Shallow and Deep Drillings

The Deep Gas Project started off with a series of seven shallow core drillings to characterize the uppermost part of the impact structure (down to ca. 800 m). Their locations are indicated in Fig. 2. Five drill holes were located on the central rise and two in the eastern sedimentary ring basin. From these investigations information was obtained of the porosity of the involved rocks and their physical properties (density, magnetic susceptibility and remanent magnetization). From rock magnetic measurements on drill cores from the centrally located test hole, it was found that below 240 m depth, the remanent magnetization made a relatively larger contribution to the magnetization below

240 m depth as compared to normal crystalline rocks, where it generally is very low. In Table 3 q-values are given for Jarna granite from the exterior, the outer part of the central uplift and from its central part, respectively. The most shock affected crystalline rocks are located in this central part of the uplift.

The first deep drilling was located at Gravberg in the northern part of the central uplift (No 1 in Fig. 2) and within a large gravity low that was thought to be related to the impact. The drilling reached a depth of 6957 m, where it was terminated due to increasing technical difficulties. The Gravberg-1 hole is, thus, among the deepest drillings made in Europe. The second drill hole was more centrally located at Stenberg (No 5 in Fig. 2) and reached a similar depth (6529 m). Information about the Stenberg well was reported in Aldahan et al. (1993).

The two deep drillings aimed at, among others, to identify the nature of 3 seismic reflectors. It was found that they are related to the occurrence of swarms of dolerite sills with thicknesses ranging from 4 to 60 m. The sonic character is similar of deeper, not drilled reflectors, which then also can be interpreted as dolerite sills. These sills are important reference structures for the estimation of the amount of uplift at the two localities (the ring rise and the central part of the structure, respectively). From drill cuttings, about 15 such sills could be identified, however only the larger ones would be recognizable in the sonic data.

The borehole gravity data indicated a porosity of 2 to 3 % in the upper part of the central core.

Relatively fresh water was found down to at least 4 km depth and highly saline fluid was recovered below 5.5 km depth. The low resistivity modeled from magneto-telluric data indicates that a similar porosity related to fracturing of the rock also occurs below 6 km depth, which correlates well with the estimates made from the sonic log.

Only occasionally deflection cores were taken and most of the remaining rock material is a huge amount of drill chips. However a large body of data exists from measurements made in the Gravberg-1 drill hole. They are reported in the scientific summary report by Juhlin (1991). The geothermal gradient could be established to 16.1 Kkm-1 from the surface to 5.2 km depth. Shock effects (planar deformation features in quartz) were reported down to almost 3 km depth. Highly fractured rocks occur at the ring rise down to ca 1.2 km depth. A characteristic fracture zone spacing of 90 - 150 m prevails in the entire central uplift area as indicated by airborne VLF data (Henkel 1992) and by resistivity logging of the Gravberg-1 well (Juhlin 1991).

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