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

Phys. Chem. Earth (A), Vol. 25, No. 5, pp. 447-453, 2000

Pergamon

© 2000 Elsevier Science Ltd All rights reserved 1464-1895/00/$ - see front matter

PII: S 1464-1895 (00)00069-7

EPR and Magnetic Susceptibility Studies in Well Samples from some Venezuelan Oil Fields M. Diaz l, M. Aldana 2, V. Costanzo-Alvarez 2, P. Silva I and A. P6rez 3 1Centro de Ffsica, Instituto Venezolano de Investigaciones Cientfficas, IVIC, Apartado 21827, Caracas 1020-A, Venezuela 2Departamento de Ciencias de la Tierra, Universidad Sim6n Bolfvar, Apartado 89000, Caracas 1081-A, Venezuela 3Escuela de Ffsica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47586, Caracas 104 l-A, Venezuela

Received l June 1999; revised 28 February 2000; accepted 4 May 2000

Abstract. Electron Paramagnetic Resonance (EPR) and Magnetic Susceptibility (MS) measurements were carried out in near-surface samples of 6 wells (producers and nonproducers) 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. © 2000 Elsevier Science Ltd. All rights reserved.

From their studies in the Arbuckle Group in Southern Oklahoma, Elmore et al. (1987) have suggested that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite, which causes a net increase in magnetization. Some other studies have tested the basic premise of this approach, namely that hydrocarbons cause precipitation of authigenic magnetic phases. Elmore et al. (1987) and McCabe et al. (1987) have hypothesized that microbial activity may play an important role in the precipitation of authigenic magnetite in oils associated with sedimentary rocks. Bailey et al. (1973) and McCabe et al. (1987) pointed out that asphaltenes tend to be the major components of highly biodegraded oils. Studies of hydrocarbon-impregnated Permian calcite speleothems from southwestern Oklahoma have been conducted by Elmore et al. (1993) in order to quantify extractable organic matter (EOM) and asphaltene levels. Their results showed a positive relationship between EOM and natural remanent magnetization (NRM), but there was no correlation between percentage of asphaltenes and NRM in the speleothems. Thus a chemical process, and not biodegradation, appears to be the mechanism for magnetite authigenesis in speleothems. On the other hand, analyses of sedimentary organic matter, using the Electron Paramagnetic Resonance (EPR) technique, have included studies of coal (Petrakis and Grandy, 1978; Bresgunov et al., 1990a, 1990b), isolated coal macerals (Morishima and Matsubayashi, 1978) and kerogens (Bakr et al., 1988, 1990; Dickneider et al., 1995; Qiu Nansheng and Wang Jiyang, 1998), because this technique is ideal for the detection of flee radicals of organic matter. In this study we report magnetic susceptibility (MS), scanning electron microscopy (SEM) and EPR measurements of drill cuttings taken at near-surface levels from six (6) different wells of some Venezuelan oil fields. EPR has been used to precisely determine the organic matter free radical concentration (OMFRC) in these samples. A review of this technique is presented in §2.2.

I Introduction Aeromagnetic surveys over oil fields (Foote, 1984, t992, 1996; Saunders and Terry, 1985) as well as magnetic susceptibility measurements in soils, sediments, and drill cuttings, have been suggested as possible means to locate anomalous magnetizations associated with hydrocarbon microseepage (Foote, 1987, 1992, 1996; Saunders et al., 1991). These magnetic contrasts have been hypothesized as the result of the existence, at shallow levels, of abundant diagenetic magnetite, the alteration by-product of primary Fe oxides in a reducing environment induced by the H2S released by the underlying reservoir (Foote, 1984). However, the appearance, origin and exploration importance of some magnetic mineralizations associated to oil accumulations is still debatable (e.g. Gay, 1992; Schumacher, 1996).

