|
DGGS continues Cook Inlet research Scientists evaluate potential oil source rocks and reservoir rocks on the southwest side of the inlet and upper Alaska Peninsula Alan Bailey Petroleum News
A team of scientists led by Alaska’s Division of Geological & Geophysical Surveys, or DGGS, has been turning its attention to some of the older rocks of the Cook Inlet basin seeking evidence that can help the search for new oil and gas resources in the basin, according to a new report issued by DGGS describing the status of research the division is carrying out in the basin.
The division’s Cook Inlet research program began in 2006, initially focusing on the relatively young Tertiary strata of the basin, the strata that host all of the producing oil and gas fields of the basin. In its Tertiary research, the team primarily assessed the potential to find new fields in what are called “stratigraphic traps,” hydrocarbon traps formed as a result of the way in which rocks have been deposited — sand bodies laid down in meandering river channels, for example.
Significant potential But older Mesozoic rocks in the basin also have significant oil potential. In fact, rocks of the Tuxedni group, of Jurassic age in the Mesozoic, are known to have sourced most or all of the oil in the Cook Inlet oil fields. Mesozoic strata underlie a thick sequence of Tertiary strata under the upper Cook Inlet, where the Cook Inlet oil and gas fields are located. To the south, under the lower Cook Inlet and down into the Shelikof Strait, the Tertiary sequence is much thinner. And Mesozoic rocks occur in surface exposures on the southwest side of the Cook Inlet and along the southeastern coast of the upper Alaska Peninsula.
Geologists have long speculated about the possibility of finding commercial quantities of oil in the Mesozoic. In addition to known oil source rocks in the Tuxedni, there is in the Mesozoic an older potential source rock, Triassic in age and broadly equivalent to one of the most prolific source rocks on Alaska’s North Slope.
But no one as yet has made a serious attempt at drilling for oil in the deeply buried Mesozoic rocks of the upper Cook Inlet. And there have only been 11 exploration wells drilled into the Mesozoic of the lower Cook Inlet, with none of these wells encountering commercial pools of hydrocarbons.
Mesozoic research In parallel with its investigations into the Cook Inlet Tertiary geology, the DGGS team has been phasing in research into the Mesozoic of the basin. And in 2011 the team examined some Mesozoic rock outcrops near Iliamna Volcano, on the west side of Cook Inlet. In its 2012 field season the team focused its efforts on the Mesozoic, examining the surface rocks on the western side of the lower Cook Inlet and on the Alaska Peninsula coast, according to the new status report for the DGGS Cook Inlet program.
By conducting independent research and making its findings public DGGS hopes to encourage oil and gas exploration in Alaska, while also providing useful data to companies interested in the potential of regions such as the Cook Inlet.
The status report describes work carried out in 2012 and presents some preliminary findings.
Potential source One area of research in 2012 involved the field examination and sampling of the Red Glacier formation, a potential oil source rock in the Tuxedni group — the Red Glacier formation occurs in surface exposures near the Iliamna Volcano. A laboratory analysis of rock samples from the formation at this location indicated that at some time in the past the rocks had been heated to temperatures higher than those conducive to the formation of oil, the report says. It is not clear whether this heating resulted from the burial history of the rock, from volcanic heating at some time in the past, or from some combination of these two factors, the report says.
However, the existence of injected layers of igneous rock in the Red Glacier formation would tend to add credence to a theory that the rocks were heated through past volcanic action.
Reservoir evaluation The team also examined surface exposures of the Naknek formation, a potential oil reservoir rock of late Jurassic age, and found evidence suggesting that the Naknek has a quite variable composition. Rocks of the Naknek exposed in the Iniskin Peninsula and Tuxedni Bay region exhibit poor reservoir quality, while oil-stained sandstone found on the southern shore of Kamishak Bay shows more promise. Rocks of the Cretaceous Kaguyak formation also generally showed poor reservoir quality.
However, pervasive fracturing throughout the Mesozoic rocks exposed on the west side of Cook Inlet suggests that underground fractures might have provided pathways for oil to migrate into relatively porous rocks of the Mesozoic and Tertiary rock sequences — an analysis of fracture data collected in the field will provide insights into the fracture history and distribution, the report says.
Oil bearing Two widely separated sandstone intervals in the Naknek and Kaguyak formations contain oil-bearing strata, an indication that at some time in the past oil had migrated through rocks in the region, the report says.
In particular, the discovery of oil-stained cobbles and boulders within the Kaguyak suggests that this formation contains rocks that could form viable oil reservoirs, connected through an oil migration route to oil sources in the older and deeper Tuxedni group, the report says.
“The presence of several other locations in the lower Cook Inlet where upper Cretaceous strata are reported to be oil bearing suggests that they overlie regionally extensive, thermally mature source rocks and might have potential as conventional (oil) reservoirs,” the report says.
A U.S. Geological Survey laboratory analysis of hydrocarbon samples and samples of oil-stained rocks will shed light on the origin of the oil in the rocks.
The team also investigated surface exposures of the Bruin Bay fault, a major geologic fault that marks the northwestern boundary of the Cook Inlet basin. Field evidence suggested a more complex movement history than previously thought for the fault. And detailed studies of rock-type variations in the Naknek formation suggest that the fault was actively moving and influencing sedimentation at the time that the material in the Naknek was being deposited, the report says.
|
