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A large gas prone sedimentary basin USGS geologist reviews geology of the Susitna basin north of Anchorage; has potential for natural gas generated from coal and shale Alan Bailey Petroleum News
The Susitna basin, a broad sedimentary basin under the Susitna Valley to the north of Anchorage, presents an enticing target for hydrocarbon exploration relatively close to major population centers and the southern Alaska transportation infrastructure. But, with attention having been focused on the nearby Cook Inlet basin, with its oil and gas fields, the Susitna basin has seen little in the way of exploration effort.
On May 22 during the American Association of Petroleum Geologists Pacific Section annual conference Richard Stanley, a geologist with the U.S. Geological Survey, overviewed current knowledge of the Susitna basin geology. The basin appears prospective for natural gas but has little potential for an oil discovery, Stanley said, presenting results from studies conducted by scientists from the USGS and the Alaska Department of Natural Resources.
Distinct basin Particularly intriguing is the close proximity between the Susitna basin and the Cook Inlet basin to the south. In fact, at first blush, it appears as if one basin is simply a continuation of the other.
But the two basins are separated by a major geologic fault, the Castle Mountain fault, a geologic feature that is fundamental to an understanding of how the Susitna basin appears to have formed. And the history of the basins indicates some basic differences between them, differences with important implications for petroleum potential.
The Cook Inlet basin is what geologists refer to as a forearc basin, a region of subsidence between an oceanic trench and a volcanic arc, in a region where one plate of the Earth’s crust is subducting beneath another. The basin contains a thick sequence of Mesozoic and Tertiary sediments, infilling the basin as the crust beneath it sinks. The Mesozoic sequence includes rocks of the Jurassic-age Tuxedni group, the rocks which sourced much of the oil in the Cook Inlet oil fields.
Basin movement The Castle Mountain fault bounds the northeastern side of the basin. The lining up of features on either side of this fault indicates that since the late Jurassic the rocks on the north side of the fault have moved about 130 kilometers to the east, Stanley said. Reconstructing the situation in the late Jurassic, before the movement on the fault began, suggests that the Cook Inlet basin, the region of the current Matanuska Valley and the Copper River basin to the east all formed one continuous forearc basin.
But the embryonic Susitna basin, at that time far to the west of its current location, would have formed as a separate entity to the north of the Cook Inlet basin.
Perhaps importantly, Tertiary strata known to exist in the Susitna basin would have been laid down on impervious metamorphic rocks, rather than on the Mesozoic sedimentary strata, including the Tuxedni group, of the Cook Inlet basin. That could explain the absence of evidence for oil in the Susitna basin, in comparison with the abundant oil found in the Cook Inlet basin, Stanley suggested.
Shortage of data Piecing together the geology within the Susitna basin is challenging, given the paucity of accessible surface rock outcrops and a shortage of well and seismic data - only about seven wells have ever been drilled in the basin. About 425 line miles of 2-D seismic data, all collected by the oil industry several decades ago, were available for analysis.
The team of scientists investigating the Susitna basin constructed a stratigraphy for the basin using data and rock samples from three wells in the basin: the Trail Ridge No. 1, the Pure Kahiltna No. 1 and the Sheep Creek No. 1. Radiometric dating of the deeper sediments from the Trail Ridge well gave an age range of 54 million to 57 million years, an age range within the late Paleocene of a Tertiary sequence. Examination of pollen and spores from younger, shallower rock from this well indicated Eocene and younger ages.
The older part of the sequence includes interstratified volcanic and sedimentary rocks. And seemingly analogous rocks exposed at Willow Creek, near the Parks Highway, include layers of sandstone, conglomerate and basalt. Radiometric dating of the basalt, a volcanic rock, gave an age of 56.2 million years. The ages of these volcanic rocks found in the basin overlap with those of volcanic rocks found in the Talkeetna Mountains to the east - the volcanism likely resulted from the extension of the Earth’s crust under the effect of crustal subduction, Stanley commented.
The younger rocks penetrated by the Trail Ridge well predominantly consist of sandstone, siltstone and coal, Stanley said.
Structure of the basin Interpretations of the seismic data, together with the use of some aeromagnetic data, reveals the general structure of the Susitna basin. The basin appears to be bounded by steeply dipping faults and is 4 to 5 kilometers thick at its deepest point. There are numerous geologic faults in the basin, predominantly oriented north-south.
Analyses of 106 samples of Paleocene coal, shale and mudstone from wells indicated that these rocks contain material capable of generating oil and gas. However, the analyses also indicated that the rocks had never reached temperatures at which oil might have formed, Stanley said. In fact, the top of the temperature window in which oil could form is estimated to be at a depth of 5.1 kilometers in the region of the basin, a depth below the deepest basin floor, he added.
There is one area in the southwestern margin of the basin where geologic faulting may have buried sediments in the basin to depths below that 5.1 kilometer threshold. That gives rise to a highly speculative oil play in that area, although no oil seeps have ever been observed there, Stanley said.
In general, the Susitna basin is shallow and cold. The basin has petroleum potential, mainly for natural gas that originated from microbial action. The potential source rocks are coal and carbonaceous shales, with possible reservoirs in Eocene and later sandstones formed from river systems, Stanley said.
The story of the Peters Hills basin The Peters Hills basin, an area containing river-lain sediments in the region of the Peters Hills and the Kahiltna River, on the north side of the Susitna Valley, is sometimes thought of as a sub-basin of the huge Susitna basin to the south. But an examination of the geology of the region indicates that the Peters Hills basin constitutes a distinct structure, probably formed as sediments eroded from the emerging Alaska Range became trapped behind movement on a major geologic fault that is thought to run through Broad Pass to the south of the range, Peter Haeussler, a geologist from the U.S. Geological Survey, told the American Association of Petroleum Geologists Pacific Section annual meeting on May 22.
In fact the Peters Hills basin is elevated about 300 meters above the Susitna basin and is now being eroded by river drainage through it, Haeussler said. Gravity data suggest that the sediments laid down in the basin attain a maximum thickness of about 2 kilometers, he said. Surface rock outcrops include sandstones, siltstones and conglomerates, all of which are poorly consolidated and tend to crumble.
The thickness of the basin suggests that movement on the Broad Pass fault would have thrust rocks over a distance of about 3 kilometers, Haeussler said.
Plate collision The collision between the northward moving Yakutat plate, a relatively small plate of the Earth’s crust, and what is now the southern edge of Alaska is essentially responsible for the system of relatively recent and in many cases still active regional faults connected with the basin structures. That collision began about 30 million years ago, Haeussler said.
The supposed Broad Pass fault may link into the Denali fault system, a major fault system that runs through the Alaska Range. That fault system is active, as is evidenced by a major earthquake associated with movement on the fault in 2002. Earlier crustal movement in this structural system gave rise to the mountain massifs of Denali and Mount Foraker - there is evidence that the rocks in the upper tracts of Denali began cooling about 6 million years ago, presumably as a consequence of the rapid exhumation of the massif from the subsurface at that time, Haeussler said.
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