High-wave-number magnetic anomalies measured as part of an airborne magnetic survey over the Cement oil field, Oklahoma, are interpreted as reflecting abundant near-surface magnetite formed by the reduction of hydrated iron oxides and/or hematite as a direct result of petroleum microseepage.

Airborne geophysical methods provide an economically sound way to prospect for resources (Craib, 1972; Maxim and Cullen, 1977). In this report we suggest a correlation between high-wave-number anomalies observed on profiles from an airborne magnetic survey over the Cement oil field, Oklahoma, and the near-surface diagenetic formation of magnetite as a direct result of hydrocarbon microseepage from underlying reservoirs. This is especially interesting because, for most petroleum exploration applications, magnetic anomalies originate in crystalline basement rocks beneath the sedimentary cover.

The Cement oil field is located in the southeast part of the Anadarko basin in Caddo and Grady Counties. The Cement structure is a northwest-southeast doubly plunging asymmetric anticline that has two distinct domes (East Cement and West Cement); it is bounded on the north flank by a large thrust fault. Oil and gas production is from Permian sandstone and Pennsylvanian carbonate and clastic rocks which range in depth from about 600 to 2,270 m. The reservoirs at Cement were shown previously to have undergone long-continued microseepage resulting in diagenetic alteration of the shallow overburden (Donovan, 1974; Donovan and Dalziel, 1977).

Flight-line profiles of airborne measurements (flown at 120 m) of the earth's total magnetic field over the Cement area display anomalous high-wave-number peaks suggesting a shallow source (Fig. 1). Because the magnetic susceptibility of sedimentary rocks is largely a measure of their magnetite content, we conducted a systematic search for magnetite in borehole cuttings collected during the development of the oil field. Ferromagnetic material was separated magnetically from crushed samples taken from cuttings from the uppermost 300 m of five boreholes and composited at ~30-m intervals. Samples from depths less than about 80 m were not preserved. X-ray diffraction of the separated material confirmed it to be magnetite. Over the anticline, the amount of magnetite in the rocks appears to increase near the surface (Figs. 2, 3).

We suggest that the slow but long-continued leakage from a petroleum or natural gas reservoir of hydrocarbons and/or associated compounds causes a chemical reducing environment in the overlying rocks and the consequent reduction of hydrated ferric oxides and hematite to form magnetite. Hematite and hydrated ferric oxides are common as grain coatings and bonding agents in clastic sedimentary rocks. Many ferric oxides are stable in the oxidizing zone, but in reducing environments they undergo reduction and release the more soluble ferrous iron in solution. A relatively large loss of iron through reduction and dissolution results in rocks that appear "bleached" or tinted. However, not all ferric iron may undergo reduction and dissolution. We postulate, schematically, that a sequence of a teration may take place as follows:

hydrated ferric oxides ^rarr hematite (Fe₂O₃) ^rarr magnetite (Fe⁺² Fe₂⁺³ O₄)

Comparison of the Cement aeromagnetic data with aeromagnetic data from the smaller but

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Fig. 1. A, Selected segments of magnetic profiles from Cement oil field aeromagnetic survey. Cement structure is located on northeast flank of Anadarko basin and magnetic profiles probably reflect rise of magnetic basement to north (compare with map of Jones and Lyons, 1964). High-wave-number peaks are readily observable in profiles over crest of structure presumably where maximum formation of diagenetic magnetite took place. For comparison, ²¹⁴Bi/²⁰⁸T1 profile (flight line ML34) from airbornegamma radiation data collected simultaneously with magnetics is also shown (bold line). Anomalously high ²¹⁴Bi/²⁰⁸T1 ratios along crest owe their origin to late diagenetic uranium mineralization associated with petroleum microseepage (McKay and Hyden, 1 56; Donovan et al, 1975; Al-Shaieb et al, 1977). B, Oil field structure contoured on top of Hoxbar Formation (Pennsylvanian) modified from Donovan (1974), flight-line segments for magnetic profiles, and boreholes sampled for magnetite. Contour interval is 1,000 ft (~333 m).

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equally densely developed Davenport oil field of Lincoln County, Oklahoma, indicates that the phenomenon is not due to cultural interferences (pipelines, storage tanks, casing, etc), because no high-wave-number anomalies are seen in the Davenport data.

Supporting evidence for the diagenetic formation of magnetite exists at Cement. The evenly red-colored Permian Rush Springs Sandstone is remarkably bleached at the surface, in a pattern that closely coincides with the area of oil and gas production; within this tinted area it is relatively depleted of iron. Along the crest of the surface-expressed anticline, the normally friable Rush Springs is strongly impregnated with carbonate cements (mostly calcite), whose C¹³/C¹² isotopic ratios indicate a petroleum-derived carbon source. A thin overlying gypsum bed has been converted to calcite with similarly indicative C¹³/C¹² ratios (Donovan, 1974).

Pirson (1975) proposed that magnetoelectric effects may occur at the interface between the chemically reduced rock overlying a seeping hydrocarbon deposit and the oxidized rock surrounding the deposit, that is, an active process occurs whereby a giant fuel cell generates magnetoelectric effects. The phenomenon we describe has a different origin; namely, the diagenetic production of a ferromagnetic mineral that is a "fossil" indicator of microseepage. Search elsewhere for anomalous concentrations of diagenetically produced materials could be useful to explorationists.

Fig. 2. Relations between magnetite (plotted as weight percent of total sample) and drilled depth for five boreholes at Cement oil field. Position of wells is shown in Fig. 1B. Samples were composited from drill cuttings collected ~15 m above and below plotted center point.

Fig. 3. Probable range of magnetite with depth in near-surface rocks at Cement oil field along and near anticlinal crest. Only data from four boreholes along crest are plotted.

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