Porosity in the Ivishak sandstone on the North Slope is frequently cited as an example par excellence of secondary, or leached, porosity. In an attempt to corroborate and expand this contention, 100 thin sections were examined from representative wells in the Prudhoe Bay field and in NPRA. Monocrystalline quartz and biogenic chert are dominant framework grains in the two areas. Minor amounts of polycrystalline quartz, and sedimentary and metamorphic rock fragments are also present. Most of the chert in NPRA is nonporous, whereas Prudhoe Bay chert contains microscopic pores where siliceous biogenic debris, chalcedony, or dolomite formerly existed.

The diagenetic sequence of the NPRA samples includes: (1) initial porosity loss by either compaction or siderite, chlorite, or pedogenic calcite cementation; (2) minor carbonate dissolution; (3) localized chert dissolution; and (4) quartz overgrowth formation. The diagenetic sequence of the Prudhoe Bay samples includes: (1) chert dissolution; (2) quartz overgrowth, kaolin, pyrite, and siderite formation; and (3) siderite dissolution.

Some workers have suggested that an early carbonate cement maintained the large intergranular pore volumes in the Ivishak at Prudoe Bay. A more likely possibility is that a long, initial period of shallow burial maintained a loose packing density without early cementation. Later chert and siderite dissolution increased porosity. By contrast, the Ivishak in NPRA was subjected to deeper burial early in its history. This caused porosity reduction by relatively early compaction. The later diagenetic events, which ultimately control porosity and permeability variations between the two areas, are, in part, also controlled by the different burial regimes.

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