EPR study of kerogens from three Alaskan North Slope wells

~ Pergamon 0146-6380(94)00122-7 Org. Geochem. Vol. 23, No. 2, pp. 97-108, 1995

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0146-6380/95 $9.50 + 0.00

EPR study of kerogens from three Alaskan North Slope wells

TRUDY A. DICKNEIDER, *l JEAN K. WHELAN: and NEIL V. BLOUGH "~ ~Department of Chemistry, University of Scranton, Scranton, PA 18510 and -'Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.

(Received I0 August 1993: returned for revision 29 November 1993: accepted 3 Norember 1994)

Abstract--Whole rock and isolated kerogens from the Seabee, Inigok, and Ikpikpuk wells of the Alaskan North Slope have been analyzed by electron paramagnetic resonance (EPR) spectroscopy. The sample suite includes representatives of the Torok, Kingak, Lisburne, Sadlerochit and Endicott formations. Power saturation studies show a definite correlation with the downhole maturation, the extent of aromaticity of the sample as defined by ~3C NMR, and with the beginning of the oil window for the Kingak formation. Strong correlations within formations across the depositional basin are seen. Abrupt changes in EPR parameters indicate a change in the nature of the organic matter present and may reflect a facies change. Peak widths and spin densities correlate with maturity as indicated by vitrinite reflectance and sample aromaticity as measured in solid state ~3C NMR spectra.

Key word~---electron paramagnetic resonance spectroscopy, kerogen, Alaskan wells, maturation, vitrinite reflectance, spin densities, power saturation behavior

INTRODUCTION

This study was undertaken to determine if EPR could be used to reveal useful information about maturational trends within a basin, and to contribute information about the content and nature of the free radicals present in the kerogens from the Alaskan North Slope.

Previous applications of EPR to the analysis of sedimentary organic matter have included studies of humic acids (Steelink, 1966), coal, isolated coal macerals, and kerogens. Studies of coal have concen- trated on correlations of EPR parameters with coal rank. It has been shown that linewidths and the spin concentrations vary in a regular fashion with increasing maturation as indicated by rank (Retcofsky et al., 1968; Petrakis and Grandy, 1978; Kwan and Yen, 1979). This correlation has been ascribed to the existence of highly extended aromatic systems in the higher rank coals, with these aromatic systems able to support larger numbers of delocalized free radicals. Recent studies utilizing high field and multifrequency EPR techniques have confirmed this interpretation (Bresgunov et al., 1990a, 1990b). EPR analysis of Argonne Premium Coals has demon- strated that organic radicals can interact with paramagnetic inorganic species and that samples for EPR analysis should be mineral free (Silbernagel et al., 1991). EPR studies of isolated coal macerals have shown that each type has its own characteristic signal pattern (Silbernagel et al., 1984; Morishima and Matsubayashi, 1978; Austen et al., 1966).

*To whom correspondence should be addressed.

Kerogens, due to their rigid matrix and appreciable aromaticity, contain free radicals significant in both their number and information content. Three sources of free radicals have been proposed (Austen et al., 1966) which probably contribute to the number and types of radicals observed.

Diagenesis

Diagenetic reactions would be expected to produce free radicals, especially enzyme mediated reactions with and without aerial oxidation.

Pyrolysis

Reactions occurring during catagenesis, possible extending into metamorphism (including all the oil and gas thermal windows) involving the homolytic cleavage of methyl, methoxy, hydroxyl, and carbonyl bonds to produce water and carbon dioxide would generate free radicals.

Radiolysis

The energy released from the radioactive decay of elements within the mineral matrix, such as uranium, thorium, and potassium-40, would produce free radicals directly from organic compounds in the sediments (Lewan et aL, 1991). An alternate view is that the decay reactions provide energy which would serve as the needed energy of activation for the metamorphic pyrolysis reactions.

There are two main mechanisms for the stabiliz- ation of free radicals in solid samples. It is likely that both contribute to the preservation of radicals in any kerogen sample.

97

98 Trudy A. Dickneider et al.

Trapping. The free radicals in the kerogen would be trapped in the solid matrix of the sample, preventing their movement and holding them at a distance too large to prevent reaction between them (Silbernagel et al., 1991; Austen et al., 1966).

Delocalization. The free radical may be stabilized by being delocalized over an extended conjugated system. Delocalization is used to interpret differences in EPR characteristics such as spin density and peak width. It is assumed that as an aromatic system becomes more extensive it would support a large number of radicals, resulting in an increase in spin density (Silbernagel et al., 1991; Hessley et al., 1986; Aizenshat et al., 1986). The delocalization could also affect the peak width through exchange narrowing of the signal (Retcofsky et al., 1968).

Typical EPR spectra for whole rock and extracted kerogen are shown in Fig. 1. The signal for the whole rock sample is usually broadened by the presence of transition metals in the mineral matrix of the shale. Therefore, a signal measured from extracted kerogen can be more reliably interpreted. An EPR spectrum provides information about both the number of radicals and their environment via the following parameters.

