Calcite beef veins in oil shale, Bohai Bay Basin, China

Fibrous veins are frequently observed in organic-rich shales in sedimentary basins worldwide. The formation mechanism of these fibrous veins remains a subject to debate. This study, based on core and microscopic observations, and employing techniques such as XRD, SEM, EPMA, fluid inclusions, stable isotopes, and rock pyrolysis, investigated the fibrous calcite veins in the Eocene of the Dongying Sag, Bohai Bay Basin, and analyzed the main controlling factors for their emplacement. The fibrous veins are bed-parallel, consisting of parallel aligned, fibrous calcite crystals. The median zone is composed of granular calcite containing wall-rock fragments and bitumen. The fluorescence colors of hydrocarbon inclusions in the fibrous veins are mainly orange-yellow, yellow, and yellow-green, with homogenization temperatures (Th) closely ranging from 74.5℃ to 86.4℃. The δ¹³C_VPDB values of the fibrous veins and the wall-rocks are between -2.37‰ and -5.02‰ and between -2.31‰ and -3.97‰ respectively, while the δ¹⁸O_VPDB values are between -9.59‰ and -11.56‰ and between -6.70‰ and -9.75‰. Our results suggest that the fibrous veins were formed through the combined effect of hydrocarbon-generation-induced overpressure and crystallization pressure. The hydrocarbon generation-induced overpressure drives the opening of the vein to form the middle zone. The fibrous crystals grow in an antitaxial direction from the middle zone towards the wall rock. They grow continuously driven by the chemical potential gradient as the vein forming materials diffuse and migrate from the wall rock to the vein surface.  Bedding, TOC, and mineral composition are the controlling factors for the formation and development of the fibers. fibrous veins are more likely to form in intervals dominated by carbonate minerals, with high TOC (＞2wt.%) and well-developed laminations.