Interpretation of core and well log physical property data from drill hole UPH‐3, Stephenson County, Illinois

Abstract

Laboratory and well log physical property measurements show variations in the mineralogy with depth in UPH‐3. Gamma ray values generally decrease with depth in the drill hole, corresponding to a decrease in the felsic mineral components of the granite. Correspondingly, an increase with depth in mafic minerals in the granite is indicated by the magnetic susceptibility and gamma ray measurements. These mineralogic changes indicated by the geophysical well logs support the hypothesis of fractionation during continuous crystalization of the intrusive penetrated by UPH‐3. Two fracture zones and an altered zone within the granite penetrated by drill hole UPH‐3 are defined by the physical property measurements. An abnormally low magnetic susceptibility response in the upper portion of the drill hole can be attributed to alteration of the rock adjacent to the sediments overlying the granite. Fracture zones can be identified from the sonic velocity, neutron, and resistivity measurements. A fracture zone, characterized by low resistivity values and low neutron values, is present in the depth interval from 1150 to 1320 m. Low magnetic susceptibility and high gamma ray values indicate the presence of felsic‐micaceous pegmatites within this fracture zone. An unfractured region present from a depth of 1380 m to the bottom of the hole is characterized by an absence of physical property variations. The magnetic susceptibility and gamma ray measurements indicate a change in the amount of mafic minerals at the base of this otherwise homogenous region of the drilled interval. Abrupt changes and repeated patterns of physical properties within the drill hole may represent interruptions in the crystalization process of the melt or they may be indicative of critical temperatures for specific mineral assemblages within the intrusive.