Correspondence to: Dra. Marisel Diaz. Centro de Fisica, 8424 NW 56 STREET SUITE CCS 00205. Miami Florida 33166. U.S.A. 447

448

M. Dfaz et al.: Studies in Well Samples from some Venezuelan Oil Fields

2 Experimental

2. t Samples In this study we have worked with: LVT-4x, GF-lx, GF-2x (producer wells), and: La Ceibita, Agua Linda and Carlo Mur3oz (non-producer wells). LVT-4x is located in La Victoria oil field. GF-lx and GF-2x are in the Guafita oil field. Agua Linda and Carlo Mufioz are in the Guafita environs, and La Ceibita is in northeastern Venezuela, far away from the rest of the wells analyzed. A regional map showing the location of these fields is presented in an accompanying article by Costanzo-Alvarez et al. (2000). Drill cuttings (unconsolidated rock samples) were taken at intervals of about 15 meters from the first 1200 meters of these wells. In La Victoria, Guafita, Agua Linda and Carlo Murloz, located at the Apure-Barinas basin (southwestem Venezuela) all the samples belong to a single geological group of molasses of fluvial-deltaic provenance. These molasses had their origin during the uplift of the Andean Range between Miocene and Mid-Pliocene times (Guayabo group, Rio YucaJParangula formations). Sample contamination has been precluded (Costanzo-Alvarez et al., 2000). The hydrocarbons of La Victoria have gravity values ranging from 30 ° to 36 ° API (units used as standards by the American Petroleum Institute). LVT-4x produced more than 3000 BPPD (Barrels of Petroleum per day) of 36 ° API in the Upper Cretaceous levels. GF-lx proved 2900 BPPD of 29.5 ° in the basal sandstones of the Carbonera formation, whereas GF-2x proved, in an interval of the same section, 900 BPPD of 29 ° API. Geochemical analyses, carried out in different wells from the Apure-Barinas sedimentary basin, indicate that the hydrocarbons have kerogen type III, according to the hydrogen index (mg Hc/gr C org) and to the content of organic carbon (medium to medium-high) with respect to the weight percentage of the whole rock. 2.2 EPR measurements An electron in an atom has an intrinsic spin angular momentum and an orbital angular momentum due to circulation about the nucleus. Both of these angular momenta may have only quantized values in an atom and can be assigned specific quantum numbers. Each of these motions of the electron gives rise to a magnetic moment. The total magnetic moment is the vector sum of these separate magnetic moments. To be paramagnetic an atom must have one or more unpaired electrons. When a paramagnetic sample is placed in a magnetic field, the unpaired electrons distribute themselves between low and high-energy states. This is due to the Zeeman effect that removes the degeneracy of the energy levels. Nevertheless, it is possible to remove all the degeneracy of the energy levels before the application of the external field due to the action of the very strong electric field present in the sample. This means that the environment seen by the unpaired electrons could modify their energy levels. When only the Zeeman effect is considered, the energy

separation between states is equal to gl3H where g is the electron g-value, 13 is the Bohr magneton constant and H is the strength of the applied field. When a microwave of fixed frequency v is applied to the sample, low energy electrons are excited to the high-energy state. Energy absorption is described by the resonance condition hv = g13H where h is Planck's constant. To achieve resonance the strength of the magnetic field is swept to satisfy the above equation. The EPR spectrum consists of the first derivative of the energy absorbed as a function of magnetic field strength. Figure 1 shows an energy-level scheme for the simplest system, i.e. free electron or free radical (by definition, free radicals have unpaired electrons) as a function of the applied field H, showing an EPR simulated spectrum (derivative absorption curve).

a)

E ~

Ms =-1/2

H H

b) :

:

AHpp Fig. 1. a) Energy-level scheme for the simplest system (i.e., free electron or free radical) as a function of the applied magnetic field H, showing EPR absorption at H = Hr. b) Simulated EPR spectrum, showing linewidth (AHpp) and resonance field (Hr).

The information obtainable experimentally from electron spin resonance includes linewidth (AHpp), saturation behavior, electron spin concentration, and g-factor. The linewidth is defined as the distance (in Tesla) between the maximum peak and the minimum peak of the derivative absorption curve. The number of spins in an unknown sample may be obtained by direct comparison with a sample of known concentration. The quantity that measures the number of spins is the area beneath the absorption curve. Detailed discussions of EPR spectroscopy appear in standard texts and articles (e.g., Weil et ai.,1994; Saraceno et al., 1961). In this work, EPR spectra were recorded at room temperature using a Varian E-line spectrometer working in the X-band (v = 9.3 GHz), with a cylindrical cavity, homemade coaxial microwave coupler and 100 kHz modulation. Experimental conditions (microwave power and modulation amplitude) were adjusted to avoid saturation effects. OMFRC were calculated by reference to the 4-(2-iodoacetamide)-2,2,6,6-tetramethylpiperidinoxyl (as a standard sample). The standard and the samples were measured using the same EPR conditions. For each well analyzed, from the whole set of samples to which MS

