The Garn Formation sandstone on the Halten Terrance exhibits higher porosity than expected at burial depths exceeding 4000 meters (between 10 and 10.5 % in the two studied wells). The reason for these relatively high values cannot be addressed to one single process, but the combined influence of the depositional environment, diagenetic processes, burial history and the presence of reservoir preserving and/or enhancing mechanism. The resulting vertical and lateral variations in reservoir quality is studied by sedimentological, petrophysical, petrographic, mineralogical and geochemical analysis. The lateral extensiveness of porous and permeable zones and facies show a correlation between the two studied wells. Two porous and permeable zones (Zone A and B, respectively) are identified and suggested to form from two different mechanisms: - Early emplacement of heavy oil in the shallowest part of the Garn Formation - Interaction between reworking by waves and shallow meteoric flushing in proximal part of delta In addition, early hydrocarbon emplacement of a lighter hydrocarbon phase is proposed to cause the moderate amounts of quartz cementation in the remaining part of the studied Garn Formation (average values around 12 %). Geochemical evidence of the heavy oil phase could not be obtained due to sample contamination of oil-based drilling mud. The high frequencies of stylolites indicated these interfaces to be the primary source of quartz cement. The inconclusive relationship found between stylolites frequency and amount of quartz cement indicate that the studied Garn Formation is a precipitation-controlled system. Illite is the dominating clay mineral, and cause severe reduction in permeability in zones of extensive illitization. Petrographic observations suggest illite form from kaolinite and K-feldspar. Retention of kaolinite observed in Zone A is assumed to be associated with the presence of heavy oil and the subsequent reduction in diffusion.