Chapter 15: Determining Migration Path from a Seismically Derived Gas Chimney: A South Africa Case History*
Fred Aminzadeh, Tim Berge, 2013. "Determining Migration Path from a Seismically Derived Gas Chimney: A South Africa Case History", Hydrocarbon Seepage: From Source to Surface, Fred Aminzadeh, Timothy B. Berge, David L. Connolly
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Chimney analysis can help us assess reservoir risk in a gas field. Seismically derived gas-chimney volumes can be used to determine migration paths and relate them to surface seeps and mud volcanoes. From the chimney cubes, we observe the vertical hydrocarbon migration paths that can be interpreted from a source into reservoir traps and then to the near surface (shallow gas) and surface (seeps). Using data from the Ibhubesi field in Orange River Basin, South Africa, we examined many applications of chimney cubes, including unraveling the hydrocarbon history and migration paths; ranking prospects; detecting reservoir leakage, spill points, and sealing versus nonsealing faults; identifying potential overpressured zones and drilling (shallow gas) hazards; and revealing areas of seafloor instability. We exploited the principle of directional attributes to highlight areas in the seismic volume that are likely gas chimneys. Aside from conventional single-trace attributes such as amplitude, frequency, and energy, directional attributes such as dip-angle variance with different step-outs, similarity measures, and dip-azimuth-based contrast enhancement can be used in the neural network. Similar ideas are used to detect chimneys as well as other objects and interfaces such as faults, stratigraphic bodies, direct hydrocarbon indicators, and time-lapse objects. Chimney cubes are produced by running a selected and appropriately weighted set of attributes through a supervised multilayer perceptron (MLP) neural network. The weights are determined by training the network from available information and geologic interpretation.
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With the increased resolution power of many geophysical methods, we are seeing direct evidence of seeps on a wide variety of data, including conventional seismic. New methods and technology have also evolved to better measure and detect seeps and their artifacts and reservoir charge and to map migration and remigration routes. In addition, detection of seepage is important for minimizing the risks associated with shallow gas drilling hazards, ensuring platform stability, and preventing well blow-outs.