Overview of results from deep drilling in the Siljan Ring impact struct

A meteorite impact in Devonian time, about 360 million years ago, resulted in the formation of the 52 km wide Siljan ring impact structure. As part of the Swedish Deep Gas Drilling project two deep boreholes to nearly 7 km depth were drilled within the crater rim (Gravberg-1 and Stenberg-1). Several shallower boreholes were also drilled. Both deep boreholes penetrate mainly granitic rock to total depth. Prior to drilling, extensive geophysical surveying was carried out in the area. These surface investigation together with data from the boreholes provide a large amount of information on the geological conditions in the upper crystalline crust in the area. Main results are (1) high amplitude reflections from the granitic bedrock correspond to sub-horizontal dolerite sills, (2) the upper 1-1.5 km of crust is highly fractured with more competent rock below, (3) hydraulic conductivity is low at about 10^-9 - 10^-10 m/s, but not insignificant, (4) highly saline fluids (salinity of 10-15%) are present below 6 km, (5) isotope data on calcite indicate groundwater may percolate to great depth, (6) the temperature gradient is nearly constant to 7 km at 16 degrees/km, and (7) the stress field is anisotropic with the maximum horizontal and vertical stresses being about equal and the minimum horizontal stress being about 2/3 of the vertical. Although the rock is relatively intact below 1.5 km, fracture zones are present at varying intervals down to total depth in both boreholes. In the deeper parts of the Gravberg-1 borehole, relatively large concentrations of helium and nitrogen are present. The isotopic signature and concentration levels of the helium indicate that the water at these depths has been stagnant on the order of hundreds of millions of years.