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 δ¹⁵N_ex-bulk (δ¹⁵N of bulk rock after extraction) continuously increased, with the largest change reaching 2.4‰. The δ¹⁵N_kerogen (δ¹⁵N of kerogen) basically remained constant until 400 °C (R_o < 1.78%), and the δ¹⁵N_oil (δ¹⁵N of expelled oil) changed by only 0.6‰ throughout the oil generation phase. A comparison of the data for δ¹⁵N_ex-bulk and δ¹⁵N_kerogen reveals that the relationship between them is affected by thermal evolution. Below 400 °C, the δ¹⁵N_kerogen value is greater than δ¹⁵N_ex-bulk, and these differences decrease with increasing temperature. However, above 400 °C, the relationship reverses to δ¹⁵N_kerogen lower than δ¹⁵N_ex-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 δ¹⁵N_oil and δ¹⁵N_kerogen reveals that thermal evolution has little effect on the δ¹⁵N_oil, which indicates that the δ¹⁵N_oil has the potential to preserve the original information on organic matter.