Caprock and reservoir assessment for CO2 storage in Lower Palaeozoic Formations of the Baltic Sea subsurface: Textural, structural and mineralogical characterization using X-ray Computed Tomography, geochemistry and microscopy

Abstract

This study presents a mineralogical, textural and structural assessment of the Cambrian Faludden Sandstone Member (Borgholm Formation) and Lower Silurian mudstone (unspecified formation), for their potential use as reservoir and caprock, respectively, for deep carbon storage in the Swedish sector of the Baltic Sea subsurface. The study was performed on two drillcores from the island of Gotland, Nore-1 and Nore-2, and a combination of methods was used for the assessment, including visual inspection, SEM, XRF geochemistry, EDS, microscopy and the novel method of X-ray Computed Tomography (XCT). The results show that the stratigraphic succession poses great potential as a reservoir-caprock pair, being homogenous and with no major compositional and/or diagenetic anisotropy. The Faludden Sandstone constitutes a massive quartz arenite with high porosity (approximately 17-21%) and permeability (>1000 mD), framboidal pyrite, dolomite cement and limited fractures filled with late crystalline calcite. The Silurian mudstone is a dense rock unit with consistently high gamma-ray signal, bentonite layers and organic-rich facies, which imply a mostly impermeable formation. Apart from the Silurian mudstone, the thin suite of Ordovician limestone that separates the reservoir and main caprock succession includes two major clay-rich horizons that are herein included in the general caprock complex. The method combination was of great importance
to the assessment made, especially the addition of XCT, which provides important insight into the structure and mineralogy. The XCT must however be combined with visual and geochemical data to avoid misconceptions due to the similarities in the densities of the present minerals, which make them appear almost identical in the 3D tomography attenuation colours. This study brings Sweden a step closer to CCS in the Baltic Sea subsurface.

Popular Abstract

Assessing rock formations for Carbon Storage potential in the Baltic Sea subsurface
Global temperatures are rising due to the greenhouse effect, and CO2 emissions produced by human industrial activity are the main reason. To mitigate these emissions and avoid hazardous weather extremes and dislocations of entire populations, the need for viable solutions is more important than ever. Carbon Capture and Storage (CCS) is one of the most promising options while humanity is switching to fully renewable energy use.
One of the storage options for CO2 is that of injection via wells into deep geological formations with high permeability (reservoir), which are capped by impermeable formations (caprocks). One can think of the reservoir rock as a sponge, able to accommodate an injection of CO2 and the caprock as a bottle cap that will not allow for the uprising CO2 to escape upwards into overlying formations or the surface. Favourable candidates for geological storage are, among others, deep saline aquifers. In this master thesis, two rock formations are going to be assessed for their value as reservoir and caprock, respectively, for potential carbon storage in the Baltic Sea subsurface, south of Gotland island off Sweden. The two candidates are the Faludden Sandstone of Cambrian age and overlying Lower Silurian calcareous mudstone for reservoir and caprock, respectively. While they are promising for carbon storage and meet the basic criteria, they need to be assessed in depth to understand their mineralogy, texture and structure to ensure maximum safety and avoid leakage scenarios.
Methods and Results
The methods used to assess the rocks include visual inspection, microscopy (SEM, EDS), geochemistry (XRF) and X-ray Computed Tomography (XCT) on two deep drill cores from the southernmost part of Gotland. The range of these methods ensure a deep insight into potential variations in microfacies or diagenesis, secondary minerals, micro-fractures or any other anisotropic features that might be of importance to the CO2 trapping mechanisms. In addition, these methods are also going to be evaluated for their usefulness in this study.
The results show that the Faludden Sandstone is homogenous with high porosity and minimal impurities such as pyrite and few vertical fractures filled with crystalline calcite, overall a very promising reservoir rock. The Lower Silurian calcareous mudstone presents a very promising caprock as it constitutes a very dense, homogenous formation with no irregularities, deformations or fractures standing out. Ten thin layers of bentonite act as additional impermeable layers, while organic-rich layer interbedding add to the very low vertical permeability of the formation. Pyrite enhances the sealing capacity, as, if it comes in contact with CO2 it will react to form minerals such as gypsum that would further reduce the mudstone’s porosity. The multi-parameter methods performed, and especially the combination of XCT with geochemical signals, provided a great insight into the mineralogy and structure that could not be otherwise obtained. Thus, it is proposed that this method is performed in similar studies.

• author: Kardakos, Nikolaos
• supervisor: Mikael Calner; Paula Lindgren; Peter Dahlqvist; Mikael Erlström
• organization: Department of Geology
• course: GEOR02 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Carbon capturing and storage, caprock, reservoir, Faludden Sandstone Member, Borgholm Formation, Ordovician, Silurian
• publication/series: Dissertations in Geology at Lund University
• report number: 704
• language: English
• id: 9196168
• date added to LUP: 2025-06-10 17:19:25
• date last changed: 2025-06-10 17:19:25