Identification and analysis of Salado Formation amplitude anomalies in the Midland Basin

Careful mapping of shallow drilling hazards is key to operating an oil field safely and economically. In the Midland Basin, historical information indicates that shallow gas in the Salado Formation may result in blowouts. By using recently acquired higher-resolution 3D seismic surveys, we can identify circular high-amplitude anomalies that we interpret to be nitrogen-filled gas pockets. The larger pockets can be mapped and avoided before drilling a well. Detecting gas pockets in the Salado Formation may require modern 3D coverage or higher-resolution seismic drill site surveys if older low-fold seismic data are available at the drilling locations. The smallest pockets may not be detectable by any means. Although some of the larger anomalies are visible on older 3D surveys, most only became visible with the advent of higher-fold denser 3D seismic surveys. The anomalies sometimes follow a linear pattern. However, most are isolated in nature and appear as circular features in plan view and usually lenticular or columnar in section view. The anomalies range in size from a few traces in width (tens of feet or meters) to more than 1000 ft (300 m) in width or length. Seismic modeling suggests that the anomalies are very low P-wave velocity zones (about half of the velocity of the surrounding salt) and exhibit longer seismic traveltimes or push-down effects on reflections underneath the larger anomalies.