Baishali Roy

Seismic and potential-field imaging of the Guichon Creek batholith, British Columbia, Canada, to delineate structures hosting porphyry copper deposits

The Guichon Creek batholith (GCB), located in south‐central British Columbia, contains several large, low‐grade copper deposits of considerable economic importance. The surface geology of the Guichon batholith and its surrounding region have been well mapped; however, little information about subsurface features is available. The batholith consists of four major phases, emplaced radially outward, which can be separated on the basis of their texture and composition. Previous interpretation of gravity data suggests a mushroom‐shaped structure for the batholith. Data from Lithoprobe seismic reflection line 88-11, acquired across the batholith in 1988, reveal weakly coherent east‐dipping reflections on the west side and west‐dipping reflections on the east in the upper 10 km. To determine if these are related to structures associated with the batholith, we reprocessed the upper 6 s with particular emphasis on applications of signal enhancement techniques (e.g., pattern recognition methods, refraction statics,...

Recent applications of turning-ray tomography

Recent applications of 2D and 3D turning-ray tomography show that near-surface velocities are important for structural imaging and reservoir characterization. For structural imaging, we used turning-ray tomography to estimate the near-surface velocities for static corrections followed by prestack time migration and the near-surface velocities for prestack depth migration. Two-dimensional acoustic finite-difference modeling illustrates that wave-equation prestack depth migration is very sensitive to the near-surface velocities. Field data demonstrate that turning-ray tomography followed by prestack time migration helps to produce superior images in complex geologic settings. When the near-surface velocity model is integrated into a background velocity model for prestack depth migration, we find that wave propagation is very sensitive to the velocities immediately below the topography. For shallow-reservoir characterization, we developed and applied azimuthal turning-ray tomography to investigate observed apparent azimuthal-traveltime variations, using a wide-azimuth land seismic survey from a heavy-oil field at Surmont, Canada. We found that the apparent azimuthal velocity variations are not necessarily related to azimuthal anisotropy, or horizontal transverse isotropy (HTI), induced by the stress field or fractures. Near-surface heterogeneity and the acquisition footprint also could result in apparent azimuthal variations.

Analytic correction for wavelet stretch due to imaging

The presence of wavelet stretch due to imaging presents serious difficulty in AVO or inversion analysis, especially for 3-term wide-angle analysis. Wavelet stretch significantly alters the gradient and wide-angle coefficient and reduces resolution of stacks. In this paper we present a method for correcting wavelet stretch that is exact for any v(z) (layered) medium. It does not depend on an underlying AVO/AVA approximation and is therefore applicable for 2or 3-term AVA analysis. The required input is an extracted wavelet from any known reflection angle. The resulting correction operator is stationary over the time coordinate of the angle domain and is robustly implemented by a Weiner-Levinson method. This filter corrects angle gathers for wavelet stretch, producing improved resolution in subsequent angle stacks or gradient computations. Wavelet stretch correction is essential for linear inversion for density.

Imaging oil-sand reservoir heterogeneities using wide-angle prestack seismic inversion

Mass density, due to its linear relationship with porosity, has long been recognized as a potential seismic indicator of fluid saturation. Given its dependence on mineral composition, density can also be diagnostic for lithology. In this paper we discuss some key aspects of a wide-angle processing and density inversion workflow, and apply it to a bitumen reservoir in Canada for imaging reservoir heterogeneities, such as shales, that can potentially act as permeability baffles. In this field, intrareservoir shales typically have higher densities than surrounding reservoir sands. This wide-angle workflow yields stable density estimates, from reflected P-waves alone, at a resolution suitable for mapping the intrareservoir shales.

Integrated characterization of hydraulic fracture treatments in the Barnett Shale: The Stocker geophysical experiment

AbstractWe integrated several independent geophysical and geologic methods to examine the effects of stratigraphic and structural heterogeneities on the growth of hydraulic fracture networks from two horizontal wells in the Barnett Shale, Fort Worth Basin, Texas. Our data set included time-lapse 3D seismic surveys, microseismic data, wireline logs, and distributed temperature sensing (DTS) data. We first created a local stratigraphic framework using wireline logs. In our area, the lower Barnett Shale consists of siliceous mudstones (the primary reservoir) intercalated with carbonate submarine fan deposits. The latter are low porosity (i.e., nonreservoir) and, if thick enough are potential baffles to the growth of hydraulic fractures. We used stochastic inversion to define the 3D distribution of fan lobes with much better resolution than could be obtained using deterministic inversion and obtained a geologically reasonable lithology prediction. The lowest of the fan lobes partially overlies the two horizon...

