Isotopic Signature of Nitrogen - An Indicator for Timing N2 Generation and N2 Charging of Dutch Natural Gas Accumulations

This paper provides data on the isotopic composition of nitrogen in natural gases of offshore and onshore Netherlands and proposes a process-based interpretation of the observed regional variation in isotopic characteristics. The nitrogen isotopic data clearly reveal a distinct regional distribution which - in combination with the regional differences in geological history - strongly suggests that differences in isotopic signature are related to regional differences in: 1. timing of nitrogen generation and accumulation: - Early generation and accumulation of hydrocarbon and nitrogen gas in northern offshore area 1 (before Late Cretaceous inversion); - Late generation and accumulation of hydrocarbon and nitrogen gas in southern area 2 comprising inverted basins (principally during Tertiary and Quaternary); 2. origin and liberation process of the nitrogen: - In area 1: main origin of nitrogen is the inorganic fixed nitrogen in clay minerals in Carboniferous shales; liberation of nitrogen during rifting period (probably Late Jurassic-Early Cretaceous rifting) is triggered by devolatization reactions during very deep burial, by short-term thermal processes (magmatic activity/igneous intrusions), and/or fluid-rock interaction; - In area 2: main origin of nitrogen are organic sources; liberation of nitrogen during maturation of the Westphalian coals and carbonaceous shales.