Boron concentrations and B/Cl ratios of formation waters from oil and gas fields in Japan, which are mainly of fossil seawater origin, are graphically shown using published data. Most of the boron concentrations range from a little to 100 mg/l except for the Tempoku area and the B/Cl ratios are mostly between 0.001 and 0.1. Taking account of studies on hot springs and pore waters in deep sea sediments, sources of boron are probably derived from decomposition of organic matters and diagenesis of clay minerals in addition to the seawater.
