Direct indication of hydrocarbons from airborne magnetics

Abstract

In the search for hydrocarbons a technique is now available that will not only just define favourable structure or stratigraphy, but that also appears to be responsive to a manifestation of the presence of the hydrocarbons. It has the added advantage of being usable from an airborne platform, thus making it a rapid coverage technique especially suitable for reconnaissance of large sedimentary basins. It is based on the hypothesis that leakage of hydrocarbons, which always occurs to some extent, causes a reducing zone to form vertically above the hydrocarbons. One of the several effects of this zone is the formation of diagenetic magnetite from the postulated reduction of any hematite present. The latter acquires a chemical remanent magnetization and is located in depths at or near the water table. The effect is observed as relatively low amplitude, high wavenumber magnetic anomalies. Such anomalies have been observed over the Cement field, Oklahoma, where anomalous concentrations of magnetite were also measured at shallow depths in the oil wells. Confusing effects such as variations in the diurnal field or micropulsations can best be discriminated by the use of horizontal gradiometer measurements. The gradiometer cancels all common mode variations and is also more sensitive to anomalies with shallow sources, as in the case with diagenetic magnetite occurrences. Geometrics have investigated the usefulness of this technique by examining results of previous flying at 120 m altitude over petroleum basins in the USA for the US Department of Energy. In several disparate areas the correspondence of oil fields and the characteristic anomaly pattern due to this effect is remarkably good, with no similar patterns occurring where there are no oil fields. A theoretical model based on the magnitude of susceptibility obtained from the magnetite content in the Cement field, Oklahoma, confirms the order of magnitude of the amplitude and the wavenumber as consistent with those observed.