The reduction in the conventional resources and insistent request for hydrocarbons and energy lead us to look forward for the increasing interest in unconventional resources by exploring new technologies for the national economy and local utilization in Malaysia. Upper Mesozoic-Cenozoic is considered the age of Central Sarawak Formations. This paper attempts to evaluate the clay mineralogy, depositional environment, gamma ray evaluation and the relationship between total organic carbon and uranium concentration of the Nyalau, Tatau, Begrih, Liang and Belaga Formation which eventually comprise black shales and mudstones using spectral gamma ray (SGR), field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), XRD clay separation and total organic content (TOC). The results from SGR indicate that the formations composed mainly of kaolinite, smectite and chlorite. These results are confirmed by XRD, XRD clay separation and FESEM with the presence of illite as a dominant clay mineral component. Based on Th/U ratios which vary between 0.42 and 3.95, the formations are expected to be deposited in the reduced marine environment. Results from calculated API (90.76�233.32 API) reveal the ability of the clay minerals to absorb uranium and thorium through cation exchange capacity. The relationship between TOC and U cross plots also show a significant positive correlation. The presence of high U concentrations in the formations is possible because of their relatively higher clay contents and the existence of organic matter. © 2019, The Author(s).