Characterising the vertical separation of shale-gas source rocks and aquifers across England and Wales (UK)

Shale gas is considered by many to have the potential to provide the UK with greater energy security, economic growth and jobs. However, development of a shale gas industry is highly contentious due to environmental concerns including the risk of groundwater pollution. Evidence suggests that the vertical separation between exploited shale units and aquifers is an important factor in the risk to groundwater from shale gas exploitation. A methodology is presented to assess the vertical separation between different pairs of aquifers and shales that are present across England and Wales. The application of the method is then demonstrated for two of these pairs—the Cretaceous Chalk Group aquifer and the Upper Jurassic Kimmeridge Clay Formation, and the Triassic sandstone aquifer and the Carboniferous Bowland Shale Formation. Challenges in defining what might be considered criteria for ‘safe separation’ between a shale gas formation and an overlying aquifer are discussed, in particular with respect to uncertainties in geological properties, aquifer extents and determination of socially acceptable risk levels. Modelled vertical separations suggest that the risk of aquifer contamination from shale exploration will vary greatly between shale–aquifer pairs and between regions and this will need to be considered carefully as part of the risk assessment and management for any shale gas development.     

The role of social capital,personal networks,and emergency responders in post-disaster recovery and resilience: a study of rural communities in Indiana

The factors that explain the speed of recovery after disaster remain contested. While many have argued that physical infrastructure, social capital, and disaster damage influence the arc of recovery, empirical studies that test these various factors within a unified modeling framework are few. We conducted a mail survey to collect data on household recovery in four small towns in southern Indiana that were hit by deadly tornadoes in March 2012. The recovery effort is ongoing; while many of the homes, businesses, and community facilities were rebuilt in 2013, some are still under construction. We investigate how households in these communities are recovering from damage that they experienced and the role of social capital, personal networks, and assistance from emergency responders on the overall recovery experience. We used an ordered probit modeling framework to test the combined as well as relative effects of (a) damage to physical infrastructures (houses, vehicles, etc.); (b) recovery assistance from emergency responders (FEMA) as well as friends and neighbors; (c) personal network characteristics (size, network density, proximity, length of relationship); (d) social capital (civic engagement, contact with neighbors, trust); and (e) household characteristics. Results show that while households with higher levels of damage experienced slower recovery, those with recovery assistance from neighbors, stronger personal networks, and higher levels of social capital experienced faster recovery. The insights gained in this study will enable emergency managers and disaster response personnel to implement targeted strategies in facilitating post-disaster recovery and community resilience.     

Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep,tectonically active margin,southern California,USA

Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection data over >570 km² of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment 5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80% of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation. Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits.     

Experimental formation of massive hydrate deposits from accumulation of CH4 gas bubbles within synthetic and natural sediments

In order for methane to be economically produced from the seafloor, prediction and detection of massive hydrate deposits will be necessary. In many cases, hydrate samples recovered from seafloor sediments appear as veins or nodules, suggesting that there are strong geologic controls on where hydrate is likely to accumulate. Experiments have been conducted examining massive hydrate accumulation from methane gas bubbles within natural and synthetic sediments in a large volume pressure vessel through temperature and pressure data, as well as visual observations. Observations of hydrate growth suggest that accumulation of gas bubbles within void spaces and at sediment interfaces likely results in the formation of massive hydrate deposits. Methane hydrate was first observed as a thin film forming at the gas/water interface of methane bubbles trapped within sediment void spaces. As bubbles accumulated, massive hydrate growth occurred. These experiments suggest that in systems containing free methane gas, bubble pathways and accumulation points likely control the location and habit of massive hydrate deposits.     

Earth''s earliest continental lithosphere, hydrothermal flux and crustal recycling

