This version is available at https://strathprints.strath.ac.uk/65335/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. The degree to which a fault will impede fluid flow is only as great as its most permeable point. Processes that determine areas of the fault surface containing transmissible fault rocks must be utilized to produce reliable predictions of cross-fault fluid flow. We use a study site in Miri, Malaysia, to investigate in detail the fault-core thickness variations along-strike and down dip, and to quantify the risk of discontinuities in the clay-rich fault core. Four fault-core types have been identified: foliated clay-rich fault core, chaotic clay-rich fault core, anastomosing sandy shear zones and sandy breccia. We performed a geostatistical analysis, showing a correlation over 3 m scale, suggesting the presence of 'patches' of thin and thick fault core generally less than 3 m in length in profile.We interpret this geometry as superimposition of two or more different deformation processes at a smaller and a larger scale. We speculate about the processes that could produce the observed distribution of thickness and composition, and in particular, processes that could have disrupted the through-going clay-rich core.