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DGGS reports on Susitna basin study Division continues to evaluate the geologic history and natural gas potential of a little explored region to the north of Anchorage ALAN BAILEY Petroleum News
Alaska’s Division of Geological and Geophysical Services has published some initial results from fieldwork that the agency conducted in the Susitna basin in 2014.
The basin, which lies under a huge area of lowland lakes and muskeg in the Susitna Valley, to the north of Anchorage, is considered prospective for natural gas, especially given the known existence of coal seams in some parts of the basin. But, with few surface rock exposures and few wells drilled, the geology of the basin is difficult to figure out. Since 2011 DGGS has been conducting field surveys and associated laboratory work, investigating the basin - one of the division’s missions is to conduct geological research in regions of Alaska thought prospective for oil and gas, with the research results becoming available to people exploring for hydrocarbon resources. With funding from the state Legislature, the DGGS-led team includes scientists from Alaska’s Division of Oil and Gas, from the U.S. Geological Survey and from Purdue University.
Initial work confirmed the existence of two sub-basins, the Susitna and Yentna “depocenters,” within the overall Susitna basin. And fieldwork focused on reconnaissance surveys of rock exposures along the edges of rivers and in the hills around the basin margins.
Better definition Field studies in 2014 aimed at better defining the stratigraphy of the basin, the origins of the sediments deposited in the basin and the potential hydrocarbon reservoir quality of Cenozoic rocks seen in outcrops along the basin’s western and northern periphery, the new DGGS Susitna basin report says. These rocks are broadly equivalent in age to hydrocarbon bearing strata of the prolific, neighboring Cook Inlet basin. The field team also examined selected exposures of older Cretaceous bedrock, the report says.
The team took samples from all of the major accessible coal seams known to be exposed at the basin’s surface and analyzed these samples to quantify their potential both as an energy resource and for their ability to hold methane, the principle component of natural gas. Additional research included an analysis of data relating to geologic faults on the western margin of the basin and of data collected for the assessment of the linkages between periods of rock uplift, rock deformation and sediment deposition.
From measurements and observations of the Cenozoic rock stratigraphy at three locations at Fairview Mountain and Johnson Creek, on the northwest and west sides of the basin, the team found coal-bearing sediments, thought to be equivalent to the Tyonek formation in the Cook Inlet basin. These sediments appear to have been deposited from sinuous streams that traversed the ancient landscape. Sandstones at all locations appear to have good hydrocarbon reservoir potential, depending on the extent to which the rocks become compacted by burial. Interbedded mudstones could act as effective hydrocarbon seal rocks, the report found.
Mesozoic rocks By contrast, Mesozoic rocks, observed at five locations in the Peters Hills, Dutch Hills and Yenlo Hills, appeared to have poor reservoir quality. These rocks tended to contain clay material, with beds of fine-grained sandstone. The strata in the Yenlo Hills tended to be massive, with a higher sand content than at other locations.
Some of the laboratory analysis of coal samples remains to be completed. However, the coals range in grade from lignite to sub-bituminous. The cleats, or fractures, in the coal appear to cluster into two principal directions, the report says.
The analysis of geologic faults on the western side of the basin suggests two distinct periods of rock deformation, driven by different directions of shortening or extension of the Earth’s crust, the report says.
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