Challenges of core-log-seismic integration in metamorphic rocks: A case study for the ICDP drilling project COSC-1, Sweden

Driven bysignificant economic benefits, the hydrocarbon industry developed sophisticated methods for the integration of geophysical and geochemical measurements with direct core observations. However, these techniques were developed primarily for sedimentary settings and have been applied only seldom in metamorphic settings. One outstanding example for comprehensive geophysical and borehole data integration is the COSC-1 borehole in the central Scandinavian Caledonides. It was drilled in 2014 and resulted in an extensive dataset to shed light on deformation during continental collision. Our study combines data from downhole logging and zero-offset vertical seismic profiling at COSC-1, with 2D and 3D seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. We show that there are powerful tools to distinguish between mafic and felsic lithologies in log-core correlation but that metamorphic settings bear special challenges for core-log-seismic integration. In contrast to sedimentary basins, reflections in the Seve Nappe Complex are not as distinct but we can link several of them to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. It appears that in spite of the metamorphic overprint around 417+/-9 Ma, the original configuration of the volcanic passive margin is partly preserved in the Seve Nappe Complex and it thus outlasted continent-continent collision including the nappe emplacement. Integration of borehole and three-dimensional geophysical data reveals lithological changes that can then be extrapolated in three dimensions to arrive at a better understanding of the compositionand geometry at mid-crustal levels. Furthermore, our results suggest that mid-crustal reflectivity at COSC-1 is primarily a function of pre-orogenic lithological variations which has to be considered when deciphering mountain building processes