Faulting in siliciclastic – carbonate sequences: an improved characterization for de-risking faulted CCS sites (Flexible Funding 2023)

Dr Emma Michie, University of Liverpool, was awarded funding in the UKCCSRC’s Flexible Funding 2023 call to look at “Faulting in siliciclastic – carbonate sequences; an improved characterization for de-risking faulted CCS sites”.

Project Overview

The main project aim was to reduce uncertainty when analysing fault seal behaviour within siliciclastic – carbonate (mixed) sequences within the subsurface, benefiting CO₂ storage site assessment. Several proposed CO₂ storage sites contain mixed sequences within the overburden, with mudstones interbedded with carbonate units.  To increase worldwide stored CO_2, is it likely that we will utilise complex sites influenced, or bound, by faults cutting these mixed sequences. However, we currently have no tools to accurately predict their sealing potential. Further, the influence of carbonate interbeds on fault seal is currently unknown.

This research project focused on outcropping analogues of faulted siliciclastic-carbonate sequences with the attempt to start to fill the knowledge gap of how to characterise these structures within the subsurface.  Previous studies have focused efforts into understanding fault seal potential either within siliciclastic sequences (sand-shale) or carbonates, but rarely combining the two, despite the commonality for the heterogeneous sequence. Outcrop analogues utilised for this study are in the northern and central Apennines.

What We Did

This project generated new collaborations with researchers from Italy, bringing their wealth of knowledge into local geology, suggesting suitable outcrop analogues.  Together, we came up with a work programme for sampling outcrops that most accurately represent potential CO₂ storage sites.

We focused on two outcrop analogues in the northern and central Apennines, Italy, collecting 20 samples from the host and the fault zone. The samples were measured for porosity and permeability, as well as generating thin section for associated microscopy analysis using optical and SEM methods.

Some outcrop localities were in easy reach.  Others meant a more laborious hike, which included sliding back down due to a downpour that occurred whilst at the top of the hill, turning the road into a mud slide!

Whilst conducting fieldwork, I couldn’t resist indulging in some nearby geotourism. I explored the lingering devastation from the 2016 Amatrice earthquake, a poignant reminder of nature’s force, and marvelled at the chance to touch the well-preserved K-Pg boundary in Gubbio—a tangible connection to one of Earth’s most dramatic events.

Key Findings

Complicated fault zone architectures were recorded within these mixed carbonate-marl sequences, creating wide zones with multiple slip surfaces. Where carbonate lithologies dominate, the fault zone narrows slightly, localising deformation to create (proto)cataclastic rock fabrics on the slip surfaces.  Where marl lithologies dominate, multiple slip surfaces are observed, distributing the strain.  Significant veining is observed in more competent lithologies that have been juxtaposed.

Measured permeability values do not seem to vary significant from host to fault rock samples within the marl units. However, the permeability is recorded as increasing within the fault rock of the more competent carbonate units, associated with the observed veining. Such results will help refine subsurface modelling. This increased permeability may have implications for storage site assessment.

What’s Next

This project has now reached completion, and one of the research papers from this project will be submitted early 2025.  Research will continue, with further analyses of the collected samples, including XRD analysis and XCT image analysis.

Read more on Emma’s Flexible Funding 2023 project page.