We have investigated tectonic structures in several hundred meters of continuous core from five boreholes located in the Ordovician/Silurian shale formations of the Baltic Basin in Poland. Detail core examination of the natural tectonic structures and mechanically significant features comprised: mineral veins, fractured veins, bare fractures, slickensides, fault zones and bedding planes. All of our observations were compared with available, geophysical logging data. The most common tectonic structures are steep, rather small strata-bound fractures and veins with very low kinematic aperture. They create an orthogonal joint system locally disturbed by minor faults. For these regular joints, we have calculated average fracture intensity parameter by averaging fracture area per cubic meter of rock. We also constructed rose diagrams of fractures orientation weighted with height, aperture or percentage of cracked veins and also taking into account an orientation error. Obtained weighted diagrams enhance dominant sets of fractures and are more relevant for reservoir characterization than common diagrams considering only numbers of fractures. Finally, we compared results received from core with those from borehole scanner interpretation. This comparison reveals some interesting discrepancy between fracture profiles based on borehole scanner logs and direct borehole core profiles.

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