oil & gas journal 2012

EXPLORATION & DEVELOPMENT

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

nana 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 grav-ity data and a gravity-aeromagnetic basement interpretation support these basin depths and show the general basin ar-chitecture.

Geochemical (RockEval) analyses from Nunivak-1 high-light marginally mature Late Paleocene coal and coaly shale with excellent source potential. These coals generate impres-sive RockEval S

2 peaks up to 189 mg/g (average 90 mg/g)

and HI up to 379 (average 240).Using the temperature gradient observed in the Nuni-

vak-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 pro-lific expulsion, surface soil cores and lake bottom sediment

Gerald K. Van KootenPetrotechnical Resources AlaskaAnchorage, Alaska

Michael RichterMichael Richter Exploration ConsultingFranktown, Colorado

Pierre A. ZippiBiostratigraphy.com LLCGarland, Texas

Oil and gas production in Alaska has historically been lim-ited to the northern and southern margins of the state, pri-marily in the North Slope and Cook Inlet, but recent drill-ing, seismic, and exploration data in Interior Alaska suggests potential for large oil and gas fields in rift basins near Ne-

Top Bottomdepth depth Leco Tmax, S1/–––––––– ft –––––––– Sample Description TOC S1 S2 S3 °C. HI OI S2/S3 TOC*100 PI Ro, % Source1

7,210 7,240 Cuttings Siltstone 2.33 0.14 2.22 2.36 420 95.3 101.3 0.9 6.2 0.0 This study7,330 7,360 Cuttings Coal NA2 0.85 73.66 15.79 417 –– –– 4.7 –– 0.1 This study7,510 7,540 Cuttings Claystone 1.86 0.16 1.95 2.37 417 104.8 127.4 0.8 8.5 0.0 This study7,600 7,630 Cuttings Coal and carb. shale 15.70 1.33 39.28 4.6 417 250.2 29.3 8.5 8.5 0.0 USGS8,170 8,200 Cuttings Coal and carb. shale 52.57 2.78 113.04 9.73 415 215.0 18.5 11.6 5.3 0.0 0.46 USGS8,200 8,230 Cuttings Coal and carb. shale 61.65 2.39 122.77 11.56 413 199.1 18.8 10.6 3.9 0.0 USGS8,320 8,350 Cuttings Coal and carb. shale 51.24 2.92 125.3 8.83 415 244.5 17.2 14.2 5.7 0.0 0.46 USGS8,680 8,710 Cuttings Coal and carb. shale 35.80 1.82 85.4 5.5 417 238.6 15.4 15.5 5.1 0.0 USGS8,800 8,830 Cuttings Coal and carb. shale 53.23 2.65 145.09 5.94 416 272.6 11.2 24.4 5.0 0.0 USGS9,130 9,160 Cuttings Clayst, mudst, siltst 13.52 0.63 27.1 2.79 413 200.4 20.6 9.7 4.7 0.0 USGS9,160 9,190 Cuttings Mudstone and coal 23.32 0.82 47.46 3.46 414 203.5 14.8 13.7 3.5 0.0 USGS9,190 9,220 Cuttings Mudstone and coal 11.55 0.30 21.29 3.24 420 184.3 28.0 6.6 2.6 0.0 USGS9,310 9,340 Cuttings Coal 56.31 0.78 89.13 12.68 414 158.3 22.5 7.0 1.4 0.0 This study9,340 9,370 Cuttings Coal 58.36 3.86 113.43 6.08 412 194.4 10.4 18.7 6.6 0.0 0.49 USGS9,370 9,400 Cuttings Coal 56.02 2.18 131.21 5.35 415 234.2 9.5 24.5 3.9 0.0 USGS9,400 9,430 Cuttings Coal and carb. shale 43.10 2.19 122.5 4.26 412 284.2 9.9 28.8 5.1 0.0 USGS9,430 9,460 Cuttings Carbonaceous shale 19.53 1.13 43.57 2.43 416 223.1 12.4 17.9 5.8 0.0 USGS9,460 9,490 Cuttings Coal and carb. shale 24.60 2.27 58.29 2.73 412 236.9 11.1 21.4 9.2 0.0 USGS9,490 9,520 Cuttings Mudstone 32.11 2.52 69.42 3.21 409 216.2 10.0 21.6 7.8 0.0 USGS9,640 9,670 Cuttings Coal and mudstone 47.92 3.64 106.07 4.34 406 221.3 9.1 24.4 7.6 0.0 0.48 USGS9,670 9,700 Cuttings Coal and mudstone 32.25 3.02 73.55 2.56 411 228.1 7.9 28.7 9.4 0.0 USGS9,700 9,730 Cuttings Coal, mudst, siltst 26.52 1.66 54.26 2.61 414 204.6 9.8 20.8 6.3 0.0 USGS10,150 10,180 Cuttings Coal 61.32 1.00 140.14 11.37 418 228.5 18.5 12.3 1.6 0.0 This study10,270 10,300 Cuttings Coal and carb. shale 39.90 3.34 102.59 2.65 416 257.1 6.6 38.7 8.4 0.0 USGS10,300 10,330 Cuttings Coal and carb. shale 12.55 0.96 34.85 1.9 420 277.8 15.1 18.3 7.7 0.0 USGS10,330 10,360 Cuttings Carb. shale and coal 25.41 3.35 65.84 2.32 415 259.1 9.1 28.4 13.2 0.0 USGS10,390 10,420 Cuttings Coal and mudstone 23.50 2.79 61.55 1.88 416 261.9 8.0 32.7 11.9 0.0 0.57 USGS10,420 10,450 Cuttings Carb. shale and coal 32.95 3.50 85.47 2.32 417 259.4 7.0 36.8 10.6 0.0 0.62 USGS10,690 10,720 Cuttings Siltstone and coal 43.42 1.92 129.72 9.62 419 298.7 22.2 13.5 4.4 0.0 This study10,810 10,840 Cuttings Coal 49.88 2.34 189.06 7.49 420 379.1 15.0 25.2 4.7 0.0 This study11,080 11,110 Cuttings Coal 46.00 2.54 129.28 6.48 414 281.1 14.1 20.0 5.5 0.0 This study

1The USGS analyses are from Stanley and Lillis (2011). Analyses from “this study” were performed by Weatherford Labs, Houston. 2NA = not analyzed.

Table 1ROCKEVAL ANALYSES OF CUTTING SAMPLES FROM THE NUNIVAK-1 WELL, NENANA BASIN, ALAS.


samples contain thermogenic oil from C2 to C

6 in parts per

million quantities.Fluorescent spectral analysis of surface samples detects

surface oils of moderate gravity in the C6-12

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.

StructureSeveral 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 con-sists 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 sed-imentary 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 devel-oping 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 as-sumed 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 aero-magnetic 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 po-tential 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. Hydrocar-bons 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.

A L A S K A

NENANA AND YUKON FLATS BASINS, SUBBASINS IN INTERIOR ALASKA

Nenana basin

Yukon Flatsbasin

N o r t h S l o p e

Fairbanks

Anchorage

Trans-Alaskaoil pipeline

Cook Inletbasin

FIG. 1


analyses detected C6 to C

12 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, genera-tion, and migration from the deep basin region onto the sur-rounding margin. The surface anomalies may occur where migration pathways intersect structural elements (for exam-ple, faults) associated with structural highs and divert to-wards the surface. Similar thermogenic hydrocarbon anom-alies 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.

StratigraphyOnly one deep test has been drilled to date in the Interior Alaska rift basins. The Nunivak-1 well, drilled in 2009, en-countered entirely nonmarine fluvial and lacustrine rocks in the Nenana basin.

The upper 4,500 ft in Nunivak-1 is Nenana gravel, a clas-tic unit of predominately sand with occasional gravel or con-glomerate (Fig. 7). These sediments are thought to be de-posited 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

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 ap-proximates 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 ma-ture sediment. A seismic line through the Birch Creek sub-basin is shown in Fig. 6.

Surface soil cores and lake bottom sediments were sam-pled in the Nenana basin and Yukon Flats. In the Yukon Flats, C

2 through C

6 thermogenic hydrocarbons were detect-

ed in lake sediment samples from the Birch Creek subbasin at the parts per million level (Fig. 5). Fluorescent spectral

Nunivak-1, TD 11,136 ft, is located along the eastern �ank of the basin. The Nenana-1, TD 3,062 ft, andTotek Hills-1, TD 3, 590 ft, are west and south of the main basin depocenter. Black lines show completedseismic coverage, and the dark outline is the area included in an exploration license from the state of Alaska.Gravity map produced by Thompson Solutions, 2011.

NENANA BASIN BOUGUER GRAVITY

0 5Miles

0 8Km

WellsiteCompleted seismicState license

FIG. 2

Gravity inversion depth map using sedimentary ll density observed in the Nunivak-1 welland basement density of 2.80 g/cc. Gravity inversion by Getech. The dashed seismic line is shown in Fig. 4.

Feet

FIG. 3NENANA DEPTH TO BASEMENT GRAVITY INVERSIONS


immature (est. Ro = 0.4), although C1

to C4 trace gas shows are present be-

low about 7,000 ft. Visibly outgassing coals were encountered from 7,600 ft to 7,900 ft. Coals from the lower Usi-belli Group have source potential for oil and are discussed below.

The Usibelli Group rests on a ma-jor unconformity surface. Age assign-ments 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 uncer-tain how much material was depos-ited and subsequently eroded, but the unconformity likely records compres-sion accompanied by uplift. Compres-sional folds formed during uplift may be truncated by the erosional uncon-formity and subsequently overlain by fine-grained seals of the lower Usibelli Group, potentially forming attrac-tive trapping geometries. Subtle drape folds, formed by differential compac-tion, may also overlie deeper compres-sional folds.

Late Paleocene rocks are pres-ent 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 represent-ing 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 sand-stone 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, fluo-rescence, and cut in several horizons. Gases from C

1 to C

4

were noted throughout most of the section, and C5 was de-

tected over a 200-ft interval near 9,800 ft. Sands are arkoses but become increasingly lithic-rich below 10,000 ft. The in-crease in metamorphic lithics near TD may signal proximity to basement or a fault.

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 fresh-water lacustrine depositional environment. Claystones also may represent lacustrine deposition and the upper and low-er Usibelli Group is dominated by fine grained deposits (Fig. 7). Color of pollen and spores indicate the Usibelli Group is

This dip line shows structural interpretation and deep basin re ectors. The red line at bottom is the depthto basement interpreted from gravity and aeromagnetic data. The Nunivak-1 well is projected about 2 milessouthwest onto the seismic line. Horizons: green = Grubstake formation; orange = Suntrana formation;blue = Sanctuary formation; purple = Late Paleocene unconformity; red = basement depth fromgravity-magnetic interpretation. Line location is given in Fig. 3.

0

0

3Miles

4.8 Km

0

500

1000

1500

2000

2500

3000

3500

0

500

1000

1500

2000

2500

3000

3500

5,000

10,000

15,000

20,000

25,000

Est.

dept

h, ft

Nunivak-1

SEISMIC DIP LINE ACROSS THE NENANA BASIN

z10

06

21

OG

J

FIG. 4


Estimates of thermal maturity indicate the rocks in Nuni-vak-1 are mainly immature. Table 1 shows six vitrinite re-flectance (Ro) analyses with the deepest sample at 10,420 ft yielding Ro = 0.62. Spore color suggests thermal immaturity with colors ranging from pale yellow at shallow horizons to orange below 8,080 ft. The small RockEval S1 peaks (Table 1) suggest the deepest coals in Nunivak-1 (11,080 ft TVD) are still immature. Trace oil shows noted on the mud log below 9,200 ft are thought to reflect the beginnings of oil generation.

Based on measured and reconstructed well temperatures, the geothermal gradient in Nunivak-1 is estimated to be about 1.5° F./100 ft, a value that approximates continental average. This is based on a surface temperature of 35° F. and a reconstructed formation estimate of 205° F. at 11,075 ft.

Source rocksIdentification 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 infor-mation, but subsurface samples are free from surface alteration and oxida-tion and are likely more closely related to rocks in the basin center.

Table 1 shows RockEval analyses of cuttings from the Nunivak-1 well. Twenty-seven Late Paleocene samples below 8,110 ft are lithologically coal, coaly shale, or coaly siltstone. Total organic carbon varies from about 11% to 61% (average 38%, Fig. 8), and S2 peaks range from 27 to 189 mg HC/g org and average a remarkable 90 mg/g.

Hydrogen index (HI) ranges from 158 to 379 and averages 240. HI in-creases with depth in the well (Fig. 9), and eight samples of Late Paleocene rocks below 10,200 ft TVD have an average HI = 284.

On a modified van Krevelen dia-gram (Fig. 9), the coaly samples form a hydrogen-rich trend indicating poten-tial to generate either oil or mixed oil and gas.

A Miocene siltstone (7,210 ft) and claystone (7,510 ft) plot midway be-tween inert (Type IV) and gas-prone (Type III) in Fig. 9. Most of the coaly samples can be characterized geo-chemically as excellent oil-prone source rocks.3 The rich coal at 10,810 ft with TOC = 49.88% and HI = 379 highlights the potential for very attractive oil source rocks in deeper parts of the Nenana basin.

Fig. 10 shows visual estimates of kerogen composition in cuttings from Nunivak-1. Visual estimates in general con-firm the geochemical results. Kerogen in the Nenana gravel is mostly Type III, whereas Type II predominates, sometimes strongly, in the Usibelli Group and Late Paleocene.

Coals and coaly shales from Nunivak-1 are compared with a worldwide dataset of more than 500 oil-prone humic coals4 in Fig. 11. Nenana basin coals have low maturity, as measured by T

max, and moderate to high HI values. Based on

this data set, Petersen4 estimates the effective oil window for Cenozoic oil-prone coals begins at Ro = 0.65. At this maturi-ty, Petersen4 suggests Cenozoic oil-prone coals are saturated with hydrocarbons and oil expulsion begins.

DEPTH TO BASEMENT MAP AND HYDROCARBON ANOMALIES

Birch Creeksubbasin

C3+C4+C5+C6Concentration (ppb)

54 - 3104

3105 - 3641

3643 - 46327

4628 - 9381

0 10Miles

0 16Km

This map shows depth to basement interpreted from gravity and aeromagnetic data over the Birch Creek subbasin of the Yukon Flats.The solid black line outlines land holdings of Doyon Ltd. The dashed black line outlines the area deeper than 13,000 ft. Total concentrationof thermogenic hydrocarbons in parts per billion from C3 to C6 are shown and includes 502 samples. The red line is the location of the seismicline shown in Fig. 6. Depth map produced by Getech. Sediment collection and geochemical analysis by Vista Geoscience.

Birch Creek subbasin, Yukon Flats, AlaskaFIG. 5


this source will expel oil at about 1,959 bbl/acre-ft. If each of the 27 coaly RockEval samples from Nunivak-1 represents 1 ft of source rock, 27 ft of source will expel about 34 mil-lion bbl/sq mile. The gravity inversion depth map (Fig. 3) shows about 240 sq miles where sedimentary rocks are be-low 15,000 ft in the part of the Nenana basin north of the Nunivak-1 well, and 27 ft of source rock over 240 sq miles would expel 8.1 billion bbl of oil or oil equivalent. These large expulsion volumes are clearly sufficient to potentially charge giant fields.

