Parallel structural interpretation of drill cores and microresistivity scanner images from gas-bearing shale (Baltic Basin, Poland)

Abstract

Having access to drill cores and microresistivity scanner images from five shale gas exploration boreholes, we were able to compare the results of structural interpretation based on two data sets. The most frequent structures observed in shale complexes are subvertical strata-bound joints that commonly create calcite veins. We have applied a modified approach for statistical analysis of strata-bound fractures taking into account their height. For comparison of cores and scanner image log structural interpretations, we used the fracture number and fracture intensity parameters. We found significant discrepancies between results of cores and image log interpretations. The much greater number of fractures recognized in the image log than in the core is explained by differences in the observation space related to the core and borehole diameters. To predict which fracture that was visible in the scanner image should be represented in the core, we introduced a “critical angle” parameter and used it in the filtering procedure, which gave satisfactory results. In general, the systematically observed superiority of fracture intensity in the scanner image over the core profile is explained by a large number of tiny noncracked veins that are better recorded by a scanner then are visible by the unaided eye. The most striking difference was found in carbonate-rich formations, in which noncracked veins are more numerous. On the contrary, fracture intensity in intervals enriched in total organic carbon (TOC) is always higher in core than in the scanner image, due to a resistivity enhancement related to gas presence. We also compared a record of en echelon arrays of open fractures that allow us to discriminate enhanced natural fractures from borehole-induced tensile fractures. A major difference in the bedding fracture density between the core and image log we attribute to core relaxation during its extraction to the surface. A tectonic inversion phase was also possible to recognize based on the integrated core and scanner interpretation.