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Michael A. Abrams1, Stefan S. Boettcher1
(1) Exxon Production Research Company, Houston, TX

Abstract: Mapping migration pathways using geophysical data, seabed core geochemistry and submersible observations in the Central Gulf of Mexico

The occurrence of hydrocarbon seepage along specific faults and expulsion features can be used to delineate effective hydrocarbon migration pathways. Integration of geophysical data, seabed core geochemistry, and submersible observations provides a means of establishing the relationship of hydrocarbon seepage to subsurface faults and reservoirs.

In the Green Canyon (GC) 204 area, four major fluid expulsion features were identified on 3-D seismic data as circular seafloor mounds overlying zones of seismic wipeout. The –1km wide mounds occur above deep migration pathways (faults and salt) directly linked to seismic amplitude anomalies associated with the Genesis field. Using a submersible, we were able to observe and collect the products of hydrocarbon leakage and relate hydrocarbon occurrences at the seafloor to processes at depth. We found both active (gas/oil in core, fresh mineral precipitate, bacteria mats. live clams, mud cloud) and relict seepage features (carbonate blocks, dead clams, mud mounds).

These observations and geochemical analyses lead us to conclude: 1) Hydrocarbons sampled from localized leakage points within larger seafloor fluid expulsion features have similar molecular characteristics to the reservoired Genesis Field hydrocarbons. 2) The seepage intensity (total UCM, maximum TSF intensity, and total headspace gas concentration) varies greatly within and between major fluid expulsion features. 3) Slow localized seepage in the GC-204 area is punctuated by periods of episodic venting. The spatial relationship among near-surface hydrocarbon seepage seabed features, subsurface geophysical anomalies and the faults supports an interpretation of hydrocarbon charge along seafloor cutting faults.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana