Alaska's Interior rift basins: a new frontier for discovery

Gerald K. Van Kooten
Petrotechnical Resources Alaska
Anchorage

Michael Richter
Michael Richter Exploration Consulting
Franktown, Colo.

Pierre A. Zippi
Biostratigraphy.com LLC
Garland, Tex.

Oil and gas production in Alaska has historically been limited to the northern and southern margins of the state, primarily in the North Slope and Cook Inlet, but recent drilling, seismic, and exploration data in Interior Alaska suggests potential for large oil and gas fields in rift basins near Nenana and Yukon Flats.

Seismic depth calibration to the Nunivak-1 well, drilled to 11,136 ft (11,075 ft true vertical depth) in 2009, suggests the Nenana basin is about 25,000 ft deep. Inversion of gravity data and a gravity-aeromagnetic basement interpretation support these basin depths and show the general basin architecture.

Geochemical (RockEval) analyses from Nunivak-1 highlight marginally mature Late Paleocene coal and coaly shale with excellent source potential. These coals generate impressive RockEval S₂ peaks up to 189 mg/g (average 90 mg/g) and HI up to 379 (average 240).

Using the temperature gradient observed in the Nunivak-1 well, basin geohistory modeling and calculations suggests coaly source rocks below 13,000 ft have expelled billions of barrels of oil. Supporting the potential for prolific expulsion, surface soil cores and lake bottom sediment samples contain thermogenic oil from C₂ to C₆ in parts per million quantities.

Fluorescent spectral analysis of surface samples detects surface oils of moderate gravity in the C_6-12 range. A robust source system appears to be in place. Only one deep well has been drilled in these rift basins, and additional seismic and drilling is needed to evaluate their hydrocarbon potential.

Structure

Several nonmarine rift basins formed in interior Alaska (Fig. 1) in response to crustal extension (Nenana basin) and strike slip movement along curved, regional faults (Yukon Flats).

The Yukon Flats includes about 12,000 sq miles and consists of several rift subbasins that formed on the northern margin of the right-lateral Tintina fault. Within the Yukon Flats, large normal faults have localized subsidence, but the entire region has also experienced structural sag and sedimentary deposition, probably intermittent, since the Late Cretaceous.

Judging by surrounding outcrops, the sedimentary rift basins developed on plutonic and metamorphic basement rocks. The Nenana basin, located southwest and separate from the Yukon Flats, is a structural half-graben with a large, normal basin bounding fault on the southeast margin. This fault forms an asymmetrical basin with a relatively steep southeast and a gentle northwest flank.

New surface gravity data collected in the northern part of Nenana basin are incorporated into the Bouguer gravity map and show the basin geometry (Fig. 2). The Nenana basin is elongate in a NE-SW direction, covers about 2,000 sq miles, and widens in the north. The Bouguer gravity map indicates that the deepest part of the Nenana basin is generally in the north. The depth of the Nenana basin is important in developing a robust and prolific oil kitchen. A gravity inversion basement map shows the maximum basin depth is about 30,000 ft (Fig. 3).

The density inversion used density indicated by logs from Nunivak-1 (11,075 ft TVD) for sedimentary fill and an assumed basement density of 2.80 g/cc. Increasing basement density to a less reasonable 2.95 g/cc decreases the basement depth to about 21,000 ft. An integration of gravity and aeromagnetic data to yield a depth to basement interpretation (not shown) is consistent with depths shown in Fig. 3.

The asymmetry of the Nenana basin is evident on the dip-oriented seismic line in Fig. 4. From an exploration point of view, the gentle western flank is generally considered more prospective than the steep eastern flank because of the potential for larger volume under closure and larger fetch areas.

Fig. 3 shows a structural nose extending to the NE in the northern basin between two sedimentary deeps. Hydrocarbons could potentially migrate onto this nose from three sides, and this feature is an attractive lead. Past exploration efforts, discussed below, focused on the southern part of the basin.

The structure of the Birch Creek subbasin in the Yukon Flats is shown in Fig. 5, a depth to basement interpretation based on gravity and aeromagnetic data. It shows a large deep in the southwest with basin depths to about 21,000 ft.

Using the depth contours on Fig. 5, sediment below 13,000 ft occurs over about 360 sq miles. This depth approximates the beginning of hydrocarbon expulsion in the Nenana basin, also discussed below. Using a depth of 6,000 ft for the top of the oil window, as suggested by Rowan and Stanley,1 dramatically increases the volume of thermally mature sediment. A seismic line through the Birch Creek subbasin is shown in Fig. 6.

Surface soil cores and lake bottom sediments were sampled in the Nenana basin and Yukon Flats. In the Yukon Flats, C₂ through C₆ thermogenic hydrocarbons were detected in lake sediment samples from the Birch Creek subbasin at the parts per million level (Fig. 5). Fluorescent spectral analyses detected C₆ to C₁₂ hydrocarbons that have similar spectral patterns to medium gravity oil.

In Fig. 5, no surface anomalies occur over the deep Birch Creek subbasin, but anomalies are associated with updip structural highs. These surface hydrocarbons are viewed as evidence of an active oil system with maturation, generation, and migration from the deep basin region onto the surrounding margin. The surface anomalies may occur where migration pathways intersect structural elements (for example, faults) associated with structural highs and divert towards the surface. Similar thermogenic hydrocarbon anomalies were detected near Stevens Village, another subbasin in the Yukon Flats, and the Nenana basin. Oil source rocks are thought to be thermally mature in each of these areas.

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.

Source rocks

Identification and characterization of adequate hydrocarbon source rocks is critical for evaluating the hydrocarbon potential of a sedimentary basin and providing a rationale for further work. Surface outcrops provide useful information, but subsurface samples are free from surface alteration and oxidation and are likely more closely related to rocks in the basin center.