Alternatively, the eight deepest samples, all below 10,800 ft, have an average HI of 284. An integrated source model of 85% Type II and 15% Type III kerogen has an HI = 285 and expels oil at 2,338 bbl/acre-ft. If each sample represents 2 ft of source rock, then 5.7 billion bbl of oil or oil equivalent is potentially expelled in the northern Nenana basin. Deeper portions of the basin south of Nunivak-1 were not assessed and have additional potential.

Regional potentialThe presence of excellent oil-prone source rocks in the Ne-nana basin allows a more optimistic evaluation of other inte-rior Alaska rift basins. Deep nonmarine rift basins, such as the oil-rich Bohai basin in northeastern China, often contain oil-prone lacustrine source rocks deposited in deep, strati-fied and anoxic lakes. Similar lacustrine source rocks may

Geohistory and basin potentialPlatte River Associates used the stratigraphic and temper-ature data from Nunivak-1 to construct a 1D thermal and maturation model of the deepest part of the Nenana basin.

The stratigraphy in Nunivak-1 was expanded to populate a basin center pseudowell to 25,000 ft. Heat flow of 48mW/m2 from 65 to 13 million years was used, then increasing to 59mW/m2 at present. The source characteristics from Nuni-vak-1 (Table 1) were used to calculate oil volumes.

The maturation results from the basin center pseudowell are shown in Fig. 12. Early maturation around 55 million years depends on about 5,000 ft of sediment deposition and subsequent erosion in the early Eocene, but this history is very poorly constrained. Thermal maturation of most hori-zons occurs after about 20 million years and continues to the present. From Fig. 12, the top of the transformation window today is estimated at about 12,000 ft.

Source oil expulsion calculations were made by Platte River Associates using 40% Type II kerogen with maturation kinetics similar to the Paris basin and 60% Type III kerogen with maturation kinetics similar to the oil-prone coals of the Mahakam Delta, Indonesia. This integrated source rock was assigned HI = 240, a value identical to the average coal and coaly shale in Nunivak-1.

Assuming TOC = 25%, a conservative value less than the average of 38% for Late Paleocene samples from Nunivak-1,

The seismic data are shown only over Doyon lands. The red dot is the location of surface geochemical anomalies shown in Fig. 5 projected about 3 miles southeast onto the seismic line.The seismic interpretation indicates a deep sedimentary basin exceeding 3.5 sec two-way time.

Birch Creek subbasin

BIRCH CREEK SEDIMENTARY DEPOCENTER AND NENANA BASIN MARGINS

Seismic line 88-2, Yukon FlatsDoyon lands

SW NE

TWT,

mse

c

0

500

1000

1500

2000

2500

3000

3500

4000

0 6Miles

0 9.6Km

FIG. 6


at 70 ft along the southern rim of the Yukon Flats is 1 ft thick with TOC = 34%, HI = 379, and T

max = 442° C., indicating

oil window maturity. This coal is an excellent oil source rock and is assigned “Late Paleocene or (?)Eocene,” a similar age

exist in the Alas-ka rift basins, possibly in addi-tion to the coaly source rocks e n c o u n t e r e d in Nunivak-1. Deep, stratified and anoxic lakes are most likely to form during the initial rift-ing phase when fault movement is greatest. The presence of la-custrine source rocks in the deepest parts of these basins would add to the oil charge.

