Print this pageDetecting Hydrocarbon Migration Pathways in Seismic Data from the Brazilian South Atlantic Margin
Vertical hydrocarbon migration is frequently observed in South Atlantic basins. This migration can be detected in the seismic record as zones of vertically chaotic, low energy data or “gas chimneys”. These chimneys have often been observed associated with shear along transform fault boundaries. They provide conduits for hydrocarbons derived from syn-rift lacustrine source beds to migrate through fractured Cretaceous evaporites, carbonate, and marls into younger reservoirs, indicating active petroleum systems. A more rigorous study of gas chimneys can provide critical clues to the sealing potential of pre-salt prospects and the charge and seal risk for post-salt prospects. Many gas chimneys are very obvious in the seismic record. However others are often difficult to distinguish on normally processed seismic data. Thus a method for detection of gas chimneys in post stack 3D data is needed too improve the identification of gas chimneys in seismic data, to map their distribution, and to allow them to be visualized in three dimensions. A method is described to create a “chimney probability” volume in 3D seismic data using directional seismic attributes trained on suspected chimney picks with a neural network. Following the creation of a chimney probability volume, the results must be validated as true vertical HC migration pathways using the following criteria: 1) are there DHI’s, shallow gas reservoirs, or chemosynthetic build-ups associated with the chimney; 2) does the chimney link to shallow sea-bottom indications of hydrocarbon seepage such as piston core data or radar; 3) are the locations of the chimneys supported by basin modeling; 4) do the chimneys have a circular pockmark morphology on time slices? Once the chimney data is validated, it can be interpreted to determine where the hydrocarbons originated, how they migrated into a suspected trap, and whether they leaked from this trap to form seeps on the sea floor. This approach has been used to successfully predict hydrocarbon charge and seal in many trans-Atlantic basins and in other salt related basins, such as the Gulf of Mexico and North Sea.A case study from offshore, Brazil is shown over a known producing field and dry hole to demonstrate how this technique can be used to predict hydrocarbon charge and seal prior to drilling. Chimney probability data is integrated with sea floor geochemical and basin modeling data to support the interpretation.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil