A potential linkage between excess silicate-bound nitrogen and N2-rich natural gas in sedimentary reservoirs

Abstract

Molecular nitrogen (N2) released from sedimentary rocks during metamorphism is an important component of the biogeochemical nitrogen cycles. However, the importance and variability of this metamorphic N2 flux from rock nitrogen to Earth's surface environment remains largely unexplored. Here we present a comprehensive bulk rock C-N and N2 concentration dataset from the lower Cambrian shale across the Yangtze Block. The results reveal a spatial trend of excess silicate-bound nitrogen in the lower Cambrian shale throughout the Yangtze Block, which is interpreted as partial assimilation of ammonium (NH4+) with high concentrations of NH4+ accumulating in the euxinic water column and in sediment pore waters at shelf and slope environments during sedimentation. The remarkable spatial coupling between silicate-bound nitrogen in bulk rock shale and N2 concentration in modern shale reservoirs indicates that the high proportion of silicate-bound nitrogen may act as an important control on the formation of N2-rich gas in shale reservoirs during metamorphism. These N2-rich reservoir rocks may have affected the surface environment through tectonic movement over Earth's history. Our results therefore identify a novel linkage in the nitrogen cycle and provide evidence for the importance of metamorphism on the return of rock nitrogen back to the surface environment. We further reveal that the metamorphic N2 gas flux from the geosphere to the atmosphere is dependent on environmental conditions during sediment deposition.