Sandstones of the transgressive-regressive Kugmallit sequence, a hydrocarbon-bearing Oligocene unit in the Beaufort-Mackenzie Basin, are typically poorly indurated, very poorly to moderately sorted, very fine to medium grained litharenites. Detrital components include quartz, chert, feldspar, rock fragments, chlorite and mica. The average quartz and chert content increases exponentially with increasing median grain size. Subtle petrographic differences between various deltaic subenvironments can be detected by considering both composition and texture. Three sandstone diagenetic stages are recognized: 1) early diagenesis at shallow to intermediate burial depths, especially carbonate cementation and replacement. The carbonate cements include calcian, ferroan, or calcian/ferroan dolomites and low-magnesian ferroan calcite, with dolomite much more common than calcite. Typically, carbonate chemical compositions are uniform within each sample, but in some cases differ significantly between sites only a few metres apart. 2) creation of secondary porosity by repeated precipitation/replacement and dissolution of dispersed patches or zones of carbonate cement. More than half of the preserved porosity is of the enhanced intergranular variety. 3) deep burial compaction and late stage cementation by quartz and ferroan dolomite. Hydrocarbon accumulation probably began as early as stage 2 and was completed by the late Oligocene to early Miocene.