13. Detection of Subsurface Hydrocarbon Seepage in Seismic Data: Implications for Charge, Seal, Overpressure, and Gas-hydrate Assessment

Abstract

Many basins are dominated by vertical hydrocarbon migration. On seismic data, the vertical migration paths are generally recognized as vertically aligned zones of chaotic, often low-amplitude reflectivity. These are described variously as gas chimneys, blowout pipes, gas clouds, or hydrocarbon-related diagenetic zones. Analysis of the gas chimneys can be used for geohazard prediction, basin modeling, prospect risking, and many more applications. However, the weak expressions of gas chimneys in seismic data make them difficult to map. Thus, a method for detecting gas chimneys in poststack 3D seismic data has been developed to map their distribution and allow them to be visualized in three dimensions. This chimney probability volume is produced by a neural network from multiple seismic attributes extracted at examples of gas chimneys picked by the interpreter. Not all vertically aligned, low-amplitude, chaotic seismic reflectors represent hydrocarbon migration. Therefore, the subjective selection of training locations and the resulting neural-network predictions are validated by objective criteria before the results are used in geologic applications. Gas-chimney detection methods originally were used to highlight gas chimneys in shallow intervals to detect shallow gas reservoirs and geohazards. However, the methodology was soon used to highlight subtle, deep hydrocarbon migration pathways, hydrocarbon migration related to faulting, and expulsion from source rock. Other applications of gas-chimney analysis are overpressure prediction and prediction of the quality of gas-hydrate accumulations. Chimneys are classified based on their morphology and the relative position of the trap, faults, and chimneys. This classification provides criteria for risking top seal, vertical fault seal, and hydrocarbon charge on exploration prospects prior to drilling.