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- Volume 24, Issue 4, 2018
Petroleum Geoscience - Volume 24, Issue 4, 2018
Volume 24, Issue 4, 2018
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Magmatism and extension rates at rifted margins
Authors Erik R. Lundin, Anthony G. Doré and Thomas F. RedfieldRifted margins are commonly classified as either magma-rich or magma-poor. Magma-poor margins are often implicitly related to ultraslow–slow extension. Conversely, therefore, magma-rich margins should represent more rapid extension. Although supported by numerical modelling, these relationships are based on limited data and depend on a perhaps spurious comparison between continental margins and spreading ridges. Three case studies from the Atlantic margins are therefore presented here as a local, by no means complete, examination of this concept.
Extension rates for magma-poor margins are mainly derived from offshore Iberia, while the best documented rates on magma-rich margins are probably those in the NE Atlantic. Particularly for the NE Atlantic, there is a dependence on the initial oceanic spreading rate as pre-break-up rates are very difficult to quantify. Our two southerly examples, the Central Atlantic and southern South Atlantic, are both magma-rich in parts and have been described as opening during ultraslow–slow plate separation. Both would therefore seem to contradict the positive ‘rate-magmatism’ correlation. However, on closer examination, a wide range of initial extension rates are actually possible. This is largely due to poor constraints on break-up ages. The assumption that break-up is synchronous with flood basalt extrusion is flawed, and may have caused initial extension rates to have been significantly underestimated. Additionally, averaging between widely spaced oceanic magnetic anomalies allows for a wide range of extension rates. New, well-constrained ages and event chronologies from critical areas of conjugate margins are needed to determine whether this relationship has global validity.
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The thermal structure of volcanic passive margins
Authors John J. Armitage and Jenny S. CollierOver the past ten years we have numerically modelled the properties of the magmatism generated at four of the key areas where the ‘mantle plume–volcanic margin hypothesis’ is expected to be valid: the North Atlantic, South Atlantic, India–Seychelles and Afar. Our model incorporates many of the original assumptions in the classic White and McKenzie model, with pure shear of the lithospheric mantle, passive upwelling and decompressional melting. Our model is however two- rather than one-dimensional, can capture the rift history (extension rate changes and axis jumps) and tracks mantle depletion during melting. In all four of our study areas we require the sub-lithospheric mantle to be 100 – 200°C hotter than ‘normal’, non-volcanic margins to explain the characteristics of the magmatism. In the three passive margin cases we find this excess temperature is limited to a 50 – 100 km thick layer. We require this layer temperature to drop along-strike away from the proposed sites of plume impact at the base of the lithosphere. However, we also find that lithospheric thickness and rift history are as important as temperature for controlling the magmatism. Our work therefore lends support to the hypothesis that the excess magmatism at volcanic margins is due to a thermal anomaly in the asthenosphere, albeit with consideration of extra parameters.
Supplementary material: Numerical model description is available at https://doi.org/10.6084/m9.figshare.c.3924505
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Crustal architecture of the Laptev Rift System in the East Siberian Arctic based on 2D long-offset seismic profiles and gravity modelling
The Laptev Shelf in the East Siberian Arctic represents a rare tectonic setting where an active oceanic spreading centre, the Gakkel Ridge, intersects a continental margin. The North America–Eurasia plate boundary follows the Gakkel Ridge and passes into a continental shelf; this has resulted in the development of a wide rift system that has been active since the Late Cretaceous. The new long-offset seismic profiles provide a reliable basis for deciphering the structural characteristics of this rift system. We use two new seismic profiles, along with one acquired in the 1990s, to examine the crustal architecture of the rift system. Our approach combines seismic interpretation, time to depth conversion of seismic profiles and 2D gravity forward modelling. The obtained results indicate the presence of hyperextended continental crust beneath the Ust' Lena Rift Basin and exhumed continental mantle at the base of the syn-rift succession along the rift axis. The upper crust was removed by brittle stretching, while the lower crust experienced extreme ductile thinning. Our results show that continental crust can be eliminated in the course of rifting without a considerable heat input from asthenospheric mantle.
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Shift of a rift by a transform zone: a case from the Northern Rift Zone and Tjörnes Fracture Zone of Iceland
More LessTransform zones present fascinating geometry and tectonic deformation, some of which can be confusing, especially, when rift and transform zones interact, and rift structures are displaced by transform fractures. A case study from north Iceland includes the transform zone of Tjörnes (the Tjörnes Fracture Zone) and the Theistareykir Fissure Swarm of the Northern Rift Zone. This fissure swarm contains two north-trending central graben that are apparently shifted in a sinistral motion across a WNW-trending fault of the transform zone (the Stórihver Fault). The present analysis takes into account the number of faults and their relative displacement as criteria, and demonstrates that the configuration of the north-trending rift structures is compatible with the dextral slip on this WNW blind leaky fault of the transform zone. The dextral motion along the WNW fault explains more coherently the greater number and greater throw of the north-trending normal faults in two of the blocks on the shoulder of the graben, as well as with the formation of the central graben themselves. A dextral motion is also compatible with the regional tectonics and the mechanism of transform faulting. The apparent displacement of tectonic structures at diverging plate boundaries can be misleading. Therefore, identifying the correct sense of motion is essential for exploration and structural drilling targets where rift and transform of any age interact.
