Practical AVO modeling: A case study from the south Texas Wilcox trend

Abstract

The lower Wilcox sands in south Texas typically have a lower impedance than the overlying shales. When the tops are fairly sharp, this produces a weak negative reflection coefficient. The presence of hydrocarbons further decreases the impedance, suggesting that these sands will have a Class 3 (classic bright spot) AVO response. However, there can be significant lateral variation in the Wilcox. Formation tops vary from sharp to gradational across our study area. Such gradational tops produce little impedance contrast and very little seismic response. Since the properties of the Wilcox can vary substantially, the results of inverting seismic signatures for rock properties and fluid content can be critically dependent upon the baseline rock property assumptions. Simply using Class 3 Gulf Coast sand parameters is not valid. We generated two AVO models for a lower Wilcox sand that produced significantly different results. One model suggests strong amplitude could be an indicator of hydrocarbons, while the other model suggests amplitude is only a sand indicator and tells us nothing about the presence or absence of hydrocarbons.