Marine reflection seismic data inversion is a compute-intensive process, especially in three dimensions. Approximations often are made to limit the number of physical parameters we invert for, or to speed up the forward modeling. Because the data often are dominated by unconverted P-waves, one popular approximation is to consider the earth as purely acoustic, i.e., no shear modulus. The material density sometimes is taken as a constant. Nonlinear waveform seismic inversion consists of iteratively minimizing the misfit between the amplitudes of the measured and the modeled data. Approximations, such as assuming an acoustic medium, lead to incorrect modeling of the amplitudes of the seismic waves, especially with respect to amplitude variation with offset ͑AVO͒, and therefore have a direct impact on the inversion results. For evaluation purposes, we have performed a series of inversions with different approximations and different constraints whereby the synthetic data set to recover is computed for a 1D elastic medium. A series of numerical experiments, although simple, help to define the applicability domain of the acoustic assumption. Acoustic full-wave inversion is applicable only when the S-wave velocity and the density fields are smooth enough to reduce the AVO effect, or when the near-offset seismograms are inverted with a good starting model. However, in many realistic cases, acoustic approximation penalizes the full-wave inversion of marine reflection seismic data in retrieving the acoustic parameters.
Porosity heterogeneities in carbonate reservoirs occur at all scales, from the size of a reservoir delineated by seismic data and/or well-to-well correlations, down to microscales which can only be revealed by scanning electron microscopy. The origin of porosity heterogeneities is varied; some result from the depositional system, while others are products of burial, diagenesis and tectonism. Defining the three-dimensional distribution of textures and facies in a carbonate reservoir is exceedingly difficult. At present, carbonate specialists rely on cores to provide an understanding of decimetre to micronscale textures and fabrics in carbonate reservoirs. Cores are rarely available from every well in a reservoir and usually do not cover the entire reservoir interval. Thus, the characterization of carbonate textures, fabrics, pore types, and porosity distribution using wireline well logs in uncored wells provides additional valuable information. Such an approach requires the integration of all of the available data, including acoustic, nuclear, electric, and dielectric measurements for reliable geological and petrophysical analyses. The wide range of possibilities coupled with the complexity of many carbonate reservoirs, however, generally means that both core and logs have to be acquired, at least in a few key wells. The evaluation of borehole electrical imagery in carbonate reservoirs from different depositional settings with a variety of diagenetic histories, indicates that this technique is providing valuable, and sometimes new, information about porosity heterogeneities. Electrical imagery, which has a high resolution and provides three-dimensional data, usually reveals more of the complexity of pore distributions than standard well logs are capable of. Large individual pores, fractures, and vugs are often directly visible in these images, although microscopic pores are below the resolution of the technique. The porosity fabric, or decimetre-scale distribution of microscopic intergranular, intercrystalline and mouldic pores, is routinely defined, much as oil-saturated porosity in a core can be revealed by ultraviolet light. The correlation of electrical imagery with core samples, or drill cuttings in uncored intervals, can help to quantify the porosity fabric and also the geological interpretation of electrical texture and fabrics. On a decimetre-scale of examination, porosity is sometimes distributed homogeneously but is more conunonly heterogeneous. In addition to vugs and fractures, there are four basic geometrical fabrics of decimetre-scale porosity. Layering is the most common, sometimes clearly associated with stratification. However, the thickness of porosity layers can be uniform or quite variable within the same formation. The next most common heterogeneous fabric is patchy porosity distribution. This fabric can either bc patches of high porosity within low-porosity intervals or patches of low-porosity rock within porous reservoir zones. A common porosity fabric in Cretaceous and Tertiary carbonate sh...
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