秋田•山形地域油bull;ガス田の地層水の地球化学

抄録

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, origin and alteration processes of them.
The formation waters are chemically characterized by depleted Cl^-, SO₄^2- and Mg²⁺, and enriched HCO₃^- I^-, HBO₃^2- and NH₄⁺ compared with the sea water.
They have a relatively narrow range of δD values between -10 and +1‰ and a large variation of δ¹⁸O values from -3.6 to +2.5‰, which are similar to formation waters in Niigata.
The measured Sr²⁺ contents of them range from 4.1 to 51.8 mg/l. The formation waters have Sr isotopic values ranging from 0.70622 to 0.70764, which are less than those of the sea waters from Miocene.
These chemical and isotopic data show that the formation waters were originated from the sea water entrapped during deposition and subsequently altered due to the interaction with rocks, organic matters and so on.