Prospect Risking Using 3-D Seismic Derived Gas Chimney Volumes
Most Tertiary hydrocarbon producing basins of the world are dominated by vertical hydrocarbon migration. This vertical hydrocarbon migration is often directly detected in the seismic record as zones of vertically chaotic low energy data or "gas chimneys". These chimneys have often been observed in relationship to producing fields and dry holes. However, with the exception of a few studies, these chimneys have not been studied in a systematic way to determine their relationship to risk for charge and seal.
A method is described for detecting chimneys in 3D seismic data using directional seismic attributes run through a neural network. Chimney probability volumes were processed over 60 known producing fields or discoveries and dry holes containing valid structural closures and effective reservoirs. The chimney analogs were divided into four trap classes which describe the relationship of the chimneys both above and below the trap. The first class was "Non-fault Seal", generally four way closures with no chimneys over the closure but clear chimneys in communication with the reservoir. The second class was "Fault Seal" traps which are fault closures, having chimneys below but not above the trap. The third class was "Gas Cloud" traps which have diffuse chimneys above the trap. The fourth class was "Fault Leak" traps which were fault closures with chimneys along the trapping fault. Significantly, all productive examples fell in the first three classes, while the non-productive wells fell in the fourth class. Thus this tool can be used to effectively risk prospects with associated chimneys.