The Miocene Miri Formation of West Baram delta is the most prolific formation of Onshore Sarawak Basin in terms of hydrocarbon accumulation. Despite their similarity in age and arenaceous composition, the prolific hydrocarbon occurrence in Miri Formation has not been reflected in the adjacent Belait and Lambir formations likely due to failure to identify discrete reservoir units in the formations. Thus, this study attempts to augment exploration in onshore West Baram Delta by analyzing petrographic and microtextural variations in Belait and Lambir formations using thin sections and Field emission scanning electron microscopy (FESEM) images. Organic content and anoxic condition of deposition, Iron and carbonate precipitation, tangential and floating rock fabric were shown to define the texture of the Lambir sandstones, while the Belait rocks are characterized by sutured and deformed quartz grains. The Belait conglomerates reveal euhedral quartz crystals characterized by mechanical weathering defects (fractures and striations), which are potential nucleation sites that promote the growth of quartz cement which subsequently limits compaction. Conversely, the chemical weathering features (etchings, pitting, solution pits and notches) in Lambir sandstones provide sites for pore growth, while the carbonate intraclasts, pyrite frambroids and iron oxides inhibits quartz cementation, but infill pore spaces. In all, the study showed that the Lambir are comparatively a more porous formation.