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68th EAGE Conference and Exhibition incorporating SPE EUROPEC 2006
- Conference date: 12 Jun 2006 - 15 Jun 2006
- Location: Vienna, Austria
- ISBN: 978-90-73781-00-9
- Published: 12 June 2006
1 - 20 of 462 results
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Lambda-Rho Processing - A tool to Reveal Full Hydrocarbon Potentials
Authors T. H. Larsen, C. Ojo, S. Gemelli, P. Licalsi and M. AtkinsRegional analysis of a 5,716 well log database from the Gulf of Mexico allows prediction of AVO response, as well as prediction of velocities useful to processing geophysicists. Regional trends are derived from depth profile plots of well sand / shale velocity histograms.
Four major sub-regions were delineated, based on these profiles:
1.Lower and Middle Miocene Shelf,
2.Upper Miocene through Pleistocene Shelf,
3.Upper to Middle Slope, and
4.Lower Slope.
Strong velocity inversion is common in the geopressured Miocene in the deep Lower and Middle Miocene Shelf region. The present deep water areas shows a slower velocity increase with depth throughout the lithologic section than is seen on the shelf above geopressure, but has a much narrower range in variation than is seen on overpressured rocks on the deep shelf. The Lower Slope appears to have a greater spread in sand and shale P-wave velocities than are seen in the Upper to Middle Slope areas.
This analysis also allows prediction of the velocities which should be seen by the processing geophysicist. Some features of the velocity functions seen are those usually associated with multiple reflections, and as such, often discounted by the processor.
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AVO and Seismic Processing Implications from a Regional Database of Velocity and Other Acoustic Rock Property Trends
Authors H.R. Nelson, S.D. LeRoy, L.R. Denham, P.J. Desai, M.E. Guthrie and M.A. DunnTo better understand the physical characteristics of sedimentary deposits in Lake Geneva, we acquired and processed very high resolution seismic reflection data. Accurate amplitude measurements were obtained after correction for the frequency response of the hydrophones which were individually calibrated. In a first part, we present the acquisition and the pre-processing done on these data to correct the amplitude. In a second part, we migrate the data using a preserved amplitude pre-stack depth migration, and then apply a post-migration processing in order to correct for residual move-out. Finally, AVA analysis enables to differentiate sedimentary unit boundaries.
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AVA Analysis of Very High Resolution Seismic Data for the Study of Sediments in Lake Geneva, Switzerland
Authors D. Hammami, A. Egreteau, F. Marillier and P. ThierryThis paper continues the studies presented in this conference by Loizou et al. (2006), where we have compared the AVO analyses from four wells in the west of Shetland: Foinaven oil well (204/24A-2); Laggan gas well (206/1-2) and two prospect wells (204/17-1 and Assynt 204/18-1). In this study, we apply the spectral decomposition to the data from these four wells and provide further evidence that there are also differences in the spectral characteristics between the four wells. While Foinaven and Laggan Class III AVO anomalies are consistent theoretical predictions for frequency-dependent AVO behaviours. However, the results from the Assynt prospect 204/18-1 and another prospect 204/17-1 show rather complicated behaviour in the iso-frequency sections that cannot be fully explained. In summary, though the amplitude anomalies seen in the two prospects were originally interpreted as to be similar to the Foinaven, but our study indicated they are different in both the AVO behaviour and the spectral characteristics.
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AVO Analyses from Four Wells in the West of Shetland - Spectral Decomposition
Authors E. Liu, M. Chapman, N. Loizou and X.Y. LiGeological models are currently used to aid AVO-analysis, usually thick-layered. In an actual thin-layered sub-surface medium, the AVO has some specifics, ignoring which may lead to errors in predicting hydrocarbon reservoirs. In the resonant frequency range periodic thin-layered stack, AVO inversion on the basis of the linearized Shuya formula produces an estimate of the effective Poisson coefficient much lower than one of a separate gas-saturated layer. Also, in the low-frequency range an effective Poisson coefficient estimate is much higher than one in case of a separate gas-saturated layer. Within a frequency range close to the resonant frequency (tuning thickness) for a thin-layered stack, favorable conditions occur for predicting multi-layer hydrocarbon reservoirs from the AVO-analysis. At the same time, disregarding absorption may lead to interpretation errors. A finite difference solution for the elastic wave equation and the Haskell-Thomson method in its visco-elastic variant were used to perform the investigation.
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AVO for Pre-Resonant and Resonant Frequency Ranges of a Periodical Thin-Layered Stack
Authors N. Marmalyevskyy, Y. Roganov, A. Kostyukevych and Z. GazarianA new method for computation of seismic AVO attributes such as intercept and gradient, is proposed. By combining detailed sedimentological modeling with well logs, we compute alternative net-to-gross and porosity logs. The method is developed for reservoirs consisting of ultra-thin sand-shale layers, as for instance turbidite reservoirs.