M. Dfaz et al.: Studies in Well Samples from some Venezuelan Oil Fields

measurements were performed, we select some representative levels for the EPR experiments. Samples from anomalous MS levels, adjacent non-anomalous levels and few samples far from these MS peaks were selected. The samples used in these measurements were pulverized to less than 150 mesh, homogenized and then split into two aliquots. The homogenization avoids possible local differences in the concentration of paramagnetic impurities (free radicals, minerals, etc). One aliquot was treated with chloroform, in a stirrer at room temperature for about 60 minutes in order to separate the EOM. The chloroform with the extracted EOM was filtered. The sample and the chloroform plus the EOM were dried at about 60°C to obtain the sample without EOM and the EOM itself. The other aliquot was not solvent extracted. EPR measurements were performed on both aliquots. 2.3 MS measurements The MS measurements were performed in a Sapphire SI-2 susceptometer, which determines the magnetic susceptibility of a rock by the contrast of inductances in a coil with and without the sample in. Each coil inductance is measured to better than 7 significant figures. The dynamic range is from 10 -6 to >1 cgs units. MS readings for every single sample were repeated 5 times and a standard deviation of less than 10% was found for each MS sampleaverage. The SI-2 only allows a single-frequency MS reading for a peak field of about 0.1 mT at approximately 750 Hz.

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3 Results and Discussion

MS profiles for LVT-4x (previously published by Aldana et al., 1999), GF-lx and GF-2x (presented in an accompanying paper by Costanzo-Alvarez et al., 2000) are shown in Fig. 2. LVT-4x has two peaks of high MS values at depth intervals of 534-564 m and 625-655 m (Fig. 2a); GF-lx shows two peaks, one between 200 and 230 m and the other between 310 and 350 m (Fig. 2b) whereas GF-2x has only one peak between 190 and 235 m (Fig. 2c). MS anomalous levels are defined as those depth intervals with MS values that are significantly different from the background and which may give mappable anomalies detectable by conventional magnetometry (Machel and Burton, 1991). Spherical aggregates (diameters ranging between 5 and 30 ~tm) of submicronic crystals were observed only at the MS anomalous levels of these producers wells (except at 310-350 m in GF-lx) as has been previously reported by Aldana et al. (1999) for LVT4x. Figure 3 shows a photomicrograph of such framboidal minerals identified at level 201-210 m in GF-2x (presented in an accompanying paper by Costanzo-Alvarez et al., 2000). We have named the MS anomalous levels, where spherical aggregates were observed by SEM, as anomalies A. On the other hand, MS anomalous levels where no spherical aggregates were observed are designated as anomalies B.

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Fig. 2. Profiles of bulk magnetic susceptibility (MS) (LVT-4x previously published by Aldana et al., 1999, and GF-lx and GF-2x presented in an accompanying paper by Costanzo-Alvarez et al., 2000), organic matter flee radicals concentrations (OMFRC) of the samples without organic solvent extraction (Nw) and OMFRC of the samples with chloroform extraction (Nc) versus depth, for producer wells: (a) LVT-4x; (b) GF-lx; and (c) GF-2x. The gray swathes correspond to anomalous levels for these values (i.e. MS, Nw and Nc). Depths correspond to the tops of the sampling intervals. Labels indicating MS anomalies A or B are also included.

