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Fossil Fuel and Geothermal Energy Sources for Local Use in Alaska Chapter G, Chapter G, Lower Yukon–Kuskokwim southwest of its present position, at the edge of the Bering Sea shelf. Hoare and Condon (1962) mapped surface sediments in the onshore portion of the modern delta and noted that unconsolidated deposits include silt, sand, gravel, and layers of brown peat up to several feet thick. They noted these deposits are many hundreds of feet thick. No deep wells have been drilled in the delta and details of the stratigraphy are known only from industry seismic lines. These data suggest a slightly thickened Tertiary succession underlies surficial deltaic sediment. The ancestral Yukon delta may have deposited sediment in the offshore Norton basin, located a short distance north of the modern delta, in the Federal outer continental shelf (OCS) area of the northeastern Bering Sea and Norton Sound. The Norton basin is an extensional basin filled with more than 20,000 feet of Tertiary-age marine and nonmarine sedimentary rocks (Fisher and others, 1981; Turner and others, 1983). Although outside of the Lower Yukon–Kuskokwim Energy Region, the Norton basin is included in the summary that follows. Source rocks. Outcrop studies have documented that sedimentary rocks in the Paleozoic-age Holitna basin generally contain organic carbon in amounts less than what is generally regarded as a good petroleum source rock (LePain and others, 2000). Likewise, outcrop studies have documented that Cretaceous-age sedimentary rocks in the Yukon–Koyukuk and Kuskokwim basins generally contain organic carbon in amounts less than what is normally considered a good petroleum source rock, and the organic material that is present is typically gas-prone (Lyle and others, 1982). The Nulato Unit No. 1 well, in the western part of the Yukon–Koyukuk basin and outside of this region, penetrated 12,000 feet of deformed and tightly cemented Cretaceous-age sandstone, siltstone, and shale. No information is available on the organic content of shales encountered in this well, but the drilling reports (available from the Alaska Oil & Gas Conservation Commission) suggest the siltstones and shales have poor petroleum source potential. The Napatuk Creek No. 1 well, approximately 50 miles southwest of Bethel, penetrated approximately 2,000 feet of Cenozoic-age rock and nearly 13,000 feet of interbedded sandstone, siltstone, and shale of Cretaceous age. The entire section penetrated by this well contains little organic material, and the material encountered was gas-prone (Mull and others, 1995). The stratigraphy of the Cenozoic-age Holitna basin is not known and that of the Bethel basin is known only from a single exploration well (Napatuk Creek No. 1). Outcrop studies of Cenozoic-age sedimentary rocks along the Denali– Farewell fault zone in the McGrath Quadrangle by Sloan and others (1979), Dickey (1982), and LePain and others (2003) demonstrate the presence of coal and carbonaceous mudstone. These rocks are thought to be similar to the stratigraphy of the Cenozoic Holitna basin (LePain and others, 2003). Laboratory analysis of samples collected from the coal-bearing section in the McGrath Quadrangle (LePain and others, 2003) and of samples of similar-age rocks exposed in the Middle Tanana basin near Healy (Stanley and others, 1990) demonstrate their potential as source rocks for gas and also show some potential to generate liquid hydrocarbons (condensate) if buried deep enough. Gravity data suggests that the Holitna basin may locally contain nearly 15,000 feet of sediment in its deepest part (Kirschner, 1994). If the basin has a normal geothermal gradient, then any organic-rich sediment from the deeper parts of the basin could generate thermogenic hydrocarbons. Biogenic gas, generated by microbial processes, is often considered an unconventional resource due to its method of production in coalbed methane systems (see Chapter A). However, in some basins, such as the prolific Cook Inlet in southern Alaska, biogenic methane has been known to occur in conventional reservoirs. If thick coals are present in the Holitna Basin, it is reasonable to assume biogenic gas has been generated due to the microbial breakdown of buried organic matter. However, in order for biogenic gas to migrate into a conventional reservoir, an unusual set of geologic conditions are required involving the formation of early traps, rapid burial, and finally rapid uplift (Rice, 1993). Details of the subsurface stratigraphy of the Yukon delta are poorly known. Hoare and Condon (1962) mapped the surface sediments in the delta, noted the presence of brown peat layers up to several feet thick, and stated the deltaic sediments are many hundreds of feet thick. Regional seismic data in the area suggest that a slightly thickened Tertiary- age succession may be present beneath the delta, but no information is available on the organic carbon content of these rocks and they are likely insufficiently thick to host a mature source rock (Mull and others, 1995). Eight deep wells were drilled in the Norton basin in the early 1980s. Data from the COST No. 1 well are summarized in Turner and others (1983). Cuttings are typically organically lean (low percentage of organic carbon), except where contaminated by coaly material. Organic carbon is dominantly land-derived woody and herbaceous material. This type of carbon, when present in sufficient quantities at sufficient burial depths, typically generates gas. Geochemical data demonstrate sufficient temperatures and quantities of organic carbon beneath approximately 9,500 feet to generate conventional hydrocarbons. Of the eight deep wells drilled in the basin, all had moderate to strong gas shows and three had weak oil shows, demonstrating that rocks capable of generating hydrocarbons are present in the basin and have generated some petroleum. Thermally mature, organically lean strata of Eocene to middle Oligocene age are the most likely source rocks (Minerals Management Service [MMS], 1998). Reservoir rocks. Partly dolomitized limestones in the Paleozoic Holitna basin commonly include visible porosity (LePain and others, 2000) and laboratory measurements demonstrate porosities greater than 10 percent in some Page 67 Lower Yukon–KuskokwimPDF Image | FOSSIL FUEL AND GEOTHERMAL ENERGY SOURCES FOR LOCAL USE
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