Light hydrocarbons in hydrothermal and magmatic fumaroles: hints of catalytic and thermal reactions

Volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated (alkanes), unsaturated (alkenes) and aromatic volatile hydrocarbons. Three major patterns of distributions can be recognized, apparently related to the chemical-physical environment of formation of the gas exhalations: alkane-rich, low-temperature gas emissions from recently active volcanic areas; aromatic-rich hydrothermal manifestations; and alkene-rich, magmatic fumaroles on active volcanoes. Thermodynamic data, together with theoretical and practical findings from the petroleum industry, point to two main types of reactions occurring in these volcanic environments: cracking and reforming. Cracking processes, mainly caused by thermal effects, occur when hydrocarbon-bearing hydrothermal fluids enter and mix with a hot and dry, rapidly rising magmatic gas phase. The most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures. The presence of aromatic species in hydrothermal fluids can be linked to reforming processes, catalysed by several possible agents, such as smectites and zeolites, generally present in the hydrothermally altered volcanic terranes, and facilitated by long residence times in a hydrothermal envelope.     

Assessment of organic pollutants in coastal sediments, UAE

Twenty-five stations were selected along the UAE coastal region to delineate the distribution and to determine the source of total petroleum hydrocarbon (TPH), total organic carbon (TOC), total Kjeldhal nitrogen (TKN), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). The concentrations of TPH ranged between 46 and 223 mg/kg and the level of TOC was in the range from 0.044 to 17.8 wt%, while TPAHs occupied the range of 5–102.2 μg/kg. On the other hand, TPCBs showed values between 60 and 205 μg/kg and TKN ranged from 104 to 1,073 mg/kg. The present study shows that the distribution of organic compounds in UAE sediments were within safe limits comparing with previous studies. The present study revealed that TOC can be used as an indicator of oil pollution in heavily oiled sediments. The highest values of TOC, TPH, TPAHs and TPCBs related to the stations covered in fine sand due to adsorption properties and the large surface areas of the grains. The evaporation of low-boiling point compounds from surface layers leads to the enrichment of sediments with a thick residual. Al Sharjah-1 exhibited the highest values of TPH and TOC at Ras Al-Khaima-5, Umm Al-Quwen-1 and Dubai-1, while the highest values of TPAHs were at KhorFakkan-9, the highest value of TPCBs was at Ajman-2, and the highest values of TKN were at KhorFakkan-1, KhorFakkan-6 and Dubai-1.     

The synorogenic pegmatitic quartz veins of the Guacha Corral Shear zone (Sierra de Comechingones,Argentina): A textural,chemical, isotopic,cathodoluminescence and fluid inclusion study

The quartz veins and pegmatites of the Sierra de Comechingones (Sierras de Córdoba, NE Argentina) belong to the Comechingones Pegmatite field (CPF). For the quartz veins and the zoned pegmatites related parental granites are missing. The country rock of the quartz veins are mylonitic augengneisses in granulite to upper amphibolite facies. Field relations, microscopy, cathodoluminescence, radiometric age data, fluid inclusion, chemical and isotopic composition and literature define the quartz veins as synorogenic formed during the high-temperature phase of the Famatinian (480–460 Ma) event. During the Famatinian up to the Achalian (382–366 Ma) event the synorogenic quartz veins were subjected to high temperature ductile deformation documented by folding, boudinage and finally brittle shearing. K-Ar ages of illite from the shear zones of about 166 Ma document the final cooling of the Sierras Pampeanas below 100 °C. The long lasting thermal and deformational history of the study area is reflected by very different populations of fluid inclusions in vein quartz with remarkably high contents of thermogenic hydrocarbons in the early-formed fluid inclusions. LA–ICP–MS analysis reveals very low lattice-bound trace element contents, i.e. high purity quartz.     

Lipid biomarkers in surface sediments from the Gulf of Genoa,Ligurian sea (NW Mediterranean sea) and their potential for the reconstruction of palaeo-environments

A series of molecular organic markers were determined in surface sediments from the Gulf of Genoa (Ligurian Sea) in order to evaluate their potential for palaeo-environmental reconstructions. Allochthonous input can be characterized by the distributions of n-C₂₉ and n-C₃₁ alkanes, n-C₂₆ and n-C₂₈ alkanols and branched glycerol dialkyl glycerol tetraethers (GDGTs), whose concentrations are generally highest near the river mouths. In the open basin however, terrestrial n-alkanes and n-alkanols may have an additional, eolian source. Autochthonous input is represented by crenarchaeol and isoprenoid GDGTs. Their concentrations are highest in the open basin showing the preference of Thaumarchaeota for oligotrophic waters. Indications of a significant degradation of sterols and C₃₇ alkenones exclude these lipids as reliable productivity proxies. Using terrestrial and aquatic lipids as end-members allows estimating the percentage of terrestrial organic matter between 20% and 58% in the coastal area decreasing to 1–30% in the deep basin. The spatial distribution of sea surface temperature (SST) estimates using the alkenone-based U^K′₃₇ index is very similar to the autumnal (November) mean satellite-based SST distribution. Conversely, TEX^H₈₆-derived SST estimates are close to winter SSTs in the coastal area and summer SSTs in the open basin. This pattern reflects presumably a shift in the main production of Thaumarchaeota from the coastal area in winter to the open basin in summer. This study represents a major prerequisite for the future application of lipid biomarkers on sediment cores from the Gulf of Genoa.     

