The State of Alaska’s annual areawide lease sales for the North Slope foothills and the Cook Inlet basin will be held on May 24. And, with both regions still substantially under-explored, the sales offer tracts with significant petroleum potential. Geologist Paul Decker and Commercial Analyst Tim Ryherd of Alaska Division of Oil and Gas talked to Petroleum News about the petroleum geology of the regions.
The North Slope Foothills consist of a series of rolling hills, mesas and east-west trending ridges that extends north from the Brooks Range and merges into the southern side of the North Slope of Arctic Alaska. The state’s lease sale area lies within a 7 million-acre swath of land between the southeastern corner of the National Petroleum Reserve-Alaska and the western side of the Alaska National Wildlife Refuge. Within that area, state tracts intermingle with inholdings owned by Arctic Slope Regional Corp.
The trans-Alaska pipeline passes north to south through the eastern part of the region and a future gas pipeline from the Prudhoe Bay area is likely to pass along that same pipeline corridor.
Geologically the Foothills region straddles the fold and thrust belt that marks the region where the emerging Brooks Range has pushed and crumpled rocks northward. The region also straddles the axis of what geologists term the Colville basin, a major basin of deposition of Tertiary and Quaternary sediments.
“You’re right in that transition zone from the leading edge of the thrusting, overriding the deepest part of the … (Colville) basin,” Decker explained.
The Colville basin forms a major component of the world-class petroleum system of northern Alaska.
Substantial research
Of five wells drilled in the Foothills area only one well, the Lisburne Test No. 1 drilled by U.S. Geological Survey in the western part of the area, has publicly available data. The data from remainder of the wells remain confidential.
However, geologists have done substantial research in the Foothills in recent years and the results of much of this research are available to the public. In particular, a team of geologists led by Alaska’s Division of Geological and Geophysical Surveys has been conducting fieldwork across the region. The USGS has also been studying the area.
Many publications by researchers can be found in the DGGS web site at http://www.dggs.dnr.state.ak.us/pubs/pubs.jsp. Other publications are available in professional journals, especially the Geological Society of America Abstracts with Programs.
It is well known that most of the major source rocks of northern Alaska exist in the area. Source rocks include the Triassic and Jurassic Otuk formation (in part, equivalent to the prolific Shublik formation of the central North Slope) and the HRZ/Hue shale. In addition, Julie Dumoulin and others from USGS have reported a potential source zone with total hydrocarbon content of 7 to 8 percent within the upper part of the Mississippian/Pennsylvanian Lisburne group.
Gas prone
But, because of the high thermal maturity of deeply buried source rocks, geologists have long believed the Foothills area to be gas prone rather than oil prone.
“I think that most people believe that the most likely product that they’d find out here would be gas,” Decker said.
On the other hand source rocks within the thermal maturity window for oil are found at the surface in some locations, especially in the southern part of the area near the front of the Brooks Range. Geologic upheavals associated with the formation of the Brooks Range appear to have pushed these source rocks to the surface. And there are examples of what are known as dead oil fields — ancient oil fields found at the surface that have long ago lost their oil.
“There are a lot of solid hydrocarbon occurrences out there … so you know there has been oil charge through the area,” Decker said. “It’s not out of the question that somebody at some point is going to find a preserved oil trap.”
And there is a known oil field at Umiat, at the northwestern corner of the lease sale area.
“That’s an obvious positive for oil prospectivity,” Ryherd said.
But the fact that the lower maturity source rocks outcrop at the surface raises questions about how any generated oil would have reached the reservoir rocks at depth, Decker thinks.
Lisburne play
Much recent exploration interest in the Foothills has focused on thick limestones and dolomites of the Lisburne group. In the southern sector of the lease sale area, just north of the Brooks Range, these ancient Paleozoic strata have been thrust north over younger strata of the Colville basin. This thrust faulting resulted from northward compression of the Earth’s crust associated with the formation of the Brooks Range. In places, the Lisburne strata have been thrust over themselves multiple times, to form repeated stacks of the same stratigraphic sequence.
