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[SCG46-P18] In-situ recrystallization of feldspars under supercritical geothermal conditions revealed by using drilling cuttings of Kakkonda Granite
Keywords:feldspar, feldspar geothermometer, recrystallization, supercritical geothermal reservoir
In geothermal development, estimating reservoir temperature is important for evaluating power generation potential. However, it is difficult to determine the actual borehole temperature during drilling because water is supplied to lower the borehole temperature. Here, we testified the application of feldspar geothermometers in drilling cuttings to estimate borehole temperature under a supercritical state (i.e., >~370°C). Calibration temperature of conventional feldspar geothermometers had been applied in a magmatic temperature of 700–900℃, their applicability to low-temperature conditions in supercritical state was not elucidated. In this study, we analyzed drilling cuttings of plagioclase and alkaline feldspar in granite directly collected from a high-temperature rock body exceeding 500℃, and investigated the recrystallization process of those feldspars.
The study area is the Kakkonda geothermal field in the Sengan area. The WD-1a well drilled at a depth of 3,500 meters through granite with temperatures exceeding 500℃ (Ikeuchi et al., 1998). The lithology of WD-1a at the depth of 2,860 to 3,729 m (bottom of the well) is Quaternary Kakkonda granite (Uchida et al., 1998).
Samples were selected from 24 drilling cuttings at a depth of 2985–3725 m. The reservoir temperature in this range was estimated to be approximately 380 to 580℃ (Ikeuchi et al., 1998), which was in supercritical state for water. Chemical composition of feldspars was analyzed by EPMA. A total of 40 types of two-feldspar geothermometers were applied to the obtained mineral composition pairs of plagioclase and alkaline feldspar to estimate their formation temperatures.
The grain sizes of plagioclase and alkaline feldspar are approximately 100–400 µm and 50–400 µm. Alkaline feldspar was almost recrystallized, and contains many pores smaller than 10 µm. The plagioclase showed a compositional zoning with its core relatively rich in Ca. The plagioclase rim was rich in Na, and there was sharp boundary between core and rim of plagioclase. The chemical composition of plagioclase in the Na-rich rim and the adjacent alkaline feldspar rim were measured at 463 points each.
The albite component (XAb = Na/(Na+K+Ca)) in plagioclase increases systematically from 0.73–0.92 to 0.49–0.70 from 2895 m to 3725 m depth. In addition, the orthoclase component (XOr = K/(Na+K+Ca)) in alkaline feldspar decreases systematically from 88–93% to 81–86% from 2895 m to 3725 m depth.
According to the feldspar geothermometer of Powell and Powell (1977), a temperature range of 362–439 ℃ at a depth of 2985 m and 513–614 ℃ at a depth of 3725 m. This result was close to the borehole temperature estimated by Horner plot method under supercritical conditions of 380–500℃.
Alkaline feldspar is rich in micrometer-scale pores. In addition, the rim of plagioclase in contact with alkaline feldspar were altered to albite. These textures suggest supercritical fluid reacted alkaline feldspar in the pores, recrystallizing the rim of plagioclase. The formation of pores in feldspars due to hydrothermal alteration was consistent with the creation of pores in feldspar displacement experiments at 600℃ (Nurdiana et al., 2023).
These systematic compositional changes with depth, similarity of the feldspar and borehole temperatures, and reaction textures strongly suggest that recrystallization of feldspars occurred at the current borehole temperature of 380–500℃. Therefore, the application of feldspar geothermometer using drilling cuttings could evaluate recrystallization temperatures of feldspar in supercritical geothermal reservoirs.
The study area is the Kakkonda geothermal field in the Sengan area. The WD-1a well drilled at a depth of 3,500 meters through granite with temperatures exceeding 500℃ (Ikeuchi et al., 1998). The lithology of WD-1a at the depth of 2,860 to 3,729 m (bottom of the well) is Quaternary Kakkonda granite (Uchida et al., 1998).
Samples were selected from 24 drilling cuttings at a depth of 2985–3725 m. The reservoir temperature in this range was estimated to be approximately 380 to 580℃ (Ikeuchi et al., 1998), which was in supercritical state for water. Chemical composition of feldspars was analyzed by EPMA. A total of 40 types of two-feldspar geothermometers were applied to the obtained mineral composition pairs of plagioclase and alkaline feldspar to estimate their formation temperatures.
The grain sizes of plagioclase and alkaline feldspar are approximately 100–400 µm and 50–400 µm. Alkaline feldspar was almost recrystallized, and contains many pores smaller than 10 µm. The plagioclase showed a compositional zoning with its core relatively rich in Ca. The plagioclase rim was rich in Na, and there was sharp boundary between core and rim of plagioclase. The chemical composition of plagioclase in the Na-rich rim and the adjacent alkaline feldspar rim were measured at 463 points each.
The albite component (XAb = Na/(Na+K+Ca)) in plagioclase increases systematically from 0.73–0.92 to 0.49–0.70 from 2895 m to 3725 m depth. In addition, the orthoclase component (XOr = K/(Na+K+Ca)) in alkaline feldspar decreases systematically from 88–93% to 81–86% from 2895 m to 3725 m depth.
According to the feldspar geothermometer of Powell and Powell (1977), a temperature range of 362–439 ℃ at a depth of 2985 m and 513–614 ℃ at a depth of 3725 m. This result was close to the borehole temperature estimated by Horner plot method under supercritical conditions of 380–500℃.
Alkaline feldspar is rich in micrometer-scale pores. In addition, the rim of plagioclase in contact with alkaline feldspar were altered to albite. These textures suggest supercritical fluid reacted alkaline feldspar in the pores, recrystallizing the rim of plagioclase. The formation of pores in feldspars due to hydrothermal alteration was consistent with the creation of pores in feldspar displacement experiments at 600℃ (Nurdiana et al., 2023).
These systematic compositional changes with depth, similarity of the feldspar and borehole temperatures, and reaction textures strongly suggest that recrystallization of feldspars occurred at the current borehole temperature of 380–500℃. Therefore, the application of feldspar geothermometer using drilling cuttings could evaluate recrystallization temperatures of feldspar in supercritical geothermal reservoirs.