The Kaapvaal craton in southern Africa and the Pilbara craton of northwestern Australia are the largest regions on Earth to have retained relatively pristine mid-Archaean rocks (3.0–4.0 Ga).The Kaapvaal craton covers about 1.2×10⁶ km², and varies in lithospheric thickness between 170 and 350 km. At surface, the craton can be subdivided into a number of Archaean sub-domains; some of the subdomains are also well defined at depth, and local variations in tomography of the lithosphere correspond closely with subdomain boundaries at surface.The Archaean history of the Kaapvaal craton spans about 1 Gyr and can be conveniently subdivided into two periods, each of about the same length as the Phanerozoic. The first period, from circa 3.7-3.1 Ga, records the initial separation of the cratonic lithosphere from the asthenosphere, terminating with a major pulse of accretion tectonics between 3.2 and 3.1 Ga, which includes the formation of “paired metamorphic belts”. This period of continental growth can be compared to plate tectonic processes occurring in modern-day oceanic basins. However, the difference is that in the mid-Archaean, these oceanic processes appear to have occurred in shallower water depths than the modern ocean basins. The second period, from circa 3.1-2.6 Ga, records intra-continental and continental-edge processes: continental growth during this period occurred predominantly through a combination of tectonic accretion of crustal fragments and subduction-related igneous processes, in much the same way as has been documented along the margins of the Pacific and Tethys oceans since the Mesozoic.The intra-oceanic processes resulted in small, but deep-rooted continental nucleii; the first separation of this early continental lithosphere could only have occurred when the mean elevation of mid-oceanicridges sank below sea-level. Substantial recycling of continental lithosphere into the mantle must have occurred during this period of Earth history. During the second period, at least two large continental nucleii amalgamated during collisional processes which, together with internal chemical differentiation processes, created the first stable continental landmass. This landmass, which is known to have been substantially bigger than its present outline, may have been part of the Earth's first supercontinent.The oldest known subdomains of the craton include the oceanic-like rocks of the Barberton greenstone belt. The comagmatic mafic-ultramafic rocks (3.48–3.49 Ga) of this belt represent a remnant of very early oceanic-like lithosphere (known as the Jamestown Ophiolite Complex), which was obducted, approximately 45 Ma after its formation, onto a volcanic arc-like terrain by processes similar to those which have emplaced modern ophiolites at convergent margins of Phanerozoic continents. The early metamorphic history, metamorphic mineralogy, oxygen isotope profiles and degree of hydration of the 3.49 Ga Jamestown Ophiolite Complex are similar to present day subseafloor hydrothermal systems. The ratio of ΔMg to ΔSi for hydrothermally altered igneous rocks, both present day and Archaean, are remarkably uniform at −5(±0.9) and the same as that of hydrothermal fluids venting on the present-day East Pacific Rise. This observation suggests that the process of Mg exchange for Si in hydrothermal systems was commonplace throughout Earth's history.The chemistry of vent fluids and hydrothermally altered igneous rocks was combined with an inventory of ³He in the mantle to model Earth's total hydrothermal flux. An Archaean flux (at 3.5 Ga) of about 10 times present day was accompanied by a correspondingly greater abundance of Mg(OH), SiO₂, carbonate and Fe---Mn metasomatic rock types as well as massive sulphides. Assuming a constant column of seawater since the Archaean, the average residence time of seawater in the oceanic crust was 1.65−8.90×10⁵ years in the Archaean. Assuming that ³He and heat are transported from the mantle in silicate melts in uniform proportions, the model stipulates that accretion of oceanic crust decreased from about 3.43−6.5×10¹⁷ g/yr to a present-day rate of 0.52−0.8×10¹⁷ g/yr, with a drop in heat flow from 1.4−2.6×10²⁰ cal/yr to 2.1−3.2×10¹⁹ cal/year.The total amounts of SiO₂ and Fe mobilised in marine hydrothermal systems since 3.5 Ga is less than their masses in the present exosphere reservoirs (crust, hydrosphere, atmosphere). The total amounts of Mg, K, CO₂, Ca and Mn are greater than their respective masses in exosphere reservoirs; therefore, they must have been recycled into mantle. The total mass of recycled hydrothermal components is small compared to the mass of the mantle. The flux of volatiles in hydrothermal systems is large compared to their volume in the atmosphere suggesting that the CO₂ and O₂ budgets of the atmosphere have been influenced by hydrothermal processes, especially in the Archaean.     

He diffusion in olivine

Helium diffusion in olivine (dunite xenolith) has been measured in the temperature range 1180–1460°C; a linear Arrhenius function was obtained with an activation energy of 120₋₂₇⁺³² kcal/mole, and a pre-exponential factor (D₀) = 2.2 × 10⁸ cm²/s.

Diffusion mechanisms are not a viable means of degassing He from the mantle. Olivine phenocrysts can be expected to retain previously trapped He, during cooling in extrusive basalts, provided the flow units are thinner than 50 m; xenoliths will retain mantle He signatures only if magma transport times are less than 50 years, or if the He fugacity in the magma is high enough to prevent xenolith degassing. The lower oceanic crust is probably substantially degassed of He. Trapped He will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 × 10¹⁴°C/m.y.; glass at several centimeters depth in a basalt flow (near the spherulite zone) will have cooling rates lower than this, so He loss may be significant in many basalt glass samples.     

Ocean crust vein mineral deposition: Rb/Sr ages, U-Th-Pb geochemistry, and duration of circulation at DSDP sites 261, 462 and 516

Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (site 261—121.4 ±1.6 m.y.; site 462A—105.1 ±2.8 m.y.; site 516F—69.9 ±2.4 m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10–15 m.y.), or were reactivated by off-ridge igneous activity.

Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust.

Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20–200.     

Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data,I. Theoretical methods

While the chemical structure of the earth's mantle is probably rather complex, multi-box models have been used as a first approximation to evaluate this structure. Most commonly, a three-box model is used, involving the continental crust, the upper mantle and the lower mantle. The depleted upper mantle and the continental crust are assumed to represe1nt complementary reservoirs, related by crust formation processes occurring during geologic history.Here we investigate the Rb/Sr and Sm/Nd isotopic systematics of several three-box models, using mass balance equations and the definition of the mean age of the reservoirs. The geochemical uniqueness of the models, chosen from a large family of possible models, is evaluated from elementary graph theory, and these models are then solved using a total inversion approach. This paper (Part I) describes the methodology of the procedure; the companion paper (Part II) discusses the application of this approach to multi-box mantle models.