Characterising petrophysical caprock properties from cuttings: A case study from offshore Sarawak Basin, Malaysia

Abstract

Caprock integrity is a critical requirement for the safe and long-term geological storage of CO₂, yet its assessment remains challenging due to extremely low permeability, strong mineralogical heterogeneity, and the limited availability of representative core samples. In particular, the influence of petrological properties and mineral composition on caprock porosity and permeability is still poorly constrained, especially where evaluations rely increasingly on drill cuttings rather than intact cores. This study addresses these challenges by systematically investigating the relationships between mineralogy, pore structure, porosity, and permeability in a siltstone caprock overlying a carbonate reservoir in the offshore Sarawak Basin, Malaysia. An integrated experimental workflow combining X-ray diffraction, scanning electron microscopy, particle size analysis, low-pressure gas sorption, mercury intrusion capillary pressure (MICP), nuclear magnetic resonance, helium pycnometry, and unsteady-state pulse decay permeametry was applied to both core plugs and drill cuttings. Two distinct caprock facies (Seal A and Seal B) were identified, differentiated primarily by clay and carbonate content, grain size distribution, and pore throat characteristics. Results show that clay-rich, well-cemented micro- to nanoporous fabrics strongly control permeability, resulting in nanoDarcy-scale values under in-situ stress conditions despite moderate total porosity. Porosity and permeability estimates derived from drill cuttings consistently exceed those from confined core measurements, highlighting the need for method- and stress-dependent corrections when cuttings are used for seal evaluation. The novelty of this work lies in quantitatively linking mineralogical and petrological attributes to multi-method porosity–permeability measurements while critically evaluating the reliability of drill cuttings as proxies for core-based caprock characterisation. The combined low permeability and high capillary entry pressures indicate that the studied caprock can safely retain CO₂ columns exceeding the structural closure of the reservoir. These findings demonstrate a robust, cost-effective framework for caprock assessment in data-limited settings and provide transferable insights for CO₂, hydrogen, and gas storage applications.