The composition of gases in fluid inclusions of late Permian (Zechstein) marine evaporites in Northern Germany

Abstract

In order to understand the generation of gases in Late Permian marine evaporites and the migration of gases in salt domes, 428 gas-bearing fluid inclusions of the Gorleben salt dome (Northern Germany) were analysed using Laser Raman Spectrometry (LRS). The investigations were divided in two parts. In the first part, only gas-rich inclusions were analysed involving samples from different stratigraphic layers of the Zechstein 2 (Staßfurt serie) and Zechstein 3 (Leine serie) and from different locations within the salt dome. For the second part, only the gaseous components in brines-rich inclusions of the Zechstein 2 were measured. The compositional range of all inclusions investigated can be subdivided into two main groups. With rare exceptions, the first group of N 2 and N 2–O 2 inclusions reveals a N 2/O 2 ratios close to that of modern atmosphere, thus indicating trapped paleoatmosphere. The second group is represented by inclusions that contain mixtures of N 2, CH 4 occasionally H 2 and/or H 2S. The most abundant subgroups are N 2–CH 4 and N 2–CH 4–H 2 mixtures. These gas mixtures can be produced by the decomposition of organic material under anoxic conditions. It is noted that N 2 in both main groups can also be generated by degassing of underlying Early Permian (Rotliegendes) or Carboniferous C org-rich sedimentary rocks and an entrapment in early stages of the Zechstein halokinesis. The different origin of both main groups is supported by stratigraphic correlation. In the stratigraphic layers which contain mainly mixtures of N 2 and O 2 or pure N 2, inclusions of the N 2–CH 4–H 2–H 2S-group are rare and vice versa: layers which are rich in N 2–CH 4–H 2–H 2S do not contain many N 2–O 2 inclusions. The majority of layers investigated frequently contains inclusions of the N 2–O 2 group. Only two anhydrite-rich layers of Zechstein 3 (Main Anhydrite and Anhydrite-intercalated Salt) contain mainly inclusions of the second group. The Zechstein 3 potash seam as well as secondary halites contain more or less the same population of inclusions from every main group. A comparison of the gas-rich inclusions and the gases in the brine-rich inclusions of the Zechstein 2 layer Main Rock Salt 3 shows distinct differences. Whereas, the gas-rich inclusions are of the N 2–O 2 group, the gases from the brine-rich inclusions are of the N 2–CH 4 group, emphasising different origins. The latter is a product of anhydrite-rich parts or potash seams that have migrated later into the overlying Main Rock Salt 3.