Analysis of the applicability of cuttings samples to test seal integrity, examples from the Triassic to Jurassic interval in 8 wells in Eastern Denmark

The Danish subsurface has a high technical CO₂ storage potential with multiple large structures that are currently under investigation as CO₂ storage sites following the recent opening of the CO₂ licencing round onshore Denmark. Within the structures two main reservoir-seal pairs are investigated namely the Bunter Sandstone sealed by the Ørslev and Falster Formations, and the Gassum Formation sealed by the Fjerritslev Formation. To evaluate the seal integrity direct measurements and tests to gauge the rock properties are needed. In Denmark, the onshore wells were mainly drilled in the 1950-1980’ies and many of the old wells have no wire-line log data or very poor data when compared to modern standards. In addition, cores of the seal section were rarely taken and the only source of material for full formation evaluation of the seal integrity are in most cases cuttings samples. This report focuses on the applicability of cuttings samples to test seal integrity and is based on analyses of cuttings samples from the wells Gassum-1, Fjerritslev-2, J-1X, Vedsted-1, Rønde-1, Søllested-1, Nøvling-1 and Vinding-1. Our cuttings analyses include photographing, conducting Handheld X-Ray Fluorescence (HH-XRF) measurements, determining the Total Organic Carbon (TOC, wt%), Total Carbon (TC, wt%) and Total Sulphur (TS, wt%) contents, and conducting pyrolysis (Hawk instrument) following the Rock-Eval method. Of the studied wells the Søllested-1 and Vinding-1 wells were not measured with HH-XRF due to insufficient material.

The first reservoir-seal pair includes the Lower – Mid Triassic Bunter Sandstone, Ørslev, and Falster Formations. These formations are distinct in their geological characteristics, predominantly characterized by terrestrial and evaporitic depositional settings. The second reservoir-seal pair encompasses the Upper Triassic – Lower Jurassic Gassum and Lower Jurassic Fjerritslev Formations deposited in humid fluvial to fully marine conditions. The two reservoir-seal pairs are clearly delineated by the chemical analysis and provides invaluable insights into the geological history and potential applicability of these formations for CCS, helping to make knowledge-based decisions for further sampling and testing of the seal integrity and capacity.