Milagrosa Aldana

Remagnetization and chemical alteration of sedimentary rocks

Abstract Chemical remagnetization is a very common phenomenon in sedimentary rocks and developing a greater understanding of the mechanisms has several benefits. Acquisition of a secondary magnetization is usually tangible evidence of a diagenetic event that can be dated by isolation of the chemical remanent magnetization and comparison of the pole position to the apparent polar wander path. This can be important because diagenetic investigations are frequently limited by the difficulty in constraining the time frames in which most past events have occurred. Remagnetization can commonly obscure a primary magnetization; developing a better understanding of remagnetization could improve our ability to uncover primary magnetizations. Many chemical remagnetization mechanisms have been proposed, including those associated with chemical alteration by a number of different fluids (orogenic, basinal and hydrocarbons), burial diagenetic processes (clay diagenesis and maturation of organic matter) or other processes. This paper summarizes our current knowledge of these chemical remagnetization mechanisms, with a focus on examples where there is a connection with chemical alteration.

Petrophysical seismic inversion conditioned to well-log data: Methods and application to a gas reservoir

Hydrocarbon reservoirs are characterized by seismic, welllog, and petrophysical information, which is dissimilar in spatial distribution, scale, and relationship to reservoir properties. We combine this diverse information in a unified inverse-problem formulation using a multiproperty, multiscale model, linking properties statistically by petrophysical relationships and conditioningthemtowell-logdata.Twoapproacheshelpus:1Markov-chainMonteCarlosampling,whichgeneratesmanyreservoir realizationsforestimatingmediumpropertiesandposteriormarginalprobabilities,and2optimizationwithaleast-squaresiterativetechniquetoobtainthemostprobablemodelconfiguration. Our petrophysical model, applied to near-vertical-anglestacked seismicdataandwell-logdatafromagasreservoir,includesadeterministic component, based on a combination of Wyllie and Wood relationships calibrated with the well-log data, and a random component, based on the statistical characterization of the deviations of well-log data from the petrophysical transform.At the petrophysical level, the effects of porosity and saturation on acoustic impedance are coupled; conditioning the inversion to well-log data helps resolve this ambiguity. The combination of well logs, petrophysics, and seismic inversion builds on the correspondingstrengthsofeachtypeofinformation,jointlyimproving 1 cross resolution of reservoir properties, 2 vertical resolution of property fields, 3 compliance to the smooth trend of property fields, and 4 agreement with well-log data at well positions.

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.