Organic geochemistry and organic petrology of a potential source rock of early Eocene age in the Beaufort–Mackenzie Basin

Organic Geochemistry Organic Geochemistry 35 (2004) 1039–1052 www.elsevier.com/locate/orggeochem

Organic geochemistry and organic petrology of a potential source rock of early Eocene age in the Beaufort–Mackenzie Basin L.R. Snowdon *, L.D. Stasiuk, R. Robinson, J. Dixon, J. Dietrich, D.H. McNeil Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303-33 Street N.W Calgary, AB, Canada T2L 2A7 Received 6 December 2003; accepted 26 April 2004 (returned to author for revision 2 February 2004) Available online 20 July 2004

Abstract The biological marker compounds 17a(H) and 17b(H), 23,28-bisnorlupane have been identified in a coaly interval at about 3300–3320 m depth within the lower Taglu Sequence (early Eocene) in the Immiugak A-06 well in the Beaufort– Mackenzie Basin. These biomarkers were previously only known to occur in the lower part of the Richards Formation and were thus used to infer that the Richards was the source for most of the Tertiary oils and condensates in the basin. The discovery of bisnorlupanes in older, much more organic-rich rocks indicates that the previous oil–source rock correlations may not have been correct and that the coals in the lower Taglu Sequence are likely the dominant effective source rock, especially for the Paleocene and early Eocene reservoirs in the southern part of the Mackenzie Delta. Petrographic and Rock–Eval analyses indicate that the lower Taglu Sequence coaly section should be mainly a source for gas, but with some potential to expel liquids. The level of thermal maturity inferred from Rock–Eval Tmax (432–438 °C, equivalent to about 0.65–0.75% Ro vitrinite) is slightly higher than that inferred from the measured reflectance, fluorescence and foraminiferal colouration index which all indicate that the maturity at 3300 m is about 0.60–0.65% Ro vitrinite equivalent at the Gulf et al. Immiugak A-06 well site. Ó 2004 Elsevier Ltd. All rights reserved.

1. Introduction Many previous organic geochemistry studies in the Beaufort–Mackenzie Basin (BMB) have considered source rock–oil systems. Initial studies by Burns et al. (1975), Powell and Snowdon (1975), Snowdon and Powell (1979) focussed on various chemical properties of crude oil samples and using these properties to assign the oils into various categories including biodegraded versus non-biodegraded and different genetic groups or families. Snowdon (1978) investigated the chemical * Corresponding author. Tel.: +1-403-292-7035/+1-403-2843500; fax: +1-403-292-5377. E-mail addresses: [email protected], [email protected] (L.R. Snowdon).

character of Tertiary potential source rocks as a function of depositional environment and compared and contrasted those results with similar data from various crude oil samples. Lane and Jackson (1980) used chemical data and geological arguments to determine that two source rocks, one Cretaceous (Smoking Hills/ Boundary Creek formations) and one Tertiary gave rise to two groups of hydrocarbons. The Cretaceous system was confirmed and additional details on Tertiary source rocks and reservoirs were provided in a series of publications based on work done by the Geological Survey of Canada largely in the late 1970s and early 1980s (Gunther, 1976a,b; Snowdon, 1980a,b, 1984, 1987, 1988, 1990a,b, 1991; Snowdon and Powell, 1982; Powell et al., 1982; Dixon et al., 1985, 1992; Brooks, 1986a,b, Dietrich et al., 1989a,b; Issler and Snowdon, 1990). A summary

0146-6380/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.04.006

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L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

Richards Sequence, see Fig. 2), specifically in the Unark L-24 well (Fig. 1), was used by Brooks to make an oil– source correlation with this unit. This correlation seemed to be very strong because of the high concentration of the bisnorlupane compounds despite the rather poor source potential displayed by most of the samples in the Richards Formation (Snowdon, 1984). Subsequent work by Curiale (1991) confirmed the presence of bisnorlupanes in a number of oils. However, he also noted that a number of oils in the south-central part of the delta, in Moose Channel and Reindeer sequence reservoirs (Niglintgak and Kumak discoveries) had strong affinities with the Adlartok oil and he concluded that these oils were probably derived at least in part from a Paleocene source. During the ongoing research efforts designed to refine our knowledge of the western part of the Mackenzie Delta, routine analysis and characterization of petroleum source potential has been carried out using Rock– Eval/TOC pyrolysis. These new data indicated the presence of an organic rich zone at
3300 m (TOC up to

of this work was published in a Geological Atlas of the Beaufort–Mackenzie Area (Dixon, 1996). The discoveries at Parsons/Siku/Kamik (Fig. 1) were considered by Langhus (1980) to belong to a separate source rock–oil system derived from the Jurassic Husky Formation. Although limited data were presented to support this interpretation, very little, if any, additional work has been published on these deposits. Two papers by Brooks (1986a,b) allowed for the identification of two different source rock–oil systems in the Tertiary. One of these was represented by oils from the Adlartok P-09 discovery well in the offshore in the western part of the Delta (Fig. 1), while the other Tertiary oils were interpreted to have been derived from a portion of the Eocene Richards Formation. This oil– source rock correlation was based on the presence of key biomarker compounds used to differentiate sources of the western Delta area from other Tertiary discoveries. The presence of 23,28-bisnorlupane in relatively high concentrations in the lower portion of the Eocene Richards Formation (approximately equivalent to the 1420

1410

129 1380

71

132

1350

Beaufort Sea

0

0

128

0

0

710

Gas well Oil well Oil and gas well Dry and abandoned

700

700

Adlartok P-09

Unark L-24

Alaska Yukon Terri to

ry

Immiugak A-06 50

Richards Island 690

690

680 0 142

141

N.W.T.

