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.

Clay mineral content and diagenetic processes are vital factors that affect reservoir quality, especially in tight clastic reservoirs, which are crucial for industrial and scientific purposes. The presence of clay minerals poses one of the most significant challenges in evaluating sandstone reservoirs’ quality. Even though wireline logs may provide a good indication of the reservoir quality, there have been cases where they have failed. This work focuses on the clay minerals’ impact on the porosity and permeability of a clastic reservoir. Typical outcrops from Labuan Island, Brunei–Sabah Basin, were chosen as a case study to investigate the petrophysical and petrographic parameters together with clay mineralogy profiling. The effects of the clays on the petrophysical parameters of the sandstone reservoir were evaluated through air permeability, spectral gamma ray measurements, a petrographic thin section analysis, a visual porosity estimation, and a grain size analysis. Field and petrographic studies revealed that Belait and Temburong formations contain massive, interbedded, laminated, and cross-bedded sandstones. Using an image analysis of the thin sections, porosity values ranged from 7.3% to 23.5%, with different macro and micro porosity distributions. According to the spectral gamma-ray investigation and air permeability, permeability reduction is strongly associated with clay minerals. The microporosity and permeability of the analyzed samples showed a unique pattern influenced by the grain size distribution. It was found that the textures dominated by mud grain size had a more significant impact on the air permeability and visual porosity, with coefficient of determination values of 0.83 and 0.70, respectively. The Belait Formation displayed a higher porosity and permeability compared to the Temburong Formation. This research provides new insight into the potential reservoir of Stage IV (the Belait Formation offshore equivalent) compared to Stage II (the Temburong Formation offshore equivalent).