EPR and EOM studies in well samples from some Venezuelan oil fields: correlation with magnetic authigenesis

Electron paramagnetic resonance (EPR), magnetic susceptibility (MS) and extractable organic matter (EOM) measurements were carried out in drilling fines, from near-surface levels, from producer and non-producer wells, with the purpose of examining a possible causal relationship between magnetic contrasts and underlying hydrocarbons. Organic matter free radical concentration (OMFRC) and EOM anomalies were found only at the producer wells, in the same zone where MS anomalies were observed. The EOM anomalies coincide in depth with the MS ones, while the OMFRC anomalies lie close to them. The results could be explained if a net transfer of electrons from reduced organic matter, possible induced by the underlying reservoir, to Fe(III) (e.g. hematite) occurs. This process alters the original organic matter and produces the formation of EOM and Fe(II) magnetic minerals (e.g. magnetite), with both anomalies coexisting.