GEOCHEMICAL AND GEOPHYSICAL CHARACTERIZATION OF THE ALUM SHALE FORMATION BY USE OF HIGH RESOLUTION XRF CORE SCANNING AND ACOUSTIC TELEVIEWER, ÖLAND, SOUTH SWEDEN

The Up­per Cambrian–Lower Ordovician Alum Shale is the most interesting gas prone shale formation in Sweden. The formation is also of interest by its relatively high content of metals and trace elements. Of main concern is, however, the environmental and health issues related to any exploration as well as existing radon and poor ground water quality within the areas of distribution. The Geological Survey of Sweden performs presently an overall characterization of the Alum Shale Formation, partly because of an increasing concern from the public and authorities in Sweden related to any exploration of the biogenic gas resources in the shale. Additionally, official reports in Europe and US have largely overestimated the shale gas potential in Sweden. Therefore, a national objective study, including mapping and characteriza­tion of dark shale formations in Sweden is undertaken. The work presented here includes results from a sub-study including high resolution geochemical and geophysical characterization of the Alum Shale on the island of Öland. Cores from the Segerstad-1 well and the newly drilled Grönhögen-1 wells were scanned with the Itrax Drillcore scanner (COX Analytical Systems). The cores were scanned in steps of 1 cm and 1 second/point. Over 30 different elements were recorded including the major elements; Al, Si, K, Ca, Ti, Fe, minor elements like P, Cl, S, Mn, Rb, Sr and trace elements including V, Cr, Ni, Cu, Zn, As, Br, Y, Zr, Mo, Nb, Ba, Pb, Th, and U. The XRF data were averaged over each core meter, and converted into absolute concentrations. The shale is characterized by high levels of Al, K, Ti and Rb, and typical proxies for TOC (Total Organic Carbon) like V, Ni, Mo and U. The presence of a light element fraction (carbon) in the shale part is also validated by the incoherent/coherent Ratio of scattering from the XRF data. The wells were also investigated using a combined high resolution Acoustic Televiewer and Gamma Ray log, a relatively novel method used in sedimentary bedrock in Sweden. The results visualize a detailed picture of the fracture system and structure of the rock. The combined data have resulted in a geophysical and geochemical type section for the Alum Shale on Öland, which is used for assessments of stratigraphic, diagenetic, depositional and environmental as well industrial aspects.