Origin and implications of early diagenetic quartz in the Mississippian Bowland Shale formation, Craven Basin, UK

Joe Emmings, Patrick Dowey, Kevin Taylor, Sarah Davies, Christopher Vane, Vicky Moss-Hayes, Jeremy Rushton

Research output: Contribution to journalArticlepeer-review


Silica cementation exerts a key control on the compaction and geotechnical properties of mudstones, and by
extension, the style of hydrocarbon and/or mineral systems present in a given sedimentary basin. Integrated
microscopic and bulk geochemical observations demonstrate that siliceous mudstones in the Bowland Shale
Formation, a target for UK shale gas extraction, exhibit abundant dispersed, discrete, μm-scale quartz cements,
and exhibit silica enrichment (‘excess’) above a local detrital Si/Al threshold of 2.5. Dissolution of siliceous
radiolarian tests during early diagenesis is identified as the main source of silica (opal A) required for quartz
precipitation, either via opal CT or directly to quartz, and potentially generated as a product of anoxic marine
‘weathering’ (dissolution) of reactive silicates during early diagenesis. Excess silica correlates with free hydrocarbons
(S1) normalised to total organic carbon (oil saturation index; OSI); we propose early diagenetic quartz
precipitation suppressed pore collapse (‘buttress effect’), retaining the pore space capacity to host oil. Quartz
precipitation was likely catalysed, for example via low porewater pH, elevated Al and/or Fe oxide content, and/
or abundant labile organic matter. Juxtaposition of siliceous mudstones and mudstones lacking quartz cement
indicates silica was immobile beyond the bed scale. Thus metre-scale siliceous packages likely represent more
prospective units within the Bowland Shale (in terms of unconventional hydrocarbons), on the basis of early
diagenetic biogenic-derived quartz cementation leading to improved hydrocarbon storage capacity coupled to
enhanced brittleness. These findings are relevant for shale oil and shale gas systems, specifically where oil
retained in pores subsequently cracks to generate gas. These findings also suggest the Bowland Shale is a subclass
of black shale, defined by the potential to host a relatively large volume of early diagenetic fluids,
derived from anoxic bottom waters, which were potentially S- and/or metal-bearing. This is potentially relevant
for understanding the genesis of adjacent and related Pb-Zn mineral deposits.
Original languageEnglish
Article number104567
JournalMarine and Petroleum Geology
Early online date4 Jul 2020
Publication statusPublished - 1 Oct 2020


  • mudstone
  • diagenesis
  • quartz
  • cement


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