Cavities known as vugs within the dolomites of the Lisburne often contain dark asphalt residue, indicating the existence of a petroleum system. The production of natural gas from petroleum in the cavities has probably caused asphalt to precipitate at some time in the past, Decker explained. And the vugs give the dolomite the porosity needed to act as a petroleum reservoir.
“The greatest play interest, exploration-wise, is for the dolomites,” Decker said.
The section containing the Lisburne dolomites includes shales that could act as seal rocks for an oil gas reservoir. And, in some places, the thrusting from the Brooks Range has also pushed Cretaceous shales over the top of the Lisburne strata.
“Also there is so little porosity and permeability in the non-reservoir parts of the Lisburne that it can itself act as a partial seal,” Decker said.
Cretaceous sandstones
North of the area of intense thrust faulting comes an area dominated by Cretaceous sandstones and shales of what are known as the Fortress Mountain and Nanushuk formations, deposited in the Colville basin.
“As you work your way north the world begins to change dramatically. You get out of the outcrop of Paleozoic rock and into Cretaceous rocks,” Decker said.
The subsurface structures transition from massive thrusted rock sheets to what geologists term a triangle zone, in which the thrusted rocks have pushed and lifted strata ahead of them into a distinctive triangular pattern of crisscrossing faults. Even farther to the north the faulted structures transition into more open folding of the strata.
Explorers in the region tend to focus on two main types of rock within the Cretaceous — topset sandstones formed along the edge of the evolving Colville basin and interlayered sandstones and shale known as turbidites that would have formed in the deep waters of the basin. Potential Cretaceous oil or gas reservoirs in the rock sequence include coarse sandstone and conglomerate dumped from the emerging Brooks Range.
Although the Cretaceous strata have the potential to form stratigraphic traps, geologist view the majority of the oil and gas plays as being structural, associated with the extensive faulting and folding in the area. The Umiat oil field, for example, involves a structural trap in Nanushuk topsets.
The existence of potential reservoirs and source rocks close to the axis of the Colville basin has led some geologists to postulate the presence of overpressured gas deep in the basin. USGS has examined evidence of overpressure, using logs from wells in the southern part of the North Slope, adjacent to the Foothills sale area.
Overpressure occurs when fluids in the rocks prevent the complete crushing of rocks under pressure at great depths. The overpressure can result in the preservation of substantial volumes of natural gas in rocks close to the axis of a sedimentary basin.
Cook Inlet
In contrast with the North Slope Foothills, the Cook Inlet basin has long been a producing oil and gas province, but lacks the major quantities of publicly available geological information that exists for northern Alaska.
“The issue with Cook Inlet in my mind is that the exploration that has been done has been done by the oil companies is proprietary,” Ryherd said.
In fact DNR is in the process of starting a new energy-related research program for Cook Inlet, using a similar approach to what the department has been doing in northern Alaska for more than a decade, Decker said. DGGS has recently hired a non-marine sedimentologist to research the basin, he said.
“The basin is in need of some new ideas,” Decker said.
Forearc basin
However, the general geology of the basin is well known. A thick sequence of Tertiary non-marine sediments, deposited primarily by rivers, has formed in an elongated northeast to southwest-aligned basin, sandwiched between mountain ranges to the southeast and northwest. Known by geologists as a forearc basin, the basin has formed where the Earth’s crust has flexed downwards between a subduction zone to the southeast and a still active volcanic arc to the northwest. A subduction zone occurs where one of the plates that form the Earth’s crust drives down beneath another plate.
The Tertiary strata of the basin lie over an older, predominantly marine sequence of Mesozoic rocks.
All of the existing oil and gas fields of Cook Inlet occur in Tertiary sandstone reservoirs. However, the oil is known to have originated from a Jurassic source rock in the underlying Mesozoic sequence. Almost all of the natural gas is biogenic in origin, and has derived from bacterial decomposition of organic material in the Tertiary sediments.
