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Long Offset AVO in a Midcontinent Tight Gas Sand Reservoir
The study area, located in North Texas, is within the Fort Worth Basin, while the zone of interest is the Pennsylvanian age, Atoka Sands. Characterized by high acoustic impedance generally encased within lower impedance shales, these sands were deposited in a shallow to moderate depth continental seaway. The age and relative cementation of the Atoka sands have led to conditions that exhibit a Class I amplitude variation with offset (AVO) anomaly in the presence of hydrocarbons. Also known as dimouts, Class I anomalies were difficult to recognize and evaluate owing to the inherent nature of the seismic signature and inevitable noise within a particular seismic volume. However, where encouraging geologic environments were defined, accompanied with an anomalous amplitude event, evaluating the very far offset traces relative to near traces was suggested. Class I AVO anomalies initially dim with offset. In the far offsets, they may go through a phase reversal and increase in amplitude (Hilterman et al., 2000). Thus, processing for amplitude preservation, evaluating the far traces in an offset section and applying a Class IIp (phase reversal) AVO approach, anomalous responses may be identified, interpreted, and decomposed to explain the basis for the anomaly (Ross, 1995; Hilterman et al., 2000). Furthermore, it was shown that the AVO anomaly class varies laterally owing to changes in geologic conditions. Therefore, it is essential that detailed geologic mapping and facies interpretation preclude AVO work and be incorporated as an iterative process in the AVO interpretation.
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