Hydraulic fracturing in unconventional gas reservoirs: risks in the geological system, part 2

Hydraulic fracturing is a method used for the production of unconventional gas resources. Huge amounts of so-called fracturing fluid (10,000–20,000 m³) are injected into a gas reservoir to create fractures in solid rock formations, upon which mobilised methane fills the pore space and the fracturing fluid is withdrawn. Hydraulic fracturing may pose a threat to groundwater resources if fracturing fluid or brine can migrate through fault zones into shallow aquifers. Diffuse methane emissions from the gas reservoir may not only contaminate shallow groundwater aquifers, but also escape into the atmosphere where methane acts as a greenhouse gas. The working group “Risks in the Geological System” as part of ExxonMobil’s hydrofracking dialogue and information dissemination processes was tasked with the assessment of possible hazards posed by migrating fluids as a result of hydraulic fracturing activities. In this work, several flow paths for fracturing fluid, brine and methane are identified and scenarios are set up to qualitatively estimate under what circumstances these fluids would leak into shallower layers. The parametrisation for potential hydraulic fracturing sites in North Rhine-Westphalia and Lower Saxony (both in Germany) is derived from literature using upper and lower bounds of hydraulic parameters. The results show that a significant fluid migration is only possible if a combination of several conservative assumptions is met by a scenario.     

Untersuchung von suspendiertem Material in den Hydrothermallaugen des Atlantis-II-Tiefs

Suspended matter was separated from the hydrothermal brines of the Atlantis-II Deep in the Red Sea. Contents of iron, manganese, copper, and zink collected on membrane filters were measured by X-ray-flourescence, and the main results were:

1. Metal concentrations in suspended form were lower by 10^?1 to 10^?5 compared to the dissolved concentrations of the brines.
2. Suspended copper and zink were enriched most pronounced in the deeper brine zone — markedly so in the SW-basin, values there ranged between 10 and 30μg/l, one Zn-value was as high as 60μ/l, the other basins contained mostly less than 1 to 10μg/l.
3. Iron, copper, and zink in the deeper brine zone and in few cases also in the 50°-brine were suspended in the form of sulfide compounds. This conclusion is based only on the slightly purple-, green-, and blue-colored gray hues of the material on the filters and its rapid oxidation upon contact with the air.
4. In the transition zone of the 50°/60°-brine iron hydroxides were highly concentrated with values ranging up to 1000μg Fe/l.
5. Suspended manganese is found only within the transition zone of brine to the normal sea water, where up to 200μg Mn/l in form of darkbrown manganese hydroxides were found.
6. Concurrently with the increased thermal activity since 1966 more strongly reducing conditions within the brines seemed apparent which were caused by discharge of higher amounts of Sulfides into the basins. Increased precipitation of heavy metal sulfides was found to be most pronounced in the SW-basin.

     

Rare earth geochemistry of fused ophiolitic and alpine lherzolites

Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively undepleted mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]_N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.     

Data analysis and adjustment of the first geodetic surveys in the Caldera of Teide, Tenerife, Canary Islands

To detect possible superficial deformations in the volcanic Caldera of Teide area (Canary Islands), we have set up a network composed by 17 points which we shall observe successively. In this paper, we present the analysis and reduction of data and the free tridimensional adjustment of the network for the first observation campaign.     

Review: secular tectonic evolution of Archean continental crust: interplay between horizontal and vertical processes in the formation of the Pilbara Craton,Australia

