24. ELECTRON PARAMAGNETIC RESONANCE STUDY OF THERMAL ALTERATIONOF KEROGEN IN DEEP-SEA SEDIMENTS BY BASALTIC SILL INTRUSION

Earl W. Baker and Wen Y. HuangDepartment of Chemistry, Northeast Louisiana University, Monroe, Louisiana

andJ. Graham Rankin, J.R. Castafio, J.R. Guinn, and A.N. Fuex

Shell Development Company, Bellaire Research Center, Houston, Texas

INTRODUCTION

Site 368 on the Cape Verde Rise was noteworthy forthe finding of a basaltic sill of Miocene age intrudedinto an organic-rich Cretaceous black shale. The sillextended from 957.0 to 970.5 meters and drillingterminated at 984.5 meters in the black shale.

Methane was present above the sill but little or noethane was present. Approaching the sill, ethaneincreased but propane and higher hydrocarbons werenot detected. The sill was encountered in Core 368-60;Core 368-61 was entirely basalt, and Core 368-62 brokethrough the sill. Neither gas in large amounts nor liquidhydrocarbons were detected in Core 368-62 or Core368-63.

EPR analysis in conjunction with other moreconventional measures of maturity of organic materialswas performed. We report, here, preliminary resultsand some modest speculations on the implications ofthese findings.

EXPERIMENTAL

Sample Collection

Samples analyzed from Site 367 consisted of portionsof Section 367-19-4, 0-20 cm collected for distributionby the Organic Geochemistry Panel of DSDP and 111-cm quarter core samples taken from the working halffour-months post-cruise. The 20-cm whole sectionsample is referred to as the OG sample; the othersamples from this site are referenced as RK samples.The OG sample was frozen upon collection andremained so until analysis. RK samples were taken toestablish the variability of the black shale facies at thissite.

At Site 368, an OG sample was taken at Sample 367-63-3, 130-150 cm, and frozen in a like manner as theother OG samples. A number of 1-cm quarter-coresamples (RK) were taken onboard for organic carbonand carbon:nitrogen analysis shortly after recovery ofthe cores (see Site 368 chapter, this volume). RKsamples were closely spaced (10-20 cm near the sill; 60-cm intervals farther away). Special samples 8 cmquarter-core) were taken for analysis of the thermalalteration of the organic matter near the sill.These aredesignated "TA" samples. These samples werecollected about 2 days after recovery of the cores butsubsequently were kept frozen.

Nine samples of interstitial gas were collected fordetermination of stable isotope ratios of the lighthydrocarbons. These samples (GAS) were taken bypuncturing the plastic core liner in the vicinity of a gasbubble and drawing the gas into an evacuated can. Gaschromatographic determination of the components wasmade onboard. Stable isotope analyses were carried outlater using standard methods (see stable isotopesection).

Kerogen PreparationThe kerogen used in the electron paramagnetic

resonance (EPR), vitrinite reflectance (VR), and visualkerogen analysis (VKA) was isolated by leaching withhydrochloric and hydrofluoric acids. Details are givenin Hood et al (1976).

Pyrolysis-Fluorescence and Pyrolysis-FID AnalysisThese two techniques are used to determine the

hydrocarbon potential of the sediment. Details of themethod are given in Hood et al. (1976) and Kendrick etal. (this volume).

EPR SpectraThe purified kerogen sample was placed in a melting

point capillary tube and tapped until a packed length of1 to 2 cm was obtained. The tube was heat sealed andwas placed in a 3 mm ID × 250 mm quartz tube forEPR analysis.

A Varian model V-4500 spectrometer equipped withaudio frequency magnetic field modulation and aphase-sensitive detector was used for EPR measure-ments. The spectra were recorded by dual cavity tech-nique which allows an accurate measurement of EPRparameters by simultaneously scanning both samplesand standard in identical conditions. The referenceused in this study was 4-acetamido-2,2,6,6-tetramethylpiperidino-1-oxyl (Eastman 11293). There are threeEPR absorption lines in this compound with spacingbetween adjacent peaks of 15.26 Gauss and a g value ofthe middle line of 2.00603. This method allows thesample magnetic field to be accurately calibrated by thespacing between the adjacent peaks of the standard(15.26 Gauss). In addition, the g value of the samplecan be simply calculated by comparison of the sampleto the standard according to the following equation: HX g = H' × g'; where H = Gauss reading of absorptionline of sample, g = g value of sample, H' = Gauss

839

E. W. BAKER ET AL.

reading of absorption line of standard, and g' = g valueof standard.

