12th International Congress of the Brazilian Geophysical Society

We extend prestack parsimonious Kirchhoff depth migration (Hua and McMechan 2003) (which is a fast migration), to two-dimensional (2D), two-component (2C), reflected elastic seismic data, from a P-wave source. The P-to-P reflected (PP) waves and P-to-S converted (PS) waves in an elastic common-source gather recorded at the earth’s surface are first separated into PP- and PS-wave seismograms. Source and receiver apparent slownesses (p values) are estimated for the peaks and troughs in both separated PP and PS waves. For each PP and PS reflection, a source ray is traced, in the P- (or the S-) velocity model, in the direction of the emitted ray angle (determined by the source p value), and a receiver ray is traced, in the P- or S-velocity model, back in the direction of the emergent PP (or the PS) wave ray angle (determined by the PP or PS wave receiver p value), respectively. The image point is adjusted from the intersection of the source and receiver rays to the point where the sum of the source and receiver times equals the observed two-way reflection traveltime. The orientation of the reflector surface is determined to satisfy Snell’s law at the ray intersection point. The amplitude of a P-wave (or an S-wave) is distributed over the first Fresnel zone along the reflector surface in the P (or S) image. Stacking over all the single-source P- and S-images separately gives the stacked P- and S- images, respectively. The quality of prestack parsimonious elastic Kirchhoff migration is not as good as that of full prestack Kirchhoff, or reverse-time, migration, but the computing time is reduced by orders of magnitude because the amount of ray tracing is significantly reduced. Thus, parsimonious elastic migration is most useful, when reducing computing time is more important than migration quality, such as in migration velocity analysis, which iterates migration many times.

Fracture surface characteristics have significant effect on fracture hydraulic conductivity. The available acid-fracture conductivity correlations do not consider surface characteristics or make an incipient use of it. A proper description of the acid-fracture surfaces is the initial step towards the right consideration of surface roughness in hydraulic conductivity. This paper presents an areal (3D) surface evaluation of acid-etched fractures, simulated in samples taken from whole cores of an oil producer limestone. The topography of acid-fractured surfaces was assessed using a laser profilometer. The surfaces were evaluated with a set of 3D surface parameters. The results showed that the main features of acid-etched surfaces are large roughness, negative skewness, high kurtosis, and intermediate isotropy, mostly random but with some spatial orientation. The acid-fractured surfaces can be represented by the rms height, which showed great linear correlation with most of the surface parameters. The parameters texture aspect ratio, bearing index, valley retention index, and density of summits showed low correlation with rms height. A method to calculate fracture width from surface topography was developed. An attempt to explain abnormal behavior in initial conductivity tests revealed the potential use of surface characterization for fines management in oil and gas reservoirs. Improved acid fracture correlation may be achieved using surface characterization parameters.

Shale gas is natural gas from shale formations which acts as both the source and the reservoir for the natural gas. Each Shale gas reservoir has unique characteristics. Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide permeability . For a given matrix permeability and pressure, gas production are determined by the number and complexity of fractures created, their effective conductivity, and the ability to effectively reduce the pressure throughout the fracture network to initiate gas production. Understanding the relationship between fracture complexity, fracture conductivity, matrix permeability, and gas recovery is a fundamental challenge of shale-gas development. Shale gas reservoirs almost always have two different storage volumes(dual porosity) for hydrocarbons, the rock matrix and the natural fractures .Because of the plastic nature of shale formations, these natural fractures are generally closed due to the pressure of the overburden rock. Consequently, their very low, matrix permeability, usually on the order of hundreds of nanoDarcies (nD), makes unstimulated, conventional production impossible. Almost every well in a shale gas reservoir must be hydraulically stimulated (fractured) to achieve economical production. These hydraulic fracture treatments are believed to reactivate and reconnect the natural fracture matrix .

The point of view of interpreters working with 3D-VSPs interpretation is seldom explored. We report here their experience during the interpretation of a marine and two land 3D-VSP projects.