Yukon Territory

Parsons/ Siku/ Kamik

0

138

0

129

0

1350

680 0 128

1320

Kilometers

Fig. 1. Map of the Beaufort–Mackenzie Basin showing location of the Immiugak A-06 well along with other borehole locations.

L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

SEQUENCE FORMATION

Late Early Late E M Late E L

(Delta only)

Shallow Bay

Recent Herschel Is

Iperk

Nuktak

Akpak Mackenzie Bay Mackenzie Bay Kugmallit Richards

Middle

Early Late Early

Olig.

Eocene

Paleo.

TERTIARY CRETACEOUS

(Basin-wide)

Holo. Pleist. Mio. Plio.

QUAT.

AGE

Kugmallit Richards

Taglu

Reindeer Aklak Member

Aklak Fish River

Maast.

Moose Ministicoog Mbr Channel Tent Island Mason R.

Camp. Sant.

Smoking Hills

Smoking Hills

Con.

Turon. Boundary Creek

Boundary Creek

Cen. Fig. 2. Stratigraphic chart showing both formation and sequence names for the Beaufort–Mackenzie area.

16%) in the Immiugak A-06 well (Fig. 1) with results somewhat different from those previously measured using older instrumentation. More detailed analyses were carried out in order to investigate this unit as a potential source for the Adlartok discovery and possibly other discoveries (Niglintgak, Kumak, Taglu) in lower Tertiary reservoirs. Seismic correlation and petrophysical log character indicate that these Immiugak A-06 samples represent thick coal deposits below the Eocene Richards Sequence. Identification of such a source rock would result in a reduction of exploration risk through the development of improved models of the oil–source rock systems, particularly in the south and western portions of the Mackenzie Delta where the postPaleocene cover is thinner than in the central and eastern parts of the Delta.

2. Methods and analytical results 2.1. Samples, stratigraphy, and biostratigraphy Drill cuttings from depths of 3000–3500 m from the Immiugak A-06 borehole (Fig. 1) were analyzed and

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TOC- and coal-enriched intervals were recognized visually and by Rock–Eval analysis between 3240 and 3350 m (Table 1). On petrophysical logs, the coals show up as low velocity, high resistivity units up to a few meters thick about 1600 m below the well defined base of the Eocene Richards Sequence which occurs at 1683 m. Initial interpretation based on petrophysical logs and seismic profiles indicated that the TOC-rich units occur within either the Aklak or the Taglu sequences. Subsequent biostratigraphic analysis using benthic foraminifera data indicated that the TOC-rich unit occurs within the Taglu Sequence and is likely of Early Eocene age. This was based on the occurrence of Early Eocene benthic foraminifera immediately underlying the coaly unit. The benthic foraminifera occur from 3300 m to the well’s total depth at 3800 m. The most distinctive species in the assemblage is Caronia gallagheri McNeil which has been documented previously in the Taglu Sequence in the Natsek E-56 and Natiak O-44 wells (McNeil, 1989, 1997). At the Natsek E-56 location, C. gallagheri ( ¼ Verneuilina sp. 2700 of McNeil, in Dietrich et al., 1989a,b) occurs in Early Eocene marine strata of the lower Taglu Sequence directly and unconformably overlying Late Paleocene to earliest Early Eocene coaly strata of the Aklak Sequence (Dietrich et al., 1989a). C. gallagheri also occurs in the Natiak O-44 well (McNeil, 1997) from 3265 to 3412 m, above coaly strata of the Aklak Sequence and below possible turbidite sands of the Taglu Sequence. To the southeast of Immiugak A-06, the C. gallagheri biozone also occurs in the Adgo F-28 well from 2652 to 2743 m, below coaly strata of the Taglu Sequence (McNeil, unpublished data). Biostratigraphic evidence consistently indicates that the C. gallagheri biozone and the bisnorlupane biomarker unit at Immiugak A-06 occur in the Early Eocene lower Taglu Sequence. It should be noted that the geochemically unique oils (lacking bisnorlupane biomarkers) from Adlartok P-09 occur in the Aklak Sequence, dated biostratigraphically as Middle to Late Paleocene by association with the widespread benthic foraminiferal Reticulophragmium boreale Zone. In addition to the biostratigraphic analysis, agglutinated benthic foraminifera in the Immiugak A-06 borehole were analyzed as indices of thermal maturity following the FCI (Foraminiferal Colouration Index) technique outlined by McNeil et al. (1996). McNeil et al. (1996) found that colouration in the organic cement of agglutinated foraminifera changed progressively with increasing temperature in Beaufort Sea boreholes. At 3467 m in the Immiugak A-06 borehole, FCI is 6.6. At present, there are insufficient experimental and empirical data to categorically correlate FCI with %VRo, but if the FCI and %VRo data from Amauligak J-44 (McNeil et al., 1996) are used for general comparison, then FCI 6.6 corresponds 0.6–0.65% vitrinite reflectance.

