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4th International Congress of the Brazilian Geophysical Society
- Conference date: 20 Aug 1995 - 24 Aug 1995
- Location: Rio de Janero, Brazil
- Published: 20 August 1995
121 - 140 of 330 results
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Principles of Magnetotelluric Interpretation:The World Between the Uniform Half-Space and the Anisotropic 3-D Conductor - A Tutorial
By P. WeideltThe talk will briefly review sucessful concepts and methods of magneto-telluric interpretation, which have been devised for conductivity models of increasing complexity. After introducing the basic assumptions of magnetotellurics, it starts with the induction in a uniform isotropic half-space, which gives rise to the notion of penetration depth and apparent resistivity. Then approximate and exact methods of 1-0 inversion are presented in some detail [including problems of existence and uniqueness and extremal models (best fit, minimum structure, minimal and maximal average conductivity in a given depth range)].
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MT Survey in Canoinhas, Parana Basin, Brazil
Authors M.A. Rodrigues de Ceia, J. Menezes Travassos, F. Morrison and E. NicholsA Magnetotelluric survey was conducted in the Parana Basin near Canoinhas, SC. The survey was designed in such a way that the MT line cut a gravimetric feature characterized by a high to a low Bouguer anomaly. The frequency range spans from 0.001 to 100 Hz. With the results it was possile to produce a clear picture of both the sedimentary cover and the basement.
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A Magnetotelluric Survey Over a Reported Geothermal Anomaly in Southern Portugal: Some Results and Interpretation
Authors A. Correia and F.W. JonesA magnetotelluric (MT) survey was carried out in southern Portugal in a region where a geothermal anomaly with heat flow density (HFD) values of about 200 mW/m2 near its center has been reported to exist. The objectives of the MT survey were to delineate the anomaly in electrical terms and to understand its origin. The area where it occurs is geologically complex and is crossed by two major tectonic features: the Messejana fault and the Ferreira-Ficalho overthrust. To measure the electric and magnetic fields, thirty-four sites were occupied over an area of about 2,500 km2 and, after processing the MT data, electrical resistivity maps for several depths were constructed. Interpretation of the processed data in conjunction with other geophysical information indicates that the HFD values reported for the area of the MT survey do not represent the thermal state of the crust in the region. Low electrical resistivity values are, however, found in zones that coincide at the surface with the Messejana fault and the Ferreira-Ficalho overthrust. This may be an indication that fluid flow occurs within these two tectonic features and may generate local high HFD values near the surface.
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STRUCTUrAL STYLES IN THE SUKESTE BASINS, MEXICO,
Authors G. Garcia-Molina and A. W. BallyThe Yucatan Platform bisects the NW-SE Sierra de Chiapas fold belt of SE Mexico at right angle. The outcropping Sierra de Chiapas involves Mesozoic platform carbonates, but its northwestern subsurface continuation involves mostly Mesozoic basinal and lope facies sediments in the Villahermosa folds and their offshore continuation, the Sonda de Campeche folds. The main decollement level for the folds is a middle Jurassic evaporite sequence. The pre-salt "basement" of the area is poorly defined but estimated o dip from about a depth of 6 km to the north (Campeche offshore) to 13km in the south (Sierra de Chiapas). The fold belt was formed during upper Miocene time and Is characterized by bivergent NW-SE striking folds. The amount of shortening is estimated to e in the . order of 45 km to 65 km. In the onshore and offshore subsurface the folded belt is orthogonally superposed by a late Neogene growth fault system which soles out near the base of the Neogene. This growth fault system developed on the continental slope and intercepted salt diapirs that probably emanated from the core of deep-seated folds. Much of the salt accumulated farther north in the large allochthonous mass of the Campeche salt domes.
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Continuous Magnetotellurics in a Section of the Folded Belt, Neuquen Basin, Argentina
Authors C.E. Cruz, E.E. Kozlowski and R. Fernandez-TomeIn the folded belt northwest of the Neuquen Basin in Argentina, one finds an extensive and continuous volcanic cover where the gathering of even moderately good quality seismic data is difficult. As part of an ongoing exploration program in this area, we have applied a continuous magnetotelluric (MT) profiling technique adding more than 100 km of data, spread over several profiles mostly oriented eastwest. The subsurface resistivity information obtained was associated to known geological information in the area. In this manner we were able to determine the spatial distribution and the thickness variability of the volcanic cover. In addition, very low resistivity units were associated to the fluvial sediments of the Neuquen Group and to the pelitic-evaporitic Rayoso Group. Intermediate values of resistivity are found in areas where the Mendoza Group (largely limestones, black shales and marls) are suspected to have been intruded by Tertiary volcanics. Finally, deeper sectors of the resultant geoelectric image are in close agreement with the overall tectonic behaviour of the basement, at least in this section of the Neuquen Basin. Two dimensional forward model responses generated from geological sections have yielded important similarities to the data obtained.
