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The Upper Devonian-Lower Mississippian Bakken Formation in the Williston Basin is estimated to have significant amount of technically recoverable oil and gas. The objective of this study is to identify differences in the character of the seismic response to Bakken interval between locations with high and poor production rates. The predicted seismic responses of the Bakken Formation will hopefully help achieve such discrimination from surface seismic recordings. In this study, borehole data of Bakken wells from both the Cottonwood and the Sanish Field were analyzed, including density information and seismic P and S wave velocities from Sonic Scanner logs. The Bakken Formation is deeper and thicker (and somewhat more productive) in the Sanish Field and is shallower and thinner in the Cottonwood Field. The Upper and Lower Bakken shale units are similar and can be characterized by low density, low P and S wave velocities and low Vp/Vs ratios. The Sonic Scanner data suggest that the Upper and Lower Bakken shales can be treated as VTI media while the Middle Bakken may be considered as seismically isotropic. Models of seismic response for both fields were constructed, including isotropic models and models with variations in VTI, HTI, and the combination of VTI and HTI in the Bakken intervals. Full offset elastic synthetic seismograms with a vertical point source were generated to simulate the seismic responses of the various models of Bakken Formation. This sensitivity study shows pronounced differences in the seismic reflection response between isotropic and anisotropic models. P-P, P-SV and SV-SV respond differently to anisotropy. VTI anisotropy and HTI anisotropy of the Bakken have different character. In particular, types of seismic data (P-P, P-SV, and SV-SV) and the range of source-receiver offsets that are most sensitive to variations in anisotropic parameters and fluid saturation were identified. Results suggest that bed thickness, anisotropy of the Upper and Lower Bakken shales, fractures/cracks and fluid fill in the fracture/cracks all influence the seismic responses of the Bakken Formation. The paucity of data available for "poorly" producing wells limited the evaluation of the direct seismic response to productivity, but sensitivity to potentially useful parameters was established.
The Upper Devonian-Lower Mississippian Bakken Formation in the Williston Basin is estimated to have significant amount of technically recoverable oil and gas. The objective of this study is to identify differences in the character of the seismic response to Bakken interval between locations with high and poor production rates. The predicted seismic responses of the Bakken Formation will hopefully help achieve such discrimination from surface seismic recordings. In this study, borehole data of Bakken wells from both the Cottonwood and the Sanish Field were analyzed, including density information and seismic P and S wave velocities from Sonic Scanner logs. The Bakken Formation is deeper and thicker (and somewhat more productive) in the Sanish Field and is shallower and thinner in the Cottonwood Field. The Upper and Lower Bakken shale units are similar and can be characterized by low density, low P and S wave velocities and low Vp/Vs ratios. The Sonic Scanner data suggest that the Upper and Lower Bakken shales can be treated as VTI media while the Middle Bakken may be considered as seismically isotropic. Models of seismic response for both fields were constructed, including isotropic models and models with variations in VTI, HTI, and the combination of VTI and HTI in the Bakken intervals. Full offset elastic synthetic seismograms with a vertical point source were generated to simulate the seismic responses of the various models of Bakken Formation. This sensitivity study shows pronounced differences in the seismic reflection response between isotropic and anisotropic models. P-P, P-SV and SV-SV respond differently to anisotropy. VTI anisotropy and HTI anisotropy of the Bakken have different character. In particular, types of seismic data (P-P, P-SV, and SV-SV) and the range of source-receiver offsets that are most sensitive to variations in anisotropic parameters and fluid saturation were identified. Results suggest that bed thickness, anisotropy of the Upper and Lower Bakken shales, fractures/cracks and fluid fill in the fracture/cracks all influence the seismic responses of the Bakken Formation. The paucity of data available for "poorly" producing wells limited the evaluation of the direct seismic response to productivity, but sensitivity to potentially useful parameters was established.
Observing offset-dependent seismic reflectivity has proven to be a valuable exploration tool for the direct detection of hydrocarbons. This monograph provides a comprehensive review of reflection coefficients and their approximations in isotropic media, followed by an in-depth discussion of reflection amplitudes in anisotropic media. No prior knowledge of seismic anisotropy is assumed, and considerable effort is spent to introduce wave propagation and medium parameterizations useful for surface seismic applications in the presence of anisotropy. The first anisotropic model discussed is transverse isotropy with a vertical axis of symmetry (VTI media), typically used to describe shale sequences. Then the study of VTI reflection coefficients is extended to transverse isotropy with a horizontal axis of symmetry (HTI) - the symmetry system that describes a system of parallel vertical cracks. Analysis of the "Shuey-type" approximate HTI P-wave reflection coefficient makes it possible to devise fracture-detection algorithms based on the inversion of azimuthal differences of the P-wave AVO gradient. The monograph also presents analysis of shear- and converted-wave reflection coefficients for HTI and orthorhombic models, discusses practical aspects of applying the azimuthal AVO analysis, and mentions promising recent results.
The sensitivity of seismic reflection coefficients and amplitudes, and their variations with changing incidence angles and offsets, was determined with respect to changes in the parameters which characterize marine sediments containing gas hydrates. Using the results of studies of ice saturation effects in permafrost soils, we have introduced rheological effects of hydrate saturation. The replacement of pore fluids in highly porous and unconsolidated marine sediments with crystalline gas hydrates, increases the rigidity of the sediments, and alters the ratio of compressional/shear strength ratio. This causes Vp/Vs ratio variations which have an effect on the amplitudes of P-wave and S-wave reflections. Analysis of reflection coefficient functions has revealed that amplitudes are very sensitive to porosity estimates, and errors in the assumed model porosity can effect the estimates of hydrate saturation. Additionally, we see that the level of free gas saturation is difficult to determine. A review of the effects of free gas and hydrate saturation on shear wave arrivals indicates that far-offset P to S wave converted arrivals may provide a means of characterizing hydrate saturations. Complications in reflection coefficient and amplitude modelling can arise from gradients in hydrate saturation levels and from rough sea floor topography. An increase in hydrate saturation with depth in marine sediments causes rays to bend towards horizontal and increases the reflection incidence angles and subsequent amplitudes. This effect is strongly accentuated when the vertical separation between the source and the hydrate reflection horizon is reduced. The effect on amplitude variations with offset due to a rough sea floor was determined through finite difference wavefield modelling. Strong diffractions in the waveforms add noise to the amplitude versus offset functions.