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  • 1
    Publication Date: 2003-01-01
    Description: During a regional seismic interpretation study of leakage anomalies in the northern North Sea, mounds and zones with a highly chaotic seismic reflection pattern in the Tertiary Hordaland Group were repeatedly observed located above gas chimneys in the Cretaceous succession. The chaotic seismic reflection pattern was interpreted as mobilized sediments. These mud diapirs are large and massive, the largest being 100 km long and 40 km wide. Vertical injections of gas, oil and formation water are interpreted to have triggered the diapirs. On the eastern side of the Viking Graben, another much smaller type of mud diapir was observed. These near-circular mud diapirs are typically 1-3 km in diameter in the horizontal plane. Limited fluid injection from intra-Hordaland Group sands, through sand injection zones, into the upper Hordaland Group is interpreted to have triggered the near-circular diapirs. This observed external' type of mobilization was generated at shallow burial (〈1000 m) and should be discriminated from the more common internal' type of mud diapirism that is generated in deep basins (〉3000 m). The suggested model has implications for the understanding of the palaeofluid system, sand distribution, stratigraphic prediction within the chaotic zone, seismic imaging, and seismic interpretation of the hydrocarbon plumbing' system.
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  • 2
    Publication Date: 2011-12-01
    Description: Hydrocarbon source rocks contain significant volumes of organic matter, are capable of expelling petroleum when heated, and have produced most of the world's known oil volumes. Recently, source rocks have also become recognized as unconventional economic reservoirs. Here we present a new way of identifying, characterizing, and mapping spatial distributions and variations of thick source rocks (〉20 m) that is based on seismic data only. This has a significant impact on the prospect risk assessment of petroleum plays. Rock property studies of organic-rich claystones show that the acoustic impedance (AI), which is the product of compressional velocity and density, decreases nonlinearly with increasing total organic carbon (TOC) percent. Claystones mixed with low-density organic matter (TOC 〉 3%-4%) have significant lower AI and higher intrinsic anisotropy than otherwise similar nonorganic claystones. This gives the top and base source rock reflections characteristic negative and positive high amplitudes, respectively, which dim with increasing reflection angle. In addition, the TOC profile, which is a smoothed TOC percent curve, influences the top and base amplitude responses. An upward-increasing TOC profile has the highest amplitude at the top, while the opposite asymmetry is observed for downward-increasing TOC profiles. By using seismic data, we therefore can map lateral distribution, thickness, variation in TOC profiles, and, with local well calibration, convert AI data to TOC percent. This approach to mapping source rocks may change the way petroleum systems are evaluated.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 3
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    American Association of Petroleum Geologists (AAPG)
    Publication Date: 2011-05-01
    Description: Seismic images of Upper Jurassic organic-rich siliciclastic rocks are studied along the 2000-km (1243-mi) Norwegian margin. These rocks are considered the main source rock for most of the large oil and gas fields in the North Sea and Norwegian Sea. We report characteristic seismic expressions of thin-skinned gravitational gliding structures that are strata bound to the organic-rich formations. The most characteristic structures are listric faults that offset and rotate the upper part and sole out near the base of the organic-rich zone. These may be present in large areas (10,000 km2 [3861 mi2]), but are usually restricted to tilted areas. The strike of the faults was perpendicular to the downdip direction of the movement of mass, and fault directions can therefore be used as paleodip indicators. Several types of contraction structures are observed, and all are formed at a maximum of a few hundred meters of burial. Although they are not limited to organic-rich shales, such strata-bound structures may help identify organic-rich intervals in basins where their presence is unknown. We suggest that adding organic material to clay leads to reduced permeability. Compaction-related vertical fluid flow may cause fluid overpressure to build up at the base of a less permeable organic-rich layer during early burial. This high fluid pressure zone becomes a low-friction decollement surface on which overlying sediments slide to form characteristic thin-skinned deformation structures.
