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  • 1
    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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  • 2
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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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  • 3
    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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  • 4
    Publication Date: 2011-12-01
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 5
    Publication Date: 2009-08-01
    Print ISSN: 0264-8172
    Electronic ISSN: 1873-4073
    Topics: Geosciences
    Published by Elsevier
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