Abstract

Rock physics analyses of data from a wildcat well 7117/9-1 drilled in the Senja Ridge area, located in the Norwegian Barents Sea, reveal a change in stiffness within the fine-grained Paleogene Sotbakken Group sediments, caused by the transformation of opal-A to opal-CT, and opal-CT to quartz. This change manifests as flat spots on 2D seismic profiles. These flat spots were mistaken as hydrocarbon-water contacts, which led to the drilling of well 7117/9-1. Rock physics analyses on this well combined with a second well (7117/9-2) drilled further northwest and updip on the Senja Ridge, indicate overpressure within the opal-CT rich zones overlying the opal-CT to quartz transformation zones in the two wells. The absence of opal-A to opal-CT and opal-CT to quartz flat spots on seismic in the second well is attributed to differences in temperature and timing of uplift. In AVA modelling, both the opal-A to opal-CT and opal-CT to quartz interface points plotted on the wet trend, whereas the modelled gas-brine, oil-brine and gas-oil contacts fell within the quadrant-I. These findings will be useful in understanding the nature of compaction of biogenic silica-rich sediments where flat spots could be misinterpreted as hydrocarbon related contacts in oil and gas exploration.

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