Early overpressuring in organic-rich shales during burial: evidence from fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin, UK

Abstract

Field, petrographic and geochemical analysis of fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin was conducted to investigate the formation mechanism of the veins. Bedding-parallel fibrous calcite veins, including beef veins and tabular cone-in-cone structures, are widespread in the black shales. The calcite veins consist of subvertical fibrous crystals and a dark median zone. The median zone contains scattered clays, pyrite microcrystals, skeletal fragments and amorphous organic matter. The veins exhibit moderate carbon isotope values, ranging from −1.515 to 2.732‰. The oxygen isotope composition ranges from −8.872 to −4.521‰, which is possibly too negative to reflect the primary porewater oxygen isotope signatures and indicates a porewater modification. It is interpreted that the veins mainly derive carbonates from seawater inorganic carbon and bioclasts. The veins formed as closed-system hydraulic fractures in overpressured cells during sediment degassing in the methanogenic zone. The shale beds with a high total organic carbon content could have generated abundant CO 2 , which may have resulted in either the cementing of the pores in the matrix or overpressure buildup. The skeletal fragments provide a control on the spatial distribution of veins as nuclei for calcite precipitation from supersaturated pore fluids.

Architecture, deformation style and petrophysical properties of growth fault systems: the Late Triassic deltaic succession of southern Edgeøya (East Svalbard)

“…This possible scenario is in line with a very shallow environment (few metres to tens of metres below the seafloor), between the methanogenic and the sulphate reduction zone (Anaerobic oxidation of methane or Sulphate Methane Transition; e.g. Mitterer, ), marking the boundary between a lower zone were anaerobic bacterial oxidation of CH 4 enhances calcite precipitation and CIC/BF development, from an upper zone were sulphate reduction favours pyrite mineralization (Figure b), as described by Meng, Hooker, and Cartwright (). The hydrocarbon decomposition‐aided, microbialitic origin of some sedimentary CIC structures in the same lithologies of eastern Svalbard studied by Tugarova and Fedyaevsky () is in line with the present observations.…”

Sedimentary architecture during Carboniferous rift initiation – the arid Billefjorden Trough, Svalbard

“…This possible scenario is in line with a very shallow environment (few metres to tens of metres below the seafloor), between the methanogenic and the sulphate reduction zone (Anaerobic oxidation of methane or Sulphate Methane Transition; e.g. Mitterer, 2010), marking the boundary between a lower zone were anaerobic bacterial oxidation of CH 4 enhances calcite precipitation and CIC/BF development, from an upper zone were sulphate reduction favours pyrite mineralization (Figure 11b), as described by Meng, Hooker, and Cartwright (2017). The hydrocarbon decomposition-aided, microbialitic origin of some sedimentary CIC structures in the same lithologies of eastern Svalbard studied by Tugarova and Fedyaevsky (2014) is in line with the present observations.…”

Regional‐scale development of opening‐mode calcite veins due to silica diagenesis

“…However, fracturing has been attributed to early-burial overpressure related to methanogenic bacteria [Meng et al, 2017], which feed on organic material in the shallow subsurface. However, fracturing has been attributed to early-burial overpressure related to methanogenic bacteria [Meng et al, 2017], which feed on organic material in the shallow subsurface.…”

“…The immaturity of the considerable organic material in the sediment precludes significant overpressure due to catagenesis. However, fracturing has been attributed to early-burial overpressure related to methanogenic bacteria [Meng et al, 2017], which feed on organic material in the shallow subsurface. We also speculate that the high organic content in the sediment could have reduced the effective aqueous permeability, and so magnified any overpressures which might have otherwise been efficiently bled away.…”