    Print ISSN: 0149-1423
    Electronic ISSN: 0149-1423
    Topics: Geosciences
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  • 4
    Publication Date: 2011-05-01
    Description: Seismic images of Upper Jurassic organic-rich siliciclastic rocks are studied along the 2000-km (1243-mi) Norwegian margin. These rocks are considered the main source rock for most of the large oil and gas fields in the North Sea and Norwegian Sea. We report characteristic seismic expressions of thin-skinned gravitational gliding structures that are strata bound to the organic-rich formations. The most characteristic structures are listric faults that offset and rotate the upper part and sole out near the base of the organic-rich zone. These may be present in large areas (10,000 km2 [3861 mi2]), but are usually restricted to tilted areas. The strike of the faults was perpendicular to the downdip direction of the movement of mass, and fault directions can therefore be used as paleodip indicators. Several types of contraction structures are observed, and all are formed at a maximum of a few hundred meters of burial. Although they are not limited to organic-rich shales, such strata-bound structures may help identify organic-rich intervals in basins where their presence is unknown. We suggest that adding organic material to clay leads to reduced permeability. Compaction-related vertical fluid flow may cause fluid overpressure to build up at the base of a less permeable organic-rich layer during early burial. This high fluid pressure zone becomes a low-friction decollement surface on which overlying sediments slide to form characteristic thin-skinned deformation structures. Helge Løseth received his candidate scientific degree in structural geology from the University of Bergen, Norway, in 1985. He worked for Total Norsk A/S and SINTEF (Stiftelsen for Industriell og Teknisk Forskning) Petroleum Research before joining Statoil in 1997. In his present position as a specialist in structural geology, he works with petroleum exploration research on seismic interpretation of hydrocarbon leakage, sand injectites, and organic-rich claystones. Lars Wensaas received his candidate scientific degree in sedimentology from the University of Oslo, Norway, in 1987. He joined Statoil in 1994, where he is currently working as a principal researcher at the Statoil Research Centre in Trondheim, Norway. His research efforts have focused on rock physics, pore-fluid pressure, and sealing properties of argillaceous rocks in petroleum exploration. Marita Gading received her Ph.D. in seismic stratigraphy from NTNU (Norges Teknisk-Naturvitenskapelige Universitet, also Norwegian University of Science and Tecnology), Trondheim, Norway, in 1994. She worked with seismic interpretation in regional geologic projects at SINTEF (Stiftelsen for Industriell og Teknisk Forskning) Petroleum Research from 1992 until 1997 when she joined Statoil and is currently working as a specialist in geophysics, at the Research Centre, Trondheim, focusing on rock properties and seismic interpretation of organic-rich rocks.
    Print ISSN: 0149-1423
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 5
    Publication Date: 2003-01-01
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
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  • 6
    Publication Date: 2011-12-01
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 7
    Publication Date: 2011-05-01
    Print ISSN: 0264-8172
    Electronic ISSN: 1873-4073
    Topics: Geosciences
    Published by Elsevier
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  • 8
    Publication Date: 2009-08-01
    Print ISSN: 0264-8172
    Electronic ISSN: 1873-4073
    Topics: Geosciences
    Published by Elsevier
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  • 9
    Publication Date: 2007-01-01
    Description: Methods for detection of pore fluid overpressures in shales from seismic data have become widespread in the oil industry. Such methods are largely based on the identification of anomalous seismic velocities, and on subsequent determination of pore pressures through relationships between seismic velocities and the vertical effective stress (VES). Although it is well known that lithology variations and compaction mechanisms should be accounted for in pore pressure evaluation, a systematic approach to evaluation of these factors in seismic pore pressure prediction seems to be absent. We have investigated the influence of lithology variations and compaction mechanism on shale velocities from acoustic logs. This was performed by analyses of 80 wells from the northern North Sea and 24 wells from the Haltenbanken area. The analyses involved identification of large-scale density and velocity variations that were unrelated to overpressure variations, which served as a basis for the analyses of the resolution of overpressure variations from well log data. The analyses demonstrated that the overpressures in neither area were associated with compaction disequilibrium. A significant correlation between acoustic velocity and fluid overpressure nevertheless exists in the Haltenbanken data, whereas the correlation between these two parameters is weak to non-existing in the North Sea shales. We do not presently know why acoustic velocities in the two areas respond differently to fluid overpressuring. Smectitic rocks often have low permeabilities, and define the top of overpressures in the northern North Sea when they are buried below 2km. As smectitic rocks are characterized by low densities and low acoustic velocities, their presence may be identified from seismic data. Smectite identification from seismic data may thus serve as an indirect overpressure indicator in some areas. Our investigations demonstrate the importance of including geological work and process understanding in pore pressure evaluation work. As a response to the lack of documented practice within this area, we suggest a workflow for geological analyses that should be performed and integrated with seismic pore pressure prediction. © 2007 Blackwell Publishing Ltd.
    Print ISSN: 0950-091X
    Electronic ISSN: 1365-2117
    Topics: Geosciences
    Published by Wiley
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  • 10
    Publication Date: 2007-09-01
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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