Geochemistry of formation waters from the Yabase oil field, Akita Prefecture

： Geochemical characteristics and alteration process during diagensis of formation waters from the Yabase oil field, Akita Prefecture, are discussed using published data. The formation waters are divided into three groups （ type I: shallow reservoirs, type II: deep reservoirs in the northern area, type III: deep reservoirs in the southern area ） based on chemical and isotopic compositions. Chemical and isotopic changes of these groups are transitional and reflect the alteration of the seawater trapped in sediments during diagenesis. The type III formation waters, which are characterized by HCO 3 − /Cl − ratio >1 and high δ 18 O values, are strongly affected by the maturation of organic matters and the phase transformation of silica minerals in source rocks around reservoirs. Li ＋ contents of formation waters from the deep reservoirs are less than the seawater and these Li ＋ /Cl − ratios are about the same as the seawater. Also, temperatures of the deep reservoirs calculated using the Mg-Li thermometer are slightly lower than reservoir temperatures. These matters suggest that the formation waters are not affected by fluids migrated from deeper parts. Geochemistry of the formation waters suggests that the oil migration is not related to the formation water in this field.