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Plenty of Cook Inlet gas USGS thinks basin may have 19 tcf undiscovered gas, 599M barrels of oil By Alan Bailey Petroleum News
On June 28 the U.S. Geological Survey published its long awaited assessment of undiscovered but technically recoverable oil and gas resources in Alaska’s Cook Inlet basin. And the numbers look good, especially for those concerned about future natural gas supplies for heating and power generation in Southcentral Alaska.
The agency has estimated a possible range of 108 million to 1.359 billion barrels of oil, with a mean of 599 million barrels, and a range of 4.9 trillion to 39.7 trillion cubic feet of natural gas, with a mean of 19 trillion cubic feet. The corresponding figures for natural gas liquids are a range of 6 million barrels to 121 million barrels, with a mean of 46 million barrels.
The breadths of the volume ranges indicate the level of statistical uncertainty in the results, reflecting the fact that there are many aspects of Cook Inlet petroleum geology that remain obscure, given the relatively low level of exploration in the basin.
And these estimates of undiscovered resources may be compared with cumulative production to date of about 1.3 billion barrels of oil and more than 7.8 tcf of gas from the basin, said USGS geologist Rick Stanley, a member of the team that conducted the assessment, during a presentation on the assessment on June 29.
Onshore and state waters Stanley stressed that the assessment only applies to onshore areas of the Cook Inlet basin, and offshore areas within state waters of the upper Cook Inlet. The U.S. Bureau of Ocean Energy Management, Regulation and Enforcement is responsible for resource assessments in the waters of the federal outer continental shelf, mainly encompassing the more southerly reaches of the Inlet.
The assessment does not take into account how much of the oil and gas resource might viably be developed – the assessment simply considers whether the resource could be produced using existing technologies, regardless of whether development of the resource would prove profitable. Neither does the assessment consider the practicalities of physical access to the resources, with a substantial onshore segment of the basin, for example, lying under land within the Kenai National Wildlife Refuge.
The new assessment numbers represent a major increase over the last USGS assessment for the Cook Inlet region, completed in 1995. That earlier assessment, which included the whole of southern Alaska rather than just the Cook Inlet basin, estimated 190 million to 970 million barrels of undiscovered oil and 0.68 trillion cubic feet to 2.14 trillion cubic feet of natural gas. A 2004 U.S. Department of Energy study conducted a statistical analysis of existing Cook Inlet gas fields to come up with an estimate of 10 tcf to 14 tcf for as-yet undiscovered gas.
Worked with the state Stanley said that USGS scientists had worked with staff from Alaska’s Division of Oil and Gas and the state’s Division of Geological and Geological Surveys to obtain data for the new assessment – DGGS is currently heading intensive studies into the petroleum geology of both the Cook Inlet basin and the neighboring Susitna basin. The increase in resource estimates results from new scientific data from these studies; new surface and subsurface geologic maps; the reprocessing and re-interpretation of seismic data; new gravity and magnetic modeling; new reviews of the organic geochemistry of Cook Inlet oils; and new studies of potential oil and gas source and reservoir rocks, Stanley said. USGS has also run a new model for the timing of Cook Inlet oil formation, as part of its oil generation evaluation.
The Cook Inlet basin consists essentially of two major rock sequences: the younger and shallower rocks of the Tertiary period, and the older and deeper rocks of the Mesozoic period. The reservoirs of all existing Cook Inlet oil and gas fields are located in Tertiary sandstones, with very minor amounts of oil having been produced from a Mesozoic reservoir. Geologists have determined that most or all of the oil was sourced from rocks of the Mesozoic Tuxedni group, of Jurassic age, with the bulk of the gas originating from coal seams in the Tertiary rock sequence.
Four assessment units In conducting their new assessment, the USGS scientists homed in on four assessment units, the mappable systems of rock with common geologic traits that form distinct components of the regional petroleum system. These assessment units consist of:
• Tertiary sandstone oil and gas, the play that includes all existing Cook Inlet oil and gas fields.
• Mesozoic sandstone oil and gas, a deeper and as-yet untested play where USGS thinks that undiscovered oil and gas accumulations probably exist.
• Cook Inlet coalbed gas, an unconventional potential source of gas from direct drilling into the extensive coal seams known to exist in the Cook Inlet Tertiary rock sequence. Feasible gas production would require the coal to be at depths of less than 6,000 feet, thus excluding coals in the deepest section of the basin, Stanley said.
• Tuxedni-Naknek continuous gas, a potential but untested and highly-speculative unconventional gas source in Mesozoic sandstones of Jurassic and Cretaceous age, buried to depths in excess of 20,000 feet in a relatively small area, mostly offshore.
USGS has placed all of its estimated undiscovered oil in the Tertiary and Mesozoic sandstone assessment units. And with 227 million barrels of the total mean estimate of 599 million barrels of oil thought to exist in the Mesozoic play, the USGS assessment seems to lend support to the concept of exploration drilling into Mesozoic targets, a drilling option that has been much debated in recent years but not attempted.
Much gas in Tertiary Perhaps not surprisingly, about 12 tcf of the mean estimate of 19 tcf of natural gas is thought to exist in the Tertiary sandstone assessment unit, the unit that hosts the Inlet’s prolific gas fields. USGS thinks that there remain in the Tertiary many undiscovered oil and gas traps, including traps formed from the folding and faulting of the rock strata, and so-called stratigraphic traps, such as distinct sand bodies formed in ancient river channels, Stanley said.
And Mesozoic sandstone reservoirs could hold another 1.4 tcf of gas, USGS says.
The agency has also estimated a mean of 4.6 tcf of coalbed methane in the basin. Dave Houseknecht, one of the USGS geologists who have been evaluating this resource in the basin, told Petroleum News in February that the most promising area for coalbed methane resources appears to be a large area of territory down the west side of the Cook Inlet.
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