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Seismic Modelling from Ultra-Thin Layered Reservoirs
Authors A.M. Stovas and M. LandroAs we are running out of oil, and the oil price is reaching new hights, the oil industry is increasing the focus on exploration and production of heterogeneous reservoirs. In this paper we focus on the rock physics of shaly sands, and how different types of clay configurations will affect the rock stiffness and pore fluid sensitivity.
Laminated sands and shaly sands with pore-filling clays will have different fluid sensitivity. We find that lamination will increase the Vp/Vs ratio for gas saturated sands compared to clean sands, in contrary to sands with pore-filling clay, where the Vp/Vs ratio will be relatively less affected by the clay content. This is related to the reduced shear effect induced by the micro-scale clay lamina.
Next, we study the effect of net-to-gross and scale on fluid sensitivity using the Backus Average method. Here we also observe an increase in Vp/Vs ratio. Gas saturated thin-bedded sands will have Vp/Vs ratios similar to oil-saturated clean, homogeneous sands. This is related to the patchy saturation induced by the geologic heterogeneities. We also study the effect of anisotropy on the fluid sensitivity of heterogeneous reservoirs.
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Seismic Fluid Prediction in Heterogeneous Reservoirs
Authors P.A. Avseth, A.J. van Wijngaarden, T.A. Johansen and G. MavkoAs we are running out of oil, and the oil price is reaching new hights, the oil industry is increasing the focus on exploration and production of heterogeneous reservoirs. In this paper we focus on the rock physics of shaly sands, and how different types of clay configurations will affect the rock stiffness and pore fluid sensitivity. Laminated sands and shaly sands with pore-filling clays will have different fluid sensitivity. We find that lamination will increase the Vp/Vs ratio for gas saturated sands compared to clean sands, in contrary to sands with pore-filling clay, where the Vp/Vs ratio will be relatively less affected by the clay content. This is related to the reduced shear effect induced by the micro-scale clay lamina. Next, we study the effect of net-to-gross and scale on fluid sensitivity using the Backus Average method. Here we also observe an increase in Vp/Vs ratio. Gas saturated thin-bedded sands will have Vp/Vs ratios similar to oil-saturated clean, homogeneous sands. This is related to the patchy saturation induced by the geologic heterogeneities. We also study the effect of anisotropy on the fluid sensitivity of heterogeneous reservoirs.
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Depth Trend and the Characteristics of PS AVO
By H. XuGonzalez et al. (2003) & Xu et al. (2005) showed the application of PS Elastic impedance (PSEI) for lithology and fluid prediction. To understand the effectiveness of this approach in relation to the target depth it is necessary to analyze the typical characteristics of PS AVO in a compaction trend framework. In this paper we will review the typical depth trend of sand and shale properties in a sedimentary basin and discuss the characteristics of PS AVO in several unique depth zones.
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Use of Rock Physics Principles for Inversion of Pre-Stack Seismic Data
Authors A. Mukherjee, H. Xu and N.C. DuttaFull waveform pre-stack inversion of seismic data usually provide a more accurate and reliable estimate of the elastic properties of the earth over conventional AVO analysis, as it can account for the contamination of seismic amplitudes from other wave modes. However as in all other types of inversion pre-stack full waveform inversion is also sensitive to the starting model and other apriori information. Conventionally well logs available in the area are used for this purpose. However, dipole sonic log in most cases is not available due to cost and P-wave sonic and bulk density logs are not available in the shallow parts due to large holes. In this abstract, we present a general rock physics based technique for constructing pseudo logs and then use them to build our initial models for pre-stack waveform inversion of seismic data. The pseudo-shear sonic log calculation is based on the work by Dutta and Wendt (1993). Another set of pseudo-logs are based on mechanical compaction theory and rock physics based velocity porosity relationships. We test inversion results for both the techniques on well log synthetic from deep water Gulf of Mexico.
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Physical Modeling of AVO Responses
Authors B.J. Evans, A. Wandler and C. LinkSeismic amplitude versus offset (AVO) methods are a well established practice for the direct indication of hydrocarbons, and are proven using both numerical modeling and field data. However, from a laboratory stand-point, AVO techniques have not to date been demonstrated in practice. This paper presents a first case of a physical model which was used to practically demonstrate the use of AVO for the separation of a hydrocarbon (oil) from pure water or brine. The physical model was also used to demonstrate how AVO cannot be used to separate water from a brine (30,000 ppm NaCl). A test using dissolved CO2 in water showed that an amount of 0.1 mol% was inadequate for seismic detection. The physical model was also used to demonstrate the complexities of the oil/gas/water mix, where it was possible to show how under certain circumstances, gas bubbles can remain trapped between oil and water.