Figure 4 shows an EPR spectrum from level (503-512 m) of GF-2x. In this figure, resonance lines associated with different paramagnetic species can be identified by reference to previously known spectra of various transitionmetal ions (Silbemagel et al., 1991). In addition to a narrow, intense signal near g = 2 (at a magnetic resonant field of Hr z 0.33 T) associated with the free radicals, there is also a strong signal with a g value near 4.3 (Hr ~ 0.17 T) which has been previously identified as coming from iron impurities in quartz. The most prominent signal is a broad (AHpp ~ 0.2 T) absorption centered around g = 2.3 (Hr 0.3 T). This absorption is consistent with pairs of Fe 3+ ions in the sample. A family of six absorption lines also centered near g = 2, which is associated with Mn 2+ impurities, is not

450

M. Dfaz et al.: Studies in Well Samples from some Venezuelan Oil Fields

well determined in this spectrum. The signals corresponding to the transition metals could affect the calculation of the free radical concentration. Effects of sample demineralizations will be discussed later.

high as 20%. For values greater than 2x1016/g, the error is lower than 10%. These data are compared in Fig. 2 with MS measurements. Anomalies of Nw and Nc values are observed for all the producer wells at depths close to the anomalies A. The gray swathes in Fig. 2 indicate the region where we have found anomalous values of Nw and Nc and MS peaks corresponding to A. Table 1. EPR data for the samples of producer wells Well

Depth Interval (m)

Fig. 3. Scanning Electron photomicrograph (topographic image of secondary electrons) showing spherical aggregates of submicronic crystals of magnetic minerals observed at level 201-210 m of GF-2x (presented in an accompanying paper by Costanzo-Alvarez et al., 2000).

i

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(x 1016/g )

(x 1016/g)

LVT-4x LVT-4x LVT-4x LVT-4x LVT-4x LVT-4x LVT-4x LVT-4x

396-411 488-503 533-548 625-640 716-731 823-838 914-929 1231-1246

2.3 2.8 2.5 2.6 3.2 4.8 5.4 0.8

2.6 3.3 1.5 3.0 1.5 4.7 6.9 1.3

GF-Ix GF-Ix GF-Ix GF-Ix GF- I x GF-Ix GF-lx GF-lx

183-195 204-214 223-231 268-278 332-342 369-378 442-451 606-616

1.2 1.5 2.3 22.1 17.6 1.6 3.9 5.6

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GF-2x 155-165 0.6 0.3 GF-2x 192-201 5.2 5.7 GF-2x 201-210 1.7 3.0 GF-2x 219-229 0.3 0.2 GF-2x 503 -512 1.5 1.4 GF-2x 640-649 0.6 0.8 GF-2x 914-924 0.5 0.4 a Organic matter free radical concentration (OMFRC) per gram of sample without solvent extraction bOrganic matter free radical concentration (OMFRC) per gram of sample with chloroform extraction

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The EPR results of the producer wells LVT-4x, GF-lx and GF-2x are shown in Table 1. Nw is the OMFRC (spin/g) of the samples without organic solvent extraction, hence these free radicals detected by EPR belong to the total organic matter (TOM: asphaltene, kerogen, etc). Nc is the OMFRC of the samples with chloroform extraction; hence the free radicals detected in this case belong to the kerogen in the samples (kerogen is not extractable). For concentration values lower than 2x1016/g, the intensity of the EPR signal is very low; hence, the error in these values could be as

For the producer wells GF-2x and LVT-4x, Nw and Nc have approximately the same magnitudes within the experimental e r r o r (see Table 1). The EOM of these samples has no EPR signal, but this EOM may affect the free radical signal changing the environment seen by the paramagnetic species. According to Bakr et al. (1988, 1990) the kerogen matrix is supposed to be weakly combined with the EOM molecules. This combination is pulled apart by the solvent extraction. This is a possible explanation for the small differences observed between Nw and Nc in these wells. Nevertheless, a large difference between these same values is observed at level 268-278 m of GF-Ix. The EOM of this sample has an EPR signal. This signal is produced by free radicals of asphaltene (Acevedo et al., 1997), according to the following experiment: forty (40) volumes of n-heptane were added to this EOM, the precipitate obtained after this process is asphaltene and has an EPR signal. These results suggest that the free radicals detected in the samples without solvent extraction of GF-2x and LVT-4x are free radicals of kerogen (Nw .~ Nc). However, in GF-Ix at level 268-278 m, free radicals of both kerogen and asphaltene are present.