The line widths of the EPR spectra were determinedby measuring the distance between the two deflectionpoints of the absorption line. In the blank run, using anempty capillary tube, no signal was detected at any gainlevel of the detector over the magnetic field of interest.

RESULTS

Electron Paramagnetic Resonance (EPR)Most of the kerogen samples from Sites 367 and 368

give very strong EPR signals. The spectroscopicsplitting factor, g value, and the peak width are themain concern in these measurements. It has previouslybeen shown that in many cases these parameters reflectthe structural transformation of kerogen ingeochemical environments (Baker, 1975).

In the present study, two groups of samples havebeen selected from the black shales of Leg 41 for thepurposes of investigating the nature and the effect ofgeochemical parameters on the chemical structures ofkerogen. Samples from Cores 368-59, 368-60, 368-62,and 368-63 (Table 1) were collected within 10 metersabove and below the basalt sill. The black shales in thissection have been exposed to high temperatures, thusthey serve the purpose of studying thermal effects onkerogen. In addition, one sample from a depth of 270meters (Section 368-27-2) was included for comparison.The second group of samples, which was collected fromthe Cape Verde Basin, represents the major lithologiesof the region between 630 and 780 meters subbottomdepth, Site 367.

The spectra for samples of Site 368 are presented inFigure 1 and those for Site 367 in Figure 2. The

calculated g values and the peak widths for the twogroups of samples are tabulated in Tables 1 and 2,respectively. In order to relate other physical andchemical characteristics of these samples to the resultsof EPR studies, the reflectance of the kerogen, thedepth, total carbon content, C/N ratio, FID yield,D/P, PF, and Visual Kerogen Analysis (VKA) are alsogiven in Tables 1 and 2. A positive sign preceding thedepth of each sample in Table 1 indicates the distanceabove the sill and a negative sign the distance below.

In Figure 1, a wide variation in widths, shapes, andintensities is observed in the EPR spectra. Starting as arelatively broad line of 10.23 Gauss in Section 59-2, theEPR spectra become progressively narrower down to3.25 Gauss in Section 60-3 at +360 cm (See Table 1).The g values seem to correlate with the changes of peakwidths going from 2.0037 to 2.0030 over the sameinterval. However, at 243 cm (60-3) above the sill, thetrend reverses and finally in the vicinity of the sill thesignals flatten and disappear. Below the sill the changesof EPR spectra move in a reverse direction which isevidently caused by the relatively similar heating by thebasaltic intrusion. The EPR spectra on both sides of thebasaltic sill appear as if they were reflecting each otherwith the sill as a mirror except that several minor sillsabove the major sill distort the image. One noticeabledifference is the presence of relatively weak signals insamples immediately below the sill (62-3 at -17 and -71cm). The signals in these two samples are extremelyweak, giving a noisy spectrum in which the g value andthe peak width measurements are subject to substantialerror. The cause of the weak signal is not known but, asis suggested by heat treatment studies discussed later, itis highly probable that these signals are the result ofsome liquid hydrocarbon driven from the vicinity of thesill which condensed in these sediments.

TABLE 1EPR Parameters, Visual Analysis, and Reflectivities of Kerogen Isolated From Black Shales

and Organic Contents and Hydrocarbon Yields of Black Shales From Site 368

Sample(Intervalin cm)

Distancefrom Sill

(cm)

EPR

(g-2)×104

PeakWidth

(Gauss)

Ro max%a

mean ±95% conf.