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Spatial resolution of 3-D downward extrapolation of incomplete data
Authors C.P.A. Wapenaar and W.E.A. RietveldThree-dimensional prestack downward extrapolation of a complete areal survey over a point diffractor yields a circular symmetric resolution function with the main lobe width being approximately 6>/5. In practice, however, the sampling distance of the sources and detectors never fulfils the anti-aliasing condition in the x- as well as in the y-direction. In this paper we analyze an extreme case of spatial undersampling, i.e., we consider a single line of sources along the x-axis and a single line of detectors along the y-axis. For this situation the "matched filter approach" cannot be used. We propose a "true amplitude" operator that is adapted for this acquisition configuration. Although its resolution function is no longer circular symmetric and side lobes occur, its main lobe is only approximately V2 times wider than in the case of full areal acquisition. Hence, even for this extreme case of spatial undersampling, reasonable images can be obtained. This is illustrated with a prestack migration example of physical model data.
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Lessons In 3·D Prestack Depth Migration
By S.A. Levin3-D prestack depth migration was applied by Mobil to data from offshore Nigeria (Figure 1). The use of parallel supercomputers enabled completion of the computations in a bit more than a week, where a high-performance modem workstation would have taken a full year. Ipresent the improvements this processing made for these data and discuss lessons learned from the project. A key observation is that far more time was spent building and validating each 3-D migration velocity than was devoted to the actual imaging computations. Lastly, Iproject industry trends and areas for future improvements and extensions in 3-D prestack imaging and inversion.
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Putting Prestack 3-D Depth Imaging into Production
Authors U. Albertin, W. Wiggins, P. Fowler, S. MacKay, C. Beasley and C. Koeniger3-D prestack depth migration is quickly coming of age. New plays beneath salt-injection features have received much recent attention. However, accurately defining reservoir geometries in more mature areas, such as the North Sea or the Gulf of Suez, also requires 3-D depth imaging technologies. Although prestack 3-D depth imaging is a field of active development, basic elements such as velocity definition and the migration algorithm itself are developed enough to discuss their applications on a field data set.
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A Unified Approach to Seismic Reflection Imaging
Authors M. Tygel, P. Hubral and J. SchleicherGiven a three-dimensional (3-D) seismic record in a fairly arbitrary measurement configuration and assuming a laterally inhomogeneous, isotropic macrovelocity model, a complete theory of amplitudepreserving seismic reflection imaging is formulated. It consists of (a) a Kirchhoff-type weighted diffraction stack to transform (migrate) the seismic data from the record space into the image space, and of (b) a weighted isochrone stack to transform (demigrate) the migrated seismic image back into the record space. Two integrals describe the amplitude aspects of both stacks. They are the key operations upon which the complete amplitude-preserving imaging theory is described.
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Prestack Imaging of 3-D Marine Data Sets
Authors B. Biondi, S. Fornel and G. PalacharlaThe routine application of 3-D prestack migration to the imaging of large-scale 3-D marine surveys requires the development of migration methods that are more computationally efficient than conventional Kirchhoff migration. To prestack image marine data we propose an efficient two-step procedure. The first step transforms the recorded data into effective common-azimuth data by applying a pres tack partial- , migration operator (AMO). The common-azimuth data synthesized by AMO is then imaged in depth using a new commonazimuth downward continuation operator. The main advantage of common-azimuth downward continuation is that the computations are carried out in the original 4-D space, as opposed to the 5-D space that would be required by straightforward application of full-prestack downward continuation. We applied common-azimuth migration to two synthetic data sets: the first data set was generated assuming a vertical velocity gradient, the second one assuming a velocity gradient with components along both the vertical and horizontal axes. Common-azimuth migration correctly imaged the reflectors in both cases.
