HYDROCARBON-BEARING FLUID INCLUSIONS IN CALCITE-TILLED HORIZONTAL FRACTURES FROM MATURE POSIDONIA SHALE (HILS SYNCLINE, NW GERMANY)

Hydrocarbon-bearing fluid inclusions in calcite-filled horizontal fractures from mature Posidonia Shale (0.88% R(0)) were investigated by microscopic, microthermometric and chemical methods. The fluid inclusions predominantly contain both a liquid hydrocarbon phase, which shows bright UV-fluorescence colours (yellow, brownish, light blue) and a non-fluorescent hydrocarbon gas phase. The different UV-fluorescence colours probably reflect the different gross chemical compositions of the trapped liquid hydrocarbons. The saturated hydrocarbon fraction of the fluid inclusions (mostly secondary inclusions) predominantly consists of low-molecular-weight n-alkanes (C-9-C-12), while C-12-C-22-n-alkanes are more abundant in the adjacent rock matrix. The molecular composition of the fluid inclusions appears to reflect a mobile hydrocarbon phase, which was entrapped within the calcitic fill of the horizontal fractures. Pressure-corrected trapping temperatures were estimated for primary fluid inclusions (123 degrees-162 degrees C). This temperature range is in agreement with the results from numerical simulation studies, which indicate a maximum temperature of 148 degrees C coinciding with the period of fracture formation, calcite filling, and entrapment of primary fluid inclusions.