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L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

Table 1 Rock–Eval/TOC data for Immiugak A-06 drill cuttings over the interval from 3000 through 3500 m generated using a Rock–Eval 6 (maximum oxidation temperature of 800 °C) and older data generated using a Rock–Eval II/TOC (maximum oxidation temperature of 600 °C) S3 (mg CO2 / g rock)

PI (S1/ (S1+S2))

TOC (%)

HI (mg HC/ g TOC)

OI (mg CO2 /g TOC)

Rock–Eval 6 analyses with oxidation to 800 °C 3000 20.0 433 0.15 0.68 3020 20.4 431 0.08 0.38 3040 20.0 429 0.09 0.59 3060 20.1 429 0.13 0.72 3080 20.1 430 0.16 0.69 3100 20.2 431 0.19 0.63 3120 20.2 428 0.16 0.67 3140 20.9 430 0.19 0.94 3160 20.5 425 0.14 0.62 3180 20.4 431 0.15 0.66 3200 20.8 431 0.11 0.70 3220 20.4 425 0.13 0.68 3240 20.3 429 0.61 6.51 3260 20.0 428 0.78 8.28 3280 20.1 429 0.70 5.04 3300 20.3 422 4.24 35.95 3310 20.2 419 4.62 50.35 3320 20.5 418 3.78 38.13 3340 20.5 428 1.18 8.19 3360 20.2 433 0.32 1.62 3380 20.5 436 0.44 2.23 3400 20.8 432 0.31 1.68 3420 20.4 436 0.18 0.88 3440 20.4 434 0.16 0.76 3460 20.7 425 0.14 0.66 3480 20.3 436 0.13 0.66 3500 20.8 436 0.14 0.74

4.95 2.66 1.33 1.85 1.78 3.23 1.60 1.77 1.54 2.33 2.22 1.92 2.30 2.47 4.96 4.61 2.33 4.05 3.34 3.35 3.22 3.69 1.62 1.91 1.84 1.38 2.32

0.18 0.17 0.11 0.15 0.19 0.23 0.20 0.15 0.15 0.18 0.14 0.15 0.09 0.09 0.12 0.10 0.08 0.09 0.13 0.16 0.16 0.16 0.17 0.17 0.16 0.16 0.16

1.18 0.71 0.85 1.19 1.12 1.07 1.18 1.25 0.91 1.05 1.18 1.00 5.34 6.05 4.70 24.23 21.31 19.00 6.16 2.07 2.49 1.88 1.26 1.25 0.85 1.27 1.26

58 55 89 61 62 59 57 86 89 65 59 75 123 138 108 150 238 202 134 81 92 91 71 62 84 52 59

419 375 156 155 159 302 136 142 169 222 188 192 43 41 106 19 11 21 54 162 129 196 129 153 216 109 184

Rock–Eval II analyses with oxidation to 600 °C 3000 – 437 0.18 0.66 3010 432 0.18 0.69 3020 553 0.09 0.76 3030 433 0.12 0.57 3040 575 0.12 1.20 3050 432 0.15 0.47 3060 433 0.18 0.85 3070 435 0.20 0.88 3080 441 0.18 0.88 3090 435 0.18 0.69 3100 438 0.21 0.63 3110 438 0.15 0.71 3120 435 0.20 0.74 3130 437 0.24 1.30 3140 432 0.23 1.66 3150 573 0.27 1.68 3160 585 0.22 1.43 3170 434 0.22 1.82 3180 436 0.21 0.79 3190 437 0.16 0.65 3200 434 0.16 1.05 3210 432 0.18 0.95 3220 431 0.22 1.24 3230 435 0.34 2.66 3240 432 0.81 9.58

3.83 2.34 2.56 1.75 1.53 2.31 2.28 2.01 1.74 1.83 2.67 1.23 1.78 1.36 1.65 1.46 1.53 1.61 1.97 2.27 1.95 2.34 1.87 1.99 2.09

0.21 0.21 0.11 0.17 0.09 0.24 0.17 0.19 0.17 0.21 0.25 0.17 0.21 0.16 0.12 0.14 0.13 0.11 0.21 0.20 0.13 0.16 0.15 0.11 0.08

1.14 1.16 0.57 1.00 0.73 0.96 1.15 1.16 1.12 1.13 0.98 1.08 1.13 1.51 1.26 1.10 0.95 1.35 1.09 1.00 1.18 0.93 1.03 2.22 5.64

57 59 133 57 164 48 73 75 78 61 64 65 65 86 131 152 150 134 72 65 88 102 120 119 169

335 201 449 175 209 240 198 173 155 161 272 113 157 90 130 132 161 119 180 227 165 251 181 89 37

Depth (m)

Qty (mg)

Tmax (°C)

S1 (mg HC/ g rock)

S2 (mg HC/ g rock)

L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

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Table 1 (continued) Depth (m) 3250 3260 3270 3280 3290 3300 3310 3320 3330 3340 3350 3360 3370 3380 3390 3400 3410 3420 3430 3440 3450 3460 3470 3480 3490 3500 *

Qty (mg)

Tmax (°C)

S1 (mg HC/ g rock)

S2 (mg HC/ g rock)

S3 (mg CO2 / g rock)

PI (S1/ (S1+S2))

TOC (%)

HI (mg HC/ g TOC)

OI (mg CO2 /g TOC)

429 428 433 435 431 428 422 421 429 430 433 433 436 437 433 437 438 438 436 439 439 521 565 438 437 439

0.90 0.97 0.36 0.73 0.73 5.06 5.30 4.72 1.64 1.47 1.00 0.39 0.40 0.48 0.47 0.39 0.29 0.22 0.33 0.21 0.18 0.21 0.20 0.27 0.18 0.16

11.65 11.28 2.66 4.74 6.91 48.03 59.02 49.24 13.95 12.70 5.69 2.10 2.29 2.40 2.62 1.93 1.27 0.79 1.04 1.09 1.14 1.20 1.23 1.23 1.19 0.83

1.7 1.91 2.18 3.7 2.08 4 1.62 2.91 2.01 2.7 2.7 2.07 1.88 2.12 1.51 2.43 2.09 1.36 1.99 1.66 1.22 1.93 1.72 1.57 1.99 2.03