The Archean Pilbara Craton contains five geologically distinct terranes – the East Pilbara, Karratha, Sholl, Regal and Kurrana Terranes – all of which are unconformably overlain by the 3.02‐ to 2.93‐Ga De Grey Superbasin. The 3.53–3.17 Ga East Pilbara Terrane (EP) represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3.53–3.43, 3.35–3.29 and 3.27–3.24 Ga. Each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3.72 Ga, and melting of crust to generate first tonalite‐trondhjemite‐granodiorite (TTG), and then progressively more evolved granitic magmas. In each case, plume magmatism was accompanied by uplift and crustal extension. The combination of conductive heating from below, thermal blanketing from above, and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust. These mantle melting events caused severe depletion of the subcontinental lithospheric mantle, making the EP a stable, buoyant, unsubductable continent by c. 3.2 Ga. Extension accompanying the latest event led to rifting of the protocontinent margins at between 3.2 and 3.17 Ga. After 3.2 Ga, horizontal tectonic forces dominated over vertical forces, as revealed by the geology of the three terranes (Karratha, Sholl and Regal) of the West Pilbara Superterrane. The c. 3.12‐Ga Whundo Group of the Sholl Terrane is a fault bounded, 10‐km‐thick volcanic succession with geochemical characteristics of modern oceanic arcs (including boninites and evidence for flux melting) that indicate steep Archean subduction. At 3.07 Ga, the 3.12‐Ga Sholl Terrane, 3.27‐Ga Karratha Terrane and c. 3.2‐Ga Regal Terrane accreted together and onto the EP during the Prinsep Orogeny. This was followed by development of the De Grey Superbasin – an intracontinental sag basin and widespread plutonism (2.99–2.93 Ga) as a result of orogenic relaxation and slab break off. Craton‐wide compressional deformation at 2.95–2.93 Ga culminated with 2.91‐Ga accretion of the 3.18 Ga Kurrana Terrane with the EP. This compression caused amplification of the dome‐and‐keel structure in the EP. Final cratonization was effected by emplacement of 2.89–2.83 Ga post‐tectonic granites.     

Temporal trends of dissolved weathering products released from a high Arctic coal mine waste rock pile in Svalbard (78°N)

It is well known that oxidation of sulphide-containing coal mine waste has considerable environmental impacts due to generation of acid mine drainage (AMD) containing high dissolved metal concentrations. This study is the first to evaluate seasonal trends in the release of AMD from high arctic coal mine waste rock. Runoff from an abandoned coal mine waste pile in Svalbard (78°N) was studied during the entire 3–4 month period with running water in 2005. Temporal variation in concentrations and fluxes of dissolved elements were quantified based on daily water sampling and used to evaluate weathering processes and estimate element budgets on a daily, seasonal and annual basis. Apart from alkali- and alkaline earth metals; Fe, Al, Mn, Zn and Ni were found to be the most abundant metals in the runoff. Element concentrations were highly correlated and suggest that the processes of sulphide oxidation, ion exchange and silicate weathering occurring within the waste pile were linked throughout the measuring period. Observed pH values varied from 2.8 to 5.2 and SO₄ concentrations from 21 to 1463 mg L⁻¹. Manganese and Al concentrations were observed above phytotoxic levels (up to 4 and 23 mg L⁻¹, respectively) and were considered the most critical elements in terms of environmental impact. Throughout the summer a total dissolved quantity of 58 kg Mn, 238 kg Al and 13,700 kg SO₄ was released from the pile containing approximately 200,000 m³ of pyritic waste material (<1% FeS₂). The highest concentrations of metals, lowest pH values and a very high daily release of H₂SO₄ (up to twice as high as the following month) were observed during the first week of thaw. This is considered a result of an accumulation of weathering products, generated within the waste pile during winter and released as a pollution-flush during early spring. Similar accumulation/flush sequences were observed later in the summer where rain events following relatively long dry periods caused high daily metal fluxes and on some occasions also elevated dissolved metal concentrations. Despite highly variable weather/climate conditions during the rest of the summer the investigated waste rock pile acted like a relative constant pollution-source during this period. Future investigations regarding the environmental impact of mine waste in the region should include measurements of bioavailable metals in order to provide further details on the seasonal trends in environmental impact.     

Fossil footprints from the Dakota Group (Cretaceous) John Martin Reservoir, Bent County, Colorado: New insights into the paleoecology of the Dinosaur Freeway

