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

Werelocate303microseismiceventsrecordedin1998bysensorsinasingleboreholeintheNorthSeaValhalloilfield.Asemiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly toa50-m-thickzonejustabovethereservoir,andlocationuncertaintiesarehalfthoseofpreviousefforts.Multipletanalysisidentifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and waveform similarities reveal two faultlike structures that match well with north-northwest‐south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differencesbetweenmultipletgroupslocatedonthesefaultscanbeattributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. Wealsofindacorrelationbetweenmultipletgroupsandtemporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic rechargeanddissipationofcap-rockstressesinresponsetoproduction-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanismsovertime.

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