Up to what temperature is petroleum stable? New insights from a 5200 free radical reactions model

The discovery of crude oils and condensates at ever higher temperatures casts doubt on the validity of the usual geochemical modelling approach, that uses empirical reactions and rate constants. The solution used to account for such a high thermal stability is presently to adjust the rate parameters, but the physical meaning and scientific value of such a strategy can be questioned. We have developed a mechanistic model consisting of 5200 lumped free radical reactions to describe the thermal evolution of a mixture of 52 organic species meant to represent light petroleum. Rate constants used are those available in the literature or estimated using well established thermochemistry-reactivity correlations. Chemical structures included in the model are linear, branched and cyclic hydrocarbons, hydro- and alkyl-aromatics, PAHs, and three heteroatomic compounds. Reactions include cracking and alkylation chains and inhibiting and accelerating reactions from the various reactants. This model has been applied to several mixtures with various proportions of reaction inhibitors and accelerators, and to a composition representing a light mature oil. From the results obtained, we conclude that mature oils will be stable up to 240–260 °C, depending on their composition, and that the thermal cracking of oil to gas is not possible under reasonable basin conditions. The kinetics of petroleum cracking are thus much slower than generally recognized.     

Geochemical arguments for identifying second‐order sea‐level changes in hemipelagic carbonate ramp deposits

Second‐order transgressive–regressive (T–R) cycles, previously recognized using sedimentological criteria in Lower Jurassic hemipelagic deposits from northern Spain, are distinguishable based upon bulk‐rock organic geochemistry [total organic carbon (TOC) and hydrogen index (HI)] and the stable carbon isotope compositions from belemnite rostra. There is a coincidence between regressions and decreasing δ¹³C_bel, TOC and HI values, and between transgressions and increasing δ¹³C_bel, TOC and HI values. The δ¹⁸O and Mg/Ca records from the belemnite rostra are not always in phase with the T–R cycles. The δ¹⁸O_bel record reveals, however, a prominent excursion towards higher values within the spinatum Zone that correlates, according to our results, with a regression and with negative shifts in Mg/Ca, δ¹³C_bel and TOC. On the other hand, an excursion in the δ¹⁸O_bel towards lower values in the serpentinus Zone also correlates with a peak transgression and with positive shifts in Mg/Ca, δ¹³C_bel and TOC. These two excursions have been identified in other European regions as geochemical perturbations of the same characteristics. This suggests a link between second‐order relative sea‐level changes and variations in seawater geochemistry that may reflect local and regional palaeoceanographic perturbations in sea‐water temperature, salinity and water circulation during the Early Jurassic. Terra Nova, 18, 233–240, 2006     

Fractionation of grain size in terrestrial sediment routing systems

Sediment is fractionated by size during its cascade from source to sink in sediment routing systems. It is anticipated, therefore, that the grain size distribution of sediment will undergo down‐system changes as a result of fluvial sorting processes and selective deposition. We assess this hypothesis by comparing grain size statistical properties of samples from within the erosional source region with those that have undergone different amounts of transport. A truncated Pareto distribution describes well the coarser half of the clast size distribution of regolith, coarse channel bed sediment and proximal debris flows (particularly their levees), as well as the coarser half of the clast size distribution of gravels that have undergone considerable amounts of transport in rivers. The Pareto shape parameter a evolves in response to mobilization, sediment transport and, importantly, the selective extraction of particles from the surface flow to build underlying stratigraphy. A goodness of fit statistic, the Kolmogorov–Smirnov vertical difference, illustrates the closeness of the observed clast size distributions to the Pareto, Weibull and log‐normal models as a function of distance from the depositional apex. The goodness of fit of the particle size distribution of regolith varies with bedrock geology. Bedload sediment at catchment outlets is fitted well by the log‐normal and truncated Pareto models, whereas the exponential Weibull model provides a less good fit. In the Eocene Escanilla palaeo‐sediment routing system of the south‐central Pyrenees, the log‐normal and truncated Pareto models provide excellent fits for distances of up to 80 km from the depositional apex, whereas the Weibull fit progressively worsens with increasing transport distance. A similar trend is found in the Miocene–Pliocene gravels of the Nebraskan Great Plains over a distance of >300 km. Despite the large fractionation in mean grain size and gravel percentage from source region to depositional sink, particle size distributions therefore appear to maintain log‐normality over a wide range of transport distance. Use of statistical models enables down‐system fractionation of sediment released from source regions to be better understood and predicted and is a potentially valuable tool in source‐to‐sink approaches to basin analysis.     

