Beef veins record fluid overpressure during oil primary migration in source rocks

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression, Bohai Bay Basin, East China. Based on the study of vein petrography and fluid inclusions features, we demonstrate the vein was the product of hydrocarbon generation and expulsion from organic-rich shales. Consequently, the primary inclusions in the fibrous calcites recorded the fluid conditions during maturation of these source rocks. In most cases, the calcite-hosted primary inclusion assemblages are composed of the two-phase (oil + gas) hydrocarbon inclusions, with or without coexisting aqueous inclusions. Less commonly, the assemblages are made up of inclusions with only liquid hydrocarbon (i.e., monophase, high-density petroleum inclusions). In addition, many bitumen-bearing oil inclusions could also be observed in the fibrous calcite veins. By modelling the isochores of two-phase oil inclusions and coexisting aqueous inclusions, in light of the burial history for the basin, we conclude the fluid overpressure up to approximately twice (2x) the hydrostatic value (i.e., ~0.5–0.6x lithostatic) are the most common during the hydrocarbon generation and primary migration. The highest degrees of overpressure are recorded by the rare monophase petroleum inclusions. The resulting isochores of these highest density inclusions project to pressures that overlap with the lithostatic gradient. Thus, the monophase inclusions indicate pressures approaching and in some cases exceeding lithostatic. Our results indicate that fluids present during hydrocarbon generation and expulsion in organic-rich shales were indeed overpressured, but that lithostatic pressures were not the norm and evidently not a prerequisite for vein dilation, which means the fluid pressures during dilation of horizontal veins are not necessarily equal to the overburden throughout the history of the opening. This suggests that at least some of the vein dilation is accommodated and offset by concomitant narrowing of the adjacent wall rock laminae, likely by scavenging (dissolution/reprecipitation) of CaCO₃ from the adjacent wall rock, owing to the positive pressure dependence of calcite solubility, and presence of organic acids as byproducts of hydrocarbon generation.