Strontium isotopic composition of oil-field and gas-field waters, Japan

The 87Sr/86Sr ratios of 10 formation waters in petroleum and natural gas reservoirs along the western coast of northeastern Honshu, Japan, ranged from 0.7052 to 0.7084, and are distinctly lower than the ratio in seawater. This forms a marked contrast to oil-field brines from the U.S.A. which have higher 87Sr/86Sr ratios. Taking into consideration that petroleum and natural gas in Japan accumulate in volcanic rocks derived from intense submarine volcanism in the Middle Miocene, the 87Sr/87Sr ratios of the water samples are explained in terms of the isotope exchange between waters containing seawater Sr and reservoir rocks with lower ratios. (author)