The Yukon Flats contains subbasins simi-lar in depth (~25,000 ft) but c o n s i d e r a b l y larger than the Nenana basin. Outcrop cores from the 1980s around the mar-gin of the Yu-kon Flats found coal with similar HI contents as Nunivak-1. For example, one coaly shale cored

NUNIVAK-1 STRATIGRAPHY

Rampart Nunivak-1

Nenanagravel

Usib

elli

Gp (M

ioce

ne)

Grubstakefm

LigniteCreek

fm

Suntranafm

Sanctuaryfm

Healy Creekfm

LatePaleocene(Cantwell

fm)

Coal

Sa

nd

Silt/

shal

e

Coal

Sa

nd

Silt/

shal

e

4,500’

5,050’

5,890’

6,850’

7,870’

8,110’

TD 11,136TD 11,075’ TVD

z10

06

21

OG

J

FIG. 7 TOC CONTENT OF ROCKEVAL SAMPLES

Nunivak-1 well, Nenana basin70

60

50

40

30

20

10

0

Tota

l org

anic

car

bon,

wt

%

Coal occurrence is from the mud log, sand and silt/shale are determined using a sand cutoff on the gamma ray log.An unconformity spanning about 31 million years occurs at the base of the Miocene Usibelli Group (8,110 ft).

3a

FIG. 8

900

800

700

600

500

400

300

200

100

0

NUNIVAK-1 COALS WILL GENERATE OIL AND GAS

Modi�ed van Krevelen diagram,Nunivak-1, Nenana basin

Hyd

roge

n In

dex

0 50 100 150 200

Oxygen Index

n = 28, TOC = 11-61%

n = 2, TOC = 1-2%

I

II

III

IV

FIG. 9


AcknowledgmentsMany people and companies contributed to this study. Jim Mery, John Woodman, and Jeff Filut (Doyon Ltd.) provid-ed support, data, encouragement, and discussion. Alicia Orange, Greg Jurkowski, Peggy Gallagher, and Tom Walsh (Petrotechnical Resources Alaska) provided critical data, discussion, and review. David West collected field samples for surface hydrocarbons, and David Seneshen (Vista Geo-science) analyzed the results. Gary Thompson (Thompson Solutions) produced the gravity map and gravity inversion and made the Birch Creek depth interpretation (Getech). Charlie James (Platte River Associates) performed the basin geohistory analysis and expulsion calculations.

References1. Rowan, E.L., and Stanley, R.G., “The Yukon Flats Cre-

taceous (?)-Tertiary Extensional Basin, East-Central Alaska: Burial and Thermal History Modeling,” US Geological Sur-vey, SIR 2007-5281, 2008, 19 pp.

2. Frost, G.M., and Stanley, R.G., “Compiled Geologic and Bouguer Gravity Map of the Nenana Basin Area, Central Alaska,” USGS Open File Report 91-0562, 1991, 30 pp.

3. Stanley, R.G., and Lillis, P.G., “Preliminary Interpre-tation of Rock-Eval Pyrolysis and Vitrinite Reflectance Re-sults From the Nunivak 1 Well in the Nenana Basin, Central Alaska,” AAPG Search and Discovery Article #90125, AAPG Pacific Section Meeting, Anchorage, Alas., May 8-11, 2011.

4. Petersen, H.I., “The petroleum generation potential and effective oil window of humic coals related to coal com-position and age,” Coal Geology, Vol. 67, 2006, pp. 221-48.

to oil-prone coals in the Nenana basin. A slightly deeper coal has TOC = 52.1% and HI = 342, while several additional coals were not analyzed.

The surface microseeps in Nenana basin and Yukon Flats may be sourced by these coaly source rocks. Oil volume in the microseeps is so far too small to geochemically type to the oil source, but correlation may be possible if more con-centrated surface seepage is found.

Visual analysis of kerogen from cuttings of the Nunivak-1 well, Nenana basin, Alaska. Type 2 kerogen,indicating a mixed oil and gas source, dominates in most of the deeper section. Hydrogen Index shows agradual increase with depth, indicating an increase in oil-prone kerogen.

