Hydrocarbon-induced magnetic contrasts in some Venezuelan and Colombian oil wells

Vincenzo Costanzo-Alvarez,Germán Bayona,Marisel Díaz,Carolina Ayala,Milagrosa Aldana

doi:10.1186/bf03352636

Abstract

Abstract In this work we present a review of our current research on the subject of near-surface hydrocarbon-induced magnetic contrasts. Samples are drill cuttings from number of Venezuelan and Colombian oil fields, and some soils from an oil prospective area. Results from rock magnetic, electronic paramagnetic resonance (EPR), extractable organic matter (EOM) and scanning electron microscopy (SEM) are discussed in light of new experiments. Two types of magnetic susceptibility (MS) anomalies have been recognized in oil well samples (i.e. A and B-like). A-like anomalies are probably associated to a reducing environment caused by the underlying reservoirs, whereas B-like anomalies might reflect lithological contrasts. Samples from A-like anomalous levels reveal the restrictive presence of authigenic spherical aggregates of submicronic Fe-rich magnetic crystals. Peaks of organic matter free radical concentration (OMFRC) and EOM are also observed at depth levels that lie close to those with A-like anomalies. Hysteresis ratios J rs/Js and H cr/Hc could be used as additional criteria to recognize A-like magnetic contrasts. In soil samples, MS and OMFRC anomalies coincide with peaks of ethane concentration and an area underneath where source rock has reached appropriate conditions of pressure and temperature to generate hydrocarbons (Cretaceous kitchen). Different types of crude oils and subsurface structures could have also a direct effect on the observed amplitude of A-like anomalies.

Highlights

In the last few decades, high resolution aeromagnetic surveys over oil fields, and direct magnetic susceptibility (MS) measurements in soils, sediments, and drill cuttings, have been proposed as possible methods to detect low amplitude anomalous magnetizations
In this work we present additional rock magnetic results for some of La Victoria and Guafita oil wells, namely MS log for oil well GF-3X, thermomagnetic susceptibility curves for oil wells LVT-4X, GF-3X and GF-8X, low temperature susceptibility curves for LVT-4X and GF-3X and hysteresis ratios Jrs/Js and Hcr /Hc, measured in whole rock drill cuttings from a number of Guafita oil wells
Concluding Remarks It has been hypothesized that magnetite, in a variety of sedimentary environments, may have resulted from organic matter oxidation coupled with Fe (III) reduction (Zhang et al, 1997)

Summary

Introduction

In the last few decades, high resolution aeromagnetic surveys over oil fields, and direct magnetic susceptibility (MS) measurements in soils, sediments, and drill cuttings, have been proposed as possible methods to detect low amplitude anomalous magnetizations. 2. Sampling and Experimental Techniques In view of the fact that our research has focused on the near-surface expression of hydrocarbon migration, the drill cuttings analysed in La Victoria, Guafita (Venezuela) and Colombian oil wells have been taken far above from the reservoirs. Key data to study and characterize magnetic contrasts over oil fields are room temperature MS logs, EPR to precisely determine OMFRC, EOM and SEM analyses coupled with X-ray energy dispersion (EDX). Some of these results are complemented with S-ratio logs (IRM−0.1 T/SIRM+3 T), X-ray diffraction, petrographic analyses at magnifications of 8X to 66X, IRM acquisition and high-temperature and low temperature susceptibility curves. We present new rock magnetic results from the two Colombian wells Castilla 30 and Cano Limon 3, namely MS log and continuous heating susceptibility curves

Geological Setting of La Victoria and Guafita Oil Fields

Findings

Geological Setting of Colombian Oil Wells in the Llanos Basin