Interpretation of geophysical well logs from the Nesjavellir geothermal field, Iceland

Abstract

Porosity and SiO2 content as a function of depth has been calculated using the data of caliper, natural gamma and neutron-neutron logs of twelve wells from the Nesjavellir geothermal field, SW-Iceland. The obtained values have been interpreted and compared with other available geological data. A unimodal distribution of the porosity, with a maximum of 19%, and a good correlation between relatively high porosity zones and hyaloclastite layer locations have been found. It seems that the major feedzones in the wells found during drilling are located in relatively high porosity zones. The SiO2 content distribution is also unimodal, with a maximum of 44.5%. Even though the SiO2 logs show several peaks which can be associated with intermediate and acidic magmatic intrusions, there is not a clear correlation between the wells. In the lower part of the rock sequence the mean porosity in the central part of the well field is higher than in the outer parts. The porosity contour maps are elongated from southwest to northeast, associated with the fracture trend in the field. This result is similar to the behaviour shown by other parameters of the field such as pressure and temperature. A very good correlation in the porosity categories has been found for the depth interval 600-1000 m b.s.l. between the results of the porosity logs and the porosity categories used in the modelling of this geothermal field in 1992. Even though there is a discrepancy in the porosity categories between wells NG-10 and NJ-11, there is also, in general, a good correlation in the results for the depth interval 1000-1800 m b.s.l.