Association between magnetic susceptibilities and hydrocarbon deposits in the Barinas-Apure Basin, Venezuela

Abstract

We have measured the magnetic susceptibility of 5425 drill cuttings, coming from 20 oil wells distributed in eight fields of the Petroliferous Barinas-Apure Basin (Barinas, Venezuela), seeking evidence of magnetic anomalies associated with the presence of hydrocarbon deposits. The rock samples are located between the near earth’s surface and the basement’s top (approximately 4000 m). In the magnetic susceptibility profiles, we observed high magnetic susceptibilities at the top of the sedimentary units corresponding to reservoir and source rocks of oil-producing wells, whereas in the case of nonproducing wells we observed low magnetic susceptibilities. A basic statistical analysis of the magnetic susceptibility measurements shows a significant difference between the mean values across producing and nonproducing wells at the correspondent oilrelated formations, with a ratio of 4∶1 of producing to nonproducing wells. In the producing wells, we have found a relation between the magnitudes of the magnetic susceptibility and the age of the formation; the larger the magnetic susceptibility, the younger the geological stratum. The age and depth of the formations where the cuts were obtained exclude the presence of cultural (human origin) contamination of the samples, suggesting the evidence of authigenic origin of the magnetic minerals due to the reductive effect of hydrocarbons in rocks.