Characterizing hydrocarbon-bearing fluid inclusions using CLSM-based microspectrometry with application to the Upper Permian rock salt in the Gorleben and Morsleben sites, Germany

Hydrocarbon-bearing fluid inclusions (HCFI) in halite from the Gorleben and Morsleben sites (Germany), petroleum from Gorleben, and epoxy resins have been analysed in-situ using a combined approach of confocal laser-scanning microscopy (CLSM), incident light, and transmitted light fluorescence microscopy. The HCFI contain vapour and liquid hydrocarbon phases and their optical appearance varies widely and distinctively. 405 nm based excitation allowed for in-situ CLSM-associated acquisitions and distinction of HCFI emission spectra for both sites. In contrast to transmitted light microscopy equipped with barrier filter >515 nm, the CLSM-related emission >410 nm employed full emission spectra essential for observation of the entire emission spectrum of HCFI. While maximum emission intensity (λ_max) of HCFI at the Gorleben site peaks at 455–456 nm, the corresponding λ_max at the Morsleben site ranges from 485 to 495 nm, indicating that emission spectra of HCFI at Gorleben site are “blue-shifted” in comparison to those at Morsleben. It is implied that the blue-shift is caused by a shift to a less aromatic/polar composition of the HCFI at Gorleben site. Both, HCFI and petroleum differ noticeably in spectral properties from epoxy resins indicating no contamination of epoxy resin in the examined thick sections. Coarse approximations of °API (a standard for petroleum liquid density), based on red/green coefficient (Q_max) and empirical correlation equation as well as direct measurements of °API, suggested markedly different °API gravities for Gorleben and Morsleben sites with higher °API values, of up to 52.4 for the first. Collectively, the results of this research study demonstrated the applicability and adaptability of CLSM as a well-advanced method to acquire in-situ spectral properties of HCFI in salt rock, as a basis for understanding hydrocarbon generation and migration in sedimentary environments.