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A Perspective on 3-D PSDM in Compressive Tectonics
Authors A. Banna, H. Calandra, P. Esquier, J.P. Goetschy, C. Lafond, T. Lecoq, Y. Kerdraon, G. Noual, X. Payre and E. Robein3-D Prestack Depth Migration (3-D PSDM) is becoming the ultimate tool for seismic imaging in complex areas. Now generally available, this technique is becoming more efficient and cheaper to use, thanks to the ever growing power of today's massively parallel supercomputers, the availability of large memory storage and very fast VO capabilities. However, one should bear in mind that 3-D PSDM cannot be used as a black box; in fact, its success depends on three procedures : 1) deriving and updating an appropriate velocity model, 2) performing careful QC at every step during the imaging process, and 3) using a robust method for computing traveltime tables and other Green function attributes. In areas such as the Gulf of Mexico, where the velocities are well-known and varying smoothly, 3-D PSDM does not always need to follow these three steps and can be applied with success in almost a blind way with most algorithms. In more structured areas such as the North Sea, where large horizontal heterogeneities exist in the velocity field, one should be more cautious: the main difficulty there is to derive an appropriate layer-based velocity model. Finally, in very complex areas such as compressive zones, the challenge is still ahead. In those areas, if not used carefully, 3-D PSDM can yield very poor results and is not cost -effective. In this case, we propose to use first an alternative, cheaper 3-D imaging method to evaluate the difficulties involved. If 3-D PSDM is then justified, we recommend following very carefully the three steps mentioned above.
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Interdisciplinary Approach To Maximize Benefits of Prestack Depth Migration
Authors R. Gonzalez and D.E. WillenThe advent of massively parallel computers has made the use of fast, efficient, accurate, 3D prestack depth migration (PSDM) a viable option for the exploration and production of hydrocarbons. However, it is still a significant investment in terms of computer resources, time, money, and personnel to carry out this process. Therefore, we need a methodology to intelligently utilize this new tool and to manage the associated technical risks and expectations. In this paper, we suggest an integrated, staged approach to both predict where it's beneficial, and to anticipate its results within an acceptable confidence level. Thus we avoid the temptation to abuse PSDM as a brute force tool on all imaging problems and instead maximize our potential benefits by knowing how to apply it judiciously.
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The Pace of Change in the Latin American Oil Patch
Authors G.E. Jones and R.L. ConroyThe social, political and economic forces impacting the traditional worldwide energy companies are now, by necessity, being addressed by the Latin American state owned oil companies. The changes brought about by the restructuring have been examined to determine how they may affect the general interdependence within the Western Hemisphere. Available competitively priced energy is, and will continue to be a prime ingredient in the rate of development of the Western Hemisphere trading partners. A country by country evaluation of available production and proven reserves indicates that adequate energy exists to meet the projected Western Hemisphere demands. The defming factor is how well the required restructuring of the oil industry in Latin America is managed by the principal stakeholders in matching the needed technology, fmancial resources and talent to the resource development needs. The current status of change in each country is discussed. State oil monopolies are fmding ways to attract private capital where needed, international oil companies are assuming service roles in some areas and service companies are being required to take new responsibilities and risks. All these changes must be accomplished in a rapidly evolving social, political and business environment.
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Modelaje de Densidad a Partir del Sonico en Litologia Complejas, Subcuenca de Barinas, Venezuela
Authors P. Calderon, D. Flores and Y.M. JordanLa correlación de registros de densidad y sónico de cinco campos de la Subcuenca de Barinas permitió derivar una relación que da resultados consistentemente mejores que los obtenidos con la relación de Gardner. Su aplicación permite derivar valores de densidad mas confiables a partir de la velocidad del sónico para aplicaciones sísmicas y gravimetricas, empleando coeficientes empíricos obtenidos de la correlación de datos de campo con un sistema petrofísico interactivo, que permita discriminar lítologícas a partir del registro de rayos gamma e información de micIeos. Multiplicando por un factor de 1.42 el coeficiente de reflexión a partir de los contrastes de velocidad en la aproximación de Peterson, se toma en cuenta los efectos de la densidad sobre la velocidad. El estudio demuestra la necesidad de modificar las relaciones empíricas de aplicación general, como la de Gardner, cuando se desea obtener resultados mas confiables en cuencas con litologías complejas, como la mencionada.
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Abrupt Onset of Geopressure: An Artifact of Sampling in Multi-Well Composites of Pore Pressure vs. Depth Data
Authors M.L. Greenberg, Z. He, T.K. Kan, L.Y. Kuo and G.J. MitchTwo-dimensional basin models were constructed to study geopressure variations in the Gulf of Mexico. Two transects, 51.7 and 100 km long, were calibrated to pore pressures estimated from sonic logs. 97% and 90% of variance in pressure data from the two transects, respectively, was accounted for with a compaction disequilibrium mechanism for geopressure. Simulated multi-well composites of pressure measurements could be interpreted as indicating vertically abrupt onset of geopressure, however, this is shown to be an artifact of sampling. Multi-well composites of measured pore pressures may best be understood with families of predicted pressure vs. depth profiles that are based on a compaction disequilibrium mechanism.
