Prediction of reservoir quality is challenging due to complexities caused by pore geometry. Here diagenesis plays an important role in providing a better understanding of the controls on reservoir quality. This research focuses on determination of pore geometry and diagenetic alterations in the Nyalau Formation, Borneo. This has been achieved by a combination of traditional, macroscopic field sedimentology and microscopic thin-section study, along with grain size analysis and porosity-permeability determination on 26 samples taken from outcrops. Represented outcrops from the study area show sedimentary features such as herringbone lamination, cross-bedding, flaserbedding, wavybedding and bioturbation structures. The outcrop samples are mainly very fine to fine grained, with petrographic visible porosity ranging from 15 to 35, indicating reservoir quality of the outcrop is good to very good. Permeability of the sandstone facies ranges from 50 to 1000 millidarcies (mD). Petrographic analysis shows the sorting to be very poor to fair depending on facies type, with common quartz overgrowths indicating diagenetic reduction of reservoir quality. Integrating the results of these analyses shows which factors affect the diagenesis of the tidal sandstones in the Nyalau Formation, which is considered as an analogue for a good quality hydrocarbon sandstone reservoir. © 2018 Geological Society of Malaysia. All rights reserved.