Permeability is a petrophysical parameter that can indicate the economical value of sandstone as a hydrocarbon reservoir. In presence of Clay minerals in sandstone composition, permeability can be affected significantly according to the Clay mineral type. Therefore, understanding the impact of Clay on the permeability of sandstone has a big importance in scientific research and industry as well. Although well-logs can provide very good data quality, it is not cost-effective as a preliminary tool. However, field investigations and observations are basic tools to collect valuable data in a short time at a very low cost. In this paper, 30 intervals from 10 outcrops in Labuan island were selected to be described and investigated in the field using air permeameter and portable gamma-ray spectrometer with no additional lab investigations except 3 thin-sections for validation. Tinyperm II was used to determine the permeability, while gamma-ray spectrometer was used for Clay minerals typing. Results of lithofacies, permeability, and gamma-ray have shown a lack of relation between facies and permeability in the selected outcrops while showing a significant impact of the authigenic Clay minerals on permeability according to their types and morphologies. This paper can provide a good reference for preliminary reservoir field studies. © Published under licence by IOP Publishing Ltd.