Abstract
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.
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 language | English |
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Article number | 104567 |
Journal | Marine and Petroleum Geology |
Volume | 120 |
Early online date | 4 Jul 2020 |
DOIs | |
Publication status | Published - 1 Oct 2020 |
Keywords
- mudstone
- diagenesis
- quartz
- cement