抄録
We analyzed azimuthal anisotropy of seismic velocity using 3D land seismic data in a Middle Eastern oil field, for predicting anisotropic horizontal stress and/or possible fractures. By applying sufficient correction, we extracted trustworthy distribution of azimuthal velocity variation from the 3D seismic data, which would represent distribution of the in-situ horizontal stress.
The 3D data was acquired with a rectangular geometry, in which the inline maximum offset was 3.6km and the crossline maximum offset varied from 1.8 km to 2.2 km periodically. The nominal fold was 36 for a 25×25 meter bin. For using sufficient volume of data for individual azimuthal angles, we first generated super-gathers with 400m x 400m super-bins. To minimize the influence of structural dip on the super-bin that presupposes flat reflectors, we applied a dip correction, which do not harm the seismic anisotropy information of the data.
Subsequently, we extracted azimuth dependence of seismic velocity at each super-bin. This analysis delivered 1) intensity of velocity anisotropy (the ratio of highest and lowest velocities) and 2) orientation of high velocity. These outputs were visualized by vectors at each super-bin, whose directions and lengths corresponded to the orientation and strength of velocity anisotropy. The result of velocity anisotropy presented that the highest velocity orientation was predominantly along NE-SW, which turned out to be consistent with the in-situ maximum horizontal stress orientation interpreted from borehole breakout of the resistivity image log data. Besides, the magnitude of seismic velocity anisotropy increased towards NE, i.e., to the proximity of active mountain range, which may indicate lateral variation of in-situ stress magnitude.