Most of the existing Cook Inlet oil and gas fields were found during exploration for oil back in the 1950s and 1960s. And although at that time there was little interest in finding gas, the discovery of substantial volumes of gas in the basin led to the development of a Cook Inlet-based gas industry.
However, with gas supplies in the region starting to run short and gas prices rising, there has been an ever-increasing interest in gas exploration around Cook Inlet. Some of the more active plays that companies are pursuing now are bypass plays, developing gas that was discovered but ignored in wells drilled to explore for oil, Decker said.
And a 2004 U.S. Department of Energy report suggesting that substantial volumes of natural gas remain to be discovered in the basin, has also encouraged exploration.
Difficult exploration
However, exploring the Cook Inlet basin can be quite challenging. Fields typically contain multiple small reservoirs that may be difficult to find. And deposition of the Tertiary sediments from rivers and river fans, spreading from the ancient mountains surrounding the basin, has given rise to rock units that are often discontinuous. River channels and fans can be difficult to differentiate using single rock samples from wells, thus giving rise to issues such as knowing how far a particular reservoir may extend, Decker said
“There’s been a lot work over the years by the industry to try to predict reservoir continuity,” Decker said.
Also, perhaps because of clay content of the sediments, it is often difficult to recognize pay zones using well logs, Ryherd said. It is not always obvious where the gas sands are, even if you drill right through the middle of a gas reservoir, he said. And acquiring high quality seismic in the basin is notoriously difficult, in part because of the complex structures in the basin. The coal seams in the Tertiary sequence also tend to absorb seismic energy, Decker said.
Two trends
Despite the difficulties, oil companies have had considerable success over the years in locating oil and gas fields in the basin. But a quick inspection of a map of these fields shows that they follow two main trends on either side of the basin axis — one trend passes up the west side of the Kenai Peninsula and the other trend passes up the west side of Cook Inlet. Oil and gas leases have also tended to follow these trends.
The trends lie on either side of the central axis of the basin.
“If you look at a map of the well plots there’s very few in the core, along the axis (of the basin),” Ryherd said.
Interestingly, the Kitchen oil prospect, where Escopeta plans to drill soon from a jack-up platform, is on the axis of the basin, east the Middle Ground Shoal and Trading Bay fields. Decker also commented that much of the drilling in the Cook Inlet basin has focused on the crests of the major structures in the basin. Decker thinks that there is scope for exploring the flanks of the structures, where fluids will likely have migrated up the structures. There is also scope to explore for stratigraphic traps.
“So far the basin has only really been explored for structure, not for stratigraphic traps,” Decker said.
Another possible play involves tight gas — gas in reservoirs with relatively low permeability — especially in the southern part of the Susitna Valley, at the northeastern end of the state’s sale area.
“Some people talk about tight gas up in that area — that is a possibility,” Ryherd said.
Mesozoic possibilities
The possibility of finding oil and gas in the Mesozoic, beneath the Tertiary basin, has also intrigued geologists, especially since the Cook Inlet oil originated from the Mesozoic. And Cretaceous rocks in the Mesozoic sequence exposed at either end of the Cook Inlet basin show evidence of oil formation, Ryherd said.
However, geologists have also been concerned about the potential for minerals called zeolites to clog the pores of potential reservoir rocks — the chemistry of the Mesozoic rocks tends to be conducive to zeolite formation.
But Decker thinks that the nature of the Mesozoic under the basin is not well understood. In fact he hopes that the Mesozoic oil and gas potential will become one focus of the new DNR Cook Inlet research program. The program might establish better knowledge of how and when the zeolites might have formed, compared with the timing of oil and gas generation and migration, he said.
“Those are just some of the ideas we’ve talked about internally here … between ourselves and DGGS,” Decker said.
And high oil and gas prices are creating considerable interest in Cook Inlet. Participation in the past few sales has been very high, some new companies have recently bought into the area and there is talk of more companies coming to explore, Decker said.
High oil and gas prices have also been encouraging a surge of drilling in the Cook Inlet basin, Ryherd said.
“If you’re looking at it from the explorer’s perspective it’s good times, I think,” he said.
