Residual gas in extensive stratified Miocene Izura carbonate concretions exhibiting thermogenic origin and isotopic fractionation associated with carbonate precipitation

Abstract

A major carbonate-cemented strata and carbonate concretions intercalated in the Early Miocene Kokozura Formation are common in the Izura area along the Pacific coast, in north Ibaraki, Japan. Chemotrophic mollusk fossils occur frequently with the Izura carbonate concretions. Significant quantities of archaeal biomarkers such as pentamethylicosane, crocetane, and squalane are detected in the carbonate concretions. Residual gas from pulverizing the carbonate concretion contains mostly CO2 with subordinate amounts of CH4, C2H6, and C3H8. The CH4/(C2H6+C3H8) ratio and the δ13C values of CH4 and carbonate indicate the Izura carbonate concretions originate from thermogenic methane. The extent of isotopic fractionation during formation of the carbonate concretions is determined by a novel dataset of δ13C values of CO2 in the residual gas and carbonates. Largescale leakage of thermogenic methane due to the deformation, fissuring, and destruction of the paleo-Izura petroleum deposit produced the vast Izura carbonate concretions. This deformation is associated with the tectonic separation of Japanese Islands from the paleo-Eurasian continent in the late Early Miocene.