Promising indicationsExploration of Alaska Interior basins points to much potential for oil and gas Alan Bailey Petroleum News
While two deep exploration wells drilled in Alaska’s Nenana basin in recent years failed to find commercial quantities of oil or gas, the wells did provide tantalizing evidence for a petroleum system.
Findings included hydrocarbon source rocks, excellent reservoir sands and shales with the potential to trap hydrocarbon pools, exploration consultant Michael Richter told the 2014 Alaska Geological Society Technical Conference on May 16. Testing of rock samples from the wells, coupled with surface geochemistry, has demonstrated the existence in the subsurface of natural gas and natural gas liquids, while the analysis of potential source rocks, coupled with estimations of the subsurface temperatures, has pointed to the possibility of oil in the basin, Richter said.
Sediment filled basins The Nenana basin is one of several sediment-filled basins, formed by the pulling apart, or “rifting,” of the Earth’s crust in Alaska’s Interior.
Doyon Ltd., the Native regional corporation for the Interior, has been spearheading the exploration of two of the basins, the Nenana basin, southwest of the city of Fairbanks, and the Yukon Flats basin, to the city’s north. Both basins show broadly similar geology, containing huge thickness of non-marine sediments with abundant coal seams, Richter said. An analysis of gravity data has indicated maximum basin depths of 20,000 to 25,000 feet, with seismic data for the Nenana basin supporting this depth determination, he said.
Doyon’s exploration program has included the recent drilling of the two deep Nenana basin wells, the Nunivak No. 1, drilled to 11,100 feet in 2009, and Nunivak No. 2 well, drilled to 8,667 feet in 2013. The basin is shaped rather like a northeast-southwest oriented dog bone, with the two wells being drilled in the narrowest part of the bone shape, near the village of Nenana, Richter said. The No. 1 well was drilled quite close to the fault that marks the eastern boundary of the basin, near the village of Nenana, while the No. 2 well was drilled farther to the west.
Nenana rock sequence The shallowest and youngest of the sequence of sediments in the Nenana basin consists of 3,000 to 4,000 feet of gravel, washed down from the Alaska Range in Pliocene and recent times, Richter said. Under these gravels lies the Usibelli group, a sequence of sediments, Miocene in age and of interest from the perspectives of both coal and petroleum potential. Beneath the Usibelli group is an unconformity, a break in the sequence of sediment deposition, below which are sediments of Paleocene age.
Both of the Nenana basin wells penetrated a sand unit called the Suntrana sand within the Usibelli group. This sand, 500 to 700 feet thick and with excellent hydrocarbon reservoir potential, consists almost entirely of quartz grains where it was encountered by the No. 2 well. But, where penetrated by the No. 1 well, the sand contained more rock fragments, a phenomenon that presumably reflects the relative proximity of the basin margin, Richter said.
The Suntrana sand exhibits porosities in excess of 20 percent, even at depths of 7,000 to 8,000 feet, he said.
Gas saturation A rock sequence of particular interest within the Usibelli group lies deeper than the Suntrana and is called the Healy Creek formation, a formation that includes sand bodies 70 to 80 feet thick, shale horizons and coal seams, Richter said. Sands of this formation found at the bottom of the Nunivak No. 2 well showed a porosity of 20 percent or more, with uniform gas saturations of 25 to 30 percent, he said, adding that the reason for this uniform distribution of low gas saturation within the sand remains something of a mystery.
The level of gas saturation was presumably too low to constitute a commercial discovery. But the gas was found to be very “wet,” containing sufficient natural gas liquids to yield some 15 to 30 barrels of liquids per million cubic feet of gas, Richter said. The hydrocarbons had clearly migrated from a source rock and were clearly “thermogenic,” formed from the action of subsurface heat on organic debris rather than from bacterial action, Richter said.
Oil potential And, in terms of potential oil and gas sources, the wells encountered coals and shales with high organic contents, with these rocks also exhibiting high hydrogen contents appropriate to potential oil generation. One shale penetrated by the No. 2 well was 120 feet thick, with a total organic carbon content of 5 to 6 percent, Richter said. Laboratory testing of samples of potential source rocks obtained from the drilling demonstrated the potential generation of gas and medium to light oil, he said. And the geochemical analysis of lake bed samples from the region showed hydrocarbon signatures characteristic of natural gas liquids.
Evidence of the subsurface temperature history of the rocks suggests that the top of the “oil window,” the depth range within which heat might have generated oil, would be about 12,000 feet in the middle of the basin, rising to about 8,000 feet towards the basin margins, Richter said. And a basin analysis has suggested that the potential source rocks could have generated a very large volume of oil, he said.
The interbedding of sand and shale strata, in conjunction with faults known to cut the basin, provides many opportunities for the trapping of generated hydrocarbons, he said.
Although coal-bearing, terrestrial sediments are more commonly associated with the formation of natural gas than oil, there are known oil-bearing coal deposits, Richter said. For example, a coal source is associated with some known oil seeps in Southeast Alaska, and a major oil basin in Australia has coal and shale as oil sources, he said.
Yukon Flats The Yukon Flats basin, to the north of Fairbanks, actually consists of several sub-basins, Richter said. Doyon has conducted both a 2-D and a 3-D seismic survey over one of these sub-basins, the sub-basin in the area of Stevens Village. Seismic data elsewhere in the basin are old and sparse. The geochemical analysis of lake-bed sediments indicates that the Stevens Village sub-basin may be oil prone. However, analysis of similar sediments in proximity to another sub-basin, the Birch Creek sub-basin, suggests that this basin may be more prospective for gas, Richter said.
In 2013 Doyon said that its sees the potential for a major oil find in the Yukon Flats basin, and that it is using its 3-D seismic data from near Stevens Village to identify potential drilling targets. The corporation also said that it hopes to conduct further seismic surveying in the Nenana basin in the winter of 2014-15.
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