- Copyright © 2000 Society of Exploration Geophysicists
For more than 17 years, AVO has been a primary tool for predicting a reservoir's rock type and pore-fluid content. However, the success of an AVO interpretation (recognition and validation of an anomaly) is constrained by the rock properties of the reservoir and its surrounding media. Class 3 and 4 anomalies are recognized as amplitude bright spots on stack data, and pore-fluid prediction is routinely accomplished by anomaly/background amplitude analyses. For Class 2 anomalies, AVO interpretation of prestack data is often needed just to recognize potential reservoirs. Finally, AVO interpretation in Class 1 environments is difficult because hydrocarbons don't yield bright spots or amplitude brightening with source-receiver offset. In short, success in recognizing an anomaly and validating its composition is best for Class 3 and 4 environments and poorest for Class 1.
However, the exploration risk associated with Class 1 anomalies could be reduced if they could be treated like Class 2 anomalies because the latter can be interpreted by examining CDP traces with large-angle offsets (Hilterman et al., TLE 1998). Could this apply to Class 1 anomalies if source-receiver offsets are extended to distances that are twice the target depth?
This possibility led Fugro-Geoteam and SEI to acquire a 2-D seismic survey in offshore Texas using a 9000-m (29 520 ft) streamer to evaluate Miocene sands. Preliminary AVO modeling revealed that deeper sands in the area have sufficient velocity contrast with surrounding shales so that potential hydrocarbon plays fall between Class 1 and Class 2. Two case studies will demonstrate some preliminary findings.
Offshore Texas had major sand deposits during two geologic epochs. A shale-prone area separates these sand deposits (Figures 1 and 2). The shale prone area is sparsely populated with wells (it is situated around the Wanda Fault System notation in Figure 1). As can be observed …