Aligned fractures in carbonate rocks: laboratory and in situ measurements of seismic anisotropy

By vertical seismic profiling and shear wave analysis we show that a packet of carbonate reservoir rocks, found at nearly 3000 m depth in the North Ge rman Basin, is seismically anisotropic. For vertical paths of wave propagat ion the estimated velocity difference of the split shear waves is 10%. No s hear wave birefrigence is observed within the hangingwall which, therefore, has to be regarded as isotropic or transversely isotropic. Additional labo ratory investigations of the petrography of drilled carbonate samples and o f their seismic velocities show that the anisotropy is most probably caused by subvertical fractures with preferred azimuthal orientation. The strike direction of the aligned fractures determined by analysis of split shear wa ves is approximately N55 degrees E. This value agrees with recently publish ed directions of maximum horizontal tectonic stress in pre-Zechstein sedime nts in the eastern part of the North German Basin, but it is in contrast to the world stress map.