The investigation of greenfield terrains through the airborne geophysics dataset provides valuable information about the geologic context and, mainly, the metalogenetic understand. Almost exclusively, there are available regional (low resolution) airborne geophysical data, like gamma-ray spectrometry and magnetic data, and, in this context, some information about the geology can be interpreted. But, the use of high resolution data, like the gradiometer magnetometry, and remote sensing images increases the potential of applications for the characterization of these terrains, like the vast area of the Amazon craton. The Tapajós Mineral Province, located in the states of Amazonas and Pará (Brazil), was selected as a case study in this paper for the recognition of the potential targets for mineral exploration.

Mafic dykes’ swarms of the Tapajós Mineral Province western portion were investigated by use of airborne geophysical data characterized by different resolutions. Additionally, geochemistry data related to amount of gold in soil and in rock were integrated with geophysical images and interpretations were done. Specifically, the relationship of gold anomalies with the approximation of mafic dykes was observed, like in Rosa de Maio, Bandeirante and Maués prospects, besides the deformations of this mafic geologic unit, which give new insights for interpretations for this area.

Fracture detection technique based on 3-D P-wave seismic dataset has been a powerful tool for fractured reservoir characterization in recent years. However, as the most economical method of fracture detection, the accuracy and resolution of this method has been doubted sometimes. The major aspect of this paper is to prove the effectivity of this method. Traditionally, fracture information comes from well data, in particular image log and core. The purpose of this article is to prove that P-wave azimuthal anisotropy technique not only can describe the fracture character in thin limestone reservoir to regions between the wells but also can get result with good accuracy and resolution. The results indicated that the final fracture orientation and intensity got from 3D P-wave azimuthal anisotropy are extremely consistent with FMI/CMI data from drilled wells.

Walkaway VSP data provides information for interval anisotropic parameter estimates for transverse isotropy with vertical symmetry axis (VTI). For any depth tomographic/migration velocity analysis, we need an adequate initial depth velocity model. Dix-type approach provides us with explicit inversion formulas for velocity model estimation. To estimate these interval parameters, using Dix-type inversion approach, we need to estimate non-hyperbolic term of NMO function. In walkaway VSP data, NMO function can be obtained from first breaks, while in surface seismic it is estimated through non-hyperbolic semblance analysis. In both cases, determination of non-hyperbolic term is the most unstable procedure in interval VTI inversion. Because exact explicit formula for reflected time in a layered (even isotropic) media is unknown, we have to use some approximation. Different approximations have different accuracy in different geology. Here we investigate accuracy of different approximations on four models with weak and strong anisotropy and modest and large vertical velocity changes.

The characterization and environmental monitoring of Municipal Solid Waste (MSW) deposits are an increasing area, in terms of the interdisciplinary studies involved. Whatever the solution adopted for the MSW treatment, its final containment on landfill is not the end of the problem. Rather, the need for continuous monitoring, tracking the evolution of the waste mass in all its surroundings, will contribute to evaluate the environmental consequences of the existence of a deposit of MSW. The assessment of contaminants dispersion in Controlled Dumps (CD) of MSW is possible through the combination of geophysical, geochemical and geostatistical methods as described in this paper. The methodology applied in this study will contribute to evaluate the environmental consequences of the Matosinhos CD (N Portugal), which is set in a granitic crystalline geological context with a permeability controlled by differential weathering associated with fracturing. The statistical methods here described (Lepeltier, 1969; Matschullat et al. 2000), more commonly applied to geochemical data, were adapted to be used with our set of 2D resistivity profiles in order to relate them and thus delineate the contamination anomalies in the CD surroundings. Groundwater samples were collected in piezometers, upstream and downstream of the CD, with the aim of directly confirming the presence of contaminants indirectly detected by the geophysical and geostatistical methods. The combination of these different approaches allows an approach towards the detection and delineation of contaminant plumes from these deposits.

The geophysical methods are usually used as auxiliary tool in geological mapping. In the Rio Pardo de Minas sheet, located in the northern state of Minas Gerais, was used to detail mapping to 1:50.000 the magnetometry and gamma-spectrometry. The maps of Analytic Signal and Vertical Derivative were used to determine guidelines and bodies of the region, maps of K, Th, U and their composi-tions are used to dertermine a lito-geophysics regions. This work has aims to aid the geological mapping of the region.