M. Dfaz et

al.:

Studies in Well Samples from some Venezuelan Oil Fields

In order to pinpoint the effects on the OMFRC calculations o f the minerals present in these samples, demineralization treatments were performed in some cuttings from two producer wells. An increase in the OMFRC was found; nevertheless the relative variations of these values with depth is the same, namely a maximum is observed in the OMFRC at the same depths with or without demineralization treatments. Because in this work we are interested in the relative behavior of the OMFRC with depth, and not in the absolute values of the OMFRC, we may argue that demineralization o f the samples does not affect our results. (a) Agua Linda '

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Carlo Murloz (Fig. 5b) and 900-1020 m for La Ceibita (Fig. 5c). It is important to point out that all these anomalous levels are anomalies B and no spherical aggregates of submicronic magnetic minerals were detected by SEM. The EPR results for the non-producer wells are given in Table 2 and compared with MS measurements in Fig. 5. None of these wells show OMFRC anomalies. The EOM o f these samples has no EPR signal, indicating the absence of asphaltene. The Nw and Nc values obtained for all these samples are low (< 1x10 ]6 spin/g). Only two samples in the non-producer wells show differences greater than 0 J x 1 0 s6 /g between these values. In the producer wells, differences greater than 0.5x1016/g are common. This is due to the fact that the EOM fraction in the non-producer well samples is about 80% lower than the EOM fraction obtained from the producer wells. Also, as has been previously indicated, the EOM affects the environment seen by the free radical and accordingly its EPR signal. The EPR results and the absence of magnetite framboids for the non-producer wells seem to suggest that anomaly B is not genetically related to the presence of hydrocarbons but rather to lithological contrasts that affect directly the magnetic mineralogies of the studied strata (Gay, 1992). X-ray analyses performed in samples where anomaly B was detected, indicate the presence of hematite and magnetite o f possible detrital origin (Costanzo-Alvarez et al., 2000). Table 2. EPR data for the samples of the non-producerwells

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Fig. 5. Profiles of bulk magnetic susceptibility (MS), organic matter free radicals concentrations (OMFRC) of the samples without organic solvent extraction (Nw) and OMFRC of the samples with chloroform extraction (Nc) versus depth, for non-producer wells: (a) Agua Linda; (b) Carlo Mufloz; and (c) La Ceibita. Depths correspond to the top of the interval. All the MS anomalies observed at these wells are anomalies B. MS profiles for non-producer wells Agua Linda, Carlo Murloz and La Ceibita are shown in Fig. 5. MS anomalies were observed at depth intervals o f 110-165 m and 630-780 m for Agua Linda (Fig. 5a), 500-630 m and 800-860 m for

Depth Interval

Nw~

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(m)

(x 10u'/g)

(x 10~'/g)

Agua Linda Agua Linda Agua Linda Agua Linda Agua Linda Agua Linda

110-125 320-335 530-545 686-701 722-737 896-911

0.3 0.6 0.3 0.2 0.2 0.1

0.4 0.6 0.2 0.2 ~0.2 0.1

Carlo Murloz Carlo Murloz Carlo Mufioz Carlo Murloz Carlo Murloz Carlo Murloz

213-228 381-396 503-518 549-564 838-853 945-960

0.9 0.6 0.8 0.6 0.3 02

1.1 0.6 0.8 0.5 0.3 0.2

La Ceibita 390-405 09 0.8 La Ceibita 719-734 0.8 0.5 La Ceibita 911-926 0.4 0.4 La Ceibita 1003-1018 0.9 0.8 La Ceibita 1213-1228 0.3 0.4 Organic matter free radical concentration (OMFRC) per gram of sample without solvent extraction bOrganic matter free radical concentration (OMFRC) per gram of sample with chloroformextraction It seems that the presence of OMFRC peaks (anomalous Nw and Nc values) is associated with the presence of anomaly A and both are detected only at the producer wells. However (except for the anomalous region at GF-Ix) asphaltene was not found in these samples. As it was previously indicated, the free radicals observed in GF-2x and LVT-4x samples are of kerogen and not of asphaltene. Asphaltenes are exceedingly resistant to microbial attack

