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Open AccessArticle

Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK

1
Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Jane Herdman Building, 4 Brownlow Street, Liverpool, Merseyside L69 3GP, UK
2
Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3FX, UK
3
Geosciences Barcelona, CSIC, Lluis Sole Sabaris s/n, 08028 Barcelona, Spain
4
School of Geosciences, University of Edinburgh, Grant Institute, West Main Road, Edinburgh EH9 3JW, UK
5
Institute of Geoenergy Engineering, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(8), 691; https://doi.org/10.3390/min10080691
Received: 27 May 2020 / Revised: 7 July 2020 / Accepted: 30 July 2020 / Published: 3 August 2020
(This article belongs to the Special Issue Characterisation of Mudrocks: Textures and Mineralogy)
Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large-scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low-permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site, so the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g., the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here, we assess the characteristics of the Rodby and Lista Shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques, with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and is calcite rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of approximately 16% a mean permeability of 225 nD (2.21 × 10−19 m2), and is calcite free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data show that Young’s modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young’s modulus increasing as shale content decreases and as cementation (e.g., calcite) increases. Our work has shown that Young’s modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young’s modulus from routine well log data could form a valuable element of CCS-site top-seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of approximately 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast, the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2. View Full-Text
Keywords: Rodby Shale; Lista Shale; carbon capture and storage; North Sea; wireline logs; splitting tensile stress; mercury intrusion porosimetry; SEM-EDS; quantitative mineralogy; geomechanical properties; Young’s modulus; CO2 column height Rodby Shale; Lista Shale; carbon capture and storage; North Sea; wireline logs; splitting tensile stress; mercury intrusion porosimetry; SEM-EDS; quantitative mineralogy; geomechanical properties; Young’s modulus; CO2 column height
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MDPI and ACS Style

Worden, R.H.; Allen, M.J.; Faulkner, D.R.; Utley, J.E.P.; Bond, C.E.; Alcalde, J.; Heinemann, N.; Haszeldine, R.S.; Mackay, E.; Ghanbari, S. Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK. Minerals 2020, 10, 691. https://doi.org/10.3390/min10080691

AMA Style

Worden RH, Allen MJ, Faulkner DR, Utley JEP, Bond CE, Alcalde J, Heinemann N, Haszeldine RS, Mackay E, Ghanbari S. Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK. Minerals. 2020; 10(8):691. https://doi.org/10.3390/min10080691

Chicago/Turabian Style

Worden, Richard H.; Allen, Michael J.; Faulkner, Daniel R.; Utley, James E.P.; Bond, Clare E.; Alcalde, Juan; Heinemann, Niklas; Haszeldine, R. S.; Mackay, Eric; Ghanbari, Saeed. 2020. "Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK" Minerals 10, no. 8: 691. https://doi.org/10.3390/min10080691

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