Interpreter's Corner—Seismic amplitudes from low gas saturation sands

Abstract

In Tertiary age sediments, P-wave seismic amplitudes are generally regarded as the most robust direct hydrocarbon indicators. Exploration prospects based on a sound geologic model and supported by seismic amplitude anomalies are highly prospective and are usually assigned a high probability of success. However, a fraction of such prospects, perhaps 10–30%, result in dry holes. Postdrill appraisal can usually assign these results to one or more of the following factors: When a dry hole cannot be explained by any of these causes, the seismic amplitude anomaly is often ascribed to low gas saturation in the reservoir. Seismic amplitudes are strong indicators of the presence of gas in a reservoir, but poor indicators of gas saturation. As a result, a reservoir with low gas saturation will have a seismic amplitude response comparable to that of a reservoir with high, commercial level saturation. The problem with this explanation lies in the difficulty in measuring low gas saturation in-situ; any independent measurements to confirm the presence of low gas saturation in the reservoir (implied by seismic amplitude analysis) are rare. This raises the question of whether such low gas saturation sands occur in nature with the appropriate rock properties to yield an anomalous seismic amplitude response. In a study of amplitude-supported prospects in deepwater Gulf of Mexico, Gross and Oliver (AAPG Bulletin , 1998) reported that 67% of these prospects were successful, encountering a net hydrocarbon sand interval of 15 ft or greater. However, 18% failed due to low gas saturation, which they were able to identify based on wireline logs. This paper presents …