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Stratigraphy
Only one deep test has been drilled to date in the Interior Alaska rift basins. The Nunivak-1 well, drilled in 2009, encountered entirely nonmarine fluvial and lacustrine rocks in the Nenana basin.
The upper 4,500 ft in Nunivak-1 is Nenana gravel, a clastic unit of predominately sand with occasional gravel or conglomerate (Fig. 7). These sediments are thought to be deposited during the Pliocene and possibly latest Miocene by coalescing alluvium fans during uplift of the Alaska Range to the south.2 Although the GR curve indicates occasional siltstone above 1,000 ft, lack of seals in the Nenana gravel limit the reservoir potential of this section.
The Miocene Usibelli Group underlies the Nenana gravel and is composed of five individual formations (Fig. 7). Coal occurs at the top and intermittently throughout the Usibelli Group and is surface mined at the nearby town of Healy. Sandstone, mudstone, and coal represent swamp, lake, and fluvial environments associated with south-flowing streams before uplift of the Alaska Range.2
The mud log notes five horizons from the Usibelli Group contain micrite (lithified carbonate mud), indicating a freshwater lacustrine depositional environment. Claystones also may represent lacustrine deposition and the upper and lower Usibelli Group is dominated by fine grained deposits (Fig. 7). Color of pollen and spores indicate the Usibelli Group is immature (est. Ro = 0.4), although C1 to C4 trace gas shows are present below about 7,000 ft. Visibly outgassing coals were encountered from 7,600 ft to 7,900 ft. Coals from the lower Usibelli Group have source potential for oil and are discussed below.
The Usibelli Group rests on a major unconformity surface. Age assignments based on palynology indicate missing section with ages from 54.8 to 23.8 million years, producing a time gap of 31 million years covering the Eocene and Oligocene. It is uncertain how much material was deposited and subsequently eroded, but the unconformity likely records compression accompanied by uplift. Compressional folds formed during uplift may be truncated by the erosional unconformity and subsequently overlain by fine-grained seals of the lower Usibelli Group, potentially forming attractive trapping geometries. Subtle drape folds, formed by differential compaction, may also overlie deeper compressional folds.
Late Paleocene rocks are present in Nunivak-1 from 8,110 ft to TD (Fig. 7). This section correlates in time to the Paleocene Cantwell formation,2 but in Nunivak-1 these rocks are composed exclusively of nonmarine clastic units representing fluvial, lacustrine, swamp, and possibly alluvial fan environments. Igneous rocks common in outcrops of the Cantwell formation, such as extrusive volcanic rocks, intrusive sills, and dikes, and plutonic granitoids, are missing in Nunivak-1. Other Cantwell outcrops are composed of nonmarine clastic rocks, and Nunivak-1 is considered correlative to these.
The Late Paleocene is dominated by fine grained rocks (Fig. 7) with occasional sand, such as an 80-ft thick sandstone at about 9,300 ft. Coal is common and has excellent oil source potential, as discussed below. The mud log notes four occurrences of micrite in the Late Paleocene, indicating freshwater lacustrine deposition, and trace oil stain, fluorescence, and cut in several horizons. Gases from C1 to C4 were noted throughout most of the section, and C5 was detected over a 200-ft interval near 9,800 ft. Sands are arkoses but become increasingly lithic-rich below 10,000 ft. The increase in metamorphic lithics near TD may signal proximity to basement or a fault.
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