Epigenetic versus syngenetic magnetite as a cause of magnetic anomalies

Abstract

Studies carried out a decade ago at Cement Field, Oklahoma, suggesting evidence of epigenetic (diagenetic) magnetite are shown to have been flawed for two reasons. Aeromagnetic surveying over the field could not have yielded the type of broad, low‐amplitude anomalies expected from hypothesized diagenetic magnetite sources because of the occurrence of numerous high‐amplitude magnetic spikes over cultural sources. Applied Geophysics, Inc., flew a low level (approximately 30 m above ground) aeromagnetic profile along the long axis of the field in 1983 and encountered nearly 40 such spikes due to culture. Additionally, studies of drill cuttings from 23 wells in the field showed them all to be contaminated by rust, scale, and bit shavings, in approximately the same percentages as had been postulated for diagenetic magnetite in 5 wells examined in the prior study. Subsequent studies in support of the diagenetic magnetite hypothesis in other areas have relied on the occurrence of “high‐wavenumber” (i.e., high‐frequency) aeromagnetic or ground magnetic anomalies as indicators of such magnetite. These studies do not take into account the possibility of detrital magnetite as the cause of the high‐frequency anomalies. Detrital magnetite is of near‐universal occurrence in small quantities in both unconsolidated sediments and sedimentary rocks and is easily concentrated by fluvial processes which would affect ground magnetic surveys in particular, but airborne surveys as well. In rocks of Mesozoic age or younger, detrital magnetite occurs in sufficient quantity in some areas to give quite detectable airborne magnetic anomalies. Four such localities have recently been documented, one in the Utah‐Idaho‐Wyoming USA thrust belt; and two in the western United States and one in Central America. However, recent measurements of soil magnetic susceptibility over oil fields do present intriguing evidence for anomalous near‐surface magnetization associated with hydrocarbon leakage plumes. Based on previous experience with sulfide mineral deposits, I postulate that small quantities of pyrite, and perhaps lesser quantities of other sulfide minerals that are sometimes precipitated by rising hydrocarbons, are oxidized above the water table in the vadose zone to secondary magnetic iron oxides. This thin layer of weak anomalous magnetization, when properly distinguished from magnetization due to detrital magnetite, would be recognizable with shallow magnetic susceptibility measurements, and possibly ground magnetic surveys, but probably not with airborne surveys, except under exceptional circumstances.