Fluid overpressures are widely expected during hydrocarbon generation and expulsion from source rocks, yet direct evidence for this phenomenon is lacking in the case of organic-rich shales. Here we show that formation of bed-parallel fibrous calcite veins in mature laminated organic-rich shales in the Eocene Dongying depression, Bohai Bay Basin, east China, occurred in direct response to fluid overpressure due to hydrocarbon generation. The evidence for overpressure is recorded by coexisting primary aqueous and petroleum inclusions in the calcite fibers. Our results show that all analyzed fluid-inclusion assemblages record variable degrees of overpressure during vein dilation, ranging from only modestly in excess of hydrostatic, to approaching and perhaps exceeding lithostatic. Thus, our results indicate that fluid pressures during dilation of horizontal veins are not necessarily equal to the opposing force of overburden throughout the history of opening. This suggests that at least some of the vein dilation is accommodated by concomitant narrowing of the adjacent wall-rock laminae, likely by scavenging (dissolution and reprecipitation) of CaCO3 from the adjacent wall rock.

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Trapped ancient microorganisms in halite fluid inclusions are of special interest to the understanding of biology and ecology in salt lake systems. With the integration of petrologic, microthermometric, and Raman spectroscopic analyses, this study utilizes fluid inclusions from Chaka Salt Lake, eastern Qaidam Basin, NW China, to assess the possibility of microorganism‐trapping by fluid inclusions. Here, we report that the solid phase of some primary fluid inclusions contains carotenoids, which is interpreted as evidence of Dunaliella algae, and that the coexisting liquid phase comprises SO42‐. The homogenization temperatures of single‐phase primary fluid inclusions indicate that the precipitation temperature of the Holocene halite in Chaka Salt Lake ranges from 13.5°C to 36.4°C. This suggests that fluid inclusions in halite are a good medium for trapping and preserving ancient microorganisms and organic matter in salt lakes, and that Raman spectroscopy has good potential to identify halophilic archaea.

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