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Lithological control on fracture cementation in the Keuper marl (Triassic), north Somerset, UK

Qingfeng Meng, John Hooker and Joe Cartwright
Lithological control on fracture cementation in the Keuper marl (Triassic), north Somerset, UK
Geological Magazine (November 2018) 155 (8): 1761-1775

Abstract

The spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the Triassic Keuper Marl Formation (UK), a caprock for hydrocarbon reservoirs and CO (sub 2) sequestration. The marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins. The nodular gypsum in evaporite horizons provides excess gypsum for nodule-rooted horizontal gypsum veins. Our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls. We interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks, based on stratigraphic correlations, and much lower Sr concentrations than gypsum nodules. Gypsum was transported to adjacent veins mainly through diffusion in the low-permeability marls. The localization of gypsum veins and varied Sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non-evaporite beds. This results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements. The study implies that the cementation of natural fractures in low-permeability rocks can highly depend on the presence of cement minerals in the host rock.


ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 155
Serial Issue: 8
Title: Lithological control on fracture cementation in the Keuper marl (Triassic), north Somerset, UK
Affiliation: University of Oxford, Department of Earth Sciences, Oxford, United Kingdom
Pages: 1761-1775
Published: 201811
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 76
Accession Number: 2019-005249
Categories: Sedimentary petrologyEconomic geology, geology of energy sources
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch map
N51°10'00" - N51°10'60", W03°21'00" - W03°19'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Cambridge University Press. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2019
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