Macro- and microscale distribution of hydrocarbons in the Stassfurt Hauptsalz of the Gorleben salt dome

In context of further exploration of the Gorleben salt dome (March 2011 to November 2012) mineralogical and geochemical investigations have been conducted on hydrocarbon-bearing samples of the "Knauelsalz" (z2HS1), particularly the oldest part of the "Hauptsalz" (Stassfurt Series z2). Investigations focus on genesis, composition and macro-/micro-structural distribution of hydrocarbons. These exploration works continue investigations of Gerling & Faber (2001) and Gerling et al. (2002). For this purpose 45 entire cored sampling boreholes supplemented with 20 packer boreholes in Crosscuts 1 West (Q1W) and 1 East (Q1E) have been drilled between March 2011 and March 2012 at the 840 m level of the Gorleben exploration mine in order to get new faultless and unaltered samples beyond the excavation damaged zone. Drill cores of hydrocarbon sampling and packer boreholes have been investigated under ultraviolet light (lambda = 254 nm) for indications of the existence of fluorescent (aromatic) hydrocarbons. The results were documented and visualised in the geological 3D model of the Gorleben salt dome. Microscopical studies of thin and thick sections show that hydrocarbons were located at the grain boundaries of halite and/or anhydrite crystals, in newly formed microcracks due to drilling respectively preparational works, in microcapillary tubes of anhydrite crystals and rarely in micro-porous parts of the Hauptsalz.