Behavior of Nitrogen Contents and Isotopes during Thermal Evolution and Hydrocarbon Generation in Shale: Insights from Hydrothermal Experiments in a Semiopen System

Nitrogen is an important component of petroleum systems. However, nitrogen loss and fractionation processes during the thermal evolution of organic matter are not conclusive. Moreover, current studies have focused mainly on single components, such as bulk rock, kerogen, or crude oil, but the relationships between them have not been systematically studied. A series of hydrothermal experiments were conducted, and data were collected from 69 geological samples. The results illustrate that the nitrogen isotope compositions (δ15N) of different nitrogen-containing components vary with simulated temperature. The δ15Nex-bulk (δ15N of bulk rock after extraction) continuously increased, with the largest change reaching 2.4‰. The δ15Nkerogen (δ15N of kerogen) basically remained constant until 400 °C (Ro < 1.78%), and the δ15Noil (δ15N of expelled oil) changed by only 0.6‰ throughout the oil generation phase. A comparison of the data for δ15Nex-bulk and δ15Nkerogen reveals that the relationship between them is affected by thermal evolution. Below 400 °C, the δ15Nkerogen value is greater than δ15Nex-bulk, and these differences decrease with increasing temperature. However, above 400 °C, the relationship reverses to δ15Nkerogen lower than δ15Nex-bulk, and the difference between them increases with the temperature. The difference between them can serve as a rough maturity indicator. In addition, the comparison of δ15Noil and δ15Nkerogen reveals that thermal evolution has little effect on the δ15Noil, which indicates that the δ15Noil has the potential to preserve the original information on organic matter.