Wide-angle inversion for density: tests for heavy-oil reservoir characterization

Summary Mass density, due to its linear relationship with porosity, has long been recognized as a potential seismic indicator of fluid saturation. Given its dependence on mineral composition density can also be diagnostic for lithology. In this paper we discuss some key aspects of a wide-angle processing and density inversion workflow, and apply it to a heavy-oil reservoir. In this field intra-reservoir shales typically have higher densities than surrounding reservoir sands. This wide-angle workflow yields stable density estimates, from reflected P waves alone, at a resolution suitable for mapping the intra-reservoir shales. Our field test indicates that data from an additional 15°20° of reflection-angle aperture, beyond the conventional 40° far-angle limit, act as an effective constraint on the inversion density solution. However, these wide (60°) reflection angles cannot be successfully processed and inverted with conventional workflows. Some of the key wide-angle considerations include anisotropic prestack imaging, a wavelet stretch correction and regularization of the inversion solution based on appropriate statistical constraints. Anisotropic imaging is required to position reflections from beyond 40° consistent with their corresponding small-angle image time. However, imaging introduces about 50% wavelet stretch in a 60° angle stack. A subsequent waveletstretch deconvolution reduced the stretch to about 13% (equivalent to stretch at 30°) in this test. These stretchcorrected angle gathers form the input data for a constrained linear prestack inversion. The constraining term characterizes statistical correlations between elastic properties measured in wellbores local to the field. This improves scaling and continuity of the density amplitudes, over and above the stability achieved by inverting wideangle data.

Prediction of Frequency Dependent Velocity in Porous Reservoirs

Summary This paper presents a mathematical technique to predict sonic logs at the seismic frequency from well logs. A correlation approximation technique has been developed that takes into account the effect of 1-D scattering from random layered media with inhomogeneous inclusions. The results demonstrate that the dynamic characteristics of porous media are substantially dependent on the inclusion sizes. The dependencies of velocities and inverse quality factor on frequency have maxima whose locations are governed by the inclusion size. Predicted velocities at seismic frequencies derived using the correlation approximation indicate major differences from the Backus averaging technique near heterogeneous inclusions.

Introduction to special section: Monitoring reservoir and overburden changes

Time-lapse geophysical technologies, including 4D seismic methods, are key enablers for improved hydrocarbon recovery and more cost-effective field operations. By analyzing differences in multiple geophysical surveys and by integrating those analyses with conventional reservoir monitoring data, time

Attenuation: Advances in analysis and estimation — Introduction

Attenuation analysis can provide valuable information about lithology, presence of fluids, and physical properties of subsurface rocks and fluids. In addition, other properties such as permeability, mobility of fluids, and fluid saturation, that cannot be inferred from velocity or amplitude-variation-with-offset (AVO) analysis, could possibly be extracted from attenuation analysis. Despite the many potential benefits of attenuation analysis, it has yet to become part of routine reservoir characterization primarily because of two reasons: first, challenges involved in its estimation from reflection seismic data, and second, understanding of the physical mechanisms and the relation to rock properties. Since estimating interval attenuation from reflection seismic data is challenging, most studies estimate the effective attenuation or use direct arrivals from crosswell and vertical seismic profile (VSP) data to estimate the local interval attenuation. Although interval attenuation estimation using layer-stripping approaches have been introduced recently, they are mostly confined to layer-cake subsurfaces. However, recent advances in …

Near-Surface Complexity Could Masquerade As Anisotropy

Near-surface velocities could vary with azimuth, impacting seismic data processing and interpretation. In this study, we developed a methodology to investigate the variations of near-surface velocities with azimuth, using 3D turning-ray tomography. The input data are the first arrivals selected from pre-defined azimuth sectors in terms of shot-receiverpair directions. The output velocities from tomography correspond to the selected azimuth sectors. A near-surface tomography study based on seismic data from a shallow heavy-oil reservoir in Canada has suggested that the observed azimuthal traveltime variations are not necessarily related to azimuthal (HTI) anisotropy induced by the stress field or fractures. It could also be caused by the nearsurface heterogeneity or acquisition footprint. Near-surface complexity could masquerade as anisotropy. Potentially this can influence statics and prestack imaging.