Vincenzo Costanzo-Álvarez

Paleomagnetism in Mesozoic rocks of the Northern Andes and its Implications in Mesozoic Tectonics of Northwestern South America

New paleomagnetic data isolated in Upper Triassic to Aptian rocks exposed in the Colombian Andes and west of the Guyana craton, in conjunction with paleomagnetic data from the Andes of Venezuela and the South American craton, permit the interpretation of along-margin northward translations of Andean Colombian terranes during the Early-Middle Jurassic. Field tests and comparison with reference paleopoles for South America indicate that characteristic components uncovered in red-siliciclastic and igneous rocks are primary, or near-depositional, and they are carried dominantly by hematite, magnetite and Ti-magnetite. Difference in declination values of characteristic components isolated in fault-bounded blocks document counter-clockwise rotations previous to syn-extensional deposition. The Jurassic tectonic scheme proposed here for the northwestern corner of South America shows an Early Jurassic, linear subduction-related magmatic arc evolving to Late Jurassic rift-related setting associated to the opening of the Proto-Caribbean Ocean and westward retreat of the subduction zone. A similar tectonic evolution for the Jurassic has been proposed for southwestern USA and the Nazas arc in Mexico.

Paleomagnetism of alkaline complexes and remagnetization in the Kapuskasing Structural Zone, Ontario, Canada

The Shenango and Nemegosenda alkaline complexes of the southern Kapuskasing Structural Zone (KSZ), an uplifted crustal block in northern Ontario, preserve primary thermoremanent magnetizations, with mean directions D=112°, I=−46° (k=13, α95=13°, N=9 sites) and D=304°, I=+51° ( k=31, α95=10°, N=7 sites) for Shenango and D=124°, I=−57° ( k=13, α9520°, N=4 sites) and D=312°, I=+54° (k=17, α95=13°, N=7 sites) for Nemegosenda. Their average paleomagnetic pole is 45°N, 189°E (dp=6°, dm=8°), falling on the Keweenawan track of the North American apparent polar wander path (APWP) between 1098–1107 Ma reverse-polarity (R) and 1087–1098 Ma normal-polarity (N) poles of Lake Superior lavas and dikes. Unlike the Keweenawan paleopoles, which have a reversal asymmetry of ≈30°, the Shenango and Nemegosenda R and N poles are almost 180° reversed, which was found also by Lewchuk and Symons (1990b). This symmetry of R and N poles casts doubt on the previously accepted predominance of asymmetric reversals of the Earths field in Keweenawan time. The Archean Shawmere anorthosite is overprinted by a B magnetization with mean direction D=97°, I=−68° ( k=12, α95= 10°, N=18 sites). The Shawmere B paleopole at 38°N, 226°E (dp=13°, dm=l6°) falls between the carbonatite poles and ≈1850 Ma cratonic poles, and the B magnetization seems to be a composite of middle and late Proterozoic overprints. Sites in the western part of the Shawmere body close to the “carbonatite line” of faults linking the various alkaline complexes are severely altered, with an abundance of secondary magnetite and hematite. Virtual geomagnetic poles for these sites fall close to the Keweenawan track and have an average pole of 189°, 53°N (dp=15°, dm=20°) suggesting that the B NRM at these sites is an ≈1100 Ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion. More easterly sites are less altered. Their VGPs resemble ≈1850 Ma cratonic poles and average to 235°E, 30°N (dp=15°, dm=17°) These B natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle Proterozoic hydrothermal activity associated with intrusion of early alkaline bodies, fault movement on the Ivanhoe Lake fault zone, or late-stage uplift of the KSZ.

EPR and magnetic susceptibility studies in well samples from some Venezuelan oil fields

Abstract Electron Paramagnetic Resonance (EPR) and Magnetic Susceptibility (MS) measurements were carried out in near-surface samples of 6 wells (producers and non-producers) from some Venezuelan oil fields. EPR measurements were performed to determine the organic matter free radical concentration (OMFRC) in the samples. MS anomalous levels, where the restrictive presence of microscopic framboidal magnetic minerals has been recognized, were found only at the producer wells. Moreover for these same wells, anomalies of OMFRC were observed at depths close to MS anomalous levels. These preliminary results seem to suggest a relationship between the presence of OMFRC and MS anomalies, possibly associated with the underlying reservoir.

Magnetic and mineralogical studies to characterize oil reservoirs in Venezuela

High-resolution aeromagnetic surveys over oil fields and magnetic susceptibility (MS) measurements in soils, sediments, and drill cuttings have been proposed as complementary or alternative means of exploration and assessment of hydrocarbon reservoirs. Some micromagnetic anomalies detected by these studies could be the likely result of the existence, at shallow depths, of authigenic magnetite and/or Fe-sulphides. It has been hypothesized that such magnetic minerals are the chemical byproducts of primary Fe-oxides exposed to a reducing environment induced by the underlying reservoir. Secondary magnetite could appear as spherical aggregates (framboids) of submicronic crystals identifiable by scanning electron microscopy (SEM). The formation of these framboids seems linked either to inorganic chemical processes associated with the presence of hydrocarbons or to oil biodegradation. In the last few years, we have examined the causal relationship between framboids of magnetic minerals and hydrocarbons via rock magnetic and electron paramagnetic resonance (EPR) analyses of drill cuttings. The samples used have been taken at different depths (within the first 1500 m) of producing and nonproducing wells from two oil fields in southwestern Venezuela. We have complemented MS results with S-ratio logs, SEM, X-ray energy dispersion (EDX), IRM acquisition curves, and high-temperature susceptibility measurements. More recently we have also studied soil samples from an oil prospective area in western Venezuela, where rock magnetic and EPR results were compared with ethane gas analyses and remote sensing data. Rock magnetic and EPR studies have been performed in 14 oil wells from La Victoria (3) and Guafita (11) fields (Figure 1). Since our research has focused at the near-surface expression of hydrocarbon migration, the samples analyzed (drill cuttings) were taken from different depth at intervals of about 15 m between 100 and 1500 m. All samples belong to a single group of molasses from strata that lie far above the producing levels …

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.

