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PGCE 2011
- Conference date: 07 Mar 2011 - 08 Mar 2011
- Location: Kuala Lumpur, Malaysia
- Published: 03 July 2011
101 - 104 of 104 results
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Recent Trends in Offshore Exploration: More Data, Less Model
Authors Guillaume Cambois and Maz FaroukiNew marine acquisition techniques – such as wide- and multi-azimuth, over-under and dualsensor – provide additional data that complement conventional narrow-azimuth towed streamer data. These new data help reduce uncertainties in velocity model building and ultimately lead to a more accurate image of the subsurface. It is a well-known aspect of the general inverse theory that ill-posed problems need additional constraints to be resolved. These constraints often take the form of an a priori model from which the
solution is required not to differ too much. This model represents an initial guess that must obviously be close to the exact solution if we want the correct answer. An alternative approach is to collect more independent data to reduce the under-determination of the system. Imaging in complex geology where pre-stack depth migration is required to correctly reveal the subsurface structure is such an ill-posed problem. Common exploration targets include sub-salt, sub-basalt, and beneath gas plumes. The complex structures and the high velocity contrasts in these regimes combine to diffract seismic waves in all directions. The little energy that gets recorded by the relatively small streamer spread does not contain enough information to fully reconstruct the complex
structures. In addition, noises (such as multiple reflections) further distort the already weak signals. Consequently, imaging in these complex geology regimes leaves a lot to interpretation. To reduce under-determination more independent data must be collected. The industry started to gradually increase the streamer spread, reaching typically 9km in length and up to 1.3km in width. This comparatively small width was first addressed by acquiring surveys in multiple directions. Later techniques extended the width using additional source vessels. An alternative approach is to acquire ocean-bottom seismic, which provides wide-azimuth as well as potentially multi-component data, but at a significantly higher cost.Recent developments, such as dual-sensor streamer and 3D over-under gather more independent data and offer a no-compromise bandwidth extension on the receiver side. On the source side, over-under and multi-level arrays also increase low-frequencies without loss of high-frequencies. The methods listed above will be further developed and illustrated with various examples from around the world.
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A Towed EM System Test Survey
Authors Johan Mattsson, Lena Lund, Jostein Lima, Marit Ronæss, Folke Engelmark and Maz FaroukiA newly developed towed EM system has been tested offshore in the North Sea. We show that the measured electric field data are of sufficient quality and signal-to-noise ratio for successful detection and inversion of the high resistivity reservoir area including distinction of some of the shallow gas accumulations above the reservoir. 1D inversion in the frequency domain is performed on individual common mid points (cmps) along a survey line across the reservoir with robust results as well as 2.5D inversion. A 3D resistivity model is also built from seismic data and interpreted horizons. This model is manually fine-tuned by comparing resulting 3D forward modeling data with the measured data. Finally, the estimated resistivity structure is investigated with respect to available vertical resolution from the data. This is accomplished by reformulating the inverse problem to a boundary value problem with solutions that approximately give the vertical resistivity structure at each cmp. The motivation for developing a towed EM system is to significantly increase the acquisition efficiency compared to existing stationary systems. Efficient EM data acquisition increases the range of applications as better spatial coverage can be achieved at lower cost. In the test survey, an electric
current dipole source and an EM streamer were simultaneously towed along a 12km long survey line from one vessel in a speed of 4 knots. 1D and 2.5D inversions are performed on the frequency response data along the survey line. In both cases, the reservoir is clearly observed, which agrees well with the seismic information. At shallower depths, there is an increase in resistivity above the reservoir, which probably originate from the thin gas pockets above the reservoir. This is also supported by the 3D modeling. The estimated sea-water resistivity also agrees well with the values from in-situ measurements. The EM towed system has provided electric field data of sufficient quality and signal-to-noise ratio for successful detection and inversion of the highly resistive reservoir area, including distinction of some of the shallow gas accumulations above the reservoir, using an acquisition method of significantly greater efficiency than stationary systems.
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Separation of Seismic Diffractions and Specular Reflections: A tool for Improved Processing, Imaging and Interpretation
Authors Riaz Alai, M Hafizal Mad Zahir, Amar Ghaziah and Eric VerschuurComplex subsurface structures often generate complex seismic data that may produce inaccurate seismic images affecting the risk factor and success in exploration of oil and gas reservoirs. Therefore, it is important to analyze the character of seismic reflection data and to facilitate this analysis; we can divide the total recorded data into suitable sub-components. As seismic waves reach local discontinuities in the subsurface, new energy is initiated and waves are generated as if a pseudo secondary point source is buried in the subsurface and is emitting diffracted energy. On the other hand, when there is a change of rock type in the subsurface giving an impedance contrast, waves will be partly reflected back at the interface and partly transmitted further into the subsurface. In detailed seismic reflection data studies in the search for coherent energy, one can observe two kinds of “seismic data events” that are generated in the subsurface and being recorded during seismic data acquisition: 1) seismic diffractions and 2)specular reflections. For better identification and understanding of recorded seismic data, it is beneficial to categorize the effect of subsurface discontinuities into these two categories and process them separately (several researchers have developed methods for this separation). Seismic interpretation is often done on specular reflection data and the effect of diffractions might be neglected in many cases, especially when they are not separated from the data and conventional seismic data processing is applied on the total recorded data. On the other hand, the diffraction energy can be instrumental in fault and fractures identification and characterization. In this abstract we review a method and implementation for the separation of diffracted energy from specular reflections and illustrate the successful application on a deepwater marine data with a complex seabottom surface. The field data example illustrates improved velocity picking on specular reflections in comparison with velocity analysis on the total recorded data (without separation). The examples confirm that a systematic methodology to separate seismic diffractions from the total recorded wave fields is essential and provides better control of locating faults and fractures optimally.
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Determination of AVO Attributes for Acoustic Impedance Zones of Malay Basin: Fluid Factors
Authors Uzir Alimat, Goh T.L, Shaidin Arshad and M. Izzuljad Ahmad FuadBased on previous study on Acoustic Impedance (AI) characteristics of the end member of two clastic rocks i.e. sandstone and shale, the Malay Basin can be generally divided into 2 major zones (Uzir et. al, 2009). It was observed in that study that the similar AI characteristic was displaying distinct distribution pattern, which later was postulated to be much related to the tectonic setting and depositional environment (Figure 1). Changes in AI pattern will directly affect the AVO response and its attributes. This paper is analyzing one of the most important AVO attributes which is the fluid factor and it is highly desirable for a hydrocarbon indicator (Smith and Gidlow, 1987). The typical published fluid factor (F =1.252A+0.580B) was derived from the combination of two well known equations i.e. Castagna mudrock equation and Gardner equation. The indicator should be negative for shale over gas-sand interfaces and significantly more negative than for shale over brine-sand interfaces (Castagna and Smith, 1994). The respective values of A and B were the intercept and the gradient attribute of reflection amplitude versus sin2 plot. The A and B values can also be calculated from Shuey Approximation equation (Shuey, 1985). In this paper, the fluid factor equations were derived based on rock physical trend lines of Vp versus Vs plot and density () versus Vp plot, which were obtained from 48 well logs data that have been rigorously conditioned (Table 1).
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