Org. C(% wt) C/N

FID yieldwt% as HC D/P

PFFluorescence

Units

Visual

Ac

Kerogen

L<1 He

Analysis13

59-2, 0-859-3,98-10360-2, 55-6360-3,25-3360-3,142-15060-4, 55-6360-4,110-11860-5, 28-3660-5, 88-9662-3, 18-2662-3,72-8062-3, 120-12862-4,44-5262-4, 118-12663-1,68-7663-2, 44-5263-3, 147-150863-4, 90-98

+ 1000+737+480+360+243+ 188+ 133

+65+5

-17-71

-121-193-267-367-493-741-839

37.032.929.430.133.5

30.130.431.231.432.531.236.3

10.239.466.463.25

19.32

5.526.696.407.348.366.12

10.28

3.33 ±.231.95 ±.090.99 ± . l l1.42 ±.300.68 ±.04ND0.48 ±.050.44 ±.030.50 ±.05

0.971.48

5.092.183.730.20

0.33

11.72

1.551.112.91

10.414.5

42.738.418.2

1.1

14.1

19.2

13.1

20.3

0.0730.0110.0360.0040.0070.0090.0140.0290.0040.0040.0110.0140.2631.2371.9541.1440.0981.290

0.0620.3060.3440.7373.2103.6842.5634.1190.4660.4571.1040.5390.1100.5090.1070.0190.0130.006

200000030040

111057590

3120

aVitrinite reflectance.bVisual kerogen analysis numerical abundance scale and percentages (by area): 1 = 0-1%, 2 = 1-5%, 3 = 5-10%, 4 = 10-25%, 5 = 25-50%, 6 = 50-75%, 7 = 75-100%.cAmorphous.dLiptinitic.eHumic.c'Reworked humic and inert.ëOG sample. All other samples in this table are TA. See text under sample collection for sample designation.

840

STUDY OF KEROGEN IN SEDIMENTS

59-3

60-2

f60-3

60-4

I

60-5

60-5

63-3

63-1

Figure 1. ££K spectra of kerogen from Site 368. For sample identification refer to Table 1. Except for 62-3 and62-4 which were recorded with 250-Gauss instrument sweep range and for 27-2 which was recorded with a1000-Gauss range, all other spectra was recorded with 100-Gauss sweep range.

841

E. W. BAKER ET AL.

A 19-4

Figure 2. EPR spectra of kerogenfrom Site 367. For sample identi-fication refer to Table 2. The spec-tra were recorded with 100-Gausssweep range.

It is worth noting that kerogen Samples 60-4 at +133cm and 60-5 at +65 and +5 cm behave very differentlyfrom other samples when subjected to EPR analysis.These samples absorb a great deal of energy suggestingthat inorganic paramagnetic materials may be present.

Sections 63-3 and 27-2 represent kerogen occurringat two quite different depths of burial; however, theEPR spectra of these samples are unique among all ofthe samples studied. An extremely broad EPR signal isobtained for Section 27-2. The absorption of thissample spans over 400 Gauss, in contrast to the peakwidths of the rest of the samples which range fromabout 4 to 20 Gauss. The depth of this sample isrelatively shallow (270 m) in comparison with theremainder of the samples which were at least 900 metersdepth. The sample is light gray and there is no evidenceof heating in the surrounding area. Apparentlytransformation in kerogen structure has not occurredas it did in the remainder of the samples.

In 63-3, two absorption lines are observed. One witha g value of 2.0028 is superimposed on an extremelybroad line which is similar to the absorption of 27-2.However, this sample at 980 meters depth is located 741cm below the sill. It has a similar color to 27-2 andcontains 1.11% total organic carbon. It is very likelythat the thermal stress created by the intrusion hasgenerated the second EPR absorption line. A similareffect has been obtained in the laboratory by heattreatment of 27-2 (see following section).

Three samples from Site 367 (a composite of 18-1 and18-2; 19-4 at 20-25 cm and a composite of 20-2 and 20-3) also exhibited two EPR absorption lines. One with abroader peak width (7 to 14 Gauss) is superimposed ona sharper line of lower g value. Because the twoabsorption lines are only partially resolved, themeasurements of g values and peak widths is rendereddifficult (see Figure 2). It is interesting to note that therelative intensity of the two absorption lines variessystematically with depth. In the spectra, the narrowline moves toward the lower g values with increasingdepth, whereas the broader line either shifts slightly inthe opposite direction or remains stationary. As aconsequence, the two lines are better resolved at greaterdepths. Only a single EPR absorption was observed in acomposite of 23-1 and 23-2. It has a g value of 2.0032and peak width of 8.36 Gauss. The presence of thesharp second EPR absorption in the three samples isinteresting and probably can be attributed to a differenttype of material comprising the kerogen.

Heat Treatment of KerogenThe kerogen samples collected from Site 368

demonstrated that EPR spectra vary with thermalstress. In order to verify the observed relationship andto obtain some measure of the severity of thermal stressnecessary to produce the observed changes, heatingexperiments of a few selected samples were conductedin the laboratory. The results of these studies are givenin Figure 3 and Table 3.

Section 59-2 was the farthest from the diabase silland presumably the least thermally stressed. Prior totreatment it had a broad EPR absorption line (10.23Gauss) and high g value (2.00370). The kerogen wassealed in a melting point capillary tube and heated at250°C for 16 hr. After treatment, the peak width had

842

STUDY OF KEROGEN IN SEDIMENTS

TABLE 2EPR Parameters, Visual Analysis, and Reflectivities of Kerogen Isolated From Black Shales

and Organic Contents and Hydrocarbon Yields of Black Shales From Site 367

Sample

(Intervalin cm)

18-1,97-9818-2, 144-14519-1,134-13519-2, 24-2519-4, 20-25819-4,44-4520-2, 7-820-3, 12-1321-3,70-7122-3, 145-14623-1,32-3423-2, 83-86

Subbottom

Depth(m)

637.5639.5649.2649.7652.8653.0688.1689.1696.2725.0784.3786.3

EPR

Peak

Width(g-2)× 104 (Gauss)

9.90

7.02

14.21

32.1 8.36

Ro max%a

mean +95% conf.

0.19 ±.02

0.18 ±.01

ND

0.30 ±.03

Org. C(% wt)

37.21

4.0918.5416.29

C/N

44.7

34.125.4

FID yieldwt% as HC

20.38413.625

1.80917.3842.514

13.1938.281

19.9722.2010.951

D/P

0.0190.0140.0140.0140.0060.0100.010

0.0080.0070.007

PF

FluorescenceUnits

31502700

1033300

2403000255027005100

6009082

Visual

Ac

7

7

7

6

Keroj

Ld

2

2

4

2

;en Analysis"

He

2

2

2

5

Rf

2

2

2

4

aVitrinite reflectance.bVisual kerogen analysis numerical abundance scale and percentages (by area): 1 = 0-1%, 2 = 1-5%, 3 = 5-10%, 4 = 10-25%, 5 = 25-50%, 6 = 50-75%, 7 = 75-100%.cAmorphous."Liptinitic.eHumic.

'Reworked humic and inert.

ëQG sample. All other samples in this table are RK. See text under sample collection for sample designation.

become 6.35 Gauss and the g value 2.00286 (see Figure3 and Table 3). The result of the heating experimentsuggests that thermal stress was responsible for thereduction of the g value and peak width of the EPRabsorption lines in samples near the sill at Site 368.

After being heated at 250°C for 22 hr, an additionalEPR line was observed to be superimposed on theoriginal broad line in Section 27-2. Using 100 Gauss ofmagnetic sweep range, the EPR spectrum looksremarkably similar to that of Section 63-3, althoughboth the g value and peak width are a little larger (seeTables 1 and 3). Tentatively, these results suggest that abroad EPR absorption (a few hundred Gauss)represents the undisturbed thermally unstressed kero-gen. Possibly the generation of "normal" free radicalsin kerogen does not occur before certain physical-chemical conditions are severe enough to cause therupture and/or rearrangement of some chemical bondsin the kerogen structure.

The second sharp EPR line in Sections 19-4 andcomposite of 18-1 and 18-2 disappeared after thesamples were heated at 200°C for 24 hr. The resultingEPR signals of the two heat-treated samples are similar,varying only by a slight difference in peak width (Table3). Upon further heating at 320°C for 2 hr, Section 19-4gave a sharper EPR line and the g value was reducedfurther to 2.00296. In composite of Sections 20-2 and20-3, the effect of heating follows a quite different pat-tern. Instead of the disappearance of the sharp peak asin the previously mentioned two samples, the sharppeak is increased in intensity after 15 hr of heating at200°C. Prolonged heating at the same temperature upto 40 hr caused a minor increase in the g value from2.0028 to 2.0030. After further heating at 320°C for 2hr, the EPR spectrum changed abruptly. A broadintense signal appears and overwhelms the previoussharp one. The transformation apparently has not beencompleted in this treatment because a trace of the sharpEPR signal is still retained in the spectrum (Figure 3).For this sample, some condensed liquid within the

capillary tube was seen after the heating. An EPRspectrum of the condensate showed a weak but sharpabsorption line (20d in Figure 3).

Stable Isotope Analyses

Stable carbon (δCπ) and hydrogen (δD) isotopeanalyses on the methane in selected gas samples aregiven in Table 4. The isotope data are expressed as permil deviations relative to the PDB standard for carbonand SMOW standard for hydrogen. Methane wasseparated chromatographically from the rest of thegaseous components prior to its combustion over hotcopper oxide in preparation for isotope ratiodetermination.

The methane δC13 values for the Leg 41 sediment gassamples (range: -70.9 to -54.8°/oo) are comparable tothose found for similar gases from DSDP Legs 10, 11,13, 14, 15, 18, and 19 by Claypool et al., 1973 (range:-89 to -47°/oo for 65 gas samples). The intrusion of adiabase sill at Site 368 partially metamorphosed at leastsome of the organic matter in the adjacent shale. Thisthermal alteration of organic matter is reflected in boththe component and isotopic chemistry of the gassamples at 956, 972.5, and 977 meters. In these samplesthe percent ethane is significantly larger and themethane δC13 significantly heavier than gas samplesfrom shallower depths in the same hole, and henceunaffected by heat from the intrusion. These three gasesmay be interpreted as mixtures of bacterial gas, similarto that found at shallower depths in Hole 368 andthermal gas, which was generated from the organic-richshale by heat from the igneous intrusion. The thermallyderived portion of these gases would contain significantamounts of ethane and higher hydrocarbons, as well asmethane which is isotopically heavier compared tobacterial methane. Thus, such a mixture can accountfor the observed properties of these gases.

The methane δD values for the Leg 41 sediment gasesare almost identical to previous measurements on

843

E. W. BAKER ET AL.

/\

\

1

\\

27-2A

Figure 3. EPR spectra of selected kerogen samples from Sites 367 and 368. The ker-ogen used for experiments 20A, 20B, 20C, and 20D is composited of 20-2 and20-3. For details of heating conditions see Table 3 and text.

844

STUDY OF KEROGEN IN SEDIMENTS

TABLE 3EPR Parameters of Heat-Treated Kerogen

Samples From Black Shales of Sites 367 and 368

Peak WidthSample Heating Conditions (g-2) × 10" 4 a (Gauss)

59-2

27-2AB

18-118-2

19-4 AB

20-220-3A

BCD

250"

200°250°

200°

200°320°

200°200°320°320°

C

CC

C

CC

CCCC

22 hr

24 hr16 hr

24 hr

24 hr2hr

15 hr40 hr

2hr2hr

28.6 6.35

31.5

34.3

33.729.6

21.929.9

9.98

10.28

8.837.87

3.263.26

30.6 4.15b

aHeat treatment times are cummulative.

^Condensed liquid in end opposite to kerogen.

TABLE 4Isotopic Analyses of Selected Gas Samples

Designation

(DSDP)

367-194368-25-1368-39-6368-60-4

Depth in

Sediment (m)

649371575956

Percenta

Methane

96.593.096.497.0

Percent

Ethane

0.060.06

<O.Ol0.25

diabase sill at 957.0 to 970.5 meters in Hole 368368-62-4368-63-2369A-41-3369A-44-2369A-47-5

972.5977423452484

94.195.882.569.280.1

0.140.280.040.040.11

Methane Isotopic Composition

δ C l 3 (PDB)

-70.9°/oo-63.6-57.0

54.8

-55.9-55.1-69.5-68.2-64.9

δ D (SMOW)

-188°/ooND-176-187

-184-181-179ND-177

CO2 contents of these samples ranged from 0.01% to 0.7%, with the balance of thesamples being mainly undifferentiated "air."

DSDP gas samples. Lyon (1974) measured methanehydrogen isotopic compositions of eight DSDPsediment gases from the Cariaco Basin (Leg 15, Site147) and obtained values between -183 and -173°/oo,which he considered essentially identical withinexperimental error. The δD values of the sediment thusappear to be independent of both geographic localityand subbottom depth. This may be the result of arelatively constant fractionation associated with thebacterial generation of methane, plus a fairly uniformconcentration of deuterium in marine organic matterand/or interstitial sediment water. On the other hand,the uniformity of deuterium concentration in DSDPmethanes might be indicative of a postgenerationisotopic equilibration with interstitial water, which canbe assumed to have a uniform composition, close toSMOW. However, a state of isotopic equilibrium hasapparently not been reached, as calculated equilibriumtemperatures differ by about 40°C from the estimatedsediment temperatures (Lyon, 1974). It is also possiblethat the very narrow range of methane δD values issimply fortuitous.

CHN Analyses, Pyrolysis Studies,and Vitrinite Reflectance Measurements

The samples from Site 367 (Table 2) are rich inorganic matter with high hydrocarbon potential but are

quite immature. There is considerable variation withinthe sequence (4% to 37% organic carbon; 1% to 20%FID yield). The richest organic content is near the top(Core 367-18) of the black shale facies becoming lessrich near the bottom (Core 367-23). Visual kerogenanalysis indicates an abundance of amorphous kerogenwhich is in keeping with higher pyrolysis FID results.Vitrinite reflectance measurements indicate that allsamples analyzed at this site have reflectivities typicalof that found in the lignite stage of coal.

The samples near the basaltic intrusion in Site 368(Table 1) from 957-970 meters subbottom showconsiderable thermal alteration. Vitrinite reflectancevalues as high as 3.3% and texture, as seen through themicroscope, indicate "coking" of the sediments by thesill. Pyrolysis-FID yields are very low as is the percentorganic carbon. Farther away from the sill (about 2 m)pyrolysis-FID yields remain low although organiccarbon values are high. This may be the result oflocalized generation of hydrocarbons, as noted by thehigher distillable/pyrolyzable ratios (D/P), withsubsequent expulsion to nearby sediments.

A profile of the organic carbon results of the CHNanalysis of RK samples near the sill is given in Figure 4.These results are taken from Site 368 chapter (thisvolume).

DISCUSSION

The g values observed in the kerogen samples of thepresent work are, except for a few unusual cases whichwill be discussed separately, generally low (2.0029-2.0037) and similar to the EPR spectra reported forpetroleum asphaltene, suggesting typical aromaticsystems with delocalized electrons. The narrowing ofline widths and the decrease of g value as the samplelocations approach the heat pocket caused by the sillare consistent with the results of earlier studies onkerogen in natural environments (Hwang and Pusey,1973), asphaltene (Yen et al., 1962), and coal (Austen etal., 1965). The unusual broading of EPR spectral linessimilar to that seen in the sample about 250 cm abovethe sill was reported by Baker (1975), but theobliteration of EPR spectra as observed in the samplesimmediately above and below the sill has not beenpreviously reported. The study of petroleum asphalteneby Yen et al. (1962) shows that heating up to 460°Conly causes narrowing of line widths and increasesrather than decreases spin concentrations. Totalelimination of free radicals did not occur in coalmaceral up to 750°C (Austen et al., 1965). It is notknown whether the extreme heat supplied by theintrusion alone has destroyed the aromatic structures ofthe kerogen which are presumably the free radical sitesor whether the presence of inorganic minerals ispartially responsible for the disappearance of the freeradicals.

In several cases atypical EPR spectra were observedin these samples. In one case, a previously unreportedspectral set consisting of two absorptions wasdiscovered and we now discuss these atypical sets.

One of the supposed transient forms in the diagenesisof kerogen is the geminal formation of an aromatic-type free radical. In Section 27-2 such a transition mayhave been observed. The germane portion of thespectrum (Figure 1, 27-2) is seen at the point where thecurvature reverses from convex to concave. Careful

845

E. W. BAKER ET AL.

PERCENT ORGANIC C2 3 4 5 6 7 8 9 10 11 12

Figure 4. Total organic carbon plotted against the distancefrom the sill.

perusal shows that imposed on the broad sweepingband is a tiny signal with a g value nominally that of a"normal" aromatic radical. Interestingly, progressiveheat treatment of this kerogen produces progressiveenlargement of the tiny signal into a major one (Figure3, 27-2 A, B, C). These experiments suggest that thegeneration of a "normal" free radical has been ob-served and that in the native kerogen, incipient radicalformation had just been initiated.

It has been proposed that narrowing of band widthof EPR spectra is related to the maximum temperatureof exposure (Hwang and Pusey, 1973). In the samplesequence 59-2, 59-3, 60-2, and 60-3 we observed anatural illustration of this effect (Figure 1, first fourspectra). In concert with the narrowing of the bandwidth, the g value decreased. Laboratory heatingexperiments confirmed the interpretation of theseobservations. Section 59-2, upon modest treatment,was converted such that its spectrum very closelyresembled 60-2 (compare Figure 3, 59-2 with Figure 1,60-2 and Table 3, 59-2 with Table 1, 60-2). Laboratoryheating apparently duplicated the effect of the heatingby the intrusion.

In the case previously discussed, 27-2, the "normal"aromatic EPR absorption was seen superimposed on avery broad band possibly not of organic origin. In

(G-2)x 10'4

Figure 5. (A) EPR peak width (Gauss) of kerogen from im-mediately below sill (Site 368) plotted against log of dis-tance from sill. (B) G value [(g-2) X lO~^J of kerogenfrom immediately below sill (Site 368) plotted againstlog of distance from sill.

contrast, a group of kerogens was observed with twoabsorptions in the region normally allocated to organicradicals. Such two banded spectra are illustrated inFigure 2. The narrow absorption band in these samplesdeserves note, because of the heat labile nature of thekerogen component represented by it (compare Figure3, 20-2 a and b, 18-1 with Figure 2, 20-2, and 18-1). Thecontrast with the broader band which is probably a"normal" aromatic-free radical absorption is evident,since after limited heat treatment the signal is no longer

846

STUDY OF KEROGEN IN SEDIMENTS

observed. These signals then most likely represent adifferent type of organic structure with a g value lowerthan the "normal" aromatic kerogen signal. Possiblythey contain heteroatoms and are less aromatized thanmature kerogens and thus lack the stability of the freeradical normally associated with such structures.

Variation in EPR Parameters and Other Characteristicsof Kerogen and Shale in the Vicinity of the Intrusion

A plot of g value versus log of the distance from thesill gives a good fit (Figure 5), except in the case of 63-3which appears to be a different type of kerogen (Table2, VKA). If the observed g value is a measure of theintegrated time-temperature exposure of the kerogen(shale) and the temperature falls exponentially withdistance then such a plot is not unexpected.

Interestingly, a similar plot using band width insteadof g value versus log distance gives a nonlinear best fit.Again, Section 63-3 falls far off the line. Since thefactors making up the band width are very complex, nosimple explanation for the observed trend can beoffered. It is, however, felt that the linear case presentedby Hwang and Pusey is probably a narrow segment of asimilar curve (Hwang and Pusey, 1973).

Bostick (1970) concluded that the effect on vitrinitereflectance for sills less than 10 meters thick wasconfined to an interval about equal to the sill thicknessabove and below the intrusion. The thermal effect seenat this site, as indicated by EPR parameters, vitrinitereflectance, pyrolysis fluorescence, and pyrolysis-FID,decreased rapidly and is minimal greater than 6 metersbelow the sill. As mentioned earlier some minor sillsabove the major sill complicate the "mirror image"effect for samples above the sill.

ACKNOWLEDGMENTS

The present work was supported by National ScienceFoundation, Grant GA-43359X and DES-74-12438 AO1.

The authors are indebted to Dr. R.E. Smith for technicalassistance with the EPR spectrometer. They also thank Dr. T.Strom of Mobil Research and Development for his suggestionof using 4-acetamido-2, 2, 6, 6-tetramethyl piperidino-1-oxylas a reference for EPR measurement, and Dr. S.E. Palmer forhelp which was greatly needed.

REFERENCES

Austen, D.E.G., Ingram, D.J.E., Given, P.H., Binder, C.R.,and Hill, L.W., 1965. Electron spin resonance study ofpure macerals: Adv. Chem. Ser., no. 55, p. 344-359.

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