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

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 Jrs/Js and Hcr/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.

An integrated rock magnetic and EPR study in soil samples from a hydrocarbon prospective area

Abstract Magnetic susceptibility (MS) and organic matter free radical concentration (OMFRC) determined by electron paramagnetic resonance, have been measured in soil samples (≈1.5 m depth) from an oil prospective area located at the southern flank of the Venezuelan Andean Range. S-ratios close to 1, as well as high temperature susceptibility analyses, reveal magnetite as the chief magnetic phase in most of these samples. Ethane concentrations, MS and OMFRC normalized data have been plotted against the relative position of 22 sampling sites sequentially arranged from north to south. Although there is not a linear correlation between MS and OMFRC data, these two profiles seem to vary in like fashion. A MS and OMFRC southern anomaly coincides with the zone of highest ethane concentration that overlies a “Cretaceous kitchen”. OMFRC highs could be linked to the degradation or alteration of organic matter, the possible result of hydrocarbon gas leakage, whose surface expression is the stressed fern observed by remote sensing studies previously performed in the area. Ethane anomalies are associated to this seepage that also produces changes in the magnetic mineralogies detected as MS positive anomalies.

Rock magnetic characterization of early and late diagenesis in a stratigraphic well from the Llanos foreland basin (Eastern Colombia)

Abstract We have carried out rock magnetic characterizations of different lithofacies along the stratigraphic well Saltarín 1A, in order to learn about the various diagenetic events that could have affected the Miocene sequence of the Llanos foreland basin (Colombia). Thermomagnetic and low-temperature susceptibility measurements performed on some selected samples were complemented with analyses of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and isothermal remanent magnetization (IRM) acquisition curves. The identification of the magnetic mineral assemblages at each depth level analysed, as well as their relative concentrations, were determined from a direct signal analysis (DSA) of the IRM curves. Samples from the top of the Guayabo formation reveal the presence of hydrocarbons-related microscopic framboids of pyrite with partial replacement of magnetite. The bottom of the Guayabo formation shows hematite and goethite and appears to record a thoroughly documented Middle Miocene global regression. In samples from the León formation, pyrrhotite could have resulted from an early diagenesis that took place in a lacustrine environment via sulphate reduction. Traces of crude oil in samples from the Carbonera formation, and the additional occurrence of hematite and magnetite, suggest that a hydrocarbons-mediated late diagenesis could also have affected the lowermost levels of Saltarín 1A.

Preliminary dielectric and rock magnetic results for a set of prehistoric Amerindian pottery samples from different Venezuelan Islands

Potsherds from 7 Venezuelan islands have been studied using a two-fold magneto/dielectric technique in order to identify clay sources and characterize different stages of pottery craftsmanship. This is the first study of archeological material using this technique. Petrographic analyses appear to agree with the clusters of data identified in scatter plots of initial magnetic susceptibility versus saturation isothermal remanent magnetization (SIRM), and natural remanent magnetization (NRM). Thus, these magnetic parameters appear to be suitable for describing clay source characteristics. Effective magnetic grain sizes, investigated via stability analyses of SIRM and anystheretic remanent magnetization (ARM) upon alternating field (AF) demagnetization, and SIRM acquisition and AF demagnetization crossover plots, seem to be related to different steps of pottery craftsmanship, namely clay preparation, finishing and firing. Thermomagnetic curves might also provide valuable information about original firing conditions. A scatter plot of SIRMs intersections versus maximum current depolarization temperatures and average activation energies, shows a coarse correlation due perhaps to the fact that these rock magnetic and dielectric data are both associated to pore-related features.

Rock magnetic and dielectric characterizations of a formational contact in cretaceous strata (eastern Venezuela)

Abstract Rock magnetic and dielectric studies have been conducted in sedimentary rocks close to a lithological contact (Chimana and Querecual formations) at the Pertigalete sedimentary sequence (Cretaceous northeastern Venezuela). Rock magnetic results suggest that the presence of secondary magnetic minerals has been conditioned by the transitional zone between Chimana and Querecual. In fact, this contact might have acted as a conduit for fluid circulation and a focus of chemical alteration on most of its adjacent strata. Some of the secondary magnetic minerals (e.g. magnetite) must be the outcome of the anoxic conditions that prevailed during times when the sedimentary basin reached its maximum temperatures when oil and gas were generated (Lower Miocene). Some others (e.g. hematite) are the likely byproduct of alteration dating from times when oxidizing conditions were dominant. The Thermally Stimulated Depolarization Current (TSDC) technique is used here to dielectrically characterize the same contact. The complex TSDC spectra obtained in the temperature range from 77 to 230 K for the samples studied, indicate a notable difference between the Chimana (CH) and Querecual (QC) samples. Drying and rehydration treatments performed on these samples show that the whole depolarization spectrum is mainly due to moisture trapped in the microporosity of the material. The broad spectra, separated by using Direct Signal Analysis, reveal the presence of four overlapping peaks. TSDC results indicate a progressive change in the mean re-orientation energy with the lithology for the four relaxations. This variation, due to the change in the environment seen by the water dipoles hydrogen-bonded to different sites in the material, is observed for the same lithologies where magnetic anomalies have been reported. Thus, rock magnetic analyses, combined with the TSDC technique, seem to serve as a sensitive and alternative means of characterizing lithologies and stratigraphic discontinuities in a sedimentary sequence.