Biological Markers in the Neogene Gas Field around Nagaoka

Abstract

Cuttings materials from Katagai Gas Field in the vicinity of Nagaoka City, Niigata Prefecture, were analyzed for biological markers (triterpenoid and steroid hydrocarbons) in order to evaluate their validity as maturity indicies, with the following results.
(1) Isomerization of both steranes and triterpanes are recognized from immature to oil generation zone.
(2) The most reliable criteria in the immature zone and the oil generation zone are inferred to be the isomerization of hopane nucleus and the epimerization of stigmastane side chain, respectively.
(3) Change of monoaromatic steroid hydrocarbons into triaromatics seems to occur from the immature to oil generation zone.
(4) Cracking reactions in the side chains of C_{26, 27, 28}-triaromatic steroid hydrocarbons into C_{20, 21}-triaromatics appear to take place from the late stage of oil generation zone to the overmature zone. This reaction may be useful as a maturity index of crude oils or gas condensates, which cannot be measured for vitrinite reflectance.