0.07 0.08 0.12 0.13 0.10 0.10 0.08 0.09 0.11 0.10 0.15 0.16 0.15 0.17 0.15 0.17 0.19 0.22 0.24 0.16 0.14 0.15 0.14 0.18 0.13 0.16

6.22 6.25 2.51 4.24 4.53 14.92 12.96 16.13 7.24 6.89 3.46 2.09 2.38 2.38 2.49 1.85 1.40 1.15 1.46 1.25 1.36 0.72 1.05 1.40 1.33 1.23

187 180 105 111 152 321 455 305 192 184 164 100 96 100 105 104 90 68 71 87 83 166 117 87 89 67

27 30 86 87 45 26 12 18 27 39 78 99 78 89 60 131 149 118 136 132 89 268 163 112 149 165

A sample weight of 100  2 mg was used for all samples but these sample weights were not preserved with the archived data.

2.2. Organic geochemistry Rock–Eval/TOC analyses were carried out on 100 mg aliquots of washed cuttings samples on a 10 m spacing from the Immiugak A-06 well (Table 1, lower section). The original analyses had been carried out using a Rock–Eval II/TOC instrument in which the sample is pyrolyzed to 600 °C and then oxidized at the same temperature of 600 °C. Analyses of a few of the high TOC samples using a newly acquired Rock–Eval 6 instrument in which the oxidation oven was programmed to ramp temperature between 400 and 800 °C yielded higher TOC estimates. However, the continuous CO2 and CO traces available from this instrument indicated that the oxidation was still not complete even at the high temperature. A re-analysis of smaller samples (about 20 mg) with the same oxidation temperature maximum of 800 °C yielded even higher TOC contents (Table 1, upper section). For all the Rock–Eval analyses, the S2 pyrolysis response was essentially similar. Selected samples with high TOC contents and high HI values were extracted using a Soxhlet extractor and azeotropic solvent system (chloroform:methanol, 87:13). The extracted bitumen was fractionated into saturates, aromatics and polar compounds (NSOs and asphalt-

enes; Table 2) using open column chromatography and a series of elution solvents of increasing polarity. Subsequent analysis of the hydrocarbon fractions was done using gas chromatography (GC); (Fig. 3) and GC-selected ion mass spectrometry (GC–MS, Fig. 4). Additional analyses of the saturated hydrocarbon fraction were carried out using GC-full scan mass spectrometry to try to identify the unusual, high intensity peaks in both the gas chromatograms (Fig. 3) and the mass chromatograms (Fig. 4). This technique collects a full mass spectrum (Fig. 5) at a rate of about one per second through the GC run and thus provides complete electron impact fragmentation data for each of the GC peaks. GC analysis was carried out using a Varian 3400 GC equipped with a DB-1 column (30 m  0.25 mm ID) with a 0.25 lm coating thickness. GC–MS analyses were done on a Micromass Autospec mass spectrometer coupled to a HP 6890 GC equipped with a DB-5MS column (30 m  0.32 mm ID) with a 0.25 lm coating. 2.3. Organic petrology Further characterization of the lower part of the Taglu Sequence was carried out on four samples (3240, 3300, 3310 and 3320 m) using organic petrographic

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L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

Table 2 Soxhlet extract and column chromatography fractionation data for selected samples of drill cuttings for Immiugak A-06 Depth (m)

TOC (%)

Extract yield (mg/g TOC)

Hydrocarbon yield (mg/g TOC)

HC (%)

Sat/arom

pr/ph

3240 3300 3310 3320

12.19 31.47 21.43 16.60

52.2 123.7 85.1 121.0

11.8 21.7 19.2 40.9

22.6 17.5 22.6 33.8

0.56 0.26 0.20 0.27

3.8 7.7 6.4 4.3

‘‘Yield’’ parameters are normalized to TOC. Sat/arom is the ratio of saturated/aromatic hydrocarbons. pr/ph is the peak area ratio of pristane/phytane.

pristane

#X09407 Immiugak A-06 3240m

C 21

C 23 C 25

5

10

15

20

25

#X09406 Immiugak A-06 3310m

30

C 27

35

40

45

50

55

60

65

5

10

15

20

25

30

35

40

#X09405 Immiugak A-06 3300m

45

50

55

60

65

#X09404 Immiugak A-06 3320m

bisnorlupane

5

10

15

20

25

30

35

40

45

50

55

60

65

Retention time (minutes)

5

10

15

20

25

30

35

40

45

50

55

60

65

Retention time (minutes)

Fig. 3. GC traces of saturate fraction of extracts. The large peak eluting near n-C29 at about 38 min is interpreted to be 17b(H)-23,28bisnorlupane on the basis of its chromatographic elution time and mass spectrum (Fig. 5). Additional peaks eluting near n-C19 to n-C20 have been identified as tricyclic diterpanes derived from plant resins.

techniques in order to help identify the organic matter type, thermal maturity and probable depositional environment of the coal-enriched units. Samples were prepared for incident light microscopy by hand-picking and then placing coaly cuttings particles (
5–10 mm diameter) into a 2.5 cm diameter Teflon mold (3 cm deep) and impregnating with epoxy. After hardening, the sample was ground using carborundum grits of decreasing coarseness, and then finally polished using slurries of alumina (0.3 lm and then 0.05 lm) and isopropyl alcohol.

