Abstract
AbstractIt has been proposed that the high‐frequency, low‐amplitude magnetic anomalies found over some hydrocarbon deposits are due to long‐term microseepage of hydrocarbons into iron‐rich sedimentary roof rocks, with subsequent precipitation of diagenetic magnetite or pyrrhotite at or near the water‐table. Aerogeophysical data sets, comprising both magnetic and gamma‐ray spectrometer measurements, over the Formby oil‐field, Lancashire, U.K., have been analysed for hydrocarbon‐related anomalies. Detailed ground magnetic traverses were also made to investigate some of the aeromagnetic anomalies.No hydrocarbon‐induced magnetic anomalies were detected. The majority of the high‐frequency events occurring in the aeromagnetic data correlated with cultural features, others were attributed to artefacts of the data processing. In particular there were no extensive areas of high‐frequency, low‐amplitude anomalies as might be expected from authigenic magnetic minerals.Borehole chippings from inside and outside the oil‐field were examined. High magnetic susceptibilities were due to contamination during drilling. Magnetite, which was identified by mineralogical analysis, was found to be of detrital origin. If any diagenetic magnetic material was present it was in such small quantities as to be insignificant.The oil‐field did not possess a characteristic gamma‐ray spectrometric signature. Anomalies were found to be related to hydrological conditions and to the distribution of surficial deposits. Attempts to suppress the influence of the drift geology had only limited success.It is concluded that the effectiveness of high‐resolution aeromagnetic onshore surveys for direct hydrocarbon detection has yet to be established. In particular, the anomaly found over the Formby oil‐field was caused by the cumulative effect of borehole casing. Similar cultural contamination by oil‐field equipment may explain some of the anomalies discovered over hydrocarbon deposits in North America. It is also unlikely that the spectrometric method can be applied diagnostically in any but the most simple and drift‐free geological environments.
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