The 1998 Valhall microseismic data set: An integrated study of relocated sources, seismic multiplets, and S-wave splitting

摘要

We relocate 303 microseismic events recorded in 1998 by sen-sors in a single borehole in the North Sea Valhall oil field. A semi-automated array analysis method repicks the P- and S-wave ar-rival times and P-wave polarizations, which are needed to locatethese events. The relocated sources are confined predominantlyto a 50-m-thick zone just above the reservoir, and location uncer-tainties are half those of previous efforts. Multiplet analysis iden-tifies 40 multiplet groups, which include 208 of the 303 events.The largest group contains 24 events, and five groups contain10 or more events. Within each multiplet group, we further im-prove arrival-time picking through crosscorrelation, which en-hances the relative accuracy of the relocated events and revealsthat more than 99 of the seismic activity lies spatially in threedistinct clusters. The spatial distribution of events and wave-form similarities reveal two faultlike structures that match wellwith north-northwest–south-southeast-trending fault planes in-terpreted from 3D surface seismic data. Most waveform differ-ences between multiplet groups located on these faults can be at-tributed to S-wave phase content and polarity or P-to-S ampli-tude ratio. The range in P-to-S amplitude ratios observed on thefaults is explained best in terms of varying source mechanisms.We also find a correlation between multiplet groups and temporalvariations in seismic anisotropy, as revealed by S-wave splittinganalysis. We explain these findings in the context of a cyclic re-charge and dissipation of cap-rock stresses in response to produc-tion-driven compaction of the underlying oil reservoir. The cy-clic nature of this mechanism drives the short-term variations inseismic anisotropy and the reactivation of microseismic sourcemechanisms over time.