Short- and long-term effects of shrub management on vegetation in the Monte, Argentina

A comparison was made of plant cover, carrying capacity and diversity in areas of Monte vegetation at the Biosphere Reserve of Ñacuñán, Mendoza, Argentina, in 1982, 1984 and 1995. Treatments applied in 1981 were: control (untreated plots), selective hand-cutting, roller-chopping, and root-plowing. By 1995, woody cover was lower in root-plowed plots (27%) than in the others treatments (41–51%), and herbaceous cover was similar in treated (28–39%) and untreated areas (26%). Carrying capacity of treated areas (6–8 ha per Large Stock Unit (LSU)) was higher than in the control areas (12 ha LSU⁻¹). Woody species diversity, quantified by the Shannon index, for both areas was similar in the study period (0·62), except in root-plowed sites in 1982 (0·0) and 1995 (1·3). Herbaceous species diversity was similar on treated (0·81) and untreated sites (1·2) in 1995. We could recommend less drastic treatments to improve carrying capacity.     

Geochemical mobility of arsenic in the surficial waters from Argentina

The presence of arsenic (As) in surface water constitutes an important environmental risk, where mobility and adsorption processes are responsible for its behavior in the sediment–water interface. Therefore, the assessment of adsorption, mobility and water availability of arsenic in freshwater sediments, with agricultural, livestock and urban soil uses was performed. Arsenic concentrations in sediments ranged from 5.4 to 15.9 mg kg^?1 (total) and 2.8 to 6.5 mg kg^?1 (labile), and those of iron and manganese were 11,563–23,500 and 140.6–662.1 mg kg^?1, respectively. The As levels in water were significantly lower than those of sediments. Results would suggest that As co-precipitation and adsorption on Fe oxides are probably the major route of immobilization, determining its low lability. Manganese did not present an outstanding contribution to the retention, and cation-exchange capacity, pH and organic matter of sediments did not show an influence on the mobility of As.     

Water masses in Thermaicos Gulf,north-western Aegean Sea

Physical oceanographic data were collected during September 1975 at stations in the shallow water (max. depth 45 m) embayments of Salonica Bay and Thermaicos Gulf, and comparisons are made with data collected during other seasons. The distribution of temperature and salinity in the water column indicated the following features: fresh surface water predominantly related to river discharge; an intermediate water mass, apparently formed by mixing between the fresh and open sea waters; a thermocline, and deep bottom water open sea characteristics. The summer distribution of density in the surface and bottom waters, and the seasonal variation in the pattern of surface salinity, suggested an anti-clockwise water circulation throughout the embayments.     

Characterization of Colliery Spoils in León: Potential Uses in Rural Infrastructures

One of the bases of sustainable development is the responsible management of natural resources, and this is the approach which has been adopted by developed countries as regards management of all kinds of waste, through the establishment of management strategies based on prevention, reuse and recycling. This article present the results of a physical, chemical and geotechnical characterization of colliery spoils stored in slag heaps in the Ciñera-Matallana coal mining area in León, Spain. The primary objectives of this research were to determine the suitability of this mining waste for use as granular material in rural and civil engineering infrastructures and to classify it according to national and international road construction standards. On the basis of the results obtained, they are proposed several possible uses (foundation and core layers in track and road embankments, granular sub-base for light traffic and top layers of embankments), although it should be noted the need for mechanical treatments as crushing or sieving and the recommendation of use of sulphate resistant cement to avoid the possible problems.     