452

M. Dfaz et al.: Studies in Well Samples from some Venezuelan Oil Fields

and therefore tend to be the major components in highly biodegraded crude oils (Bailey et al., 1973). Since we have not found asphaltene in these two wells, and providing that spherical aggregates of magnetic minerals are indeed responsible for anomaly A, then oil biodegradation does not seem to be a likely process involved in framboidal precipitation at GF-2x and LVT-4x. The OMFRC anomaly can be quite wide (see figures 2a and 2b) encompassing a zone where thermochemical conditions for precipitation o f magnetic minerals are given and MS anomalies A can be observed. Due to the presence of free radicals of kerogen and asphaltene at the anomalous GF-lx region, it seems possible that two processes have taken place during precipitation of magnetic minerals in this case, namely thermochemical reactions and oil biodegradation. According to Saunders and Terry (1985), hematite in the sediments overlying petroleum accumulations is converted to magnetite by chemical reduction due to hydrogen sulfide formed by sulfate reducing bacteria in the presence of hydrocarbon gases. Machel (1995) pointed out that anaerobic sulfate-reducing bacteria are a factor in the formation of magnetic irons sulfides because they generate reduced sulfur (as H2S and HS). Where dissolved Fe 2+ is available, it reacts with the aqueous sulfur species to form diagenetic iron sulfides, e.g. pyrite framboids. Mineral precipitation is generally "inorganic" sensu strictu, that is the bacteria exude the reduced sulfur species. According to Machel and Burton (1991), iron sulfide precipitation may occur either after considerable migration of the sulfide or very close to the surface of the organisr~s. Magnetite can occur in the same shape as the result of replacement (oxidation) of pyrite framboids (Suk et al., 1990) or by direct reduction of precursor oxides such as hematite (e.g. Machel, 1995). We argue that these could be possible mechanisms for oil biodegradation in GF-lx where asphaltene was detected about 30 meters below the levels where anomaly A was observed. Three possible causes may be responsible for the anomalous Nc values detected by EPR: 1) an increase in the kerogen content of the samples at those depths; 2) a change in the kerogen type (Aizenshtat et al., 1986; Bakr et al., 1988); or 3) an increase in the free radical concentration associated with the kerogen maturity (Bakr et al., 1988, 1990; Qiu Nansheng and Wang Jiyang, 1998; Dickneider et al., 1995). The demineralization treatments previously discussed seem to discard the first possibility. At present we continue the investigation of this problem. Nevertheless, it is important to point out that for all the producer wells, in the same region where anomalies A are observed, there is also an anomaly in OMFRC, whereas such an anomaly is completely absent in the non-producer wells.

4 Summary and Conclusions In this study we report, for three producer wells located in some Venezuelan oil fields, near-surface MS anomalies that seem to be associated with the presence of microscopic framboidal magnetic minerals (anomaly A), and with the

presence of OMFRC peaks. In the non-producer wells MS anomalies have been also detected. However, at these same levels no spherical aggregates of magnetic minerals were detected. EPR measurements indicate that OMFRC anomalies are not present either. For LVT-4x and GF-2x (producer wells) the only presence of free radicals of kerogen suggest the existence of a zone where thermochemical conditions are given for the precipitation of framboidal magnetic minerals (probably associated with these anomalous MS values). According to the results for GF-lx (i.e. the presence of free radicals of kerogen and asphaltene) it seems likely that two processes, thermochemical reactions and oil biodegradation, could have taken place leading to the formation of diagenetic magnetic minerals in this well. Although these results are rather preliminary, the presence of MS anomaly A and OMFRC anomalies in the producer wells could be regarded as a step forward in establishing a link between contrasts of magnetic properties and the presence of hydrocarbons. Finally it is important to point out that this is the first time a combined study of such a kind has been undertaken in drill cuttings of different oil wells (producers and non producers).

Acknowledgments. We are grateful to CORPOVEN S.A. (now PDVSA),

specially to F61ix Castillo, Eulogio Del Pino and Diego Funes, for providing the samples, informationaboutLa Victoriaand Guafitaoil fields and their continuous interest in non-conventional geophysics. For their assistance throughoutthe different experimentalsteps, we are also in debt with: Paulo Frias (Institute of Engineering, Caracas); Daniel Vitiello (BP Venezuela, Caracas); Otto Aristeguieta (INTEVEP, PDVSA); Milexi Pacheco (Universidad de Carabobo); Justo Juanilla (PolymerLaboratory, Universidad Sim6n Bolivar) and Alejandro Mailer (Department of Material Sciences, Universidad Sim6n Bolivar). The paper benefited from thoughtful reviewsby two anonymousreferees.

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