Logging in deep water wells in central Jutland, Denmark

Abstract

During a search for unpolluted resources, new wells have been drilled to a deep confined aquifer in central Jutland. Since little is known about the extent and vulnerability of the deep reservoir, geophysical logging was used to supplement hydraulic well‐testing. Gamma‐ray, SP and resistivity logs were recorded in the boreholes. Gamma‐logs were also recorded in two cased wells. Analysis of the logs, combined with information from samples, shows an unconfined aquifer of quartz and mica sand to a depth of about 55 m at Grindsted. Below this level there is a 40 m thick aquitard with an apparent southerly dip. The aquitard is composed of interbedded sand, silt and clay. The deep confined aquifer extends from 95 to 110 m below the surface at Grindsted. Both the aquitard and the deep aquifer are provisionally dated as Early/Middle Miocene. The electrical logs discriminate well between sand, silt and clay beds. If a suitable mud is used in the well it is possible to calculate the formation water resistivities in good agreement with water samples obtained after completion. From the resistivity logs formation factors were calculated in the range 3–5 using no‐invasion departure curves. The high content of heavy mineralsin the Tertiary deposits makes it impossible to obtain precise lithological information from the gamma‐ray log. On the other hand this log provides clear markers for identifying the hydrogeological units in the well‐bores.