Dwindling oil production, tightening product deliverability and an aging infrastructure are normally the hallmarks of a mature oil and gas province. But Alaska’s Cook Inlet basin, despite its collection of aging oil and gas fields, remains in many geologists’ eyes relatively underexplored, especially given the sparse distribution of exploration wells in the region.
So, as residents of Southcentral Alaska become increasingly jittery about ever tightening natural gas supplies for power generation and heating, and as Southcentral Alaska’s main oil refinery at Nikiski on the Kenai Peninsula has to import more and more of its feedstock, are there still significant oil and gas resources to be found in the region? Or is the end in sight for the basin that triggered Alaska’s first major oil and gas production, before the discovery of oil on the North Slope?
A team of geoscientists led by Alaska’s Division of Geological and Geophysical Surveys, in a spirit of optimism about the continuing potential of the Cook Inlet basin, has been engaged in a multiyear project investigating the petroleum geology of the region and providing public domain information of help to explorers who seek new oil and gas resources. Funding for the research has come from the state and from the oil industry.
And, in parallel with the DGGS work, the U.S. Geological Survey plans to conduct a new oil and gas assessment of the Cook Inlet basin in early 2011.
Rationale
A newly revised prospectus for the DGGS Cook Inlet project explains some of the rationale behind what the DGGS-led team is trying to achieve.
The aging Cook Inlet oil and gas fields occur in porous and permeable reservoirs in large, easily found geologic structures and represent the “low hanging fruit,” picked early in the Cook Inlet basin’s commercial history, the report says.
But, as oil and gas have percolated their way from petroleum sources through the tens of thousands of feet of rock strata in the basin, there could have been many less obvious locations where petroleum resources could have become trapped, perhaps for example in ancient river channels meandering their way through the subsurface or in the sands bodies left behind by ancient river fan structures — the Tertiary rocks that host all of the producing oil and gas fields are known to have been laid down on land from ancient river systems.
These relatively small, subtle oil and gas traps, known in geologic parlance as “stratigraphic traps,” are much more difficult to find than the big structures of the low hanging fruit but could reservoir substantial undiscovered oil and gas resources. In addition, some gas could have become trapped in what are called “tight sands,” where a lack of permeability within sand units has prevented the gas from escaping.
Through geologic fieldwork and associated investigations, the DGGS-led team wants to help oil and gas explorers by addressing questions such as whether the configuration of likely stratigraphic traps would lead to relatively low-volume reservoirs, and where within the basin stratigraphic traps are most likely to be found. Other questions being addressed are the extent to which well logs and seismic data can help people find the assemblages of rocks likely to contain stratigraphic traps, where within the basin tight gas sands might be found and the extent to which chemical alteration of the rocks or the presence of pore-clogging clays might detract from oil and gas reservoir quality.
Two areas
Since the Cook Inlet research program began in 2006, investigation of the Tertiary strata of the basin has focused on two areas where Tertiary rocks are well exposed at the surface: the Homer-Clam Gulch area on the southwest coast of the Kenai Peninsula, and the Capps Glacier area, tucked against the Alaska Range on the northwest side of the basin.
The work in the Homer-Clam Gulch area has enabled the recognition of several different styles of ancient river-sand deposition, while structures within the sands suggest upheavals caused by ancient earthquakes. In the Capps Glacier area, thick rock units containing masses of pebbles and boulders, with much of this detritus consisting of volcanic material, indicate the operation millions of years ago of torrential rivers flowing into the margin of the basin from volcanic peaks to the immediate west.
The team completed more than 400 square miles of geologic mapping in the Capps Glacier area in 2009, clarifying some aspects of the rock stratigraphy and assessing the impact of major geologic faults on the deposition of the rocks.
And as the research continues moving forward in 2010, the research team is piecing together the evidence for changes in rock types between the margins of the basin and the basin interior, and how these changes impact oil and gas reservoir characteristics. Geoscientists in the team are also using rock samples to assess how the origination of the sand grains in potential reservoir sandstones influences the chemical alteration of the grains, and hence the reservoir quality.
Another line of research involves the use of available seismic and well data to track Tertiary rock units through the subsurface, to develop maps of the subsurface geology and to determine the history of rock subsidence in different parts of the basin.
Mesozoic oil
As well as investigating the petroleum geology of the Tertiary strata that host the existing Cook Inlet oil and gas fields, the DGGS-led team is investigating the geology of the tens of thousands of feet of older Mesozoic rocks that underlie the Tertiary. The Jurassic Tuxedni group, within the Mesozoic rock sequence, is believed to have sourced most of the oil in the Cook Inlet oil fields, the oil having presumably percolated upwards into the Tertiary reservoirs.
And the question of whether to drill into the Mesozoic to seek new oil fields has perplexed Cook Inlet oil explorers for many years, especially given the very high cost and high uncertainty of drilling into this deep section of the basin. In the 2010 legislative session, Alaska lawmakers passed a bill that included a $25 million tax incentive for the next Cook Inlet Mesozoic oil exploration well.
The DGGS-led team has been investigating excellent surface exposures of Mesozoic rocks on the west coast of Cook Inlet, between the Iniskin Peninsula and Tuxedni Bay, measuring rock sequences, taking rock and fossil samples, and assessing the geologic processes whereby the rock sequences were laid down.
Within the Mesozoic, there are Cretaceous sandstones that have good reservoir characteristics, but well penetrations into deeper and older rocks have found sandstones that tend to be clogged by minerals formed from the chemical alteration of sand grains. A prime objective of the DGGS research is to determine whether there may be areas of pre-Cretaceous rocks where reservoir quality is better than has been found to date. The team also wants to learn more about the ways in which the Cretaceous rocks were deposited, to evaluate potential Cretaceous reservoir shapes and sizes.
Subsurface map
And a continuing research objective will be to use available well and seismic data to evaluate which Mesozoic rock units directly underlie the Tertiary rocks of the basin, and hence produce a subsurface Mesozoic map showing the juxtaposition of oil and gas source and reservoir rocks. The team is also investigating the timing of the chemical alteration and clogging of potential Mesozoic oil reservoirs — the question of whether the rocks would have been altered before or after oil could have migrated into them is one of the keys to understanding the Mesozoic oil potential of the basin.
