Oil-bearing inclusions in vein quartz and calcite and, bitumens in veins: Testament to multiple phases of hydrocarbon migration in the Barrandian basin (lower Palaeozoic), Czech Republic

Abstract

The petroleum charge history of the Barrandian basin was investigated by analysing quartz and calcite and organic phases that occur in veins and fractures cutting dolerite sills within the Liteň Formation (Silurian). The geochemical characteristics of fluid inclusions trapped in vein quartz and calcite, vein bitumens and adjacent potential source rocks when combined with burial and thermal history data reflect the presence of at least three separate hydrocarbon charge episodes. Solid highly reflecting ( R max = 0.92–1.49%) bitumen provides information on the first and oldest episode of oil migration. The precursor oil was probably derived relatively early during diagenesis from nearby organic-rich sediments and was subsequently thermally altered to form the solid bitumen. The liquid oil preserved within the fluid inclusions in vein quartz and calcite cements represents a second hydrocarbon charge that migrated after the cooling of the sill, at diagenetic temperatures between 73 and 150 °C. This migration event probably occurred during the peak burial of the strata, during or after the Variscan orogeny. The inclusion oil is moderately matured, non-biodegraded and most likely derived from lower Palaeozoic source rocks such as the enclosing Liteň formation and/or directly overlying Kopanina and Přídolí formations. The presence of low salinity (0.2–3.2 eq. wt. % NaCl) aqueous inclusions coexisting with oil inclusions, indicates that the hydrocarbon flow had a substantial aqueous component. Brightly yellow-fluorescing, low-reflecting ( R r = 0.25 %) wax forming thin coatings on vein minerals herald the third and most recent pulse of hydrocarbons that migrated significantly after the peak heating of the strata. Indirect evidence suggests that this late-stage event may have been associated with the post-Palaeogene Alpine uplift of the basin that stimulated circulation of waxy oils and acid waters through the reactivated fractures.