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Reservoir Characterisation Using Nuclear Geochemical Logs
Authors J.F. Bristow, J.C. Lofts, P.K. Harvey and M.A. LovellAdvances in the technique of nuclear spectroscopy logging have resulted in the ability to determine continuous. in situ abundances of most major rock-forming elements. If the use of log-derived geochemistry is to be applied, widely quality assessment must be improved to cover a wider variety of environments. A case study of a producing well from the Gulf of Mexico exposes some of the problems in the processing of the geochemical data quantitatively assesses the accuracy of that data in comparison to core measurements and examines the validity of an elemental uncertainty estimate. Neutron transport simulation studies show the measurement of Al to be particularly sensitive to borehole environmental effects.
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LWD and Wireline Resistivity Logs: Correction for Time and Temperature Dependent Resistivity
Authors A.Q. Howard, K. Cozzolino, E. Decoster, D. Allen and J. SeydouxThis paper quantifies transient temperature effects on resistivity logs caused by large differences in mud and formation temperature. The differential temperature between formation and circulating borehole fluid can be large, particularly in offshore drilIing for example when several hundreds of meters of casing is exposed to the ocean and acts as a heat sink. The temperature transient gives rise to a thermal invasion profile and translates to a time-dependent monotonic radial resistivity profile, independent of whether the rock is permeable or not. The transient problem includes the heat diffusion transient with charactersitic time of a day or so depending upon the ratio K/(pC) where K (W/(moK)) is the thermal conductivity, p (kg/m3) is the density, and C (J/(kgOK) is the heat capacity of the mineral. The changes in the distribution of water saturation driven by invasion is also time dependent. A Buckley-Leverett model for multi-phase fluid· flow in porous media is used to predict time dependent water saturation Sw(p, t). The resulting radial resistivity profiles are then input into electromagnetic modeling codes to predict the transient apparent resistivity as measured by both LWD and wireline resistivity tools. Numerical results for wellbore environments within normal operating conditions show transient temperature effects alone can cause noticable LWD and wireline curve separation.
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Borehole Seismic Applications Using a Slim Well Seismic Receiver in a Production Enviroment
More LessAs the costs for searching for new oil fields continues to escalate, many companies are evaluating the economics of enhanced recovery methods for their older fields. Therefore, there exists a greater need for high resolution borehole seismic surveys to correlate existing field information with the surface seismic. Using a slim well seismic receiver, it is now possible to acquire these surveys without a drilling rig and without killing the well, saving time and money. The data is acquired inside tubing, inside casing - an environment which has not previously been utilized for borehole seismic surveys. A velocity survey and three different vSP surveys using both impulsive and vibrator sources are shown. These types of surveys acquired inside production tubing, inside casing in producing wells provide enhanced well correlation to surface seismic in areas which had little or no well velocity control. This allows important geophysical information to be acquired were it is needed and when it is needed.
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Well Log Correlation Using Backpropagation Neural Networks
Authors S.M. Luthi, I.D. Bryant and H. Gamero de VillaroelOne of the principal tasks of a production geologist is to correlate well logs in order to construct a reservoir model. This task is not only time consuming, but also difficult at times because of geological complications such as fault cutouts - where entire sections of the geological column are missing in a well due to the offset caused by a fault - or depositional complications such as facies changes, layer pinch-outs etc. Thus this task still poses a great challenge in the field ofpattem recognition. We propose in this paper an approach based on neural networks which learn from the actual patterns observed in one or more key wells, and then apply this "knowledge" to other wells. An implicit assumption to geological correlation of markers is the relative position of these marker to each other, which obey stratigraphic rules, and which we built into the neural network as a depth reference function that penalizes an attempted correlation if the distance to other markers is beyond the expected value.
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Consistent Interpretation of Galvanic and Induction Measurements by Joint Inversion Methods
Authors A.G. Mezzatesta, M.H. Eckard and K.-M. StrackAccurate estimation of residual and movable hydrocarbon requires precise estimation of the formation resistivities in the flushed and uncontaminated zones. The estimation of formation resistivities is commonly done by interpreting the data provided by galvanic and induction devices. Different physical principles govern these devices. In galvanic tools, the voltages being measured are related to the surface charge densities at the interfaces between adjacent media of different resistivities, whereas in induction tools, the voltage induced in a measuring coil results from the magnetic fields due to alternating currents flowing in the surrounding formation.
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