Micas of the muscovite-lepidolite series from the fregeneda pegmatites (Salamanca,Spain)

Summary In the Fregeneda area different types of pegmatites can be recognized by their mineralogy, morphology, internal structure and field relationships. The most common type corresponds to a simple pegmatite with homogeneous internal structure, but zoned Li-bearing pegmatites also are relatively widespread. Cassiterite-bearing pegmatites are subordinate. The pegmatites are spatially associated to the Lumbrales granite. This is a parautochthonous, fine- to medium-grained, two-mica granite, one of the syntectonic massifs which were deformed during the third phase of Hercynian deformation. Representative micas selected from the different groups of pegmatites were studied to determine wether the pegmatites can be related by a common fractionation path, and how different pegmatite types are related to the spatially associated Lumbrales granite. Compositional variations in the micas depend on the pegmatite type. Muscovite coexisting with Li-micas in the Li-bearing pegmatites is one of the richest in Al₂O₃ (35.4–37.7 wt%) and the poorest in FeO (0.2–1.5 wt%) and MgO (0–0.3 wt%), whereas muscovite of the simple discordant pegmatites shows the highest FeO (2.2–3.3 wt%) and that from the Lumbrales granite is the richest in MgO (0.5–0.7 wt%) and TiO₂ (0.6–1.1 wt%). On the other hand, Sn (70–1168 ppm), Li (< 5–22253 ppm), F (880–21470 ppm), Cs (< 5–1696 ppm), Rb (800–9181 ppm) and other trace elements seem to increase with distance from the Lumbrales granite, and K/Rb decreases. According to this ratio, the exterior Li-bearing pegmatites are the more evolved, whereas the interior pegmatites are less evolved, and are richer in Cs, Li and Zn than other pegmatite types.

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Glimmerminerale der Muscovit-Lepidolith-Serie aus den Pegmatiten von Fregeneda, Salamanca, Spanien

Zusammenfassung Im Gebiet von Fregeneda sind auf Grund ihrer Mineralogie, Morphologie, Internstruktur und Geländebeziehungen verschiedene Pegmatittypen zu unterscheiden. Am häufigsten sind einfache homogen aufgebaute Pegmatite. Zonar gebaute Li-Pegmatite sind ebenfalls weit verbreitet, Zinnstein-führende Pegmatite treten hingegen zurück, Die Pegmatite sind räumlich mit dem Lumbrales Granit, einem paraautochtonen, fein- bis mittelkörnigen Zwei-Glimmergranit, assoziiert, Dieser gehört einem der syntektonischen Massive, die während der dritten Phase der hercynischen Deformation deformiert wurden, an. Repräsentative, aus den verschiedenen Pegmatittypen separierte Glimmerminerale wurden untersucht, um zu klären, inwieweit die Pegmatite über einen direkten Fraktionierungspfad zu verbinden sind und in welcher Beziehung sie zu dem Lumbrales Granit stehen. Die Variation der Zusammensetzung der Glimmer hängt vom Pegmatittyp ab. Muscovite, die mit Li-Glimmern koexistieren, sind die relativ Al₂O₃-reichsten (32–37.7 Gew.%) und Fe- (0.2–1.5 Gew.%.) und Mg-ärmsten (0–0.3 Gew.%). Jene aus dem Lumbrales Granit sind die reich an MgO (0.5–0.7) und TiO₂ (0.6–1.1 Gew.%). Die Gehalte von Sn (70–1168 ppm), Li (< 5–22253 ppm). F (880-21470 ppm), Cs (< 5–1696 ppm), Rb (800–9181 ppm) und anderer Spurenelemente nehmen mit der Entfernung vom Lumbrales Granit zu, während K/Rb abnimmt. Auf Grund dieses Verhältnisses sind die externen Li-führenden Pegmatite höher, die internen Pegmatite hingegen geringfügiger entwickelt. Erstere sind daher auch reicher an Cs, Li und Zn.

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With 4 Figures