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Vissers R.L.M.,University Utrecht | van Hinsbergen D.J.J.,University Utrecht | van der Meer D.G.,University Utrecht | van der Meer D.G.,Nexen Petroleum UK Ltd | And 2 more authors.
Gondwana Research | Year: 2016

The Pyrenees at the Iberia-Europe collision zone contain sediments showing Albian-Cenomanian high-temperature metamorphism, and coeval alkaline magmatic rocks. Stemming from different views on Jurassic-Cretaceous Iberian microplate kinematics, two schools of thought exist on the trigger of this thermal pulse: one invoking hyperextension of the Iberian and Eurasian margins, the other suggesting slab break-off. Competing scenarios for Mesozoic Iberian motion compatible with Pyrenean geology, comprise (1) transtensional eastward motion of Iberia versus Eurasia, or (2) strike-slip motion followed by orthogonal extension, both favoring hyperextension-related heating, and (3) scissor-style opening of the Bay of Biscay coupled with subduction in the Pyrenean realm, favoring the slab break-off hypothesis. We test these kinematic scenarios for Iberia against a newly compiled paleomagnetic dataset and conclude that the scissor-type scenario is the only one consistent with a well-defined ~. 35° counterclockwise rotation of Iberia during the Early Aptian. We proceed to show that when taking absolute plate motions into account, Aptian oceanic subduction in the Pyrenees followed by Late Aptian-Early Albian slab break-off should leave a slab remnant in the present-day mid-mantle below NW Africa. Mantle tomography shows the Reggane anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below southern Algeria. Mantle tomography is therefore consistent with the scissor-type opening of the Bay of Biscay coupled with subduction in the Pyrenean realm. Slab break-off may thus explain high-temperature metamorphism and alkaline magmatism during the Albian-Cenomanian in the Pyrenees, whereas hyperextension that exhumed Pyrenean mantle bodies occurred much earlier, in the Jurassic. © 2016 The Authors. Source

Biggin A.J.,University of Liverpool | Steinberger B.,Helmholtz Center Potsdam | Steinberger B.,University of Oslo | Aubert J.,CNRS Paris Institute of Global Physics | And 8 more authors.
Nature Geoscience | Year: 2012

The Earth's internal magnetic field varies on timescales of months to billions of years. The field is generated by convection in the liquid outer core, which in turn is influenced by the heat flowing from the core into the base of the overlying mantle. Much of the magnetic field's variation is thought to be stochastic, but over very long timescales, this variability may be related to changes in heat flow associated with mantle convection processes. Over the past 500 Myr, correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late Mesozoic era, beginning about 180 Myr ago. Simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability - such as occurred between the Middle Jurassic and Middle Cretaceous periods - may have been triggered by a decrease in core-mantle boundary heat flow either globally or in equatorial regions. This decrease in heat flow could have been linked to reduced mantle-plume-head production at the core-mantle boundary, an episode of true polar wander, or a combination of the two. © 2012 Macmillan Publishers Limited. All rights reserved. Source

Van Der Meer D.G.,University Utrecht | Van Der Meer D.G.,Nexen Petroleum UK Ltd | Torsvik T.H.,University of Oslo | Torsvik T.H.,Geological Survey of Norway | And 7 more authors.
Nature Geoscience | Year: 2012

The vast Panthalassa Ocean once surrounded the supercontinent Pangaea. Subduction has since consumed most of the oceanic plates that formed the ocean floor, so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean's history since the Cretaceous period 1,2, and the Triassic-Jurassic plate tectonic evolution of the Panthalassa Ocean remains largely unresolved 3,4. Geological clues come from extinct intra-oceanic volcanic arcs that formed above ancient subduction zones, but have since been accreted to the North American and Asian continental margins 4. Here we compile data on the composition, the timing of formation and accretion, and the present-day locations of these volcanic arcs and show that intra-oceanic subduction zones must have once been situated in a central Panthalassa location in our plate tectonic reconstructions 5-7. To constrain the palaeoposition of the extinct arcs, we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic-wave tomographic models 8,9. We suggest that a series of subduction zones, together called Telkhinia, may have defined two separate palaeo-oceanic plate systems - the Pontus and Thalassa oceans. Our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the Telkhinia subduction zones and Panthalassa Ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics. © 2012 Macmillan Publishers Limited. All rights reserved. Source

Castillo H.,Baker Hughes Inc. | Scott G.,Nexen Petroleum UK Ltd | Wills P.,PW Inc Ltd
Society of Petroleum Engineers - International Petroleum Technology Conference 2014, IPTC 2014: Unlocking Energy Through Innovation, Technology and Capability | Year: 2014

This paper documents the outcome of an eight-year field study that confirmed how the consistent use of innovative conveyance methods and technologies significantly improved the success rate of fluid sampling programs while reducing the overall cost of the E&P projects in long-reach challenging wells part of the largest development project undertaken in the last 25 years in the UK, North Sea sector. The formation properties encountered by these wells along their trajectories, including long deviated sections, posed massive challenges to the acquisition of critical formation evaluation (FE) data, core samples and representative formation fluid-samples required in these wells. In the early stages of this field development, the most significant risks encountered were differential sticking of the wireline-conveyed sampling tools and differential sticking of the wireline in very permeable reservoir rocks. The direct consequence of a stuck sampling tool is the need to fish the sampling tool first and then recover the required fluid samples using pipe-assisted wireline conveyance methods, a sequence that typically takes five additional days. The additional cost to the Project Partners is $4.0 million, based on five days rig spread costs plus the deferred production revenue losses resulting from the five-day production delay of all development wells not yet drilled/completed from the same installation. During the first eight fluid sampling operations in the development phase of this field the fishing rate was as high as 25%. The development and adoption of modern methods and technologies to manage the conveyance risks identified when performing coaxial packer sampling in 2012 resulted in no wireline fishing jobs (0% fishing rate) and no pipe conveyed logging (PCL) operations required (0% PCL rate) on these UK North Sea projects. The structure of this paper follows: 1. Overview of the oil fields and the development programs subject of this paper 2. Characterization of the conveyance hazards present in the wells where the wireline logging jobs subject of these paper were executed 3. Detailed description of each of the key wireline risk-reduction technologies used through the duration of these field developments and the roles they played on the success of the wireline logging jobs 4. Conclusions and recommendations for similar field developments on wireline risk mitigation Copyright 2014, International Petroleum Technology Conference. Source

Adams A.J.,Nexen Petroleum UK Ltd
SPE Projects, Facilities and Construction | Year: 2011

For more than 30 years, the design of platform and jackup conductors has been based on Stahl and Baur's famous hypothesis that internal string loads do not contribute to buckling (Stahl and Baur 1983). It is a vital result, allowing significant weight and cost savings. However, no derivation was ever given, and the result has remained a folk theorem: widely used, but never proved. The industry has, therefore, been at risk should the result prove to be a severe approximation, or to have unduly restrictive assumptions and/or limitations. This paper provides a rigorous proof of the hypothesis. It shows that it is an approximation, though an acceptable one, and gives a thorough exposition of its meaning, assumptions, and limitations. Finally, it derives the exact counterpart of Stahl and Baurs' result. The improved result gives minor weight and cost savings. Copyright © 2011 Society of Petroleum Engineers. Source

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