A Zeiss Axioplan II microscope equipped with white and UV light sources, AxioVision camera-image capture system, photometer and grating monochromator was used for imaging and fluorescence microspectrometry of liptinite and hydrocarbon fluid inclusions (hcfi) in the samples. An epiplan-neofluor 40x water immersion objective (NA ¼ 0.95), ultraviolet G 365 nm excitation filter, 395 nm beam splitter and 420 nm barrier filter (HBO 100 W Hg UV-light source) was used. Zeiss Lambda Scan software was used to record, correct and average spectra. A black body curve was used to correct spectra

L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

Detector Response

X09407 Immiugak A-06 3240m

X09407 Immiugak A-06 3240m m/z = 177

X09407 Immiugak A-06 3240m

m/z = 191

X09406 Immiugak A-06 3310m

X09406 Immiugak A-06 3310m

m/z = 217

X09406 Immiugak A-06 3310m

D

A

Detector Response

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m/z = 177

m/z = 191

m/z = 217

C B

Retention time

Retention time

Retention time

Fig. 4. Selected ion monitoring mass chromatograms of saturated hydrocarbon fraction of extracts showing distributions of terpanes (m=z ¼ 177 and 191) and steranes (m=z ¼ 217). The tricyclic diterpane peaks are the early eluting compounds visible in the m=z ¼ 191 trace and the large peak near the centre of the trace is 17b(H), 23,28-bisnorlupane (peak D). Mass spectra of lettered peaks are shown in Fig. 5.

for background (i.e., spectral characteristics of the light source, optical imaging system, photomultiplier and monochromator). A standard tungsten lamp (3100 K) was used as a reference radiator during background correction. The system was standardized with fluorescing Plexiglas standards (kmax ¼ 504, 580 and 604 nm). Photochemical alteration of the ‘‘well-sealed’’ hcfi did not occur during spectral scanning. Two spectral parameters best characterize the fluorescence properties of liptinite macerals and hcfi: (i) Lambda max (kmax ¼ wavelength of maximum emission intensity in nm) and (ii) R=G quotient (Q ¼ intensity650 nm /intensity500 nm ). A Leitz MPV II microscope with white and UV light sources was used for evaluating per cent reflectance in oil (% Ro) of vitrinite and for point count maceral analysis. Glass standards (0.495%, 1.025% and 1.82% Ro) were used for calibrating the microphotometric system.

3. Discussion 3.1. Organic geochemistry Different TOC and Hydrogen Index values were produced by the Rock–Eval II (with TOC module) and Rock Eval 6 pyrolysis systems. The initial Rock–Eval II/ TOC high Hydrogen Indices (>300 mg HC/g TOC) were determined to be incorrect. The high HI values result from the incomplete oxidation of the coals at 600 °C.

This analytical error in the denominator of the HI parameter leads to estimates of HI that are too high. When the Rock–Eval 6 instrument was operated to 650 °C (both for pyrolysis and oxidation stages), the estimated TOC contents were higher than for those of the Rock–Eval II/TOC machine. However, the Rock–Eval 6 instrument provides a continuous trace of the concentrations of CO and CO2 and it was apparent that when the ballistic cool down of the oxidation oven began, the evolution of CO2 , and hence the oxidation, was not complete. A subsequent analysis of a smaller aliquot (
20 mg rather than
100 mg) of sample with the oxidation temperature extended to 800 °C yielded significantly higher TOC contents and commensurately lower HI values (Table 1). These last TOC estimates (up to
24%) are considered to be correct because the CO and CO2 traces had returned to close to baseline before the end of the oxidation step. The Tmax values for Rock–Eval II and 6 are within the same general thermal maturity range, although Tmax values from the Rock–Eval 6 (418–436 °C) are slightly lower than those noted in the initial Rock–Eval II/TOC analysis (421–439 °C). Smaller sample sizes commonly yield higher Tmax values in a Rock–Eval II instrument, but this weight dependence is not observed in Rock– Eval 6 instruments. The observed trend between the two machines is the reverse of that expected for different weights on a Rock–Eval II, and thus the different Tmax ranges in the data from the two machines are not likely due to the differences in sample weight. A more likely

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L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052 100

Detector Response

100 90

(A)

90

(B)

80

aX09406 Im miugak A-06 3310m Scan 137

80

X09406 Immiugak A-06 3310m Scan 178

70

70

60

60 50

50 233

40

30

20

20

10

10 247 262

0 60 100

80

100 120 140 160 180 200 220 240

233

276

0

80

260 280 300 320

(C)

120

140

160 180

200 220

240

260

280

300 320

(D)

90

80

80

70

70

60

60

50

50

40

40

30

30

X09406 Immiugak A-06 3310m Scan 652

20

192

10

369

10

369 341

341

384

0

0 60

100

177

X09406 Immiugak A-06 3310m Scan 641

20

261

100

177

90

Detector Response

40

30

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420

Mass

60

385

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420

Mass

Fig. 5. Background subtracted mass spectra of selected chromatographic peaks marked in Fig. 4. Note the – 29 ion (loss of ethyl side chain) in A and – 43 (loss of isopropyl side chain) in B–D. Also note the intensity of m=z ¼ 177 and m=z ¼ 192 relative to m=z ¼ 191 in C and D, both characteristic of lupanes.

