Study of magnetic contrasts in the Guafita oil field (south-western Venezuela)

Abstract

We have carried out magnetic susceptibility (MS) studies of near-surface samples (drill cuttings) from 11 wells at the Guafita oil field and environs (southwestern Venezuela). In order to characterize magnetic mineralogies and lithological contrasts, MS results have been complemented by S-ratio logs, scanning electron microscopy (SEM), X-ray energy dispersion (EDX), isothermal remanent magnetization (IRM) acquisition curves, visual examination of mineral compositions using a stereomicroscope of 8X to 66X and X-ray diffraction studies of whole rock, clay and magnetic fractions. Two different kinds of MS anomalies have been recognized (i.e. A and B). Anomaly A was observed only in three producer wells and seems to be related to the presence of Fe-rich spherical aggregates and low coercivity magnetic phases, probably of authigenic origin, whereas B appears in most of the producer and non-producer wells. Although B is not associated to framboids of magnetic minerals, X-ray diffraction analyses reveal the presence of quartz, kaolinite, magnetite and hematite at those depth levels where it has been detected. We argue that anomaly A could be linked to a reducing environment induced by the underlying reservoir, whereas B probably reflects a lithological contrast. This hypothesis also seems to be supported by preliminary visual examination of mineral compositions. A contour map of anomaly B amplitude values appears to indicate the sediment input and transport directions in the field. On the other hand, an average-depth contour map of this same anomaly shows the existence of a major fault that has been previously identified by geological studies in the region.