--> ABSTRACT: Remote Detection of Petroleum Seepage In Sea Floor Sediments, by K. F. Thompson; #91021 (2010)

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Remote Detection of Petroleum Seepage In Sea Floor Sediments

THOMPSON, KEITH F.

Mapping vertical profiles of electrical potential in sea floor sediments reveals anomalous areas characterized by the shallow occurrence of negative potential. Such areas exhibit concentrations of organic matter in excess of those due to natural sedimentation, due either to petroleum seepage or anthropogenic dumping.

Shipboard equipment emplaces in the sediment an insulated 3-meter probe bearing platinum electrodes, the potentials of which are measured against reference. An altimeter records depth of penetration; an acoustic beacon an an ultrashort baseline navigation system indicate position. Data are transmitted acoustically to the host vessel. Between measurement sites the probe is towed at depth.

The system seeks abnormal displacements of the Redox Potential Discontinuity. Sediments deposited in oxygenated sea water exhibit positive potential at their surface. At various depths beneath, dependent upon the natural rate of provenance of organic matter and its microbial oxidation potential decreases to zero (defining the Discontinuity), and becomes negative at greater depths. Negative potential characterizes chemically reduced sediments bearing hydrogen sulfide. In coastal waters the discontinuity occur at centimeter depths, in the open ocean at meter depths. Between, it deepens with regularity. In deep water, areas of shallow occurrence are due to th abnormal provenance of bacterially metabolizable organic matter, commonly oil or gas. Data representing petroleum seepages in the Gulf of Mexico are reviewed. At these sites the discontinuity rises from a depth of several meters to the sediment-water interface. Methane and bacterially-generated sulfide commonly support chemosynthetic communities at such seeps.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.