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.


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