Seventy sandstone samples of the Late Maastrichtian? Early Paleocene strata from the Willow Creek Formation of the southwestern region of Alberta were selected in order to characterize factors that control the sandstone’s properties and to investigate diagenetic processes related to porosity evolution. The sandstones are fine- to medium-grained, moderate- to well-sorted litharenite and have undergone a complicated diagenetic history. The diagenetic regimes of the sandstones combine several processes related to three stages of diagenesis: eodiagenesis after deposition, mesodiagenesis during burial, and telodiagenesis after uplift. The diagenetic processes recognized during eogenesis include compaction, cementation (calcite, clay minerals), and the dissolution of feldspar grains. Extensive occurrences of early calcite cement suggest that calcite was a main control on the porosity. Abundant calcite cements precipitated from pore water with a significant meteoric component, as shown by the strong negative oxygen isotopic values (−19.546 to −3.019). However, the calcite cements inhibited the later mechanical compaction of the sandstones, thus preserving an unfilled part of the primary porosity. The most important mesogenesis processes that controlled reservoir quality were the illitization and chloritization of smectite followed by dickite formation. The pore-filling illite and chlorite resulted in a considerable loss of porosity, whereas the pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of quartz overgrowths. During telogenesis dissolution of calcite cement created secondary porosity, whereas the precipitation of kaolinite caused a reduction in porosity. This study may enhance the understanding of the role of diagenesis on reservoir quality in similar aged fluvial sandstone deposits in the subsurface such as the Scollard Formation in the central Alberta plains and the Coalspur Formation in the southwestern Alberta foothills.