Spotlight on Cook Inlet
DNR team hopes to facilitate exploration by developing public-domain model
A growing supply/demand crunch for natural gas in Southcentral Alaska is catching many people’s attention, not least the Kenai Peninsula industrial gas users who have had to curtail some operations and gas consumers who have seen steady increases in their gas bills. Now, in a move to help address the gas supply part of the supply/demand equation, the Alaska Department of Natural Resources has established a team to work intensely on the petroleum geology of the Cook Inlet basin. In recent years similar DNR teams have studied the geology of the Brooks Range foothills and of the Bristol Bay basin.
Termed the Cook Inlet Basin Analysis Program, the DNR research program started ramping up its activities during 2006.
On Feb. 2, Dave LePain, the program leader and a geologist with Alaska’s Division of Geological & Geophysical Surveys, talked to Petroleum News about the objectives, plans and activities of the program.
“Our goal … is to try and take a fresh look at the geology of the basin, the geologic elements that are relevant to hydrocarbons, that is, and try and get that information out in the public domain,” LePain said.
The program team includes nine geologists and geophysicists from DGGS and the Division of Oil & Gas (most of whom also work on other projects around the state), as well as researchers from the University of Alaska Fairbanks and Purdue University, Le Pain said.
Cook InletThe petroleum bearing strata of the Cook Inlet basin lie under the waters of Cook Inlet, under the low-lying western Kenai Peninsula and under the lowlands adjacent to the west side of the inlet. The reservoirs of all 23 known Cook Inlet oil and gas fields lie in sandstones that were laid down during the Tertiary period. These rocks occupy a subsiding basin aligned northeast to southwest, bounded in part by major geologic faults along its margins. The basin was warped downwards by the forces of the Earth’s Pacific plate subducting under the North American plate. The deepest part of the basin lies under the northwest corner of the Kenai Peninsula and contains about 25,000 feet of Tertiary rocks.
Older Mesozoic strata underlie the Tertiary basin and these older rocks sourced the oil of the Cook Inlet oil fields. Natural gas found in the Cook Inlet basin originated from bacterial decomposition of coal material in the Tertiary sediments.
Oil and gas exploration of the Cook Inlet region dates back to the early 1900s but peaked in the 1960s following the discovery of the Swanson River oil field on the Kenai Peninsula. Almost all of the known oil and gas fields were discovered between the 1950s and the 1970s, with most of the later Alaska exploration focusing on the North Slope.
“When you look at the size of the (Cook Inlet) basin and you look at where all the activity has been concentrated you can’t help but conclude … that in a relative sense it’s an underexplored basin when you compare it to other mature hydrocarbon basins around the world and in North America,” LePain said.
Not only that, but Cook Inlet exploration has sought the “low hanging fruits,” in the form of what geologists term structural traps — large, obvious fold and fault structures that can trap oil and gas. However, geologists believe that the Cook Inlet basin also contains the more subtle stratigraphic traps, where reservoir rocks perhaps pinch out against impervious seal rocks, as a consequence of the way the sediments that formed the rocks were deposited.
“If you look at Cook Inlet, if you look at the history of exploration and you look at all of the discoveries and producing fields, all of those fields are associated with structural traps,” Le Pain said. “Cook Inlet is grossly underexplored for stratigraphic traps — some people would go as far as to say it is completely unexplored for stratigraphic traps.”
Lack of informationThe problem is, however, that much of the data assembled by companies who have explored the Cook Inlet basin remains proprietary.
Under Alaska law most well data does move into the public domain 25 months after a well is drilled. But those well data only include raw information such as well logs. Geologic interpretations of the raw data and the geologic models used to plan exploration efforts — data that represents major amounts of work by company geologists and geophysicists — will likely remain locked away in company files, LePain said. And most seismic data are also proprietary.
Just to compound the problem, a general shortage of surface rock exposures in the Cook Inlet basin makes the geology of the basin impossible to fully investigate without access to subsurface data from wells and seismic surveys. Even where spectacular rock outcrops occur for miles along the bluffs on the southwestern coast of the Kenai Peninsula, the exposed strata lie nearly horizontal and only show a very limited section of the complete stratigraphic sequence.
“The vast bulk of the stratigraphy of the basin is not exposed at the surface,” LePain said.
The shortage of readily available information can create a major obstacle for an exploration company new to the Cook Inlet area.
“You’ve got a lot of new players in the inlet who have no real regional experience,” LePain said. “It’s very important … for the state to get information out into the public domain to help these new players get a leg up.”
