Azimuthal Seismic AVO Responses of the Fracture Zones in the Bakken Formation

Abstract

The presence of vertical aligned fractures in the Bakken Formation results in azimuthal seismic AVO responses. In this paper, we design a realistic geologic model of the fracture zone in the Bakken Formation, and investigate corresponding azimuthal seismic AVO responses. In the model, based on results from formation images, we assume that fractures and cracks cut through all seven major geologic units in the Bakken Formation with a specific ratio of fracture intensity about 3:3:2:1:2:1:1. Corresponding anisotropy parameters are calculated based on Hudson's theory. We then employ the reflectivity method to generate elastic seismograms. AVO curves picked for four interested geologic units indicate azimuthal variations in amplitudes especially at far offset. The decrease in amplitude at far offset corresponds to the increase in azimuth, and the AVO gradient also presents a linear variation with azimuth. However, because picking of the exact amplitudes is difficult due to interference and the presence of overlying fractures, it may be appropriate to treat the formation as an integrated part and investigate azimuth variations in RMS amplitudes. Comparison results indicate that the stacked RMS amplitudes act as a better indicator of the variations in crack density and fluid saturation than the corresponding AVO gradient.