Micromagnetic seep detection in the Sudan

Abstract

It has been known for many years that entrapped hydrocarbons often coincide with shallow-sourced micromagnetic anomalies. Field studies have shown that these usually sub-nT amplitude anomalies are associated with near-surface magnetite deposits generated predominantly from microbial alteration of hematite to magnetite in the presence of hydrocarbon microseepage. These studies have demonstrated that hydrocarbon traps usually leak vertically as well as laterally. In the absence of significant cultural magnetic interference or shallow volcanic magnetic signature, and with favorable ground conditions, a high-resolution aeromagnetic survey can effectively locate seep-induced micromagnetic anomalies. One such survey has been recently performed over the Muglad Basin of the Sudd region of southern Sudan, flown with a relatively small line spacing (500 m) and low ground clearance (100 m). Analysis of these data has revealed characteristic annular micromagnetic anomalies (aureoles) that cluster along prospective high structures and coincide with the Jarayan oil field. Besides delineating exploration fairways, this method can be used to help prioritize seismic structural leads and to target non-structural hydrocarbon traps. Numerous researchers have studied this phenomenon over the past 70 years confirming its validity, and in turn have recommended the use of high-resolution aeromagnetic surveying as a valid exploration method for this particular purpose. Despite this, and highly effective complementary geo/biochemistry surface studies, the oil industry has not yet fully embraced this approach as a standard exploration tool. The micromagnetic exploration approach is likely to remain empirical due to the complex interplay of geologic, geochemical and biochemical factors involved. It is this empiricism that appears to deter oil companies. We hope that the southern Sudan case history presented here usefully contributes to the growing body of evidence that this is a valuable exploration technique that ought to be used with greater confidence in the field. The exact nature of the vertical microseepage process, …