Molecular and isotopic compositions of hydrate-bound gases collected from the Eastern Nankai Trough

Abstract:

Natural gas hydrates are crystalline clathrate compounds trapping light hydrocarbons, and have been expected as new natural gas resources. Natural gas hydrates are stable under high pressure and low temperature conditions such as deep marine environments. Hydrate-bound natural gas compositions are related to a thermodynamically stable condition, or origin of natural gas hydrates, which are important to characterize natural gas hydrate reservoirs.

The hydrate-bearing sediments were recovered from the eastern Nankai Trough area during the 2012 JOGMEC/JAPEX Pressure coring operation, aboard the RV CHIKYU. In this study, the molecular and isotopic compositions of the gases released from the hydrate-bearing sediments were analyzed.

The primary hydrocarbon component is methane for all gas samples collected from the hydrate-bearing sediments. The hydrate-bound gas contained small amounts of ethane and heavier hydrocarbons (less than 400 ppm). The concentrations of the minor hydrocarbon components tended to decrease at upper levels of the sediment core recovered, implying a compositional fractionation of natural gas during the migration of fluid from deeper level. The carbon isotope ratios (δ¹³C) of hydrate-bearing hydrocarbons were around -63‰ (vs. VPDB). No significant variation of the δ¹³C values against depth was observed. The molecular and isotopic compositions suggest that the hydrate-bound hydrocarbons are of a microbial origin.

This work was supported by funding from the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) planned by METI.