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58th EAGE Conference and Exhibition
- Conference date: 03 Jun 1996 - 07 Jun 1996
- Location: Amsterdam, Netherlands
- ISBN: 978-90-73781-07-8
- Published: 04 June 1996
41 - 60 of 604 results
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The impact of the 3D seismic reshoot on field development in the Greater Gannet Area, Central North Sea
Authors D. J. Davies, M. E. Minning, C. Grant and R. A. BealeAn improved geological subsurface image has been obtained in the greater Gannet/Guillemot area, as a result of the 1993 3D survey reshoot. This has resulted in: > Revised subsurface models in a number of fields > Revised well locations -avoiding side-tracks in areas of high uncertainty > Better definition of direct hydrocarbon indicators (dhi's) > Improved well-ties
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Fluid imaging using densely spaced OBS-data from the Vøring Basin
Authors E. W. Berg, L. Amundsen, A. Morten and R. MjeldeIn 1992 a comprehensive three component seismic OBS survey was performed in the central and northern part of the Verring Basin, offshore Norway. The data acquisition program consisted of three parts where the two regional parts provided the large basin coverage. The third part concerned a local OBS survey with 20 OBSs dropped at 1300 meters water depth along the seismic line VB-8-89. The distance between the OBSs was approximately 200 meters. The purpose of the local survey was to acquire densely spaced P-and S reflection OBS data above an observed seismic "flat spot" anomaly in order to more accurately predict if hydrocarbons could be related to the seismic anomaly.
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3D Ocean bottom cable project over the Abu Al Bukoosh Field
Authors D. Enns, J. -L. Piazza and Ph. PratA 3D dual sensor ocean bottom cable seismic survey was conducted over the ABU AL BUKHOOSH oil field to optimize the future development program which includes production wells, injection wells and enhanced oil recovery. A complete approach was taken that included a pre-survey feasibility study, processing of a 2D test line, fast track 3D cube and finally an independent full processing sequence.
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Seismic imaging in the presence of basalts wide-angle profiling in the Faroe Basin
Authors R. Masotti, P. J. Barton, K. R. Richardson and R. CameronThe frontier areas of the North Atlantic margins are overlain and interbedded with Tertiary volcanic rock's, which form a high impedance barrier that obscures deeper structure on conventionally acquired seismic profiles. However, wide angle energy, recorded by sea-bed seismometers to offsets of ten of kilometers, may be used to characterise and model structures under these acoustic barriers. Sediments trapped within or beneath basalts form a classic low velocity zone which cannot be resolved unambiguously using simplistic one-dimensional refraction analysis. However in reality these layers are always two-/three-dimensional in shape, and dense overlapping sampling with diving rays using a well designed wide-angle seismic survey allows the low velocity zone to be delineated. Wide-angle diving rays refract along curved paths through a velocity gradient, with energy at greater offset sampling progressively deeper into the Earth. A seismic phase generated by diving waves within a layer is typically continuous in offset until it is truncated abruptly at the offset corresponding to the ray grazing the base of the layer: this observation strongly constrains the seismic model. Thus the high velocity layers are detected directly, and a low velocity layer sandwiched between them may be modelled by defining its top as coincident with the base of the high velocity layer above, its base at the top of the layer below, and its velocity in terms of the delay experienced by diving rays travelling through the layer. A programme of wide-angle acquisition coupled with intensive development of data processing, analysis and inversion techniques has produced a revolution in wide-angle imaging during the past five years, so that it can truly be regarded as an exploration tool. With the high data density achieved by using tens of receivers and hundreds of sources, laborious forward modelling has been superseded by tomographic inversion, allowing the resolution of different parts of the model to be evaluated quantitatively.
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Lithology and compaction from a seismic velocity study in the Vøring Basin, Norwegian Sea
Authors S. Fanavoll, O. Hansen, H. Løseth and Ø. SyltaAs part of an extensive geological and geophysical study in the Voring Basin, a seismic velocity study was performed on 9 seismic lines evenly spread in the study area. The purpose of the study was twofold, to obtain information on compaction and/or lithology along the profiles, and to get a velocity database for depth conversion of digitised time sections. In this paper, we will present results from the lithology interpretation on three parallel, NW-SE running profiles, covering different parts of the Voring Basin.
