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Shining the spotlight on Cook Inlet reservoirs DGGS-led research is starting to shed light on some of the more elusive hydrocarbon plays in Southcentral Alaska’s Cook Inlet Alan Bailey Petroleum News
Exploration in and around Alaska’s Cook Inlet in the 1960s and 1970s uncovered many of the easy-to-find oil and gas fields in the major underground structures of the Cook Inlet basin. But now that much of that low-hanging fruit has been plucked, people are starting to look at the less obvious possibilities in the basin.
Of particular interest are what geologists term stratigraphic plays, in which oil and gas become trapped in situations where, as a consequence of the circumstances under which the rocks were formed or eroded, reservoir rocks such as sandstones abut impermeable trap rocks such as shales.
A team led by geologists from Alaska’s Division of Geological and Geophysical Services, with joint state and industry sponsorship, is engaged in a multi-year study of the Cook Inlet geology, to provide useful public-domain information about the geology and to help identify parameters for locating those elusive stratigraphic traps.
Tertiary sediments During the Tertiary period, beginning more than 60 million years ago, a series of sediments were laid down from rivers, lakes and swamps in a subsiding area of land between what are now the Kenai Mountains and the Alaska Range, to form the Cook Inlet basin. Sand bodies within those sediments constitute the reservoirs of all the producing Cook Inlet oil and gas fields.
The basin attains a maximum thickness of about 25,000 feet near the northwest corner of the Kenai Peninsula.
And, as in any petroleum province, the quality of the hydrocarbon reservoirs in the basin depends on the porosity (the ability to hold fluids) and the permeability (the ability to flow fluids) of the reservoir rocks. In sandstone reservoirs the porosity and permeability depend on factors such as the size of the sand grains in the rock, the range of sizes of the grains and the amount of material clogging the spaces between the grains.
Those factors depend in turn on the exact circumstances under which the sands and pebbles which later formed the rocks were deposited, and what subsequently happened to the rocks.
River-borne debris Geologists think that in the Cook Inlet basin, rivers carried rock debris, pebbles and sand down from the mountains that defined the northwest and southeast edges of the basin and that those lateral rivers flowed into a central river system that followed the northeast-southwest axis of the basin. Periodically rivers would overflow their banks into lakes and swamps.
The DGGS-led team thinks that by understanding the details of how this system of rivers, lakes and swamps worked geologists will be better able to determine where the more promising reservoir rocks may be found, DGGS geologist Dave LePain told the Alaska Unconventional Gas Forum in Anchorage in April.
“The devil’s in the details and many of the reservoirs … are in this axial-fluvial system, and it’s important to understand the depositional systems that were controlling the distribution of those reservoirs,” LePain said. “The big question we’re trying to address is ‘Can we predict which depositional systems … are likely to be present in the basin in space and in time?’ … We think we can.”
Rivers and valleys LePain described the team’s investigation of rock outcrops on the west side of Cook Inlet, almost due west of Anchorage. The team assessed some very thick sequences of rock containing boulders and pebbles, deposited near the edge of the basin quite early in Tertiary times and apparently associated with the movement of a geologic fault.
The geologists interpreted these rocks as having formed in a river fan system in which rapidly moving water had quickly transported and dumped rock debris across the basin edge.
“This is quite clearly an alluvial fan package that was generated from nearby basin-margin uplifts,” LePain said.
Because the debris in this type of deposit contains a wide range of fragment sizes, from boulders to sand, the smaller fragments tend to fill the spaces between the larger fragments. As a consequence, the permeability and porosity of the rock are likely to be low to moderate, thus limiting their effectiveness as hydrocarbon reservoirs.
On the other hand, the geologists have found that some other rocks of similar age farther into the basin contain a higher proportion of sand. That feature presumably resulted from the river water carrying a higher proportion of finer grained material, as the rivers flowed farther from the basin edge.
In this type of environment, the higher uniformity of fragment size leads to moderate to high permeability and porosity, so that these rocks could form good oil and gas reservoirs.
On the south-east side of the basin, near Seldovia and Homer in the southwestern Kenai Peninsula, the geologists examined some younger rocks that are well exposed along the sea coast. The team found evidence that these rocks filled ancient river valleys, where rivers had flowed into the basin from the east. As on the west side of the basin, a variety of sizes of rock fragments had been rapidly deposited, perhaps from flash flooding.
And, again, although these rocks would make relatively poor reservoirs, farther into the basin they appear to transition into more sandy river-deposited rocks with better reservoir potential.
The geologists also found evidence that the type of sediment deposition, and therefore reservoir potential, changed with time at different points within the basin. On the west side of the inlet, for example, several cycles of river sediment deposition have each culminated in river overbank flooding, the deposition of mud and the formation of coal seams.
The DGGS-led team was especially intrigued by variations in the exposed rocks along Kachemak Bay, near Homer. In that area the sand units within the rock succession become thicker and more extensive as they become younger, suggesting a transition in time into a braided river system in the ancient landscape. And these braided river sandstone units ought to make good reservoirs.
Piecing together these features of the ancient geography of the basin and determining how that geography changed, should lead to some level of prediction of where the better hydrocarbon reservoirs might be found, the DGGS-led team thinks.
For example, does the transition into a braided river system near Homer represent some far-reaching geographic change and thus a regional transition to better reservoir rocks? Or was that transition just local to the Homer area?
“We don’t know the answer to that question, but hopefully we’ll be able to shed some light on it in the future,” LePain said. “Stay tuned. That is where we’re heading.”
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