Deformation of crust in the Tibetan Plateau

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, February 1999.Includes bibliographical references.(PART I) In this thesis, we study the possibility of using P-waves to investigate properties of fractured reservoirs and the diagnostic ability of the P-wave seismic data to detect fractures. The objectives of this thesis are threefold: (1) to identify the effects of fracture parameters and background media on the NMO velocities and azimuthal AVO responses in homogeneously fractured reservoirs, (2) to investigate the scattering characteristics in heterogeneously fractured reservoirs and the diagnostic ability of these characteristics to recover fracture density information, (3) to apply these theoretical studies and techniques to the field data analyses in order to detect fractures. Fracture properties, described by fracture parameters (fracture density, fracture aspect ratio and saturating fluid), are related to anisotropic parameters of the equivalent VTI model and affect the NMO velocities. Studies show that the shear wave splitting parameter is most sensitive to crack density and is insensitive to saturating fluid content and the crack aspect ratio, while 8(v) and Ev) have different characteristics in gas- and water-saturating, fractured rocks. The effects of fracture parameters on P-wave NMO velocities are comparable with the influences of 8(v). Unlike anisotropic parameters and NMO velocities, P-wave azimuthal AVO variations are not necessarily correlated with the magnitude of fracture density. Studies show that the properties of background rocks have an important effect on P-wave azimuthal AVO responses. Azimuthal AVO variations at the top of gas-saturated, fractured reservoirs which contain the same fracture density are significant in the reservoir model with small contrasts in Poisson's ratio. Varying fracture density and fluid content can lead to variations in AVO gradients in off fracture strike directions. The presence of overburden anisotropy caused by VTI media can significantly distort AVO gradients, which suggests that the inversion of fracture parameters based on an individual AVO curve would be biased without correcting for this influence. By using primarily P-wave data, studies indicate that though studying azimuthal AVO variations could be effective for detecting fractures, the combination of other of data is more beneficial than using reflection amplitude data alone. Quantitative interpretation can be difficult when using the P-wave seismic signatures alone because of the variables discussed above. Considering heterogeneity of fracture density due to spatial variations, statistical representation is employed to model the fracture density field and build the fracture density realizations. In this study, for stochastic modeling to be practical, the fracture density field is modeled as a spatially stationary Gaussian random field. The von Karman correlation function is used to model heterogeneity in fracture density, which is specified by the function describing its amplitude, orientation, characteristic wave numbers and its roughness number. The scattering characteristics, including amplitude and frequency, are investigated at the tops and the bases of gas saturated, fractured reservoirs. Results show that the strength of the scattering field is a function of the background medium. Scattering field, are weak at the top of fractured reservoirs. The first order results are dominated by velocity anisotropy of a mean crack density field. The base of the fractured reservoir corresponds to a strong scattering field on which fracture heterogeneity has a larger effect. The strength of the scattering field at the base of a fractured reservoir is inversely proportional to the correlation length of the fracture density field and is proportional to the scatter (fracture) density. An estimation procedure is applied to field data analyses to obtain the offset-dependent attributes, the amplitude versus offset (AVO) and the frequency versus offset (FVO). These attributes are applied to determine principal orientation of fractures in carbonate fractured reservoirs located in the Maporal field in the Barinas basin in southwestern Venezuela. Our studies show that, in this area, P-wave reflectivity is characterized by a large increase of amplitude with offset (large positive AVO gradient) and a large frequency decay with offset (large negative FVO gradients) in the crack normal direction. In the crack strike direction, P-wave reflectivity is characterized by a scatter distribution of AVO gradients but a small variation of FVO gradients. Velocity attributes, inverted from near-offset and the whole range of offset stacked amplitude, are used to predict the lateral lithological heterogeneity in the reservoir zone. The variations of AVO gradients in the crack strike direction can be attributed to heterogeneity in the reservoir. (PART II) The objectives of this thesis are twofold: (1) to understand how the strength of upper and lower crust influences the topographic features, (2) to understand the tectonic evolution of the Tibetan plateau and the basic processes that control its deformation. A 3-D, large-scale analytical model is used to model continental crustal deformation in the Tibetan plateau. The idealized crust is assumed to consist of two layers, an upper layer with a uniform viscosity and a lower layer with viscosity decreasing exponentially with depth. The motions of the underlying mantle are the fundamental driving forces for crustal thickening. The parameters used in modeling are constrained mainly by observations in the India-Asia collision zone of topography. The modeled results reveal that the growth of the Tibetan plateau can be divided into two phases: the mountain building and plateau phases. With the thickening of crust, the spatial and temporal characteristics of deformation are different in these two phases. Numerical experiments yield spatial distributions of velocity, strain and rotation rate on the modeled Tibetan plateau. The modeled velocity results indicate that right-lateral motion occurs between India and south China. Large extension is located at the edges of the high plateau and the development of surface extension is proportional to the weakness of the lower crust. Rapid shortening occurs on the flanks of the plateau and is oriented roughly perpendicular to the regional topographic contours. In the central plateau, deformation is characterized by E-W extension. Much of the relative right-lateral motion is accomplished by clockwise rotation in the region of eastern syntaxis. The maximum rates of rotation about vertical axes occurs in the region of the eastern syntaxis proper.by Feng Shen.Ph.D