Interpretation of Nitrogen Isotope Profiles in Petroleum Systems: A Review

Abstract

Bulk sedimentary nitrogen isotope profiles are often used as proxies for depositional redox conditions, nitrogen cycling, and nutrient uptake in modern and ancient marine systems. The general preference in terms of analysis is that the sediments measured have undergone minimal thermal alteration, as post-depositional processes might have altered the initial δ15N signal, thus complicating the interpretation of these records. Although not a traditional proxy for petroleum evaluation purposes, recently the use of nitrogen isotopes in petroleum systems has been investigated as potential proxies to reconstruct paleoenvironmental conditions such as redox, and for organic matter evaluation. In this paper we review the use of nitrogen isotope data in petroleum systems, their interpretations, and factors that may complicate their use as proxies. We review the evidence for nitrogen isotopic fractionation during diagenesis, catagenesis, and fluid migration as determined by lab experiments, and how these might impact interpretation of δ15N data in petroleum systems. We also analyze the use and interpretation of δ15N data from petroleum-producing reservoir units, including unconventional reservoirs and lacustrine basins. Lastly, we discuss potential applications for nitrogen isotopes in petroleum systems with regards to their use as both geochemical proxies and as tools to evaluate petroleum reservoirs.

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

“…In addition, the relationship between δ 15 N oil and δ 15 N kerogen during hydrocarbon generation has rarely been reported. 22 Studies on crude oil and kerogen from Liaohe and Tarim found that the δ 15 N of crude oil was comparable to that of the corresponding kerogen. Williams et al 25 also reported that the δ 15 N of crude oil and soluble extracts of source rocks are similar at close reservoir distances 25 so the δ 15 N oil may be expected to become a new oil-source comparative analysis index.…”

“…7,32 Although the loss of nitrogen during diagenesis and catagenesis has been well documented, how these processes alter the δ 15 N signal in different nitrogen-containing compounds has not been determined. 22 Several bulk rock studies have documented an increase in C/N ratios and δ 15 N values with increasing metamorphic grade, 1,33,34 although to varying degrees and not under all circumstances. 35−37 In most cases, the net isotopic effect is on the order of 1−2‰ up to the greenschist facies, but it is often greater at higher metamorphic grades.…”

“…Nitrogen is an important element in ancient sedimentary rocks and crude oil, but the field has developed slowly due to the technical difficulty of obtaining measurements from these N-poor samples. ,, However, recent technical advances have opened the door to carrying out rapid and reliable analyses of minute quantities of N, and the application of this proxy has thus accelerated. , A series of achievements have been made in the reconstruction of ancient sedimentary environments and oil-source correlation. ,,− First, nitrogen is redox-sensitive in the field of liquid water stability. The oxygenation of Earth’s atmosphere and oceans has played a major role in the evolution of the global biogeochemical nitrogen cycle, and the isotopic composition of nitrogen in oceans is preserved in sedimentary rocks (δ 15 N sed ) during depositional processes, which makes δ 15 N a meaningful tool for paleoenvironmental reconstruction. ,− Moreover, reconstructing the biogeochemical nitrogen cycle in deep time is also important for deciphering the evolution of life over Earth’s history and major geological events. , And some research in the petroleum system reported that the δ 15 N of crude oil is correlated with δ 15 N of source rock so the δ 15 N can be expected to become a new oil-source comparative analysis index. ,, …”

“…In addition, the relationship between δ 15 N oil and δ 15 N kerogen during hydrocarbon generation has rarely been reported . Studies on crude oil and kerogen from Liaohe and Tarim found that the δ 15 N of crude oil was comparable to that of the corresponding kerogen.…”

“…It is worth considering how nitrogen isotopes change between kerogen and products during the process of hydrocarbon generation. Due to the difficulty analyzing of δ 15 N oil , data on this topic are lacking. , …”