10 20 30 40 50 60 70 80 90Type 3

10 20 30 40 50 60 70 80 90Type 2

10 20 30Type 1

Hydrogen Index

0 100 200 300 400

7000

7500

8000

8500

9000

9500

10000

10500

11000

7000

7500

8000

8500

9000

9500

10000

10500

11000

KEROGEN ANALYSIS, NUNIVAK-1Type 3

Woody fragments,gas prone

Type 2Pollen, spores,

cutinite, resinite,mixed oil and gas

Type 1Algal,

amorphous,oil-prone

Nenanagravel

Usi

belli

Gp

Grubstakefm

LigniteCreek

fm

Suntranafm

Sanctuaryfm

Healy Creekfm

LatePaleocene(Cantwell)

4,500’

5,050’

5,890’

6,850’

7,870’

8,110’

TD 11,075’TVD

z10

06

21

OG

J

FIG. 10

The authorsGerry Van Kooten ([email protected]) is profes-sor of geology at Calvin College, Grand Rapids, Mich., and a consultant on energy projects with Petrotechnical Resources Alaska. He has worked in geothermal, coal, and oil and gas exploration and development for over 30 years from ARCO offices in Dallas, Denver, and Anchorage. He also has investigated the ecological effects of natural oil seeps along the southern coast of Alaska. He has a PhD from the University of California Santa Barbara, an MS from Arizona State University, and a BS degree from the Univer-sity of Washington.

Michael A. Richter is owner of Michael Richter Exploration Consulting LLC, an oil and gas explo-ration consulting company located near Denver. He has more than 35 years of experience in oil and gas exploration, working projects for ARCO in Asia, the Middle East, Russia, Central Asia, and Alaska. He was ARCO Alaska’s vice-presi-

Reprinted with revisions to format, from the January 9, 2012 edition of Oil & Gas JournalCopyright 2012 by PennWell Corporation


dent of exploration and land when Phil-lips Petroleum purchased ARCO Alaska’s assets. He has an MS in geophysics from the University of Wisconsin-Mil-waukee and a BS in geology from the University of Wisconsin-Oshkosh.

Pierre A. Zippi is principal scientist and owner of Biostratigraphy.com LLC, a biostratigraphic service company located near Dallas. He has over 30 years’ experience in ap-plied biostratigraphy. Previously he was the lead biostratigrapher for ARCO Inter-national Oil & Gas Co. and palynologist at ARCO Alaska. He is a research professor at Southern Methodist University. He has a PhD from the University of Toronto, an MS from the University of Georgia, and BS from Pennsylvania State University.

NENANA COALS, COALY SHALES, WORLDWIDE HUMIC COALS COMPARED

(b)

HI vs. Tmax Worldwide coal data set, n = 494500

400

300

200

100

0

Hyd

roge

n In

dex,

mg

HC

/g T

OC

350 400 450 500 550 600 650Tmax, °C.

Nenana basin coals and coaly shales

Worldwide humic coal data set from Peterson, 2006.[4] The Nenana basin samples tend to have moderateto high HI and low thermal maturity.

FIG. 11

Transformation ratio showing timing of source rock maturation in the Nenana basin pseudowell. Early maturation about 55 million years ago depends on poorly constrained deposition and erosion of sediment from 60 to 40 million years. Most hydrocarbons are generated and expelled after about 20 million years.

TRANSFORMATION RATIO, NORTHERN DEPOCENTERNenana basin pseudowell

Grubstake

Lignite CreekSuntrana

SanctuaryHealy Creek

U. LatePaleocene

M. LatePaleocene

L. LatePaleocene

Pal. Eoc. Olig. Mio. Plio. Pleist.

Quaternary

Nenanagravel

75°F.

125° F.

100° F.

150° F.175° F.

200° F.

225° F. 250° F.

275° F.300° F.

325° F.

350° F.

375° F.

400° F.

425° F.

450° F.

475° F.

1

0.8

0.6

0.4

0.2

0.01

Tran

sfor

mat

ion

rati

o, f

ract

ion

0

5,000

10,000

15,000

20,000

25,000

Dep

th s

ubse

a, f

t

70 60 40 20 0Age, million years

FIG. 12

Reprinted with revisions to format, from the January 9, 2012 edition of Oil & Gas JournalCopyright 2012 by PennWell Corporation

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