Alteration of Core Samples and Geothermal System in Yuzawa-Ogachi Area, Kurikoma North, Northeast Japan

Abstract

Alteration minerals of cores from six exploration wells are identified by X-ray diffraction, thin section and SEM observations. Five alteration zones, montmorillonite-mordenite, chlorite-laumontite, chlorite-wairakite, sericite and epidote-amphibole, were identified. Three different alteration processes were identified from the distribution pattern of such alteration zones. One is simple heating by one thermal event at present. Second is same procedure but heating event was past and now in cooling stage. Final is the overlap of past alteration and present one. Chemical analyses of major elements of whole rock samples show that the zone of hot water circulation is characterlized by big chemical change and such circulation zones moving from deep to shallow. Amphibole group which has been thought as metamorphic mineral is partly assigned as geothermal products by occurrence, chemistry and thin section and SEM observations. Mineral names for both metamorphic and geothermal amphibole are tremolite-actinolite, hornblende and edennite-ferroednite. They reflect alteration conditions and must be treated more carefully. Combination of alteration mineral and chemical study give us a good way for identifying the thermal history and present reservoir estimation.