source of error would simply be temperature calibration on one or both of the machines. Both data sets clearly show significant Tmax suppression (Snowdon, 1995) for the coal-rich samples (3240–3350 m) relative to the surrounding rocks (Table 1). The level of thermal maturity may be inferred by interpolating the Tmax values from the trend in the results from the intervals surrounding the organic rich intervals. These data suggest that the maturity at about 3300 m depth in the Immiugak A-06 well are about the equivalent of 0.7% vitrinite reflectance. This level of maturity is slightly higher than that inferred from the actual measured vitrinite reflectance and from the fluorescence and the foraminiferal colouration data discussed below. The carbon-normalized total solvent extract yield ranges from 52.2–123.7 mg extract/g TOC (Table 2). Hydrocarbons comprise about 18–34% of the total extracts (11.8–40.9 mg HC/g TOC). The data for both of these parameters are consistent with marginal thermal maturity. These values fall below the empirical ‘‘potential source’’ limit of Powell (1978), with only the deepest of the samples falling within the category of ‘‘marginal’’

source potential. Note that the TOC contents reported in Tables 1 and 2 for a given depth are highly variable and this reflects the heterogeneity among separate aliquots of the cuttings samples. That is, any given aliquot of material selected from the bulk sample may contain highly variable proportions of coal particles relative to the sandstone, shale and siltstone particles that comprise the majority of the cuttings samples. The pristane/phytane ratios of 3.8–7.7 (Table 2, Fig. 3) are also consistent with marginal thermal maturity for terrestrial organic matter, as is the predominance of odd/even carbon numbered normal alkanes over the C21 –C29 range (Fig. 3). The mass chromatograms (Fig. 4) indicate the presence of significant amounts of tricyclic diterpanes in the saturate fraction. These compounds have the C-ring alkyl group attached at the 13 position and are derived from plant resins. They are distinguished from the tricyclic terpanes (cheilanthanes) which range from C19 to C45 (Peters and Moldowan, 1993, p. 143) with the alkyl group attached to the C-ring at the 14 position. Two C19 and two C20 tricyclic diterpenoid hydrocarbons have

L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

been tentatively identified based upon comparison of mass spectra and other published data (Livsey et al., 1984; Noble et al., 1986). The two C19 tricyclic terpanes are 4b(H), 19-norisopimarane (the largest peak in the mass chromatogram of m=z ¼ 177 for the sample from 3240 m) and fichtelite. Both have the molecular ion at m=z ¼ 262 indicating C19 tricyclic compounds and m=z ¼ 109 from rupture of the B ring. Norisopimarane has a significant m=z ¼ 233 ion from the loss of an ethyl group, whilst fichtelite has m=z ¼ 219 from the loss of an isopropyl group. The two C20 tricyclic diterpenoids are thought to be pimarane (or isopimarane) and abietane. Both have a m=z ¼ 276 molecular ion indicating C20 tricyclic compounds, and a dominant ion at m=z ¼ 163 ion from the rupture of the A ring. Pimarane has a significant ion at m=z ¼ 247 ion from the loss of an ethyl group and abietane has an ion at m=z ¼ 233 from the loss of an isopropyl group. Other diterpenoids are present, but are in insufficient quantities for identification based on mass spectra. While the early eluting diterpane peak is also present in significant amounts in samples from 3300 to 3320 m, the m=z ¼ 177 mass chromatograms of these latter three samples are dominated by a pair of peaks inferred to be 17a(H)- and 17b(H), 23,28-bisnorlupane (Figs. 4 and 5). This compound is also the largest peak in the saturate fraction gas chromatograms (Fig. 3). Identification of the lupane compounds was similarly based on fragmentation patterns (Fig. 5) and comparison with literature results (Rullk€ otter et al., 1982; Brooks, 1986a,b; Curiale, 1991; Nytoft et al., 2002). Key fragments include the ion at m=z ¼ 177 which is significantly higher than that at m=z ¼ 191 and the high abundence of the ion m=z ¼ 192 relative to that of m=z ¼ 191. The occurrence of very large concentrations of 23,28-bisnorlupanes in samples that are stratigraphically older, and structurally deeper, than the Richards Formation is highly significant for the interpretation of the source rock-reservoir systems in the Beaufort–Mackenzie Basin. Previously, Brooks (1986a,b) inferred that the Richards Formation (Eocene) was the most likely source for almost all of the Tertiary oil and gas discoveries in the basin because this rather unusual biomarker compound was present in many of the discoveries but was only observed over a restricted stratigraphic interval within the Richards Formation. Curiale (1991) also used Brooks’s results to infer an Eocene source for the hydrocarbons in the Kugmallit and Richards formations and the presence of 28,30-bisnorhopane and/or oleanane to indicate a Paleocene source for the crude oil in the Reindeer and Moose Channel formations. The results from the Immiugak A-06 well clearly demonstrate that the same bisnorlupane compounds are also present in high concentrations within at least one of the coaly intervals in the lower Taglu Sequence. It would now appear that the presence or absence of bisnorlupanes in

