Abstract

Seismic waveform changes, which in their most obvious form are known as 'bright spots,' have been known for some years to give direct indications of hydrocarbons. An example of successful application of waveform analysis and direct detection of gas in a shallow Lower Cretaceous formation of east-central Alberta, Canada is detailed.At a depth of approximately 1 800 ft, the Colony formation typically consists of only thin (10 ft) blanket sands interbedded with shale. However, in 1976 Hudson's Bay Oil and Gas Company Ltd. encountered a 100 ft thick occurrence of channel sand (with substantial gas pay) in this formation. After some hit and miss attempts at extending the channel trend through geologic interpretation, seismic methods were applied. A seismic line over the channel well revealed a classic bright spot. Several other lines also showed bright spots in the Colony zone.The conclusions from seismic modeling are as follows. Gas within the Colony sand is seismically detectable. The relatively low velocity of the gas sand, combined with the lateral consistency of the sediments above the Colony formation, permits detection. However, the inconsistency and complexity of sediments underlying the Colony resulted in interference patterns that prevented exact quantitative analysis of gas pays. Furthermore, other geologic phenomena provided waveform changes similar to that of gas sand. Through detailed examination of the geology and evaluation of the alternative explanations of the waveform changes, successful interpretation was accomplished. Estimations of net gas pay were generally accurate within 20 percent. In some areas, very subtle anomalies in wave character representing gas pays as thin as 5 ft can now be interpreted with confidence. Several examples are given of successful detection and prediction of gas.To date (October, 1979) seismic waveform analysis has led to the drilling of 86 wells; 67 of these are commercial gas wells in the Colony formation, representing a success ratio of 78 percent. Total reserves discovered geophysically (by Hudson's Bay Oil and Gas Co. Ltd.) to date in the Colony formation are estimated at 110 Bcf.

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