Abstract

Magnetic forward models of the Cement oil field, Oklahoma, were generated to assess the possibility that ferrimagnetic pyrrhotite related to hydrocarbon seepage in the upper 1 km of Permian strata contributes to aeromagnetic anomalies at Cement. Six bodies having different magnetizations were constructed for the magnetic models, based on geology and on petrologic and geochemical results, supplemented by rock magnetic measurements of shallow-core and outcrop samples. The column of rock through which hydrocarbons have passed is divided into three sulfide zones on the basis of pyrrhotite content, and the column is capped by a 30 m thick zone that contains ferric oxide minerals formed mainly from oxidized pyrite. Red beds unaffected by sulfidization, as well as a zone of rock depleted in hematite but lacking sulfide, surround sulfidic zones.The synthetic magnetic profiles are controlled mainly by pyrrhotite-bearing strata at depths of 200-500 m. The magnetizations of these bodies are calculated from: (1) petrographic estimates of pyrrhotite content relative to pyrite; (2) content of sulfide sulfur determined from chemical analysis; and (3) values for the magnetic susceptibility of monoclinic pyrrhotite. Total magnetizations of the bodies of highest pyrrhotite content range from about 3 X 10 (super -3) to 56 X 10 (super -3) A/m in the present field direction and yield magnetic anomalies (at 120 m altitude) having amplitudes of less than 1 nT to approximately 6 to 7 nT, respectively. Such amplitudes are much lower than those (as high as 60 nT) reported from the original total-field survey over the Cement field.Numerous assumptions were made in the generation of the models, and thus the results neither prove nor disprove the existence of aeromagnetic anomalies related to hydrocarbon seepage at Cement. Nevertheless, the results suggest that pyrrhotite, formed via hydrocarbon reactions and within a range of concentrations estimated at Cement, is capable of causing magnetic anomalies.

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