Origin of Hydrocarbons in the Bohemian Massif Revisited: New Evidence from Field Observations and Geochemical Composition

Abstract

Evidence is provided that many, if not most, small-scale hydrocarbon occurrences in the Bohemian Massif, both newly described and already known, are in fact concentrated within five distinct north-south zones, 20-−40 km wide, which cut various geological units. These linear structures provided long-lived and periodically reactivated conduits for various geofluids that include, apart from hydrocarbons, thermal and mineral waters, warm brines, uranium-bearing hydrothermal solutions, CO₂, CH₄, and He. Geological and geochemical data suggest repeated episodes of fluid and hydrocarbon migration throughout the geological history of the linear zones. A number of local present-day gas emissions and small oil seeps are probably indicative of the most recent migration stage that has resumed in the Tertiary period, following the Alpine orogeny. In some localities, the present-day peaks of CO₂, CH₄ and C₂-C₁₂ hydrocarbon emissions mirror major earthquakes that occur elsewhere in the southern hemisphere. This implies that distant earthquakes can be instrumental in releasing geofluids from the linear zones. The hydrocarbons found at the surface may point to unknown deposits and/or petroleum source rocks hidden beneath the surface; though juvenile, deep-mantle (abiogenic) contributions can be also viable. Alternatively, the hydrocarbon origin can be also explained in terms of a long-distance lateral migration from unknown sources in the Alpine realm. Regardless of the dubious source(s), our analysis indicates that the internal part of the Bohemian Massif shall be seriously considered as a possible target for future oil and gas exploration - a conclusion that is both fundamental and challenging.