SEISMIC IMAGING OF REFLECTORS IN THE SG4 BOREHOLE, MIDDLE URALS, RUSSIA

The upper crust in the Middle Urals is known to be highly reflective. However, it is not clear whether the observed reflections originate fr om lithological boundaries or from fault zones. The SG4 borehole locat ed in the Tagil Synform of the Middle Urals, drilled to 5354 m as of A ugust 1995, presents one opportunity to study the source of this refle ctivity. The hole has penetrated several different lithologies as well as numerous tectonic or fracture zones, The upper 5070 m consist of i sland are rocks which have presumably been thrust on top of basalts. T o allow correlations between available surface seismic studies and the borehole observations vertical and offset seismic profile data were a cquired in the borehole over the interval 520 m to 3940 m. Two shot po ints were used, a near offset one at 135 m from the wellhead and a far offset one at 1845 m from the wellhead. After processing, the borehol e seismic data show numerous reflecting horizons. Many reflections cor relate with major fracture zones, while others have no clear correlati on with fracture zones nor lithological contrasts. The interval betwee n approximately 4450 m and 5100 m, which is imaged below the bottom of the survey, shows a layered reflective pattern that correlates with t he lower part of a flysch unit which ends at 5070 m. The basalts below appear to be relatively transparent. A prominent east-dipping reflect or on a W-E CDP line about 700 m north of the wellhead is not clearly imaged on the borehole seismic, However, in a pilot hole which deviate s a few hundred metres in the northerly direction from the main hole, a marked low-velocity zone is observed at the corresponding depth for this reflector, indicating its source to be from a fault zone. That th e fault zone is not clearly observed on the borehole seismic data is a ttributed to 3D effects.