Implications of the New Geophysics for Hydrocarbon Production

Azimuthally varying seismic shear-wave splitting (SWS) (seismic birefringence) is widely observed above swarms of small earthquakes, in reflection and refraction surveys, and in local and teleseismic arrivals in almost all igneous, metamorphic, and sedimentary rocks throughout the Earth’s crust and uppermost ~400km of the mantle (Crampin 1994; Crampin and Peacock 2005, 2008). The splitting is caused by propagation through pervasive distributions of fluidsaturated stress-aligned vertical microcracks in almost all in situ crustal rocks, and intergranular films of hydrolysed melt in the mantle. Fig. 1 is a schematic illustration of SWS in stress-aligned microcracks. These fluid-saturated microcracks are the most compliant elements of in situ rock, so that changes of in situ stress in rock at depth modify microcrack geometry. These changes can be monitored by SWS (Crampin 1999, 2006; Crampin and Peacock 2008). The observed range of shear-wave velocity anisotropy (SWVA) is ~1.5% to ~4.5% in ostensibly-intact unfractured in situ rocks throughout the crust and mantle (Crampin 1994, 1999; Volti et al. 2003).