The formation of illite in hydrocarbon-bearing clastic sedimentary formations can have different effects on reservoir quality, as illite cement may preserve porosity by reducing quartz cementation or can degrade permeability. In case of Late Carboniferous–Early Permian Unayzah A sandstones from eastern Saudi Arabia, illite cement is widespread, but its distribution is laterally and vertically heterogeneous. For the present study, core mineralogy and formation-water hydrochemistry from the Unayzah Group were evaluated to identify processes controlling the formation of illite cement during burial. Hydrothermal experiments and geochemical modeling were performed to reconstruct the formation of secondary minerals under various burial scenarios. Mineral reactions potentially relevant to illite formation were evaluated by comparing cementation and diagenesis of a shallow field (6000 ft) with those in a deeply buried field (15,000–16,000 ft). Similar depositional environments ensure that sediment composition at the time of deposition was similar between both fields. The shallow field is characterized by tangential (smectitic) infiltrated clay coats and kaolinite cement, while the deep field is dominated by diagenetic illite grain coats and illite matrix. During burial of the deep field, illitization of kaolinite was driven by potassium liberated during feldspar dissolution. K-Ar dating suggests an age of 91.8–108.3 Ma for illite cement coats in the deep field. Grain-coating illite cement is locally lacking in the upper part of the deep field, which may be explainable by the absence of feldspar at the time of deposition, early exhaustion of K-feldspar as a result of kaolinitization of feldspar during early diagenesis, or illitization of smectitic grain coats consuming available potassium during deep-burial diagenesis. The poor development of diagenetic illite coats locally had a strong effect on reservoir quality, as these sandstones experienced pervasive quartz cementation. Geochemical modeling with SOLMINEQ.88 suggests oversaturated conditions for muscovite (as illite proxy) for shallow and deep field scenarios, whereas hydrothermal experiments with present-day formation water and sandstone core material encountered incipient illite crystallization on kaolinite grains at 150°C and 450 bar. Temperature is proven to be a major controlling parameter on illite precipitation, but local grain mineralogy (such as the presence of feldspar), early diagenetic clay assemblage (kaolinite and smectite), or grain-surface substrate may control grain-coating illite cement distribution on a field scale. This study suggests that moderate grain coverage of smectitic infiltrated clays may not always be improved by the later formation of diagenetic illite coats during deep burial if there is insufficient potassium for illitization.