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oils does not provide any constraint on whether the source is the Richards Formation or older Eocene coals. Indeed, the attribution of significant source potential to the Richards Formation has always been enigmatic in so far as the TOC content was lower than most source rocks (typically about 1.5%, rarely more than 2%), and the stratigraphic position of this unit, above the Paleocene and earliest Eocene reservoirs in the southern portion of the Delta area, added constraints to the generation and migration history of this basin. If, however, the coals in the Immiugak A-06 well represent a significant source rock for many of the Tertiary discoveries, then the apparent discontinuity between the size and number of the oil and gas discoveries and the marginal source rock quality of the Richards Formation is no longer an issue. Furthermore, Taglu and Aklak sands act as reservoirs and thus migration should not be as much of a limiting factor in the effectiveness of an Eocene-sourced petroleum system. Because the organic matter content in the coals is very high and the level of thermal maturity for the lower Taglu Sequence would be expected to be somewhat higher than the Richards Formation by virtue of its lower stratigraphic position, the source potential for the coaly section in this unit must be considered as probably being much higher than that of the Richards Formation which has typical TOC contents of about 1.5%. This is especially true for the older Tertiary reservoir units in the Reindeer and Moose Channel formations. The relative organic matter contents and maturity of lower Taglu Sequence and Richards Formation also suggest that the former must now also be considered as a potential source for many of the Tertiary discoveries. The lower Taglu Sequence must also be considered as a potential source for reservoirs in the Kugmallit Formation, above the Richards Sequence, given that vertical migration has been documented in the Beaufort– Mackenzie Basin (Snowdon, 1988). Clearly, additional work will have to be done to determine whether the bisnorlupane-bearing lower Taglu Sequence and Richards Formation can be chemically differentiated. The picture is further complicated by the observations made on the Immiugak A-06 sample from 3240 m. This sample, from only 60 m above the bisnorlupane bearing coals, is also relatively organic rich (>5% TOC) and probably represents a ‘‘coaly’’ interval, that is, one in which the coaly material is dispersed or occurs in thin beds that are not well resolved by the petrophysical logging tools. The cuttings samples analyzed in this study contain discrete coaly particles, but because of the mechanical mixing of rock material during drilling and recovery, any given sample of cuttings represents an average of the rock material over some unknown vertical interval. Regardless of this limitation, it is quite clear that organic-rich, coaly intervals close to, but above, 3300 m in the Immiugak A-06 well do not

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L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

contain significant amounts of the bisnorlupane compounds. Thus it is possible that the character of the biomarker patterns observed in any given petroleum sample may be controlled by precisely which coaly interval was most effective in expelling hydrocarbons. Thus far, the bisnorlupanes have only been observed within the lower Taglu Sequence for a single well and over a restricted stratigraphic interval within that well. Until a significant number of additional samples are systematically investigated, no definitive interpretations or conclusions regarding the real effectiveness of this and other potential Paleocene and early Eocene sources can be made.

Table 4 Petrographic maceral and mineral analysis for coal particles only in cuttings samples from 3240 to 3300 m depth in Gulf et al. 3300 m Depth (%)

5.4 32.8 6.0 22.0

8.0 39.8 4.6 20.6

Inertinite group Inertinite

2.5

3.0

Liptinite group Sporinite Resinite Cutinite Exsudatinite Matrix bituminite

5.5 5.0 2.3 1.5 2.5

5.6 5.3 2.5 1.0 2.0

13.5 1.0

5.6 2.0

Vitrinite group Vitrinite A – high fluorescence Vitrinite A – low fluorescence Vitrinite B – high fluorescence Vitrinite B – low fluorescence

3.2. Organic petrology The four coal-dominated drill cuttings samples from 3240, 3300, 3310 and 3320 m depth in the Immiugak A06 well contain similar maceral and mineral components (Tables 3 and 4). High and low intensity fluorescing telovitrinite is the dominant component in both samples (66–70%) (Fig. 6), whereas liptinites account for about 15% of the samples. The liptinites primarily consist of sporinite, resinite and cutinite macerals (Fig. 6); however minor quantities of exudatinite (i.e., solid bitumen) are also present, mainly occurring within microfractures in vitrinite (Fig. 6). One significant difference among the samples is that the 3310 m sample contains shale particles that are enriched in lipto-detrinite and Botryococcus-like, degraded alginite, and this could account for higher HI. Inertinite macerals are present in only very minor quantities ( 6 3%). Mineral matter is relatively abundant in the coal and consists almost exclusively of pyrite (Fig. 6). Habits and forms of pyrite are variable include cryptocrystalline, framboidal, massive (including infilling microfractures in vitrinite), euhedral and replacive of organic matter. One and two phase hydrocarbon fluid inclusions which occur in cements in finegrained sandstone cuttings (also enriched in terrestrial organic matter) were noted in association with the coal cuttings (Fig. 6). The crude oils in these inclusions ex-

3240 m Depth (%)

Mineral Pyrite Fe-oxides

Immiugak A-06. Percentages are based on 200 and 300 point counts, respectively.

hibit mainly a yellow fluorescence and have kmax and Q values (Table 3) generally correlative with an API gravity of
30–35 (Stasiuk and Snowdon, 1997). The maceral assemblage (i.e., high in liptinite; highly gelified fluorescing vitrinite, very low in inertinite) in the coals is characteristic of peat deposition under the influence of a high water table, and in some cases also marine-influenced coastal deposition (Taylor et al., 1998). The abundance of early organic replacive, biogenic framboids, and late euhedral to massive diagenetic sulfides in the coals is also consistent with a marineinfluenced peat palaeodepositional environment, or, at least the effects of dissolved sulfate (i.e., sea water/fluids) from a marine transgression over the peat deposit shortly after its formation.

Table 3 Optical analysis for cuttings from Immiugak A-06 Depth (m)

Vitrinite % Ro

kmax (nm) sporinite

Q – sporinite

kmax (nm) crude oil

Q – crude oil

3300.0 3300.0 3310.0 3310.0 3320.0 3320.0 3240.0 3240.0

0.53 0.61 0.51 0.59 0.52 0.59 0.53 0.61

590

1.1

540

0.65

585

0.86

–

590

0.95

–

590 –

1.0 –

510 –

(pop (pop (pop (pop (pop (pop (pop (pop

1) 2) 1) 2) 1) 2) 1) 2)

0.47

Vitrinite % Ro values were measured on two populations (see text). For explanation of fluorescence data (kmax and Q) for sporinite and crude oil inclusions see methodology and discussion in text.