The John Martin Reservoir tracksites from the Dakota Group of Bent County, in southeastern Colorado form part of the mid Cretaceous Dinosaur Freeway characterized by abundant ornithopod footprints (Caririchnium). Over 350 tracks (331 Caririchnium, 1 Magnoavipes, 22 crocodiles, and a few pterosaurs) were discovered at 10 new tracksites. All tracks were found as natural casts, including Caririchnium trackways that were still in-situ, parallel and regularly spaced, suggesting gregarious behavior. Most crocodile tracks are swimming tracks that consist of three or four scratch marks also with parallel orientations in many cases. A few pterosaur tracks consist of pes footprints and scratch marks suggesting swimming or floating activity in shallow water. This is the first pterosaur evidence from the Dakota Group. The Caririchnium size structure from John Martin Reservoir is consistent with the size structure tendency of the whole Dinosaur Freeway, which shows larger track size in the north. This tendency could be interpreted as evidence for more than one ornithopod species spread out across the Dinosaur Freeway. The alternative, that there was one ornithopod species that migrated north and south seasonally is less likely. The John Martin paleoecosystem is interpreted from the track evidence as a well-vegetated coastal plain environment with many ornithopods and a few theropods on land, crocodiles in the water, and pterosaurs in the skies and on the water.     

Seasonal variations in the loosely sorbed phosphorus fraction of the sediment of a shallow and hypereutrophic lake

On the basic of selective extractions, loosely sorbed phosphorus (ADS-P) has been shown to constitute much of the total phosphorus in the P-rich near-surface sediments of Lake Søbygaard, Denmark. The concentrations of ADS-P are seasonally variable, ranging from 0.2 mg Pg^?1 DW in the winter to more than 2 mg Pg^?1 DW in the summer. The variations can be observed as deep as 10 cm into the sediment but are most pronounced in the upper few centimeters. During the summer, lake and pore water pH levels are very high, and photosynthetic activity causes elevation to pH 10–11 in the lake. Laboratory experiments demonstrated a strong association between ADS-P and high pore water pH. It is likely that Lake Søbygaard represents an extreme example of pH control on sediment/water phosphorus equilibria in which high concentrations of internal ADS-P contribute significantly to the total P load of the Lake.     

Top-down versus bottom-up limitation in benthic infaunal communities: Direct and indirect effects

Top-down effects of predators and bottom-up effects related to resource availability can be important in determining community structure and function through both direct and indirect processes. Their relative influence may vary among habitats. We examined the effects of nutrient enhancement and predation in southeastern North Carolina to determine relative effects on benthic macrofaunal communities. Short-term nutrient additions and predator exclusions were conducted in two estuaries to examine main and interactive effects on benthic microalgae and infauna. This experimental approach was complemented by comparisons of microalgal biomass, infaunal abundance and composition, predator abundance and predator exclusion among four estuarine systems that varied in background nutrient levels. In the short-term experiments, nutrient enhancement induced increased microalgal biomass but had limited effects on abundances or sizes of infauna. Predator exclusion increased the density of sedentary and near-surface dwelling fauna, but we did not observe interactions between predation and responses to nutrient additions as might be predicted from a simple cascade model. General patterns of abundance were explained to a larger extent by interannual and amongestuary pattems. These results indicate a lack of simple trophic cascade responses for this community over a short time scale and little evidence for local interactive effects. The lack of interactive effects may reflect the opportunistic nature of the dominant infaunal species and potentially different time and spatial scales for the effects of predation and resource controls.     

Rapid climate change in the Upper Palaeolithic: the record of charcoal conifer rings from the Gravettian site of Dolní Vĕstonice,Czech Republic

Precisely how Upper Palaeolithic human ecology was shaped by changing climate during the Pleniglacial remains a matter of debate, for while this generally cold period is now understood to include complex and often rapid flux in climate, there are still considerable difficulties in resolving climatic variations at particular times and places — derived from various lines of proxy evidence — with the high-resolution proxy record of temperature changes from oxygen isotope analysis of the Greenland ice-cores.In this paper we apply the methodology of large-scale flotation to newly excavated contexts from the Upper Palaeolithic (Gravettian) site of Dolní V?stonice II, Czech Republic, to explore the potential of charcoal — as a natural archive of environmental information — to offer information on environmental change towards the end of the middle pleniglacial during Oxygen Isotope Stage 3, between c. 32,500 and 31,200 Cal yr BP. The results of an analysis of ring widths and other anatomical features — interpreted alongside micromorphological data — indicate that this charcoal may capture a higher-resolution record of the changing climatic conditions during which humans were first expanding into these hitherto marginal ecologies and, consequently, shed new light upon the complexity of the lifeways that enabled them to do so.