Tuning is not necessarily bad—Channel sand delineation from seismic data of the Upper Gharif in Oman

Abstract

Upper Gharif reservoirs contribute to a significant portion of Petroleum Development Oman (PDO) reserve and resource volumes. To drill and encounter channel sands in this formation have been historically challenging. Previous attempts to predict the location and extent of the sands were discouraging. As such, grid drilling is employed in several fields, which is costly, especially in deep reservoirs. However, a breakthrough was made in Field Z. The three existing wells show different seismic characteristics. Suspecting that focus on high-tech issues such as seismic inversion might have been a distraction instead of adding value and that thickness below tuning might not always be a bad thing, a 2D model with two sand wedges was made. The model showed that synthetic seismic amplitude varies when sand thickness varies and that seismic trace shape changes too. These observations supported a decision to drill an amplitude hot spot, where good sand presence was predicted. Results from this and three additional wells have all confirmed sand/shale lithology predictions from seismic data. Wells, synthetic data, and real seismic data converge on one conclusion: The absence of thick sands eliminates the well-known tuning-related nonunique correlation between seismic amplitudes and sand thickness. In other words, tuning becomes a good thing. Neither an innovative concept nor cutting-edge technology was used. The work might seem primitive to people who are used to good-quality seismic data where thin beds are illuminated via frequency decomposition and RGB blending in a routine workflow. However, it is the first successful case of the Gharif Formation in Oman, where previous seismic studies resulted in an overall disbelief in using seismic data for reservoir characterization. This work proved that reservoir distribution in Field Z is predictable and thus there is no need for expensive grid drilling. Going back to geophysical basics and not overlooking thin layers, this success has not only played a key role in the field-development plan study of Field Z but also has rejuvenated PDO's interest in quantitative seismic interpretation of the Upper Gharif.