Numerical Analysis of Geological Structure and Fracture System in the Kurikoma Geothermal Area

Part 1: 2-Dimensional Experiment

Abstract

A numerical experiment with two-dimensional elasto-plastic finite element method was applied to an analysis of geologic structure and fracture systems in the geothermal area. The experiment is based on the Virtual Basement Displacement Method (Kodama et a1., 1985). The virtual basement is preliminarily assumed, and deformation of upper-layer strata caused by its movement is experimentally reproduced for each step of deformation. The location selected as a model area is the northern part of the Kurikoma geothermal area, Akita prefecture, Japan, which has many active geothermal fields. The geology of this area consists of Quaternary volcanic rocks, Sanzugawa Formation (Pliocene lake deposits), Minasegawa Formation (late Miocene volcanic rocks), Doroyu Formation (middle Miocene marine sediments), and basement rocks (granite and schist). The basement rocks broadly subsided from Akinomiya to Minase River. This subsided zone is filled with thick tuff layers of Minasegawa Formation (> 1000m), and it has been considered to be a caldera (Utada and Ito, 1986). The central part of this subsided zone was raised after collapse of the caldera. This experiment was executed in a geologic section trending NE-SW, perpendicular to the general geologic structure trending NW-SE. The development processes of geologic structure are divided into the following five steps:step 1: sedimentation of Doroyu Formationstep 2: sedimantation of Minasegawa Formationstep 3: uplifting of the central area (1)step 4: sedimentation of Sanzugawa Formationstep 5: uplifting of the central area (2) Calculated deformation and fracture distribution in each step shows that the highly fractured vertical zones are formed along edges of the uplifted zone in step 3. It also suggests that radially distributed fracture systems are formed around the uplifted zone in step 5. Deep geothermal reservoirs have been found at Uenotai, located at the eastern edge of the basement uplifted zone. There fracture systems are distributed in the basement rocks and around dikes in Minasegawa Formation (Naka et at, 1987). Kawarage and Arayu are also active geothermal fields along this section. The result of this experiment indicates that underground strata at these geothermal fields have been fractured with basement uplifting in step 3 and 5. The distribution of the fracture system shown by this numerical experiment is considered to represent the actual fracture distribution. This method is useful when the fracture systems are not found through surface mapping, for example, the surface is covered with Quaternary volcanic rocks, such as the Uenotai field.