L.R. Snowdon et al. / Organic Geochemistry 35 (2004) 1039–1052

1049

Fig. 6. Digital images of maceral (organic) and mineral components in coals from the Immiugak A-06 well at 3240 and 3300 m depth. Image (a) was captured using a water immersion objective, plane polarized white incident light (ppl). Images (b)–(h) were taken under water immersion, fluorescent light. (a) Highly gelified, sulphide-rich collovitrinite; ppl. (b) Fluorescing vitrinite showing exceptional preservation of wood cellular structure possibly related to secondary cell wall thickening phenomenon. (c) Abundant resinite and exsudatinite associated with telovitrinite and cutinite. (d) Massive resinite. (e) Sporinite-enriched detrovitrinite (old term desmocollinite). (f) and (g) Cutinite-enriched vitrinite associated with fluorinite variety of resinite and fracture-filling exudatinite. (h) Fluorescing crude oil inclusions in microfractures in sandstones associated with coal cuttings.

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The telovitrinite in the coals from the Immiugak A06 well is characterized by two distinct populations based on reflectance (% Ro) and relative fluorescence intensity during UV exposure (qualitative observation). The first population has a mean of 0.51–0.53% Ro (Table 3) and relatively high fluorescence intensity. The second has a higher mean % Ro of 0.59–0.61 (Table 3) and distinctly lower fluorescence intensity. Fluorescence microspectrometry of sporinite (including pollen) macerals within the coals yields maximum emission wavelengths and Q ratios of 585–590 nm and 0.86–1.1 nm, respectively, are generally consistent with telovitrinite % Ro values on the order of 0.60–0.65 (Ottenjann et al., 1975; van Gijzel, 1981). Defining a thermal maturity/ coalification level based on vitrinite reflectance alone for coals characterized by highly variable telovitrinite reflectance with high vitrinite fluorescence, high resinite/ liptinite content and high sulfur contents is often misleading/problematic because the telovitrinite reflectance is commonly suppressed (e.g., Hutton and Cook, 1980; Wilkins et al., 1992), i.e., a lower level of thermal maturity is indicated. However, the level of thermal maturity inferred from both the measured vitrinite reflectance and fluorescence properties is consistent with that inferred by the foraminiferal colouration index (FCI, McNeil et al., 1996) at about 0.6–0.65% vitrinite reflectance. The overall geochemical and petrological observations for the studied intervals of the Immiugak A-06 well suggest that they should have some potential for generating and expelling liquid hydrocarbons despite being coaly and hence primarily a source for gas. The liptinite content of these coals is quite high (about 15%) and the vitrinite is generally fluorescent, indicating some liquid generation potential. Similarly, the Rock–Eval Hydrogen Index parameter exceeds 200 mg HC/g TOC for two out of seven samples within the coal seams at about 3300 m depth. These observations then lead to the prediction that the potential source rocks studied would be expected to produce mainly gas by virtue of their high content of terrestrial organic matter dominated by vitrinite, but they would also be expected to generate, and probably to expel, some amount of liquid hydrocarbons as oil. If the gas to oil ratio were high enough for the gas to accommodate all of the liquids, then, by definition these liquids would be present in the form of condensate. If the reactive portion of the liptinites at low levels of thermal stress were resins, the expected liquid product would tend to be dominated by two and three ring saturated and aromatic compounds. The saturate fraction GC traces for the 3300–3320 m samples show a very small hump of unresolved compounds that corresponds to sesquiterpanoids (C15 compounds), but little or no evidence for a similar hump in the diterpenoids (C20 ) region (Fig. 3). Several discrete tricyclic diterpanes are present in relatively high abundance, providing evidence

that the resin macerals have been at least somewhat effective in generating liquid hydrocarbon compounds. 4. Conclusions Early Eocene coal-rich samples from 3300 to 3320 m in the Immiugak A-06 well contain large amounts of the biological markers 17a(H)- and 17b(H), 23,28-bisnorlupane. These compounds were previously known only in the lower part of the Eocene Richards Formation in this basin and as such were considered to be a strong indicator correlating many of the Tertiary oils and condensates to a Richards Formation source rock. The discovery of large concentrations of bisnorlupanes within at least some of the coals in the lower Taglu Sequence suggests that this older unit may well be a contributing, or even dominant, source of much of the Tertiary hydrocarbons in the Beaufort–Mackenzie Basin. From an analytical standpoint, it may also be concluded that great care must be taken when interpreting high Hydrogen Index values in rocks rich in coal because of the potential for incomplete oxidation of these samples in a Rock–Eval II/TOC instrument. Incomplete oxidation in a Rock–Eval 6 instrument is easily detected by the failure of the CO and CO2 traces to return to baseline before the start of the ballistic cooling of the oxidation oven. A combination of the high liptinite content observed during petrological point counting and Rock–Eval Hydrogen Index values in excess of 200 mg HC/g TOC indicate that the coal-rich interval studied in the Immiugak A-06 well would be expected to be dominantly a gas source but with the potential for the generation and expulsion of some liquid hydrocarbons. Acknowledgements Superb technical assistance was provided by Sneh Achal, Marina Milovic, and Kim Dunn. Funding for this research was provided by Anadarko Petroleum, BPAmoco, Burlington Energy, Chevron-Texaco, ConocoPhillips, Devon Energy, EnCana, PetroCanada, and Shell Canada. Critical reviews by Volker Dieckmann and Nigel Mills provided helpful comments to improve the manuscript and are gratefully acknowledged. This is Geological Survey of Canada contribution 2003-181. Associate Editor – Rolando di Primio

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