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Impact of rift dynamics on palaeoenvironmental conditions and hydrocarbon system development (northern Upper Rhine Graben, SW Germany)
Authors M. Perner, H. Jäger, C. Reinhold, T. Bechstädt and W. StinnesbeckThe Upper Rhine Graben (URG), a classical hydrocarbon (HC) province, is part of the European Cenozoic Rift System. The rift graben development has led to a complex basin fill of terrestrial and marine Cenozoic deposits, providing several HC source and reservoir units. The aim of this study is the multidisciplinary analysis of the palaeoenvironmental conditions and source-rock development within the transgressive marine intervals, and the palaeothermal history of the graben system to improve the understanding of the HC system development. Palaeoenvironmental conditions are strongly influenced by rift-related tectonic activity. Transgressive marine intervals in times of major subsidence show high terrestrial input from the graben shoulders, leading to mainly terrigenous gas-prone kerogen, while transgressive marine intervals during weak tectonic activity are dominated by marine–brackish palaeoenvironments and oil-prone kerogen. This differs clearly from the previously suggested HC potential of these intervals. Thermal maturation analysis shows nearly constant maturation with depth, which is atypical for burial-controlled maturation. It indicates significant secondary thermal overprinting related to long-lasting very hot hydrothermal fluid systems, concentrated along fault zones. Therefore the development of the depositional setting, kerogen composition, thermal maturation and the HC potential is directly linked to the dynamics of the rift system development.
Supplementary material: detailed results of all three wells are available at https://doi.org/10.6084/m9.figshare.c.4183106
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Improved quantification of the porosity–permeability relationship of limestones using petrographical texture
Authors Shuo Zhang, Peng Lu, Dave Cantrell, Yan Zaretskiy, Dawn Jobe and Susan M. AgarThe relationship between porosity and permeability in limestones is a fundamental constitutive equation in subsurface fluid flow modelling, and is essential in quantifying a range of geological processes. For a given porosity, the permeability of limestones varies over a range of up to five orders of magnitude. Permeability of a given rock sample depends on the total amount of pore space, characterized by porosity, as well as how the pore space is distributed within the rock, which can be expressed as a probability density function of pore sizes. We investigate in this study whether the information about pore-size distribution can be sufficiently captured by the bulk petrographical properties extracted from thin sections. We demonstrate that most of the uncertainty can be explained by variations in texture, which is defined by the mud content (mass fraction of particles less than 0.06 mm in diameter). Using mud content as a quantitative texture descriptor, we used multivariable regression and neural network models to predict permeability from porosity. For a given porosity, inclusion of mud content reduces the uncertainty in permeability prediction from five to two orders of magnitude.
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Basement–cover reservoir analogue in rift-margin fault blocks; Gulf of Suez Rift, Sinai, Egypt
Authors A. Braathen, M. M. Abdel Fattah, S. Olaussen, G. Abdel-Gawad and K. OgataFaults, fracture systems, weathering profiles and cover sediments of granitic basement in the rift shoulder of the Gulf of Suez Rift (Sinai, Egypt) are useful conceptual analogues for basement–cover reservoir fields. Outcrops demonstrate that fracture intensity peaks adjacent to major faults, and declines in damage zones that stretch to the background fracturing level over distances of 150 m. In the rift shoulder, smaller faults have damage zones that are 30 – 40 m wide with 1 – 5 m-wide fracture corridors. Faults show chemical alteration extending hundreds of metres into basement, with characteristics similar to saprolite in schists, mafic rocks and granitoids. Cover sandstones fill and drape top-basement relief, as recorded by metre-thick basal fluvial coarse sandstones, hosting kaolinite both as diagenetic pore fill and clastic grains. Overlying floodplain to marginal-marine deposits consist of mature quartz arenites.
Strategies for production of hydrocarbons or groundwater from basement–cover reservoir couplets should consider a layered system with: (i) deep tight basement of minimal porosity (c. 1%) hosting producible fractures and faults in a plumbed system with potential thief zones; (ii) top-basement weathering profiles capping granitoids representing a volumetrically considerable reservoir; and (iii) draping cover sandstones showing good reservoir properties, and representing the most homogenous unit. Diagenetic modifications of saprolite, fault rocks and fractures potentially baffle recharge between layers.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)