Framboidal magnetic minerals and their possible association to hydrocarbons: La Victoria oil field, southwestern Venezuela

Abstract

Magnetic contrasts observed in aeromagnetic studies and in surface samples above oil fields have been attributed to the presence of secondary magnetic minerals produced by a reducing environment that has been induced by the underlying reservoir. However, an unequivocal relationship between these anomalies and the presence of hydrocarbons has not yet been established. We report rock magnetic and magnetic mineralogy results of a preliminary study carried out in drilling fines at different depth levels from three oil wells at La Victoria oil field. Profiles of magnetic susceptibility (MS) and bulk magnetization (|M|), corresponding to the first 1500 meters are complemented by studies of X-rays diffraction, Electronic Microscopy (Scanning and Transmission), thermoremanence and Isothermal Remanent Magnetization acquisition curves, and thermal demagnetization of composite IRMs. There is a higher concentration of spherical aggregates (framboids) of submicronic magnetic minerals in levels with conspicuous peaks of MS and |M| values. If such aggregates represent the authigenic outcome of near-surface reduction caused by hydrocarbon seepage, these results could establish a link between the contrast of magnetic properties and the presence of hydrocarbons.