Variability of petroleum inclusions in vein, fossil and vug cements—a geochemical study in the Barrandian Basin (Lower Palaeozoic, Czech Republic)

Abstract

Although no commercial oil or gas occurrences have been found in the Barrandian Basin, residual oils and petroleum inclusions give evidence about the petroleum history of this Lower Palaeozoic Basin. Petroleum inclusions are hosted in six generations of calcite and quartz cements that can be attributed to different stages of a basinal cycle and associated diagenetic events. They were analysed using an on-line crushing and an off-line crush-and-leach approach. Five different groups (PI-A to PI-E) and intermediate mixtures (PI-M) could be differentiated based on the relative distribution of n-alkanes. All oils had a calculated aromatic maturity Rc ranging between 0.9 and 1.6%. PI-A shows a molecular weight maximum in the range of n-C8–11 with a constant attenuation towards higher carbon numbered n-alkanes. This distribution is interpreted as the pristine precursor oil for other petroleum inclusion groups. PI-B has a maximum weight abundance at n-C15–20. The genetic relationship between PI-B and PI-A can tentatively be explained by mixing effects or by gas stripping. PI-C is bimodal and characterised by a molecular weight maximum at n-C32–34, and also a molecular weight maximum at n-C15–20 similar to that of group PI-B. PI-D shows a normally distributed molecular weight maximum in the range n-C25–28 and is interpreted as a wax precipitate from ascending gas saturated in n-alkanes. PI-E contains predominantly gaseous compounds with only a few higher hydrocarbons. Thompson's aromaticity values are elevated for condensates of group PI-E, and also for group PI-C that is dominated by long-chained n-alkanes. This gives evidence that fault-bound petroleum migration in the Barrandian Basin was associated with evaporative fractionation for group PI-C and PI-E. Samples of group PI-E yielded gas-range compounds only, and all come from a last generation of vein-filling whitish calcite that was formed in a late uplift phase of the basin. For other petroleum inclusion groups, only a vague preferential occurrence in specific mineralisation phases or stratigraphic intervals of the host rock has been found. This reflects the predominance of fracture-bound migration in the Barrandian Basin. Aromatic maturity values also showed no correlation between either crystal type or PI-group. In six of the investigated petroleum inclusion samples alk-1-enes are present. These terminal olefins are interpreted to be the result of the natural pyrolysis of petroleum due to the intrusion of volcanic dykes or hydrothermal processes. An artificial generation of olefins during sample work-up and analysis is unlikely. The preservation of alk-1-enes from Palaeozoic times was possible due to the protecting environment of petroleum inclusions.