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Moveout Stretch Implications for AVO
Authors P.R. Williamson and E. RobeinThe Zoeppritz equations for AVO, and their various approximations, apply strictly to a single isolated interface between two half-spaces. In practice this is almost always not the case, and effects known as AVO tuning are observed due to the apparent change in the time-thickness of beds on recorded traces with offset. While NMO or migration should equalise these time thicknesses across all offsets, the tuning effects are maintained by wavelet stretch. This stretch cannot be meaningfully avoided by 'non-stretch' processing, which simply corresponds to the isolated reflector paradigm. Meaningful AVO analysis, can, however, be achieved by effectively equalising the wavelet across all traces.
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Quantitative Calibration of Seismic to Wells and Probabilistic AVO
Authors M.F. Bee, E. Rietsch, D. Pankhurst and J. MagillWe present deterministic and probabilistic methods to quantitatively calibrate pre-stack seismic data to well data before AVO analysis. We then show how to use the calibrated seismic data with probabilistic AVO methods to analyze the prospectivity of a step-out exploration prospect several kilometers away from well control with the goal of identifying and quantifying hydrocarbons.
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Identification of Upper Cretaceous Volcanics Using AVO Attributes; Santos Basin, Brazil
Authors S. Klarner, R. Culpan, R. Fontana and B. BankheadThe deposition of the main reservoir unit in the Santos Basin, the fluvial to shallow marine Upper Cretaceous Ilhabela sandstones, took place during a period of active basaltic volcanism. The volcanic rocks have been partly eroded and subsequently deposited in the reservoir. This has been the cause of a complex diagenetic history. The results are locally occurring sandstones with complex lithologies and elastic properties which differ from the basin trend (Klarner et al, 2005). In order to predict and interpret the amplitudes and AVO behaviour of the Ilhabela reservoir it is essential to map the occurrence of the volcanics as well as the transport direction of their erosional products. We make use of the fact that the volcanic related lithologies display a significantly higher Vp/Vs ratio than the embedding clastic sequences. In this paper, an AVO approach is presented which helps to identify volcanic rocks in the section.
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Improved Seismic Imaging of Deepwater Turbidite Channels - A Case History from Offshore Angola
Authors S.P. McHugo, M.F. Francis, A. Cooke, N. Woods and J. EnglishExisting seismic data and appraisal drilling in the lower Congo basin show evidence of significant sandstone reservoirs in the tertiary section, formed by deepwater turbidite flows. The challenge is to develop and manage these complex reservoirs. To delineate the reservoirs, it is important to identify the fine details of the geometry of the reservoir sands and locate the structural and stratigraphic trapping mechanisms. The cost of drilling development wells in this area and the complex nature of the reservoirs make reflection seismic data a cost-effective tool in reservoir management. Thickness resolution obtained from existing seismic data in this area is in the order of 19 m A new survey was acquired in 2003 in an effort to obtain higher resolution.
In this paper, we describe the planning, acquisition, and processing results of a high-resolution point-receiver seismic survey. The survey was acquired with the objectives of mapping the thin sand units associated with turbidite sand channels, to provide long offset data not present in the existing data and to assess the benefits of point-receiver acquisition.
Results indicate that the objectives of the project have been met through acquisition and high-fidelity processing of point-receiver seismic data.
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Porosity and Shale Volume Estimation for the Ardmore Field Using Extended Elastic Impedance
Authors A.M. Francis and G.J. HicksDeterministic seismic inversion and extended elastic impedance (EEI) have been used to obtain quantitative estimates of porosity and Vshale over the Ardmore Field. The optimum EEI angles corresponding to porosity and Vshale were determined from the well logs, together with a lithology indicator. The prestack seismic data were then projected to the Chi angles corresponding to these three petrophysical indicators and deterministic inversions were performed to obtain three broadband EEI volumes. Three-parameter linear regressions were then performed to estimate quantitative porosity and Vshale volumes from the three EEI volumes. Regions of High porosity and low Vshale have been identified which may suggest possible future drilling locations.
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Prediction of Resistivity Using P and S-wave Properties from Prestack Seismic Inversion
Authors H. Xu and N.C. DuttaIn this paper, we study the prediction of resistivity from seismic data, based on resistivity-velocity relationships calibrated from well logs. Previous efforts in this area have included predicting resistivity from seismic amplitude or seismic-derived P-wave velocity. Here we present the prediction of resistivity from both P and S wave properties (P-impedance and Poisson’s ratio), which can be derived from prestack seismic inversion. The results can be used for identification or upgrading prospects in a mature field where resistivity logs are abundant.
