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DGGS review Susitna basin evidence Region is probably prospective for natural gas, but sparse surface rock exposure makes interpretation and evaluation challenging Alan Bailey Petroleum News
Having started a three-year research program into the geology of Alaska’s Susitna basin in 2011, a team of scientists led by Alaska’s Division of Geological & Geophysical Services, or DGGS, has published a status report for its work so far.
The basin, which lies under a huge area of lowland lakes and muskeg in the Susitna Valley, to the north of Anchorage, has long been thought by geologists to be prospective for natural gas, especially given the known existence of coal seams in some parts of the basin. But, with few surface rock exposures, the geology of the basin is very difficult to interpret and assess.
Reconnaissance work In a 10-day field program in 2011 the DGGS-led team conducted some reconnaissance work, examining rock exposures along the edges of rivers and in the hills around the basin margins. In addition to geologists from DGGS, the team included scientists from Alaska’s Division of Oil and Gas, from the U.S. Geological Survey and from Purdue University. Funding came from the state Legislature.
The new DGGS status report says that a prime motivation for research in the Susitna basin is the recognized similarity between coal-bearing strata in the basin and strata in the adjacent Cook Inlet basin, a basin that contains eight producing oil fields and more than 25 producing gas fields.
Only 12 exploration wells have ever been drilled in the Susitna basin, with half of these being concentrated in a small area near the town of Houston, in the extreme southeastern margin of the basin, the report says. Data from these wells, gravity data and some old proprietary seismic data that the DGGS team has permission to use provide the only information available about the basin’s subsurface.
Two sub-basins The report says that the basin as a whole consists of two sub-basins: the Susitna and Yentna “depocenters,” lying on either side of a ridge-like structure and containing what are presumed to be Tertiary sediments. The ridge-like structure, where it is exposed in the Yenlo Hills, not far from the remote village of Skwentna, consists of rocks of Cretaceous age, heavily altered in the past by temperature and pressure. The Susitna sub-basin, on the southeast side of the “ridge,” deepens to the southwest and contains at least 3,000 to 4,000 meters of sedimentary fill, the report says. The Yentna sub-basin, in the northwest quadrant of the Susitna basin as a whole, is smaller and shallower than the Susitna sub-basin, the report says.
A major geologic fault — the Castle Mountain fault — separates the Susitna basin from the Cook Inlet basin to the south, but little is known about the relationship between the two basins. In particular, although the geologic history of the Cook Inlet basin is fairly well understood, a lack of information about the age of the material in the depths of the Susitna basin makes the timing of the subsidence of the Susitna basin especially difficult to assess, the report says. And that in turn renders it difficult to determine whether the Susitna basin is simply a northern continuation of the Cook Inlet basin, or whether the two basins have different origins. In fact, even the history of slippage along the Castle Mountain fault is incompletely understood, the report says.
And the way in which part of the Susitna basin cuts across volcanic terrain to the north of the Cook Inlet basin may, depending on the timing of the formation of the Susitna basin, indicate that different geologic mechanisms caused the two basins to subside, the report says.
Resolving uncertainties The DGGS fieldwork in 2011 focused on trying to resolve the uncertainty surrounding the mechanism and timing of the formation of the Susitna basin, as well as using surface rock exposures to gain a better understanding of the nature of the basin fill and the stratigraphy of the fill rocks, the report says. Geologists examined rocks exposed along a series of creeks, noting rock types, making measurements and taking samples for the assessment of hydrocarbon reservoir or seal potential. The geologists also collected some pollen and mineral samples from the rocks, to determine the age of deposition of the sediments in the rock and to assess the origin of those sediments.
The rocks largely consisted of sandstones, conglomerates and siltstones laid down in an ancient river system, the report says.
From subsurface data, geologists have inferred the existence of a major geologic fault, the Beluga Mountain fault, bounding the southwest margin of the Susitna sub-basin. However, a reconnaissance survey up a deeply incised creek system across the inferred surface intercept of the fault failed to find any evidence of the fault’s existence, the report says.
Samples of the mineral apatite from bedrock exposures around the basin margin will help pin down when and how quickly the sources of basin sediment were eroded and exhumed during basin formation, the report says. And other samples from specific strata inside the basin will help correlate the sediment in these strata with their sediment sources, it says.
Return in 2013 The DGGS team did not conduct any fieldwork in the Susitna basin in 2012 but plans to return to the basin in the summer of 2013, focusing on rocks and geologic structures exposed along the western periphery of the basin while also continuing the search for evidence of boundary faults such as the Beluga Mountain fault. The goal will be to better understand the changes in sediment type parallel to the basin margin and to gain insights into the way geologic structures have controlled the basin subsidence, the report says.
This knowledge and understanding will help to better predict the distribution of potential hydrocarbon reservoir and seal rocks in the subsurface and to evaluate the potential for natural gas traps, the report says.
DGGS has permission to publish two seismic sections that intersect at right angles through the deepest part of the basin — these seismic sections, together with geologic interpretations and data from wells will enable the development of a geologic model for the Susitna basin, providing a preliminary means of assessing the basin’s potential as a natural gas province, the report says. The research team will also collect a suite of coal samples from the basin, to enable an evaluation of Susitna basin coal as a primary coal source or as a source of methane gas, the report says.
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