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Subsurface image of geopressured rocks using seismic velocity and acoustic impedance inversion
Authors N. C. Dutta and A. RayA good understanding of subsurface effective stress and fluid pressure is essential at several stages of an exploration/development program. It can be used during exploration to assess the effectiveness of a regional topseal section, to provide a map of hydrocarbon migration pathways, and to analyze 'trap' configuration and geometry of a prospective basin. In the exploration and appraisal drilling and development phase, pressure prediction is a pre-requisite for safe and economic drilling. An optimized casing and mud-program design can avoid well control problems.
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Stratigraphic interpretation below salt from 3D PreSDM AVO
Authors R. O. Lindsay and D. W. RatcliffSubsalt lithology prediction is a rather all encompassing topic. Constraints can be placed upon the concept by limiting the breadth of the field to include only; stratigraphy, seismic amplitudes extracted from 3-D prestack depth migration (3-D PreSDM) and depth domain 3-D amplitude versus offset (AVO) -- a still none too small set of tasks.
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Can we separate the effects of anisotropy and structure from P-wave travel time data?
More LessThe kinematic features of wave propagation in a general medium are affected by the presence of anisotropy as well as the deviation of any reflectors from the horizontal. There are places where these structural variations are quite common (such as salt domes). The effects of anisotropy and structure are then coupled. Current processing [Muller (1991)] and analysing methods for seismic anisotropy often assume horizontal layers and use sites where the structural influence is minimal [Lewis (1991)].
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Pore space structure and anisotropy properties of small porosity cracked media
Authors B. P. Sibiriakov, S. B. Gorshkalev and W. V. KarstenMedia, containing cracks, have very special pore space structure. They generally have large specific surface even with very small porosity. It is known that small porosity cracked media are changing their P and S velocities much more than 1-ƒ times for porosity ƒ. Evidently there are different parameters of pore space that are responsible for decreasing P and S velocities. There are models considering isolated ellipsoidals and their aspect ratio is very important parameter of pore space. These models imply strict alignment of cracks. Determinining the aspect ratio is hard since the physical reason for the large axis of the ellibsoids is indefinite.
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A Resolution analysis for anisotropic inversion / migration
Authors M. V. de Hoop and C. SpencerWe consider spatial and parameter resolution aspects of G(eneralised) R(adon) T(ransform) / A(mplitude) Versus scattering) A(ngles) inversion in anisotropic media. Particular attention is paid to the proper treatment of amplitudes within the distorted Born approximation, viz., by taking into account the radiation patterns of the contrast sources corresponding to the medium perturbations giving rise to the images. By treating the amplitudes correctly, the spatial resolution can be improved even if one leaves the parameter interpretation aside. On the other hand, the parameter resolution, through a singular value analysis, provides a hierarchy of parameter combinations with a decreasing number of symmetries.
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Far field radiation patterns in anisotropic media
Authors T. N. Teles, L. A. Diogo and I. PsencikThe effects of point source situated in anisotropic structures can be investigated by means of the radiation pattern, i.e., the magnitude of the displacement field at a unit sphere around the source. The first attempts to perform ray calculations of the radiation patterns generated by point sources in inhomogeneous anisotropic media were made only recently (Ben-Menahem et al., 1991, Gajewski, 1993).
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Ray theory Green's function in inhomogeneous anistropic media
Authors T. N. Teles and I. PsencikOne of the most efficient methods for obtaining synthetic seismograms for complicated models of laterally varying anisotropic layered media is through the use of zero order approximation of asymptotic ray theory. Although it is only approximate and with restrictions, e.g., the medium must be smooth enough and it doesn't work in the vicinity of caustic and shadow zones, it can yield results with suficient accuracy in many cases. For a long time the wave field by the ray method was computed by situating the source in an isotropic layer due to inexistence of an adequate formulation for the Green's function in general anisotropic media. Recently, various formula for its computation appeared in many ways (Cerveny, 1990; Ben-menahem and Sena, 1990; Kendall et al, 1992).
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Shear wave analysis for azimuthal anisotropy using pseudo-rotation of marine VSP
By C. MacBethResolution of seismic anisotropy for marine VSP remains poorer than for land VSP, primarily due to lack of control for converted shear-waves, and more development is required. We demonstrate that the Alford rotation can be adapted for VSP analysis provided an optimal acquisition geometry is used. This application relies upon efficient near-offset shear-wave conversion.
