P-wave traveltime tomography for a seismic characterization of black shales at shallow depth on Bornholm, Denmark

Black shales are organic-rich rocks with the potential to contain oil or gas. On the Danish island Bornholm, Lower Palaeozoic black shales at shallow depth (<50 m) have a high maturity and a high total organic carbon content. We carried out seismic measurements in the southern part of Bornholm in October 2010 along two profiles to study the seismic properties of the shallow-lying black shales. The profiles were located as near as possible to existing borehole locations for geologic control, and were arranged perpendicular to each other to allow an estimate of the 3D structure of the subsurface. We analyzed the data using P-wave traveltime tomography and obtained 2D P-wave velocity models with a good resolution up to a depth of 100 m. Based on borehole information and the geologic setting, three formations can be identified in the tomography models: (a) the Komstad Limestone Formation with a thickness of about 4 m, (b) the Alum Shale Formation with a total thickness of 33 m, and (c) the Laesa Formation consisting of sandstones. The Alum Shale appears as a low velocity zone between the Komstad Limestone at the top and the Laesa Sandstone Formation beneath. All layers dip southwestward. We observed good agreements between borehole-logging data, acoustic velocity measurements of Alum Shale samples in the laboratory, and our P-wave velocity tomograms.