| Title | Numerical Analysis of Geological Structure and Fracture System in the Kurikoma Geothermal Area |
|---|---|
| Authors | Mizugaki, K. and Kodama, K., |
| Year | 1988 |
| Conference | Japan International Geothermal Symposium |
| Keywords | |
| Abstract | In many geothermal fields, fractures in the form of faults or joints have been reported to play the role of forming paths for geothermal fluid or creating reservoirs. A numerical experiment through employment of a two-dimensional elasto-plastic finite element method was applied to an analysis of deep-stratum geologic structures and the actual fracture systems. The experiment is based on the Virtual Basement Displacement Method (Kodama et al., 1985), constructing a model of the geologic structures of the deeper parts by simulation. This method reproduces the mechanism of deformation in which the geologic units of the basin were deformed as the result of the movement of the basement. Also it shows the process of propagation of the deformation from the deeper parts to the surface. This is a type of inversion analysis method. By preliminarily assuming the basement movement, deformation of upper-layer strata caused by this movement is experimentally reproduced. Then the displacement amounts of virtual basement are regulated until differences between the reproduced results and the observed frcts are minimized, thereby obtaining the optimum basement displacement. This process is conducted for each step of deformation, incremental deformation of basements in respective steps is clarified, and then by accumulating resultant data, analyses are achieved on formative process of geologic structure up until today. The location selected as a model area is in the northern part of the Kurikoma geothermal area, Akita prefecture, Japan. This is an active geothermal area resulting from Quaternary volcanic activities. In the south of this area, Onikobe Geothermal Power Plant (12.5MW) is in operation. In the north of this area, researches and exploitation for the construction of a geothermal power plant have been conducted (Naka et al., 1987). The geology of the subject area is mainly composed of Doroyu formation, marine deposits of middle Miocene age, Torageyama formation, volcanic and clastic rocks of late Miocene age, and Sanzugawa formation, lake deposits of late Miocene to Pliocene age. On top of these formations, Quaternary volcanic rocks are overlaid. The geologic structure of this area is characterized primarily by the NM-SE trending faults and secondary by the NE-SW trending faults, and one of NW-SE trending faults is regarded as a Quaternary fault. Most of geothermal manifestations, such as hot springs or fumarloes, occur with these faults. The major rivers in this area also trend NW-SE. The basement rocks consisting of granite and schist broadly subsided from Akinomiya to Minase River, and the central part of the subsided zone was raised before and after sedimentation of Sanzugawa formation. Ths subsided zone is filled with thick tuff layers of Torageyama formation (>1000m), and it is considered to be a caldera (Takeno, 1988). In this experiment, a two-dimensional elasto-plastic finite element method was executed in a geological section. The selected model section trended NE-SW, perpendicular to the general geologic structure trending NW-SE. The experiments were conducted on the development processes of geologic structure by dividing them into the following five steps: Step 1: Deformation during sedimentation of Doroyu formation Step 2: Deformation during sedimentation of Torageyama formation Step 3: Deformation with uplifting of the central area Step 4: Deformation during sedimentation-of Sanzugawa formation Step 5: Deformation with uplifting of the central area (during Quaternary) In presuming displacement, the uppermost layers that deposited in respective steps were assumed to be the sea level (for Doroyu formation and Torageyama formation) or estimated lake water level (for Sanzugawa formation), while the restoration of ancient environmental conditions and the horizontal shift of strata were not subjected to our consideration. Calculated deformation and fracture distribution in each step are shown in Fig. 1. It is evident that the highly fractured vertical zones exist between subsided zone and uplifted zone, such as Uenotai,and Akinomiya (Fig. 1g). Radially distributed fracture systems are around the uplifted zone (Fig. lk). Deep geothermal reservoirs were found in Uenotai, located at the eastern edge of the basement uplifted area (Naka et a1., 1987). The observed fracture distribution at the Uenotai geothermal field is as follows: 1) Vertical fault zone between basement uplifted area (Kawarage) and subsided area (Kijiyama) 2) Boundary of the basement rocks and Doroyu formation 3) Around dikes in Torageyama formation This experiment indicates tha~ vertical fractured zone developed at Uenotai in Step 3, and upper part of Torageyama formation was fractured in Steps 4 and 5. The distribution of the fracture system shown by this numerical experiment is-considered to represent the fault structures actually surveyed from the field research. It might evaluate the underground fracture systems as well as its distribution based on strain distribution calculated by the numerical experiment. 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. |