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Unloaded Rocks due to Secondary Pressure Mechanisms - Cause for False Negatives?

Abstract

The exact same conditions that control the sediments in the sub-surface in terms of elastic properties also control the geo-pressure side of things. What can be said about abnormal pressures in the sub-surface is that it often means trouble. For an explorationist, it could mean blown reservoir seals and misinterpreted AVO signatures (e.g. Shaker, 2009); for a driller, it could mean excessive time spent fighting formation fluid influxes and/or drilling fluid losses. The aim of this abstract is to describe the importance of how integration of elastic properties and pressure is essential in order to better understand and constrain AVO signatures in the sub-surface, and particularly how amplitudes in the load transfer domain (Lahann et al., 2011) may differ from what is expected at a given prospect level, and thus may be overlooked as false negatives if not accounted for. This is particularly important in a regional context for exploration purposes (e.g. regional studies). Several authors (Avseth et al., 2008; Japsen et al., 2007) uses depth trends for shales and sands in order to facilitate expected AVO responses with increasing burial depth, but only focuses on chemical compaction effects related to quartz cementation in sands and cementation related to the smectite to illite transformation in shales and potential disequilibrium compaction effects, but leaves out load transfer effects. While the aforementioned authors use examples from the Norwegian Continental Shelf to address shale diagenesis, specifically from the North Sea and the Norwegian Sea, Katahara (2006) and Lahann et al. (2001) use data from the Gulf of Mexico to highlight issues related to how pressure influences the elastic properties in cross-plot space. Primary findings from both Katahara and Lahann is that the S-I transformation doesn't necessarily increase density and velocity, mostly because of secondary pressure mechanisms occur simultaneously as shale diagenesis. Lahann showed complex behaviour of how the velocity and density reacts to various mechanisms affecting these two elastic properties, and thus will alter seismic responses as a result of that. Elastic behaviour and pressure mechanisms have to be combined in a regional context in order to fully understand seismic amplitudes in exploration. Failure to do so might result in false negatives, and thus potentially overlooked HC prospects as a direct consequence.