Pyrites are usually indicators of paleoredox conditions and hydrocarbon generation from organic matter. This study investigates the mineralogy, chemical composition and reservoir quality of the pyritic sandstones found in Lambir Formation of Onshore West Baram Delta, Sarawak Basin, which was selected because the prolific occurrence of hydrocarbon in the adjacent Miri Formation has not been equally reflected in the Lambir Formation, despite their similarity in age (Miocene). X-ray diffraction of the collected samples identified the presence of quartz, hematite and pyrite, while X-ray fluorescence confirmed the silicate, iron and sulfate composition. The petrographic analysis shows the lithic samples are friable and highly porous (27%).The unstable more soluble reduced or ferrous iron (Fe2+), indicated by the pyrite content, suggest water-logged soils, euxinic marine and organic-rich saline waters, all denoting anoxic environments. The pyrite hinders quartz cementation, as indicated by the friability of the rock samples. It also fills the pore spaces and reorganizes the detrital grains in a preferential alignment to create micro-laminations. Based on the high content of syngenetic pyrite, it can be inferred that the thermochemical generation of sulfate could have led to the destruction or depletion of hydrocarbon in the Lambir Formation.