Strontium isotope constraint on the genesis of crude oils, oil-field brines and Kuroko ore deposits from the Green Tuff region of northeastern Japan

Abstract

Crude oils from Akita to northern Niigata oil fields in the Green Tuff region of northeastern Japan have distinctly uniform 87Sr86Sr ratios (0.7080–0.7082), while those from the southern Niigata oil field contain more radiogenic strontium (0.7095–0.7102). The regional variation in the strontium isotopic composition of crude oils is also reflected in their sulfur contents and sulfur isotopic compositions, and may be attributed to the regional heterogeneity of marine organic sediments from which the crude oils were ultimately derived. The 87Sr86Sr ratios of most oil-field brines (0.7061–0.7084), however, are different from and vary more locally than those of the accompanying crude oils. This finding supports the view that strontium, and by inference some other dissolved solutes in the brines, may have evolved during diagenesis by reaction of a connate and/or a meteoric water with rocks in the Green Tuff region.Barites in the sulfide ore and anhydrites and gypsums in the sulfate (sekko) ore from the Fukazawa and Kosaka Kuroko deposits in the Hokuroku district are divided by the 87Sr86Sr ratio of 0.7081 (±0.0001), which is identical to that of crude oils from nearby oil fields. This similarity in ratios lends support to the conclusion that the Kuroko base metal deposits and crude oil deposits were ultimately derived from a common organic sediment named PUMOS (Primitive Undifferentiated Metalliferous Organic Sediments).