Remote Detection of Petroleum Seepage in Sea Floor Sediments by Measurement of Electrical Potential

Offshore Technology Conference1997

K. F. M. Thompson, J. W. Holt, G.C. Kennedy

Abstract

Abstract Measurement of vertical profiles of electrical potential in superficial sediments can be employed to locate anomalous areas characterized by the shallow occurrence of negative potentials. Such areas exhibit abnormally elevated concentrations of organic matter, shown to be due to petroleum seepage. Shipboard equipment is employed to emplace in the sediment an insulated 3-met{:r probe bearing a series of platinum electrodes. The potentials of the latter are measured against a Ag/AgCl reference by an electrometer system in an associated pressure vessel. An altimeter records depth of penetration; an acoustic beacon used in conjunction with anultrashort baseline navigation system indicates position. Data are stored in a microcontroller or may be transmitted acoustically to the host vessel. Between emplacement sites the probe is towed at depth. The system seeks abnormal displacements of the Redox Potential Discontinuity (RPD, zero volts), and of the negative signals characterizing anoxic sediments in the "sulfide zone" below. Sediments deposited in oxygenated sea water invariably exhibit positive potentials at their surface. At various depths, dependent upon the natural rate of sedimentation of organic matter and its microbial oxidation, potentials decrease to zero and to levels commonly of the order of -250 mv. Such potentials characterize oxygen-depleted, chemically reduced, sediments bearing hydrogen sulfide. In coastal waters the RPD and potentials of -250 mV occur at centimeter depths, but in deep waters only at depths of several meters. Between, the sulfide zone deepens with regularity. At petroleum seepage sites in deep water the sulfide zone rises to the sediment surface due to the abnormal provenance of bacterially metabolizable organic matter (oil or gas). This facilitates the location of seepage by way of its electrical effects. Negative electrical potential anomalies characteristic of petroleum seep- ages in the Gulf of Mexico Were successfully measured with the autonomous instrument system described. Significance of Seepage Petroleum seepage, both on land and on the sea floor, has long been a valuable indicator for exploration. Seepage in Oil Creek, Pennsylvania, led to the development of the petroleum industry in the United States. Worldwide, virtually every major productive region is similarly characterized. Seepage demonstrates the presence of a petroleum source rock at depth, its adequate level of thermal evolution, and that subsurfacemigration has occurred, potentially charging subjacent reservoirs. Terrestrial seepage tends to be obscured by a soil profile, by vegetation, aggressive oxidation, and erosion. On the other hand, the quiescent state of the sea floor, low oxygenlevels, and the absence of erosion, tend to preserve evidence of seepage. On the continental slope, slow sedimentation over geological time increases the overburden load and keeps "petroleum kitchens" at depth active, generating oil and gas at elevated pressures. These fluids migrate upwards along faults, commonly reaching the surface.

Abstract

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