P- and SH-wave seismic imaging of the Chalk Group: Aspects of Acquisition and Interpretation

Research output: Book/ReportPh.D. thesisResearch

The Danish Chalk Group has been of great commercial interest for decades due to offshore hydrocarbon reservoirs in the Danish North Sea. This PhD thesis aims to gain a deeper understanding of rock physical parameters and geological structures of the Chalk Group that are determining for the seismic imaging, and investigate the distribution of the rock physical parameters in the Chalk Group. To achieve an understanding of small-scale changes within the Chalk Group, I use high-resolution P- and SH- wave onshore reflection seismic profiles with a close tie to outcrops and boreholes. The main field site is located at the Stevns peninsula in eastern Denmark. A set of P and SH-wave seismic data were acquired at the same locality and with similarly dense receiver setups. The P-wave profile was acquired with a 45-kg accelerated weight drop impulse source,while the SH-wave profile was acquired with a small seismic shaker source. A close tie to the Stevns-1 borehole provides excellent control on lithological and facies changes down to about 440m depth. Integration of the P-wave profile and the borehole information shows reflectivity strength and continuity dependencies of clay content, but no dependency of the presence of flintbands. For the first time, it was also possible to image fine layering in the perceived clean chalk member (Hvidskud) and outcrop-scale mound structures within the Danian and Upper Maastrichtian succession. The SH-wave data set shows good data quality and correlates well to the P-wave data in the most prominent reflections. The upper 200 m are significantly better resolved in the SH-wave profile and offer more detail of the near surface layering and faulting. When calculating VP/VS based on different correlated reflection horizons across the profiles, it becomes apparent that lateral changes appear to be weak whereas vertical changes correlate with changes in reflectivity strength. This observation is consistent when comparing to outcrop scale variability of porosity and density in the upper 40 m of the Danian and Maastrichtian chalk. Variations with depth align with changes in neutron porosity in the clean chalk units from wireline log data. In areas where the chalk is near the surface, compressional vibro-seis data suffer from unusually high S-wave energy. Contrary to this observation, impulse source surveys and SH-wave vibro-seis surveys do not suffer from unusually strong mode conversions. Forward modeling shows, that the cause of the observed compressional vibro-seis source performance is a combination of near-surface heterogeneity and high Poisson’s ratios.
Original languageEnglish
Number of pages128
Publication statusPublished - Sep 2019

ID: 229808771