新潟地域における油・ガス田地層水の地球化学

抄録

Chemical and isotopic compositions of formation waters from oil and gas fields in Niigata Prefecture, Japan, were examined to demonstrate their geochemical characteristics and stratigraphic change for each formation.

They have the following geochemical characteristics.

1） The formation waters are mainly composed of Na^＋ and Cl^－ ions.

2） Although they were originated from seawater （Kato and Kajiwara, 1986）, their SO₄^2－ and Mg^2＋ contents have decreased rapidly and their NH₄^＋, I^－ and HCO₃^－ contents which are derived from organic matter have

increased roughly in this order during burial diagenesis.

3） They have a relative narrow range of δD values between －15 and －3 ‰ and large variations of δ¹⁸O values from －4.8 to ＋1.6 ‰, similar to formation waters from oil and gas fields in Akita, Japan.

Their Cl^－ contents reach the maximum in the upper part of the Nishiyama Formation which is due to formation of authigenic minerals such as clay mineral and zeolite from volcanic glass, and decrease toward the underlying Shiiya and Teradomari formations. This decrease of Cl^－ content may be caused by release of waters from transformation of clay minerals and maturation of organic matter during burial of sediments.