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Now showing items 1 - 3 of 3

  • Why is Africa rifting?

    Kendall, J.-Michael   Lithgow-Bertelloni, Carolina  

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  • Subsurface fluid injection and induced seismicity in southeast Saskatchewan

    Verdon, James P.   Kendall, J.-Michael   Horleston, Anna C.   Stork, Anna L.  

    In order to mitigate CO2 emissions while continuing to use fossil fuels as an energy source, CO2 emissions from large point sources such as power stations can be captured and stored in suitable subsurface sedimentary formations. However, concerns have been raised that the injection of pressurized CO2 may alter the subsurface stress state, leading to the re-activation of faults and generating induced seismic activity. Southeast Saskatchewan has seen extensive oil and gas activity since the 1950s. This activity includes, in recent years, a boom in shale oil production entailing hydraulic fracturing. It is also home to two world-leading CCS projects, the Weyburn-Midale CO2 Monitoring and Storage Project, and the Boundary Dam/Aquistore Project. The aim of this paper is to assess whether any of the conventional oilfield operations, shale oil activity or CCS has caused induced seismicity in southeast Saskatchewan. We find that the region has a very low rate of natural seismicity, and that there is no evidence to suggest that any kind of oilfield activity has caused induced events. However, seismicity has been associated with potash mining activities in the region. It is not clear whether the potash mining-induced events are triggered by subsidence above the mined zones, or by re-injection of waste brines. It is of interest to compare the situation in southeast Saskatchewan with other areas that have seen substantial increases in the amount of injection-induced seismic activity. It is notable that in many areas that have seen injection-induced seismicity, fluid injection is into basal aquifers that are hydraulically connected to the crystalline Precambrian basement. In contrast, most oilfield activities in southeast Saskatchewan are in Carboniferous formations, while the only units to have experienced a net volume increase are of Cretaceous age. It is tentatively suggested that the lack of induced seismic activity is due to the fact that injection is hydraulically isolated from the basement rocks, although it is also possible that stress conditions in the region are less conducive to induced seismicity. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Influence of convergent plate boundaries on upper mantle flow and implications for seismic anisotropy

    Lowman, Julian P.   Pinero-Feliciangeli, Laura T.   Kendall, J.-Michael   Shahnas, M. Hosein  

    Shear-wave splitting observations in the region of the upper mantle enveloping subduction zones have been interpreted as showing extensive regions of trench-parallel flow, despite the difficulty of reconciling such behavior with a sound model based on the forces that drive mantle motion. To gain insight into the observations, we systematically investigate flow patterns around the cold downwelling sheets associated with consumed plate material in a three-dimensional numerical mantle convection model. First, we compare results from calculations employing prescribed plate geometries and kinematic plate velocities where the convergent plate boundary morphology is varied while keeping the plate velocity and convective parameters fixed. Subsequently, we examine the flow around sheet-like downwellings in a number of convection calculations featuring dynamically evolving plate velocities. All of the calculations include thick viscous plates and a stratified mantle viscosity. In all of the models examined, we find that at mid-upper mantle depths, flow directions no longer align with plate motion and the influence of buoyancy-driven downwellings clearly dominates flow solutions. In the first models analyzed, a pair of plates are included in the calculations, and the large-scale flow is generally roll-like. In the final model we investigate the interaction of four plates and a plate geometry characterized by triple junctions. We examine a sequence from this calculation that features a triple junction of convergent boundaries. In this model, large-scale flow characterized by convection rolls is superseded by a complex flow solution where flow in the mid-upper mantle neither aligns uniformly with the plate motion nor necessarily follows the forcing associated with local buoyancy sources. In this setting, upper mantle flow in the vicinity of the sheet-like downwellings featured in the solution moves orthogonal, obliquely, and even parallel to different sections of the convergent plate boundaries. In the latter case our calculations of the deformation of a fixed volume parcel of upper mantle material suggest that an olivine lattice-preferred orientation should develop that would result in a fast polarizing direction for seismic shear waves parallel to the slab. Our findings have implications for the interpretation of flow in the upper mantle based on seismic anisotropy.
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