Geographies of peak oil: The other carbon problem

This extended editorial introduction to a themed issue of Geoforum on geographies of peak oil has three objectives. First, it provides a concise account of the ’peak oil’ claim, identifying the key protagonists in the debate, and outlining different stances with regard to the timing, shape and composition (conventional vs. non-conventional hydrocarbons) of the peak. Second, after briefly characterising the limited engagement with peak oil by human geographers, it offers a provisional set of claims about what a geographical analysis of peak oil might yield. Finally, it introduces each of the papers and, in doing so, makes the case for a fuller and more sustained engagement by geography with this ’other carbon problem.’     

Weathering of fuel oil spill on the east Mediterranean coast, Ashdod, Israel

Residual fuel oil spilled into the sea from the Eshkol power station on 8 February, 1998 contaminated about 9 km of the foreshore north of the Ashdod harbour. A study of the aliphatic, polycyclic alkane and polyaromatic hydrocarbon (PAH) composition of the spilled oil shows rapid weathering in the early stages followed by gradual slowdown after about three months. Weathering of isoprenoids and PAH compounds and variation in Pr/Ph ratio appear to occur almost contemporaneously with that of n-alkanes, at a relatively moderate level of degradation, when much of the >C₂₀ n-alkane envelope is still well preserved. Depletion of various compounds in accordance with molecular size rather than molecular structure appears to imply that physical weathering processes, i.e. evaporation and perhaps flushing due to wave energy, might have played an important role in the degradation of the spilled residual fuel oil in this study case.     

Regional structure and tectonic history of the obliquely colliding Columbus foreland basin, offshore Trinidad and Venezuela

Cenozoic eastward migration of the Caribbean plate relative to the South American plate is recorded by an 1100-km-long Venezuela-Trinidad foreland basin which is oldest in western Venezuela (65-55 Ma), of intermediate age in eastern Venezuela (34-20 Ma) and youngest beneath the shelf and slope area of eastern offshore Trinidad (submarine Columbus basin, 15.0 Ma-Recent). In this study of the regional structure, fault families, and chronology of faulting and tectonic events affecting the hydrocarbon-rich Columbus foreland basin of eastern offshore Trinidad, we have integrated approximately 775 km of deep-penetration 2D seismic lines acquired by the 2004 Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region (BOLIVAR) survey, 325 km of vintage GULFREX seismic data collected by Gulf Oil Company in 1974, and published industry well data that can be tied to some of the seismic reflection lines. Top Cretaceous depth structure maps in the Columbus basin made from integration of all available seismic and well data define for the first time the elongate subsurface geometry of the 11-15 km thick and highly asymmetrical middle Miocene-Recent depocenter of the Columbus basin. The main depocenter located 150-200 km east of Trinidad and now the object of deepwater hydrocarbon exploration is completely filled by shelf and deepwater sediments derived mainly from the Orinoco delta. The submarine Darien ridge exhibits moderate (20-140 m) seafloor relief, forms the steep (12°-24°), northern structural boundary of the Columbus basin, and is known from industry wells to be composed of 0.5-4.5 km thick, folded and thrust-imbricated, hydrocarbon-bearing section of Cretaceous and early Tertiary limestones and clastic rocks. The eastern and southern boundaries of the basin are formed by the gently (1.7°-4.5°), northward-dipping Cretaceous-Paleogene passive margin of South America that is in turn underlain by Precambrian rocks of the Guyana shield.Interpretation of seismic sections tied to wells reveals the following fault chronology: (1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; (2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; (3) early Pliocene-Recent strike-slip faults parallel the trend of the Darien ridge and accommodate present-day plate motions.     

Upper Permian Main Dolomite microbial carbonates as potential source rocks for hydrocarbons (W Poland)

Various microbialite lithofacies were common in the shallow-water environments which flourished under highly saline and arid climate conditions in the Polish part of the Zechstein (Upper Permian) Main Dolomite (Ca2) carbonates. Data came from detailed analysis of 78 cores from wells located at the southern and northern margins of the Main Dolomite basin in west Poland. Microbialite lithofacies are represented by columnar, planar and domal stromatolites, clotted thrombolites and biolaminites developed in high-to-low energy environments within the upper slope, lower parts of oolitic barrier/shoal, restricted lagoon, and tidal flat and tidal channel zones. In barrier environments ooids were predominant whereas in lagoonal settings microbial oncoids and peloids were common. In basinal settings laminated dolomudstones formed which are regarded as hemipelagic and are partly pelagic microbial in origin. Microbial communities coexisted with algae and developed mainly within shallow marine environments. Microbialites are built of cyanobacterial filaments and organo-mineral coccoidal forms possibly of bacterial origin. Organic geochemistry data confirm that organic matter was transformed by cyanobacteria and bacteria which may have played a main role in hydrocarbon generation. This is confirmed by C₂₉ to C₃₅ hopanes, Pr/Ph ratio below 1.0, and the presence of gammacerane as typical for Ca2 profiles. The role and occurrence of algae, as confirmed by C₂₉-steranes, in hydrocarbon generation was possibly minor. Thus it is concluded that the Main Dolomite microbialites could be the likely source rocks for hydrocarbons, with calculated original TOC values from 0.8 to ca 2.0 wt. %. These units also form the reservoir rocks, with porosities reaching 20%.