Near-Surface Epigenetic Magnetic Indicators of Buried Hydrocarbons and Separation of Spurious Signals

Terrence J. Donovan, Douglas P. O'Brien, J. Gregory Bryan, Mark A. Shepherd

Significant geochemical alteration zones occurring over buried hydrocarbon deposits can be recognized and mapped by geophysical methods. We believe near-surface secondary magnetic minerals formed as a result of seeping hydrocarbons and associated compounds interacting with constituents of the overlying rocks. A new method is described to identify anomalous magnetic signatures associated with this mineralization, and to differentiate that signal from cultural interference and other surface, shallow, and intermediate-depth geologic sources. Using low-altitude, high-sensitivity aeromagnetic data, the separation involves detailed spectral analysis, subsequent band-pass filtering, and analytic signal transformation of the filtered data. Depicted in contour form, the analytic s gnal minimizes spatial aliasing and allows us to map the areal distribution of subtle, near-surface anomalies related to probable epigenetic magnetic mineralization. This method is illustrated using data from the Arctic National Wildlife Refuge and Cook Inlet, Alaska, and from offshore Texas, where high-resolution seismic data support the aeromagnetic interpretation and suggest important structural controls.

Correlations of published detailed gravimeter and low-altitude aeromagnetic data at the Cement oil field, Oklahoma, were coupled with interactive modeling studies. Except for the obvious extreme high wave-number spikes, cultural contamination cannot be responsible for the high wave-number signal there, and the epigenetic magnetic mineralization may be more extensive vertically than originally suggested.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.