Relationship Between Surface and Borehole Geophysics During Deep Gas Drilling in Siljan Impact Structures

Christopher Juhlin, Goran Rissler-Akesson

Pre-drilling investigations in the Siljan area revealed an anomalous geophysical structure in the upper 8 km along with numerous low amplitude reflectors, both dipping and subhorizontal. A circular gravity minimum of about 16 mgal and about 40 km in diameter is centered on the chosen drill site. Magnetotelluric measurements indicate anomalously low resistivities are present shallower in the crust than what is normally found, with the most anomalous resistivity depth profiles southwest of the drill site. A wide spectrum of borehole investigations, including VSP, natural gamma spectroscopy, sonic, resistivity, and borehole gravity logging help explain the results from the surface geophysics. Doleritic sills, fine-grained granite intrusives and fracture zones all play a role in the surface geophysical expression. An attempt has been made to characterize the fracture zones, which cause both resistivity and sonic anomalies, using the above-mentioned logging tools, as well as the micro electric scanner tool (FMS) and the geochemical tool string that includes the NGS, GST, and AACT tools. The FMS displays distinct images of the fracture zones and allows for orientation of the individual fractures within them. The geochemical tool string, provides important information concerning the chemical composition of the rock. Tentative results show that the rock is more porous than may have been expected and porosities of around 5% are possible in fracture zones at depths exceeding 4 km.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.