AVO in azimuthally anisotropic media fracture detection using P-wave data and a seismic study of naturally fractured tight gas reservoirs

In tight gas reservoirs natural fractures often control the storage and mobility of gas in tight formations. Detecting and characterizing natural fractures are critical practical problems for reservoir characterization. The objective of this research is to develop a methodology to detect and characterize natural fractures using conventional P-wave data. The thesis includes three parts.;The first part studies AVO trends in azimuthally anisotropic environments. I present simple formulas to estimate AVO responses in orthorhombic media and show that azimuthal anisotropy can indeed significantly change AVO trends. The effect of anisotropy is quadratic in the sine of angle of incidence, and linear in the difference of anisotropy between the layers. When the media are only transversely isotropic, but with a horizontal symmetry axis, the AVO trends depend not only on the contrast in Thomsen's parameter ;The second part studies possible P-wave seismic signatures of fractures. The focus is on azimuthal variations of traveltime and AVO responses. The maximum amount of P-wave traveltime variations is approximately one half of the traveltime difference between the fast and the slow shear-waves, which can be tens of milliseconds. Azimuthal traveltime analysis is useful for detecting both fluid-filled and gas-saturated fractures, whereas azimuthal AVO analysis is effective only for fluid-filled fractures. Therefore, the best fracture indicator in tight gas reservoirs is P-wave traveltime anisotropy.;The last part is a report on a 9-component seismic study of a tight gas reservoir in the Powder River Basin, east-central Wyoming. We collected about 50 kilometers of 9-component surface reflection seismic data. Throughout the field site, the fracture direction trend consistently SW-NE. The fracture intensity is highly variable throughout the site--the corresponding shear-wave anisotropy in the Frontier-Niobrara zones ranges from near zero to 7 percent.;We have also observed intriguing indications of P-wave anisotropy. Azimuthal variations of AVO response and P-wave stacking velocity were observed at two locations where lines intersect. The azimuthal velocity variation is consistent with the directions of the fracture model.