Radical density

The number of spins measured, commonly known as spin density or spin concentration, is calculated from the integrated intensity of the EPR signal. The usual range is from 10 j5 to 10 TM spins per gram up to 1020 for very aromatic coal macerals, such as inertinite (Silbernagel et al., 1984). A spin density of 102~ spins per gram should constitute a theoretical maximum, since a concentration of radicals above that value would result in radicals close enough in the sample to react with each other (Aizenshat et al., 1986).

Environment o f the radical

L&e width (Hpp). The width of the signal, measured from peak to peak of the recorded signal, shows variations due to spin-spin interactions between the electron of the free radical and protons (hydrogens) on carbon. This, then, becomes a sensi- tive indicator of changes in the nature of the environment of the radical, particularly changes in the aromaticity of the sample which would be seen as exchange narrowing of the peak width (Retcofsky et al., 1968).

Early EPR studies of kerogen have been summar- ized by Marchand and Conard (1980). Attempts have been made to correlate measured EPR parameters with the thermal history of the sample (Suzuki and Taguchi, 1981) and with pyrolysis-induced maturation (Aizenshat et al., 1986). The use of EPR to assess kerogen maturation is based on the assumption that during catagenesis the bond dissociation is homolytic, producing a greater number of stable free

Typical EPR Spectra

Whole Rock

/

Kerogen

# Fig. 1. Typical EPR spectra for whole rock and extracted

kerogen.

radicals with increasing maturation. This assumption has been supported by data showing correlation of EPR parameters and sample burial depth (Bakr et al., 1988; Bakr et al., 1990). The major focus of the current study was to determine which, if any, EPR signals were indicative of either changes in maturation or the nature of the various geologic formations.

The research described below suggests some con- straints on the origin of the free radicals, the chemical nature of the kerogen in which the radicals occur, and, most importantly, their changes during maturation.

M E T H O D S

Drill cuttings from the Seabee, Inigok, and Ikpikpuk wells were provided by the United States Geological Survey. The depth and geologic formation of the samples from each well are shown in Table 1. The cuttings were extracted with organic solvents and the kerogens were then isolated by HCI/HF demineralization (DGSI, Woodlands, TX). Whole rock samples were ground to a fine mesh. Samples were weighed into I mm quartz EPR tubes and packed to a known and uniform volume.

E P R analysis

EPR measurements were obtained on a Bruker ER-200D-SRC spectrometer. The instrument operated at a nominal frequency of 9.78 GHz and a 100 kHz modulation. All measurements were made at room temperature. Peak widths (Hpp) were measured peak to peak on the first derivative spectra recorded at a sweep width of 20 Gauss (G), 10 milliwatt (mW) microwave power and a modulation amplitude of 500milliGauss (mG). A wide scan of 5500 G was recorded for each sample at 10 mW power and a modulation amplitude of 20 G to detect the presence

E P R s t u d y o f k e r o g e n s

Table I. EPR and N M R data of Alaskan North Slope samples

99

Depth TC TOC Spin density Peak width I/P f 'a Well/formation fit) (m) (wt%) (wt%) %R o (spins/g) (Hpp Gauss) E(I/2) (NMR)

Seabee/Torok 6900 (2104) 2.5 1.56 0.79 4.46E + 17 7.4 Seabee/Torok 11,280 (3439) 2.6 1.67 1.3 5.09E + 16 9.5 Seabee/Kingak 13,830 (4216) 3 2.52 2.1 2.35E + 17 9 Seabee/Kingak 14,910 (4546) 4.22 3.15 2.4 4.72E + 17 7.8 lnigok/Torok 7030 (2143) 1.69 1.09 0.65 7.49E + 16 7.4 Inigok/Kingak 9340 (2848) 2.05 1.39 1.04 3.80E + 16 10.4 lnigok/Sadlerochit 12,580 (3835) 1.8 0.91 2.56 9.38E + 16 8.8 Inigok/Endicott 18,370 (5601) 2.62 1.27 4.22 4.73E + 16 10.2 Ikpikpuk/Torok 4490 (1369) 2.3 0.89 0.55 2.20E + 17 8.4 56,200 Ikpikpuk/Torok 6740 (2055) 0.6 0.72 lnigok/Torok 7030 (2143) 0.67 0.72 lkpikpuk/Torok 7070 (2155) 2.05 1.01 0.6 2.18E + 17 8.6 53,700 Ikpikpuk/Kingak 7460 (2274) I. 15 0.69 0.57 1.97E + 17 I 1.6 38,900 lkpikpuk/Kingak 7500 (2287) nd* 2.47 0.56 1.82E + 17 13 37,300 lkpikpuk/Kingak 7700 (2348) 2.1 1.46 0.62 1.69E + 17 12.2 56,200 lkpikpuk/Kingak 8000 (2439) 1.79 1.18 0.67 1.37E + 17 12.6 53,700 lkpikpuk/Kingak 8300 (2530) 1.82 1.03 0.64 1.24E + 17 12 41,600 Ikpikpuk/Kingak 8600 (2622) 7.2 6.41 0.7 1.01E + 17 12.2 1.01E + 05 Ikpikpuk/Kingak 8900 (2713) 3.9 3.11 0.7 1.31E + 17 11.9 1.13E + 05 Ikpikpuk/Kingak 9200 (2805) 0.74 Ikpikpuk/Kingak 9500 (2896) 2.52 1.98 0.78 1.42E + 17 11.3 1.01E + 05 Ikpikpuk/Shublik 9800 (2988) 1.65 1.18 0.75 1.54E + 17 10.2 95,700 0.73 Ikpikpuk/Shublik 9950 (3034) 0.74 0.72 lkpikpuk/Shublik 10,010 (3052) 1.16 > 0.63 lkpikpuk/Lisburne 11,660 (3555) 3.3 0.38 1.52 2.02E + 17 8.8 1.03E + 05 > 0.9 Ikpikpuk/Lisburne 11,730 (3576) 11 0.18 1.67 1.83E + 17 9.4 1.14E + 05 >0.9 Ikpikpuk/Lisburne 12,450 (3796) 7 0.22 2.1 2.58E + 17 9 76,300 >0.9 lkpikpuk/Lisburne 13,050 (3979) 5.6 0.26 2.12 2.32E + 17 9 1.47E + 05 >0 .9 lkpikpuk/Lisburne 13,050 (3979) 74,900 >0 .9

*nd, means not determined.

of transition metal signals to ensure that measured peaks were not being affected by undetected signals. Spin densities, Ns, were calculated from spectra recorded at a sweep width of 100G, microwave power of 10 mW and 50 mG modulation amplitude by comparison to a Bruker weak pitch standard (Bruker No. 8511238) with a known spin concentration of 1 x 1013 spins per gram. For power saturation studies of each sample the spectrometer was operated at a sweep width of 100 G, a modulation amplitude of 500 mG, and spectra were obtained at microwave powers ranging from 0.1 to 200 mW.

The intensity of the recorded spectra, I, was calculated by double integration of the recorded spectra of the first derivative after baseline subtraction using EPR DAS software. The integrated spectra were reproducible within 3%. Peak width measurements were reproducible within 1%.

N M R analysis

Solid-state NMR measurements were made using a Chemagnetics CMX 100/200 solids NMR spectrometer. These measurements were carried out on isolated kerogens in most cases.

An attempt was also made to carry out measurements on whole rocks with a large diameter probe. However, because of the organic-lean nature of the rocks, no signal was obtained above background.

For the kerogen measurements, carbon aromaticity measurements were made at a 13C frequency of 25 MHz using the technique of cross polarization (CP) with magic-angle spinning (MAS) and high power decoupling. A 9.5mm dia zirconia bullet

spinner was used. Spinning rates were between 3.5 and 4kHz. Other instrumental parameters were a 90R pulse width of 5.1 ms, a contact time of 1 ms, a pulse delay of I s. A 50 Hz exponential multiplier was applied to the free induction decay of each ~3C spectrum before integration. These parameters are typical for CP/MAS measurements on these types of materials (Miknis et al., 1982; Wilson et al., 1991). The number of transients varied between 43,200 and 64,800, because of the low levels of TOC in many of the "kerogen" concentrates.

Concerns about the accuracy of the carbon aromaticity measurements have been discussed (Snape et al., 1989), including the use of a single contact time for determining carbon aromaticities. Aliphatic and aromatic carbons are known to cross polarize at different rates and therefore a single contact time may not give representative aromaticities. However, Wilson et al. (1991) compared aromaticities for coaly source rocks from the Brent group (North Sea), obtained at 1 ms contact time, with aromaticities obtained by varying the contact time between 10#s and 8ms, and calculating the optimum signal intensities by curve fitting. In all but one case, the aromaticities agreed within experimen- tal error. They concluded that CP/MAS ~3C NMR aromaticity made at a 1 ms contact time could be used with confidence. Thus, a 1 ms contact time was utilized in this work.

All spectra were manually phased and integrated using the instrument's dedicated computer software. Because of the low levels of carbon in the kerogens and the large number of transients required to


obtain a spectrum, some broad signals from the NMR probe were also recorded. Consequently, a probe background spectrum was subtracted from each kerogen spectrum prior to integration.

The carbon aromaticity measurements were integrated over the range from 360 to -160 ppm. This large range was required to include any contributions from spinning side bands in the aromaticity measurements. The region between 340 and 90 ppm was considered in the aromatic region, and included unresolved contributions from any carbonyl carbons in the region 165 210 which might be present. The region between 90 and - 2 0 ppm was considered the aliphatic carbon region. The integrated intensity between - 2 0 and - 8 0 p p m was added to the aromatic carbon integral.

SAMPLES

Samples from the Alaskan North Slope Wells in the National Petroleum Reserve in Alaska (Fig. 2) were selected for study because of the vast amount of data compiled by the U,S. Geological Survey, as described in Bird (1981); Magoon and Claypool (1983, 1984); Whelan et al., 1986; and Farrington et al., 1988. These studies made it possible for us to correlate the EPR data with other organic geo- chemical maturation indicators covering several different wells (the Ikpikpuk, Inigok, and Seabee) representing different ranges of maturity, and different formations across the basin.

The location of the North Slope wells is shown on the map of the National Petroleum Reserve in Alaska, Fig. 2. The Ikpikpuk, Inigok, and Seabee Wells were selected to represent different ranges of maturity across the basin. The north-south cross-

section of the basin, Fig. 3, shows the formations contributing samples to this study: Torok, Kingak, Lisburne, Endicott, and Sadlerochit. The basin matured from north to south (Bird, 1981), so for any particular formation and specific depth, the Ikpikpuk Well represents the least mature, while the Seabee well represents the highest level of maturity. The majority of the work reported here focused on the Ikpikpuk and Seabee Wells, with the Ikpikpuk Well representing clear maturational trends and the Seabee Well migrated hydrocarbons (Whelan et al., 1986, 1988). The Ikpikpuk samples selected for this study represent a lower maturity section (Torok Formation and upper Kingak, Ro < 0.6%), a section within the oil window (Kingak Formation, Ro = 0.6-1.0), and a section in which oil generation is complete (Lisburne Formation, Ro > 1.5%).

RESULTS

The EPR analysis performed in this study consisted of:

1. A determination of the Power Saturation behavior of the samples.

2. An analysis by formation of EPR parameters as a function of sample depth.

3. The dependence of EPR behavior on the extent of maturity (i.e. aromaticity) of the samples.

4. A detailed analysis of the Kingak Formation in three of the North Slope wells.

Power saturation studies

At low powers the intensity of the signal (I) increases linearly with the square root of the power (W '2) (Bolton, 1972), with the ratio of I /W :2 being

Fig. 2. Location of wells studied in the National Petroleum Reserve, Alaskan North Slope (Magoon and Claypool, 1984).

EPR study of kerogens 101

Q Q ~ ID. JD 0 ..~ a) e- ,,1¢ ¢/}

I I BROOKS RANGE FOOTHILLS COASTAL PLAIN SHELF SLOPE

I I I I NORTH SOUTH Shale of the Colville Group Colville River Wolf Cretaceous I I / Sandstone of the Sagavanirktok o I JF°rtres~ ' ~ \ ~ Creek ~

Formation\,. - . . . ~ ::.:

lo ,ooo " _ . 4 "'

gr°upand ] ~ ' / ~ - / ' r ' ~ " : / ' " 2 " O ' F o r m a t i o n TROUGH ARCH ' ~ ~ ~ : ~0 Otuk I , ~ 'COLVlL'LE L - - - ' - - " 1 5 , 0 0 0 - L . ~ /'-? -~ -~'~/

J / / / /_~-~. Pebble shale unit / ~ , ~ , Y ~ , ~

20,000--25,000 -- Shublik~aga n d g a m m a r aY z° n e ' ~ / - ~'/t" ~°~-~'~ ~ i l K ! A U K H I v e r Fo r m a t i o n s ~ , ~ j a n a / / ~ ~ ~'~'~~'~-"~'~ ~O - EXPL TION ~ , ~ t i

30,000 - PLATFORM Vitrinite reflectance percentage

O Gas field

Fig. 3. North-south cross-section of basin studied showing rock units and approximate location of wells (Bird, 1981).

independent of power. At sufficiently high microwave powers, spin-lattice relaxation is not fast enough to dissipate the absorbed microwave energy resulting in signal saturation. As a result, the ratio of I/P m, normalized to the value observed at the lowest power used (100mW) vs log P decreases at the onset of saturation and is seen as a "rolloff" in the plots of I/P m as a function of log P (Figs 4 and 5).

Normalized saturation behavior is plotted separ- ately for specific formations from all wells, including the shallowest Torok [Fig. 4(a)], samples from the top and bottom of the intermediate depth Kingak formation [Fig. 4(b)], and from the deepest high maturity carbonate rich Lisburne formation [Fig. 4(c)]. Samples from all depths of the Ikpikpuk Kingak formation are shown in Fig. 5(b). For the Kingak formation, which is believed to be one of the major source rocks for the North Slope (Bird, 1981), a progression in the normalized power saturation curves is evident with the more mature samples lying higher than the lower maturity samples [e.g. IKP 9800 ft compared to IKP 7500 ft in Fig. 5(b)]. However, in both the generally lower maturity and organic-lean gas-prone Torok formation and the relatively high maturity carbonate-rich Lisburne formation, the rolloff curves tend to group more

together [Figs 4(a) and 4(c)] with the same flattening of the curves occurring for the higher maturity sample. This suggests that a comparison of power saturation behavior for specific formations might provide a relatively easily obtainable measure of maturity. Similar plots of samples from the top and bottom of all formations [Fig. 5(c)], the top and bottom of each formation for the Ikpikpuk well [Fig. 5(a)] vs just the Kingak samples from the Ikpikpuk well [Fig. 5(b)] suggests that a progressive trend in power saturation as related to maturity is best observed for a specific formation.

In order to minimize effects of changing lithology on the EPR parameters and to try to maximize any maturation differences [I/P I/2] at a power where saturation just starts to occur (50 mW) is plotted directly vs vitrinite reflectance in Fig. 6(a). A distinct break in the curve occurs just at the start of the oil window (Ro = 0.68%). If this trend proves to be general, power saturation behavior could be a useful parameter for detecting the oil window. (In order to simplify terminology through the remainder of this paper, [I/P 1/2] at 50mW has been abbreviated as "PS(50)", which stands for "power saturation at 50 mW". This value is used in all plots, even though it would be more traditional to plot the signal


intensity at half-saturation power. This is not possible for these samples since the more thermally mature kerogens do not saturate.)

Other EPR parameters normally plotted against vitrinite reflectance are also shown for the Ikpikpuk well Kingak formation in Fig. 6, including spin density [Fig. 6(b)] and peak width [Fig. 6(c)]. The results are discussed in the next section.

To test the generality of the "PS(50)" vs Ro behavior shown in Fig. 6(a), the same parameter was plotted

against vitrinite reflectance for other wells and formations as shown in Fig. 7. Figure 7(a) shows "PScs0~" vs Ro for all formations and all wells, while Figs 7(b)-(d) show similar data for all maturities and all wells for the individual formations. At first glance, the results appear to be disappointing, with considerably more scatter being evident in Figs 7(a)-(d) in comparison to the nice trend observed in Fig. 6(a). However, closer examination of all formations suggests that "PSi50)" first increases

Torok Formation 1.4 I I I I

~ , 1.2

1,0

o 0.8

o.6

0.4

0.2

i I I a

0

+ + x o <> +

I .5 2.0

I I I I 2.5 3.0 3.5 4.0

Log P

I I 4.5 5.0

• Ikp 4490 ft. (Ro=0.55%) 0 Ikp 7070 ft. (0.60) o Inigok 7030ft. (0.65) x SB 6900 ft. (0.74) + SB 11280 ft. (1.30)

5.5

1.4

~, 1.2 I 1.0

~ 0 . 8 -

0.6 "

0.4 ~-

0.2 1.5

Kingak Formation

I I I

+

I I I 2.0 2.5 3,0

I I

o

I I 3.5 4.O

Log P

t b

+

®~÷ ~ ® © x ® ~

© x • ©

I I 4.5 5.0 5.5

• Ikp 7500 ft. (0.56) 0 Ikp 8900 ft. (0.70) ® Ikp 9800 ft. (0.75) × Ing. 9340 ft. (1,00) q--SB 13830 ft. (2.00) zx SB 14910 ft. (2140)

&

Lisburne Formation

1 . 4 i i i i

1.2

1.0 • • [ ]

0.8

0.6

0,4

0.2 I .5 2.0

I I I

<> [ ]

I I I I I 2.5 3.0 3.5 4.0 4.5

Log P

• 11660 ft. (Ro=1,52%) [ ] 11730 ft. (1.67) 0 12450 ft. (2.10) × 13050 ft. (2.12)

I c

I 5.0 5.5

Fig. 4. Normalized power saturation behavior of kerogen samples from all three Alaskan North Slope wells from: (a) shallowest Torok formation; (b) mid-depth Kingak formation; and (c) deepest Lisburne formation. Vitrinite reflectance values for each sample are shown in parentheses. (See text for definition

of "normalized power saturation".)


to some maximum value at an Ro of about 0.8-1%, followed by a gradual decrease at maturi ty values above R o = 1%. Therefore, based on the limited data set obtained to date, the break in "PS(50)" vs R o plot at the beginning of the oil

window may also be occurring for the Torok formation, but at somewhat higher "PS(50)" values than for the Kingak formation (Fig. 7). At higher maturities ( > 1.5%), a decrease in PS occurs in all formations (Fig. 7). This is particularly evident

1.4

1.2

1.0

o~ 0.8

~ 0.6

0.4

0.2 1.5

Ikpikpuk well, all formations

I I I I I I I la

•

+

~ o ~ " _ • ~-

I I I I I I I 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Log P

• T, 4490 ft. (0.55) 0 T, 7070 ft. (0.60) • K, 7460 ft. (0.57) [ ] K, 9800 ft. (0.75) + L, 11660 ft. (1.52)

K, 13050 ft. (2.12)

1.4

~ . 1.2

~- 1.0

0.8

0.6

0.4

0.2 I 5.5 1.5 2.0

Ikpikpuk Kingak formation

I I I I

• i []

I I I b

I I I I 2.5 3.0 3.5 4.0

Log P

• 7460 ft. (0.57) [ ] 7500 ft. (0.56) o 7700 ft. (0.62) x 8000 ft. (0.67) -I-8300 ft. (0.64) n 8600 ft. (030)

8900 ft. (0.70) 9500-ft. (0.78)

• 9800 ft. (0.75)

ot [ ]

[ ]

I I 4.5 5.0 5.5

All formations, all wells

1 .0 • ~ I " ; • " . .

0.8 A ~ •

0.4

0.2 I I I I I I I 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Log P

Olkp LIs 11660 (1.52 ®lkp LIs 13050 (2.12 x Ikp Tor 4490 (0.55) -t-lkp Tor 7070 (0.60) zx Sea Tor 6900 (0.74) • Ikp King 7460 (0.57) • Ikp 9800 ft. (0.76) • SB 14910 ft. (2.4)

5.5

Fig. 5. Normalized power saturation behavior of kerogen samples from: (a) lkpikpuk well, all formations; (b) lkpikpuk well, Kingak formation; and (c) all formations, all wells. Vitrinite reflectance values for each

sample are shown in parentheses (See text for definition of "normalized power saturation".)


for the samples from the high maturity Lisburne and Sadlerochit kerogens shown in Fig. 7(d).

Other E P R parameters

Other EPR parameters, including peak width (AHpp) and spin density (Ns), were also measured. As found by others who worked with heterogeneous samples sets (e.g., Marchand and Conard, 1980), no particular trends were observed when these parameters were plotted vs vitrinite reflectance, either according to formation or maturity (Table 1). The possible exception is shown in Fig. 6(b), where a reversal in spin density may be occurring at the beginning of the oil window for the Ikpikpuk well Kingak formation.

t3 C Nuclear magnetic resonance data

Solid state ~3C nuclear magnetic resonance spectra of kerogens from the Ikpikpuk well are shown in

Fig. 8. The upfield signal above 90 ppm is considered to be the aliphatic carbon region and the downfield region below 90 ppm the aromatic carbon region (see Methods section). With increasing depth and maturity, the upfield aliphatic signal disappears while the aromatic signal remains the same or increases slightly (Peters et al., 1978; Miknis et al., 1982; Dennis et al., 1982; for a recent literature review, see Patience et al., 1992). Within the Kingak formation, the ratio of aromatic to aliphatic plus aromatic carbon atoms (f,) stays relatively constant. An increase in aliphatic to aromatic carbon atoms then occurs in the 10010 and 11210 samples, which are believed to have been conduits for expulsion and migration out of the bottom of the Kingak and the top of deeper formations (Whelan et al., 1986; Whelan et al., 1987). In deeper, more mature, formations below 11,660 ft, only aromatic carbon atoms are present (fd > 0.9). This progressive loss of

Ikplkpuk, Kingak Formation 0.55

0.60

I~ 0.65

0.70

0.75

a l @~7500 ' ' ' 7460 a

• 8300

0 8 0 0 0 890(

860O ° O -

0.60

o 0.65

0.70

0.75 0 9 8 0 0

0.8C I J t 95°°01 0.80

0 0 0 0 0 0 0 0 (~ 0 0 0 o c~ ~ ,,6 o6 .,-: ~ c:,

PS5o = I/(P)e 1/2

Ik )ikpuk, Kingak Formation 0 . 5 5 i i i i

b) • a b

I L I I u..- r.,. P.. r,-- ..,,.

Spin Density (spins~g)

Ikpikpuk, Kingak Formation 0.55

0.60

~: 0.65!

0.70

0.75

C

g o

0.80 I o q ~.

i i i i i q ~. q

0 ~ ~ 04 OJ

Peak Width

Fig. 6. EPR data for kerogens from the Kingak formation of the Ikpikpuk well: (a) Vitrinite reflectance (%Ro) vs power saturation, I/P u:, measured at 50 mW, or "'PSI50~" (see text); (b) %R o vs spin density;

and (c) %Ro vs peak width.


2.50105

2.00105

All Wells & Formations I I I I

O a -

1.501 0,5

1.00105

5.00104 0

0 0 . 5 1 . 0 1 . 5

I I I I 2.0 2.5 3.0 3.5 4.0 4.5

% Ro

2.50105

2.00105

1.50105

1.00105

5.00104

0 0.5

Kingak, All Wells I

1.0 1.5 % Ro

I

¢

0 O-

I 2.0 2.5

Torok, All Wells 2.50105

2.0010 ̀5

1 .50105

1.00105

00.44 0.8 0.8 1.0 1.2 1.4 5.00 1

% Ro % Ro

Fig. 7. Power saturation, or "PSIs0)" (see text) vs vitrinite reflectance for: (a) all samples, all formations; (b) all wells, Torok formation; (c) all wells, Kingak formation; and (d) all wells, Lisburne and Sadlerochit

formations.

Lisburne & Sadlerochit, All Wells 2.50105 I t t t I /

0 2.00105 [-- d

/ 1.5olo51 - o

1 . 0 0 1 0 5 0

5.001041-- 0 ~ I

o l I I I I I 1.5 2.0 2.5 3.0 3.5 4.0 4.5

aliphatic carbon accompanied by increasing aromatic character of kerogen with increasing maturity (Fig. 8) is typical and diagnostic of increasing kerogen maturation (e.g. Peters et al., 1978; Miknis et al., 1982). We assume that the changes in kerogen EPR power saturation behavior discussed above are also related to the changes in the aromatic structural features of the kerogen, which vary both with maturation and the chemical nature of the kerogen (for a recent review, see Whelan and Thompson-Rizer, 1993).

DISCUSSION

Potential usefulness o f power saturation as an indicator o f the oil window

The break in the PS vs o R curve just at the beginning of the oil window [Figs 6(a) and 7], if it proves to be general, could be important in providing a kerogen maturity parameter that is relatively easy to measure and less operator-dependent than those available from organic petrographic techniques. These techniques (e.g. vitrinite reflectance, spore color, etc.) are widely used for this purpose and are considered to be the most reliable maturation measurements currently available. However, all

petrographic techniques depend on the operator having special training in recognition of the maceral under consideration (see Whelan and Thompson- Rizer, 1993, for a review of potential problems). As a result, considerable variation can occur between laboratories or even within the same laboratory. Solid-state ~3C NMR is useful in providing a direct measure of oil generating potential via the ratio of kerogen aliphatic to aromatic carbon atoms. How- ever, certain types of kerogens, especially those which are more woody, organic lean, or gas prone, show very little aliphatic carbon signal even in immature samples. In addition, presence of paramagnetic impurities or free radicals in highly aromatic systems can cause the NMR signal to broaden so much that it is impossible to integrate the aliphatic to aromatic signal. Thus, the EPR PS values appear to offer a potentially useful alternative maturation measurement which is fairly simple to run.

The biggest potential advantage of the EPR PS technique is its ability to clearly define the start of the oil window [Fig. 6(a)]. In samples examined here, the results are not subtle---a very distinct break in the PSi50) vs Ro plot which occurs right at the beginning of the oil window. In addition, this break in PS(50) occurs in spite of the pyrobitumen present in the


I 1 I I l ' '

A L i s b u r n e fl" \ %Ro = 1.52

• , . 12

p p m p p m

I i J i ~ i = i - 2 0 0 I I - 2 0 0 4 0 2 0

Fig. 8. Solid-state ~3C NMR spectra for kerogens from the Ikpikpuk well, except when noted otherwise. Geologic formations are designated. See Table 1 and text for definition of terms.

9800 ft sample. This material, which we believe to be a residue of expelled and migrated hydrocarbons (Whelan et al., 1986; Tarafa et aL, 1988) is not extractable with organic solvents (J. Whelan, unpublished results). Therefore, pyrolysis T,,,x values also could not be used as a reliable maturation indicator for this section. This residue is also responsible for the relatively constant aliphatic ~3C NMR signals observed in samples from 9800 to 9950 ft (Fig. 8). Thus, ~3C NMR data is also not providing reliable maturation data for this interval. In contrast, the EPR PS(50) data appears to be unaffected by these problems [Fig. 6(a)].

Possible relation of extent o f aromatieity and saturation behavior

The solid state ~3C NMR data, as well as the known general loss of hydrogen with increasing maturation, and the known ability of aromatic rings to stabilize proximate free radicals, strongly suggests a relationship between maturity, aromatic kerogen character, and EPR behavior. With increasing maturity, kerogens become less aliphatic and more aromatic. Systematic changes in aromaticity can explain the general response of several EPR parameters with increasing maturity (Fig. 6). For example, the plot of spin density vs maturity (%Ro) for the Ikpikpuk well Kingak formation [Fig. 6(b)] shows that after an initial decrease (7460-7700 ft) the free radical concentration increases as the samples

enter the oil window (8000 ft) suggesting that the increasingly aromatic character of the kerogen can support an increasing number of free radicals. While the number of samples examined to date is limited, the data so far indicate that as the aromatic system becomes more extended, the number of radicals stabilized shows an increase throughout the oil window. At this point, several lines of evidence show that aliphatic groups are also being cleaved from the kerogen structure, probably initially at benzylic positions (see Whelan and Thompson-Rizer, 1993, for a recent review of extensive literature on this point). However, as the kerogen passes through the oil window, cleavage of aliphatic groups from the kerogen must take place on higher energy bonds, that is, at points other than on benzylic carbon atoms, so that the concentration of free radicals which can be supported by the kerogen system would not continue to increase, as is observed. This plot also shows that the minimum spin density is recorded for the samples as they enter the oil window, suggesting that there is some minimum number of free radicals that the kerogen system can support as it undergoes the changes that result in or from the generation of petroleum. It should also be noted that the highest spin densities, largest free radical concentrations, correspond to the shallowest, least mature samples. This may reflect the fact that the bond cleavages discussed above are indeed proceeding at the lowest energy sites, as well as some quenching of free radical


as the aromatic matrix of the kerogen develops due to closer proximity of the free radicals in the more rigid structure and the increasingly aromatic nature of the spin centers (Bakr et al., 1988).

Information from the power saturation behavior of the samples can be used to examine possible kerogen structural characteristics as a function of increasing aromaticity and maturity. The intensity of the integrated signal at PSs0 was examined as a function of peak width (Fig. 9). Power saturation vs peak width for this Kingak formation sample set shows two distinct regions, with each region showing little change in saturation behavior with relatively large changes in peak width. If the linewidths of these signals were determined only by homogeneous relaxational processes, one would expect a correlation between power saturation and the linewidth. However, this situation is obviously more compli- cated. Portis plots, in which the ratio of signal intensity to the square root of the power are plotted as a function of the half-saturation power, are used to distinguish homogeneous from inhomogeneous line broadening. For these samples the Portis plots indicate that the signals are inhomogeneously broadened, perhaps due to factors such as g and/or hyperfine anisotrophy. The points group into two linear sets, one a deeper set showing higher saturation and a second shallower set showing less saturation. Nevertheless, this data indicates a change in the nature and/or the environment of the free radicals in the kerogen at the depth of the break in the power saturation behavior. However, pyrolysis GC-MS patterns of these kerogens throughout this interval are virtually superimposable (Whelan et al., 1986; J. Whelan, unpublished data), implying that the kerogen structural changes over this depth interval are relatively small.

C O N C L U S I O N S

EPR studies carried out on kerogens from three Alaskan North Slope wells showed that:

1. The power saturation behavior of samples from different formations show definite maturational trends, with the most mature samples showing the least saturation. As a result, EPR studies of source rocks should always include power saturation studies. At present, most published analyses do not.

2. EPR data show the greatest correlation and yield the most information when analyzed as a function of formation rather than combining formations from one well or several wells. Formational analysis allows conclusions with respect to maturational trends across a basin.

3. EPR signal intensity over the square root of the power at a power of 50,000 mW (the power at which saturation had just begun for the kerogens examined

I/P j'2 called "PSts0K)" above, plotted here) or / fs0.000 mW), vs vitrinite reflectance, shows a distinct break at the beginning of the oil window (approximately in the

Ikpikpuk well, Kingak Formation

2.00o,o 4. t !o I I d f to o O • ¢ O tn

,~ 4.000,0~ i * ~ ~ L ; ! r~ Co

i i t -- $

~ " 6 .000 1 0

Ifo s.000 104 o 5 ~ ° o

1.000 1 0 . . . . . ! ~ -~ o ~ : = ; o~ '

,.2o0 ,oSF I r r r I i 10 10.5 11 11.5 12 12.5 13 13.5

Peak width

Fig. 9. Intensity of integrated signal at 50 mW [PSIs0~ ] as a function of peak width (Hpp) for the Ikpikpuk well Kingak

formation kerogens. Sample depths are shown in feet.

range of Ro = 0.8-1%). This parameter may provide a useful operator-independent measurement showing the beginning of the oil window. This increase in PSIsoK) did not seem to be affected by the presence of residual pyrobitumens or oil migration residues in some of the samples. The latter can present problems with a number of other maturation indicators, including vitrinite reflectance, pyrolysis T,,ax values, and solid state 13C NMR values.

4. Plots of spin density as a function of aromaticity, as reflected in vitrinite reflectance values and solid state =3C NMR values, suggest that there exists a minimum number of free radicals that can be supported by a kerogen system, as well as a theoretical maximum number.

Associate Edi tor- -R. Patience

Acknowledgements--Samples for this study were kindly provided by Dr Les Magoon of the United States Geological Survey, Menlo Park, California. We are grateful to Lorraine B. Eglinton for the vitrinite reflectance data and to Peggy Dickinson for general laboratory assistance. We thank Dr Francis P. Miknis of the Western Research Institute, Laramie, Wyoming for NMR spectra. This research was supported by a Department of Energy Grant (No. DE-FG02-86ERI3466) to JW. The solid-state NMR analyses were provided in part by Department of Energy University Research Instrumentation grant No. DE-FG05- 89ER75506 to FM at the Western Research Institute. The EPR Spectrometer was funded through the Office of Naval Research under grant No. N00014-86-G-0164 to NVB. TAD is grateful to the University of Scranton for providing sabbatical leave to perform this research and to the Woods Hole Oceanographic Institution for scientific and personal hospitality. Woods Hole Oceanographic Institution Contribution No. 8437.

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EPR study of kerogens from three Alaskan North Slope wells

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