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The Development of Low Temperature Tem Squid Systems for the Geosciences
By C.L. Le RouxSuper-conducting Quantum Interference Devices (SQUIDs) are tiny sensors that detect and measure very small magnetic fields. As part of an ATD-GRG research project, the Institut für Physikalische Hochtechnologie (IPHT) in Jena, Germany, have developed a Low Temperature SQUID (LTS) ground Transient ElectroMagnetic (TEM) system for Anglo to further strengthen the company’s mineral exploration capabilities. A brief history of the technology development is illustrated with results from various field tests. Early field trials conducted in Germany in 2002 showed good promise, but some system problems. Partial redesign and good applied science led to successful field testing and comparison of LTS, HTS and conventional coil receivers in Sweden in 2003. Further field tests on the Western Australian Nickel belt in 2003 and 2004 proved the system’s field-worthiness and that using liquid Helium as a coolant poses no serious logistical problems even in such a harsh environment. Undisputed proof of superior signal-to-noise capabilities over HTS, Fluxgate and coil sensors was again evident as well as the advantages of using LTS sensors for detecting conductive targets at depth or below conductive cover, hitherto a severely limiting constraint on exploration for conductive ore-bodies. Because of the better S/N stacking time is reduced and production is 4 to 10 times faster depending on the environment. Some spurious system response problems were more prominent during tests in resistive terrains in South Africa, but have subsequently been solved. The LTS TEM SQUID system has now been recognized as a major breakthrough with potential to give Anglo exploration teams a significant strategic advantage over competitors. An agreement has been signed with IPHT that provides Anglo with exclusive rights to the project technology for a ten year period following its development. Three systems are being deployed by our base metal exploration teams in Australia and Canada, while a fourth will be delivered later in 2005 for on-going exciting development work in Southern Africa.
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The Interpretation of Potential Field Data Using Continuous Wavelet Transforms
More LessThe continuous wavelet transform has been used with much success in the analysis of non-stationary time series. It has been used much less frequently in the interpretation of magnetic or gravity data, although several approaches have been tried. A simple method of obtaining location and depth estimates of gravity and magnetic field sources is suggested here, For gravity data the method uses wavelets based on the integer-order horizontal derivatives of the gravity anomaly from a point source (the Poisson kernel). For magnetic data the wavelet is based on the integer-order horizontal derivatives of the analytic signal of the anomaly from a step or a horizontal cylinder. The method is compared with Euler deconvolution, and is demonstrated with synthetic models and on gravity and magnetic data from South Africa.
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Contributions to the Gravity Field of Southern Africa from Crustal Thickness and Seismic Velocity Variations
Authors S.J. Webb and D.E. JamesAs a result of the Kaapvaal Project we now have unprecedented coverage of the crustal thickness and details of the seismic velocity variations in the tectosphere beneath southern Africa. By using these data and reasonable density relationships we can calculate the contribution of each of these components to the overall gravity field of southern Africa. These gravity modeling results confirm the suggestion that the high seismic velocity keel beneath southern Africa has a lower density than surrounding mantle material and is consequently highly depleted; a result in agreement with the xenolith data obtained from kimberlites. We estimate that the 1% seismic velocity variation observed between on and off craton leads to a 1% variation in mantle density due to compositional variation. This low density keel is vital to counterbalance the effect of the thinner than average crust that is observed beneath the Kaapvaal and Zimbabwe cratons. Thus in regions where the Archaean craton boundaries may not be apparent from geological mapping, seismic studies and gravity modelling could assist with delineating potentially cratonic regions with diamond potential.
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New Palaeomagnetic Data from the Main and Upper Zones of the Bushveld Complex
Authors S.A. Letts, T. Torsvik, L. Aswal and S. WebbThe Bushveld Complex (BC) has been the subject of many palaeomagnetic investigations since 1959. Despite these studies, systematic inconstancies have been found between the palaeomagnetic findings and well constrained ages determined for the Bushveld Complex. One concern is the large spread in pole positions previously determined for the different zones of the BC. This has been interpreted in prior studies to indicate that the Bushveld was emplaced and cooled below the Curie temperature of magnetite over a time span of 50 Ma. This inconsistency has prompted the current re-investigation of the palaeomagnetic results.
New palaeomagnetic data collected in the Main and Upper zones of the Eastern and Western limbs of the BC, yielded palaeomagnetic poles that eliminated the spread in the apparent polar wander path. This observation is in agreement with precise age data, constraining the time period of emplacement of the BC. Fold tests applied to each zone yielded statistically positive results supporting a primary magnetic signature and indicate that the complex was intruded in a near horizontal position.
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