And as a key step in making better information available, Diane Shellenbaum of the Division of Oil & Gas is coordinating an effort to obtain access to seismic data for DNR research around the state, including the Cook Inlet basin. DNR recently negotiated access to a large, high quality Veritas offshore spec seismic dataset — although there are some restrictions on how the state can publish these data, the state can use the data in its research and publish the results.
“We now have a license to use that dataset in our basin analysis program,” LePain said. “… It’s exciting for us because we have data now that we can use … and it’s pretty good data.”
Complex geologyThe crux of the problems associated with interpreting Cook Inlet geology is the complex nature of the Tertiary sediments. These sediments all formed in a non-marine environment in which rivers poured down from the mountainous basin margins, spreading sand and other detritus in fans towards a major river system along the basin axis. Sands deposited by rivers often pinched out over quite short distances, while swamps with dense vegetation occupied some parts of the basin. The vegetation in the swamps would later compact and decompose into coal seams. The locations of rivers and swamps would change over time, as the shape of the basin itself altered.
These variations in the terrestrial environment both over quite short distances and over time mean that quite different types of rock formed at the same time in different parts of the basin, while different types of rock also formed at the same location at different times. All of that, together with complexities that result from the way in which the strata have been distorted by folding and faulting, makes it very difficult for geologists to correlate rocks found in one well with rocks found in another well. And without those correlations, it’s impossible to determine with any certainty what is happening in the subsurface geology.
That’s where the seismic data become indispensable — by using seismic cross sections showing the underground geologic structures, geologists can trace the rock stratigraphy from one well to the next. The seismic data can also help tie geology seen in surface outcrops with what is found in wells.
And that’s exactly the approach that the DNR team is using, through a combination of field studies of surface outcrops combined with the use of all available well and seismic data. Then, by using fossil evidence to determine the ages of the various rocks at different locations, the team hopes to reconstruct models of what the Cook Inlet basin looked like over a series of time intervals, and how the forces in the Earth’s crust have changed the shape of the basin over time. A detailed model of the subsurface geology ought to help industry focus their search for those elusive stratigraphic traps.
“You can begin to reconstruct the overall basin history,” LePain said. “That’s critical to understand where we might have (oil and gas) reservoirs.”
LePain said that an understanding of the history of the structure of the basin would also help elucidate the locations of natural gas source rocks. Bacterial-formed methane in coal buried deep underground remains adsorbed in the coal because of the pressure of the overlying rocks. But if structural changes in the basin elevate the coal to a position closer the surface, the reduced pressure can release the gas from the coal.
“It’s only after those coals are uplifted along certain structures that you decrease the overburden pressure and then that methane desorbs … and it can migrate up dip into a … reservoir sand,” LePain said.
Fieldwork in SeptemberAfter some initial planning, the DNR Cook Inlet program got under way in September with some fieldwork based in Homer, studying the rock exposures along the coast in that part of the Kenai Peninsula. The Tertiary strata near Homer are almost undeformed.
“We figured that our best chance was to look at undeformed stratigraphy, to start to try to understand the potential for stratigraphic traps,” LePain said.
The team has also started assembling and analyzing well and seismic data, correlating the geology between wells in different parts of the Cook Inlet basin. In May they hope to resume the Homer-based fieldwork.
“What we then hope to do in the latter part of the summer is shift over to the northwestern side of the basin … and start looking at the stratigraphy that’s exposed at the surface over there where the basin margin has been structurally modified,” LePain said.
In perhaps two or three years time the team may extend its analysis into the older and deeper Mesozoic rocks that include the source rocks for Cook Inlet oil, LePain said.
With the research program ramping up, DNR is seeking industry sponsorship to help with funding, through an industrial consortium of the type that has successfully supported DNR research in the Brooks Range foothills. LePain is in the process of preparing a funding proposal. Benchmark Oil and Gas, an exploration company that is relatively new to the Cook Inlet, has already committed funds to the program.
“Benchmark was very interested in what we are doing and wanted to support us before we’ve even finished our proposal,” LePain said.
Meantime, the team is preparing a report on the results of its research so far, which should come out in April or May. And it will report on its progress at a public DNR geology program review in late April.
“It’s exciting. I think we’re going to get a lot done,” LePain said. “… We’re going to go out there and we’re going to look at the geology in an objective and unbiased way, and package that information into … a coherent story and get it out there for all these companies to use.”