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Determination of the principal directions of azimuthal anisotropy from P-wave seismic data
Authors S. Mallick, K. L. Craft, L. J. Meister and R. E. ChambersIn an azimuthally anisotropic medium, the principal directions of azimuthal anisotropy are the directions along which the quasi P and the quasi S waves propagate as pure P and S modes. When the azimuthal anisotropy is induced by oriented vertical fractures, two of these principal directions correspond to the directions parallel to and perpendicular to the fractures. S-waves propagating through an azimuthally anisotropic medium are extremely sensitive to the direction of their propagation with respect to the principal directions. As a result, primary or mode converted S-wave data can be used to obtain the principal directions from seismic data. Apart from the high acquisition cost, processing and interpretation of S-wave data require a special processing technology. Moreover S-wave data quality is poor in many regions. Thus, an alternative involving only P-wave data is attractive but is limited to a few qualitative studies of the amplitude-variation-with-offset (AVO) for different azimuthal directions (Mallick and Frazer, 1991; Chang and Gardner, 1993; Allen and Peddy, 1993 ; Lefeuvre, 1994 ; Lynn et.al, 1995, Rüeger and Tsvankin, 1995), and to velocity studies of selected horizons on data for different common mid point (CMP) locations (Neidell and Cook, 1986).
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Shear wave anisotropy and permeability changes in sandstones with aligned cracks
Authors M. S. King, A. Shakeel and N. A. ChaudhryAligned fractures and cracks in porous sedimentary rocks are of considerable interest to those engaged in studies of subsurface fluid flow, such as hydrolog ists and petroleum engineers. Geophysical techniques, especially seismic, are often used to determine the physical characteristics of such aligned discontinuities.
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Measurement of ultrasonic bulk properties of water-saturated porous media
Authors O. Kelder and D. M. J. SmeuldersAcoustic signals are used extensively in the oil industry to determine the physical properties of reservoir rock. In the interpretation of these signals empirical laws play a major role. To obtain a more fundamental interpretation of the recorded wavetrains, the need for a comprehensive theory for wave propagation and attenuation in rocks is obvious. For this purpose, Blot's theory (1956) turned out to be a simple but effective two-phase theory. One fundamental feature of this theory is the existence of both a fast and 'a slow compressional wave, as well as a shear wave. Preliminary measurements of ultrasonic properties of water-saturated porous media, i.e. bulk attenuation and phase speed, show the predictive power of the Biot theory (Johnson, Plona and Kojima, 1994). In this paper we present new experimental data of bulk attenuation and phase speed for all three types of bulk waves, covering an ultrasonic frequency range from 100 kHz to 1 MHz. The results are compared with theoretical predictions of the Biot theory.
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Up-scaling for geochemical modelling - Non-symmetric rigidity tensors and their symmetric approximation
Authors W. Zijl, G. Rijpsma and I. RitsemaThe rigidity is often assumed to be a symmetric tensor. A symmetric rigidity tensors has many advantages. Therefore, it is worth while to review the arguments for a symmetric rigidity tensor, and to investigate whether, and under what conditions, non-symmetric rigidity tensors may occur.
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Acoustic velocities of North Sea shales
Authors R. M. Holt, E. F. Sønstebø and P. HorsrudSeismic waves used for petroleum reservoir exploration spend most of their time travelling through shales. More knowledge of acoustic properties of shale will improve the quality of reservoir interpretation (e.g, corrections for shale anisotropy are important for AVO) as well as of basin evaluation and predicted safety/stability during drilling. Previous studies of velocities in shales (e.g. Jones and Wang, 1981; Johnston, 1987; Homby et al., 1994; Johnston and Christensen, 1995; Sayers, 1995) have demonstrated stress dependency, interpreted mainly as a result of deformation of the contact regions between clay particles. Anisotropy has been observed and modelled primarily on the basis of stress sensitive alignment of clay particles. Previous measurements have, however, often been performed with outcrop or shallow subcrop cores, and data are usually obtained only for a limited range of sample characteristics.
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Seismic attenuation in thinly layered, fluid saturated and permeable sediments
Authors S. Gelinsky and S. A. ShapiroTypical reservoir rocks are often thinly layered, porous, fluid saturated and permeable sediments. Scattering of an incident wavefield by thin layering is an important reason for the attenuation of seismic waves in sediments.
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Permeability estimation of rocks using local porosity theory
Authors U. Fauzi, A. Hördt, F. M. Neubauer and K. VozoffPermeability is one of the very important parameters in reservoir engineering and in environmental geophysics. Direct measurement of permeability however, is very expensive. Thus, the study of pore geometry that controls permeability of porous media remains an active research field both theoretically and experimentally.
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