Geochemistry of formatin waters from oil and gas fields in Akita and Yamagata Prefectures, northeastern Japan

摘要

Geochemical studies on formation waters collected from oil and gas fields in Akita and Yamagata Prefectures, northeastern Japan, were carried out in order to clarify chemical characteristics, ortin and alteraction processes of them. The formaiton waters are chemically characterized by depleted Cl~-, SO_4~(2-) and Ag~(2+), and eriched HCO_3~-, I~-, HB_3~(3-) and NH_4~- compared with the sea water. They have a relatively narrow range of #delta#D values between -10 and +1per thousand and a large variation of #delta#~(18)O values from -3.6 to + 2.5 per thousand, which are similar to formation waters in Niigata. The measured Sr~(2+) contents of them range from 4.1 to 51.8 mg/l. he formation waters have Sr isotopic values ranging from 0.70622 to 0.70764, which are less than those the sea waters from Miocene. These chemical and iostopic data show that the formation waters were orginated from the sea water entrapped during dposition and subsequently altered dure to the interaction with rocks, organic matters and so on.