The Susitna Valley, a huge, low-lying area of muskeg, lakes and rivers to the north of Anchorage in Southcentral Alaska, is perhaps best known as a destination for wilderness salmon fishing and as the location for the first day or so of the annual Iditarod sled dog race. But with known coal seams in the sedimentary basin that lies underneath the valley, could this region immediately to the west of the Alaska Railroad route and close to the state’s highway system become a new source of much-needed natural gas for the Alaska Railbelt?
Alaska’s Division of Geological and Geophysical Surveys and Division of Oil and Gas are embarking on a multiyear research program to investigate the resource potential of this sedimentary basin, known as the Susitna basin and separated at its southern end by a major geologic fault from the prolific oil and gas bearing Cook Inlet basin. The research program is the first of what DGGS anticipates as a series of investigations into the energy resources of several Alaska basins that could perhaps supply energy for rural communities as well as major population centers, as the cost of energy consumed in Alaska soars, DGGS geologist Dave LePain told Petroleum News Feb. 18.
“Energy is a huge challenge in the state right now,” LePain said.
The U.S. Geological Survey and possibly some university geologists will help with the Susitna basin program, Le Pain said.
Little knownAlthough much is known about the geology of the Cook Inlet basin and there is a well substantiated model for how that basin has formed, very little is known about the adjoining Susitna basin.
“We really don’t understand why that (Susitna) basin is there,” LePain said.
But an understanding of the history of the basin is critical to evaluating the potential presence of exploitable gas resources in the basin.
In the Cook Inlet basin a sequence of Tertiary rocks, more than 25,000 feet thick in the deepest part of the basin, were laid down as sediments deposited from rivers and lakes. And periods of lush vegetation during the Tertiary period gave rise to abundant coal seams, as the sediments sank and became buried. Those coal seams, containing vast volumes of natural gas, are being lifted up by forces within the Earth’s crust and this uplift, starting perhaps 5 million to 10 million years ago, has reduced the pressure on the coal, thus releasing gas from the coal, with the gas migrating into the sandstone reservoirs of the Cook Inlet gas fields, LePain said.
Geologists know from surface rock outcrops and from the nine exploration wells drilled in the Susitna basin that this basin contains coal seams in a similar Tertiary rock sequence to that of the Cook Inlet basin. But how pervasive are the coals, and have the strata of the Susitna basin, like those of the Cook Inlet basin, gone through a gas-releasing uplift? If coals are extensive in the basin and if they have generated and released gas, are suitable reservoirs and seal rocks present to form gas fields? These questions remain unanswered.
In addition, almost nothing is known about the older Mesozoic rocks that lie underneath the Tertiary of the Susitna basin. The Jurassic Tuxedni group of the Cook Inlet Mesozoic sequence has sourced most of the oil in the Cook Inlet oil fields. Do similar Mesozoic source rocks exist under the Tertiary of the Susitna basin?
Few rock exposuresOne of the biggest challenges for any investigation of Susitna basin geology is the general lack of exposed rock in the swamps and forests of the Susitna Valley. There are, however, some good rock exposures around the perimeter of the basin and the DGGS-led team plans to spend about 10 days in the summer of 2011 taking an initial look at those exposures, investigating the nature of the rocks and determining the quality of the coals known to exist in some outcrops, Le Pain said. Members of the team will also fly across the interior of the basin by helicopter, seeking any other locations where rock may be exposed, he said.
The team hopes to be able to use these surface investigations to start to clarify the Tertiary stratigraphy of the basin and the way in which the structure of the basin has evolved, eventually enabling a geologic comparison with the way in which gas has formed and accumulated in the Cook Inlet basin. The DGGS led team also expects to glean information about the Mesozoic rocks that underlie the Tertiary, especially from surface outcrops of the Mesozoic in the Yenlo Hills to the northeast of the tiny hamlet of Skwentna, in the northern part of the basin.
The team will also use publicly available data from the few wells that have been drilled in the Susitna basin. In addition, publicly available gravity data for the basin will provide insights into the thicknesses of the Tertiary rocks across the basin, while publicly available aeromagnetic data will provide clues about the nature of the underlying Mesozoic rocks and about major geologic structures such as faults, LePain said.
Proprietary seismicThere is some 2-D seismic data for the basin, but all of this data is proprietary and is held confidential within DNR. The agency cannot publish the data without permission from the data owners but does use it to inform surface geologic mapping in poorly exposed areas, LePain said.
One well in the Susitna basin penetrated through the Tertiary strata into the underlying Mesozoic rocks and demonstrated the existence of a 3,000- to 4,000-foot thickness of Tertiary strata, although the Tertiary is known to be thicker than that in some parts of the basin, LePain said.
“In deeper parts of the basin we think we have a thick enough section to where we could have an appreciable amount of coal,” he said.
The summer 2011 reconnaissance fieldwork, coupled with the analysis of existing publicly available subsurface data, will enable the team to put together a plan for completing its Susitna basin program. That program will take at least two years to complete, after which the team will shift its attention to other Alaska basins. For now, the Susitna basin seems the “low hanging fruit” of the underexplored basins with energy potential.
“It’s right next to the road system and close to the population centers,” Le Pain said. “It’s a good place to start.”