Geochemistry of crude oils and natural gases in the northern Fossa Magna region, Nagano Prefecture, Japan

Abstract

Three oil samples, eight gas samples, and three rock samples, collected mainly from surface seepages in the northern Fossa Magna region, Nagano Prefecture, Japan, were subjected to geochemical analysis, yielding the following results.
1) The oils are heavy and biodegraded, and are classified as Kubiki-type (Hirai et al., 1995). Geochemical characteristics of the oils indicate an origin from type II-III organic matter, and their maturity is estimated to be ∼ 0.7% Ro equivalent.
2) The gases are mainly methane (C₁); only three samples from Tenjinzawa and Nakatsuka contain heavier hydrocarbons, up to C₆, and these samples have with high wetness over 10%. The chemical and isotopic compositions of the gases indicate that they are partly biodegraded and mostly thermogenic in origin, with a maturity of 0.7%-0.9% Ro equivalent.
3) Mudstones in the Bessho Formation and the lower part of the Aoki Formation are probably the source rocks that generated the oil and gas in the areas of the Takafu and Komeji synclines as indicated by source rock analysis and by surface oil and gas seepages.
Although subsurface geologic structures remain unclear, there is hydrocarbon potential in the zone between the areas of oil and gas seepages and the synclines.