Correlation of crude oil to source rock using stable carbon isotopes. A case study in the Niigata basin.

Abstract

Stable carbon isotope ratio of 13C to 12C is applied to problems of geochemical correlation of crude oil to source rock on the assumption that isotope ratio of kerogen is heavier by ranging from 0.5 to 3‰ in δ13C values than that of oil when oils and kerogens are genetically related. The accumulated oils have δ13C values ranging from -23 to -22‰ and, consequently, those of source kerogens are expected to be in range from -23 to -19‰ in the Niigata basin. The source kerogens appear at two stratigraphic horizons. One of them is the Nanatani Formation to the lower part of the Teradomari Formation (source “D”) and the other is the top of the Shiiya Formation to the base of the Nishiyama Formation (source “S”). The source “S”, which is confirmed for the first time using stable carbon isotopes, is distributed in the area of the Niitsu, Minami Aga and Aga Oki oil fields, but it is absent in the area of Higashi Niigata and Matsuzaki gas fields. From this result, it may be inferred that the source “S” contribute to generation of the oils accumulated in the Shiiya and the Nishiyama Formations in these oil fields.