Ph.D. (1992) in geology with emphasis in geophysics from the University of Kansas. He is an associate scientist at the Kansas Geological Survey. His main research interest is near-surface material characterization by integrated geophysical tools, including shallow seismic reflection and refraction, surface wave technique, ground-penetrating radar, electromagnetics, and potential fields.