The tight gas sands of the Cook Inlet

During the Alaska Unconventional Gas Forum in Anchorage in April, Kenneth Helmold, a petroleum geologist from Alaska’s Division of Oil and Gas, talked about the results of a joint state and university study into the quality of sandstone gas reservoir rocks of the Cook Inlet basin. The study involved the analysis of 50 well-core samples and focused on assessing the potential for finding what are termed “tight gas reservoirs.” Tight gas cannot be produced at commercial flow rates without some form of artificial reservoir stimulation or the use of horizontal wells, Helmold explained.

A key issue in the Cook Inlet basin is the source of the material that later formed the sandstones, Helmold explained. The material consists of rock debris carried by rivers from mountains on either the northwest side or the southeast side of the basin.

But those two mountain ranges contained very different types of rock. On the northwest side of the basin, in the area of the Alaska Range, the rocks have been predominantly volcanic in origin, while the rocks in the area of the Kenai Mountains include some volcanic material but also contained a wide range of other rock types, including pre-existing sandstones and more silty rocks.

So sandstones originating from one side of the basin tend to have significantly different chemical and physical properties than sandstone from the other side of the basin — knowing the source of the material in the sandstone is important in assessing the likelihood of a particular sandstone reservoir being tight, Helmold said.

However, the study found that the sizes and ranges of sizes of the grains within the sandstones, coupled with the extent to which the sandstones have been compacted by deep burial, play critical roles in determining reservoir quality of Cook Inlet sandstones.

“(Grain) size does matter when you’re considering whether you can deal with a conventional and unconventional reservoir,” Helmold said.

And coarser-grained sandstones tend to preserve their porosity and permeability better under compaction — in the Cook Inlet basin the finer grained sandstones would likely form tight reservoirs at depths as little as 4,000 feet, while coarser grained sandstones remain more porous down to depths of 10,000 to 12,000 feet, Helmold said.

But the chemical and physical alteration of the rock fragments in the sandstone as a result of fluid flow through the rocks, or heightened temperatures and pressures at depth, can play havoc with the reservoir quality. The rock alteration typically results in the formation of pore-clogging secondary minerals — a wide variety of such minerals occurs in sandstones of the Cook Inlet basin, especially in the lower part of the Tertiary section and in the Mesozoic rocks below the Tertiary, Helmold said.

—Alan Bailey