--> The Relationship Between Seismic Amplitude and Hc Migration Offshore Espirito Santo Basin, Brazil, Rad, Freyd K.; Warner, Lloyd L.; Kirby, Benjamin T.; Thompson, Sarah, #90100 (2009)

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The Relationship Between Seismic Amplitude and Hc Migration Offshore Espirito Santo Basin, Brazil

Rad, Freyd K.1
 Warner, Lloyd L.1
 Kirby, Benjamin T.1
 Thompson, Sarah1

1Anadarko Petroleum, The Woodlands, TX.

Offshore
Espirito Santo Basin has experienced an increase in exploration activities since the discovery of the Golfinho Field in 2003 when Petrobras drilled the successful 1-ESS-123-ES exploratory well (Figure 1). The sedimentary succession in the Espirito Santo Basin consists of typical Atlantic Margin rift, transition, and drift sequences. Geochemical analysis of rock samples and oils recovered from wells drilled in the area suggests a mixed hydrocarbon type. Oils generated in this basin, however, are interpreted to be predominantly of lacustrine origin mixed with usually minor amount of oil from a marine source. The lacustrine oils are attributed to the Barremian-Aptian pre-salt source beds of the Nativo Group, while the marine oil is believed to be sourced by the post-salt deposits of Albian-Turonian age. Despite several discoveries, a close examination of geological/geophysical data suggests that commercially viable accumulations in the post-salt interval may exist only in areas where direct access to migrated oils from the pre-salt section has occurred - commonly referred to as salt window areas. To date, only a few wells in the Espirito Santo Basin have reached the pre-Salt section. These wells suggest that many of the areas mapped on seismic as salt windows contain a thin layer of Aptian evaporites (usually anhydrite and/or halite), in the range of 50 to 150 meters thick, that prevented the migration of oil from the underlying pre-salt source into the overlying post-salt reservoirs. A study of 3D seismic and well data suggests that seismic amplitude can provide a clue on where the hydrocarbon migration windows are likely to be present in offshore Espirito Santo. In areas where the seismic picks for the top and base of Aptian Salt meet/merge the following observations have been made: a) a strong amplitude response seems to indicate that a relatively thin evaporate layer (salt and anhydrite) is likely to be present (Figure 2); and b) a low amplitude response appear to suggest the absence of evaporites or a very thin layer of mainly anhydrite (Figure 3). The risk of hydrocarbon migration from the main source beds located within the pre-salt sequence into the post-salt reservoirs is believed to be significantly diminished for prospects located optimally in and around areas described in (b), especially if faults with mapable displacement happen to be present present.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil