Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike

Advances in Organic Geochemistry 1985

0146-6380/86 $3.00 + 0.00 Copyright © 1986 Pergamon Journals Ltd

Org. Geochem. Vol. 10, pp. 135-143, 1986 Printed in Great Britain. All fights reserved

Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike J. L. CLAYTON and N. H. BOSTtCK U.S. Geological Survey, Denver Federal Center, Denver, CO 80225-0046, U.S.A.

(Received 23 October 1985; accepted 15 April 1986) Abstract--A suite of siltstone samples from the Upper Cretaceous Pierre Shale from the contact zone of a 130-cm thick igneous dike near Wolcott, Colorado, U.S.A., was taken from the contact to 170 cm from the dike to study the effects of temperature on the organic matter. The sampled bedding interval was about 10 cm thick, so variation in lithology and type of organic matter is minimal. Vitrinite reflectance values (R0) increase from 0.4 far from the dike, to 3.3% near the dike contact. Geochemical measurements show systematic thermal effects analogous to those often observed for catagenesis and metagenesis in the depth range of 1-4 km within a sedimentary basin. The H/C ratio of kerogen and the hydrogen index (Rock-Eval) decrease most rapidly in the 0.6-1.7% Ro range, in which the transformation ratio (Rock-Eval) increases from 0.I to 0.3. Based on extraction of Cts+ compounds, the main increase of hydrocarbons and total extractable organic matter occurs between 0.6 and 1.0% reflectance. The saturated/aromatic hydrocarbon ratio increases almost twofold in this range of maturity. However, the pristane/phytane ratio is essentially constant through the hydrocarbon generation zone but decreases slightly at high levels of thermal alteration (R0 > 1.2%). The c~t3C values for aromatic and saturated hydrocarbons are about - 2 7 and -29%0, respectively, and are constant to about 1.0% R0, then both become heavier by about 2%0 at higher R0 values.

Key words: thermal maturation, igneous dike, kerogen, pristane, carbon isotope, rock temperature, organic geochemistry, organic petrology, vitrinite reflectance, hydrocarbon

INTRODUCTION The concept that petroleum hydrocarbons are generated by thermal alteration of organic matter during burial heating over geologic time is well established. M a n y studies demonstrate temperature effects on solid (kerogen) and liquid organic matter during gradual burial heating (Philippi, 1965; LaPlante, 1974; D u r a n d and Espitalir, 1976; Albrecht et al., 1976; Tissot et al., 1971). In addition, the effects of geologically rapid heating by intrusive sills, dikes, and larger plutons are known in several geologic settings (Dow, 1977; Simoneit et al., 1981; Perregaard and Schiener, 1979a, b; Baker et al., 1977; Akiyama, 1979; Aiteb/iumer et al., 1983; Spiro, 1984; Bostick and Pawlewicz, 1984; Ujiie, 1984). The current study determines the effects of geologically rapid heating on organic matter in a siltstone in a setting where possible differences in original depositional environment (organic matter type) and/or early diagenesis are negligible. SAMPLES AND ANALYTICAL PROCEDURES Samples were collected from an outcrop of the upper Cretaceous Pierre Shale near Wolcott, Colorado, U.S.A. (39.7505°N, 106.6788°W), adjacent to a 130-cm thick igneous dike. Samples were taken from an interval of bedding 10 cm thick in the bottom of a trench about one meter into the outcrop in a steep-sided gulley to avoid possible weathering effects (Clayton and Swetland, 1978). An initial suite of samples (triangle symbols on the figures) underwent

organic-petrographic and some geochemical analysis as part of an international ring analysis in 18 laboratories (Pittion and Bostick, 1981). Four years later, the excavation was opened to approximately the same depth and samples were taken at closer intervals (circles on the figures). Because the dike is thin (130 cm), heating effects extend for only about 100 cm from the dike. Therefore, rocks were sampled over short lateral intervals parallel to bedding within the shale unit (Table I) to allow resolution of changes in organic-geochemical parameters. For example, the entire peak of bitumen generation, corresponding roughly to the main phase of oil generation in sedimentary basins, occurs through just 35 cm of rock. The advantage of this geologic setting is that variation of lithology, organic matter type, and early diagenetic history is minimal over the entire sampled interval. This uniformity allows study of the effects of heating alone, without ambiguities often associated with samples collected in boreholes through a thick stratigraphic section or from a broad geographic area. Samples were collected into the unaltered zone (i.e. not affected by the dike, but typical for the regional thermal history) to establish background values. The sampled part of the Pierre Shale consists of mediumgray, calcareous siltstone. Organic carbon content is generally about 1.0% (Table 1). Rock-Eval pyrolysis indicates that the composite organic matter is type III (Tissot et al., 1974; Espitali~, 1977). Petrographic analysis of samples unaffected by the dike indicates approximately 50% vitrinite, 20% inertinite, 5% liptinite, 20% nonfluorescent amorphinite, and 5% fluorescent mixtinite or bitumen-mineral groundmass. Most of the analytical procedures used in this study are standard and have been described elsewhere (e.g. Clayton and Swetland, 1980). Organic matter was extracted with chloroform using a Soxhlet apparatus. Gas chromatographic separations of C~5+ saturated hydrocarbons 135

158-177 127-135 115-130 96-103 89 96 8~89 76-82 65-81 69-76 62-69 58-62 50455 55-58 5 2- 5 8 5 1- 5 5 46-51 45-52 4145 33-50 36-41 31-36 2.%33 2:~31 18-25 17-23 10-18 0-10

485A 675 R 485 B 675 Q 675 P 675 O 675 N 485 C 675 M 675 L 675 K 485 D 675 A 675 J 675 B 675 C 6751 675 D 485 E 675E 675 F 485 F 675G 485G 675H 485 H 4851

170 131 122 99 92 85 79 73 72 65 60 58 57 55 53 48 48 43 42 38 33 29 27 21 20 14 5

% d" (avg.)

0.384).48 -0.384).52 --0.404).54 0.40~.52 0.40~.56 0.484).58 0.53-0.66 0.54-0.76 0.484).68 0.58-0.78 -0.64-0.85 0.764).92 -1.00-1.34 1.28-1.48 1.14-1.46 1.4~1.72 1.55-1.90 1. 7 8- 2 .0 4 1.90-2.21 1.92-2.26 2.3-3.1 2.9 3.7

0.42 -0.46 --0.46 0.46 0.48 0.54 0.59 0.62 0.57 0.66 0.84 0.74 0.86 0.94 1.16 1.34 1.32 1.58 1.75 1.92 2.08 2.12 2.7 3.3 -----0.58 --0.44 -0.43 -0.39 0.37

0.78 ---0.77

0.71 -0.77 ----

H/C ~ 1.04" 1.04 1.06" 1.34 1.63 1.53 1.53 1.30* 1.37 1.47 1.28 1.12" 1.85 1.39 1.17 1.31 1.33 1.09 1.09* 1.17 1.18 I. 10" 1.19 1.01" 1.13 0.98* 1.13"

TOC a (wt%) 56 59 81 61 68 67 66 27 70 65 59 44 42 58 68 49 37 29 75 16 9 -5 -5 ---

HI e

Rock-Eval

TR / 0.10 0.06 0.15 0.09 0.10 0.09 0.08 0.41 0.09 0.10 0.12 0.t5 0.10 0.15 0.14 0.17 0.19 0.22 0.44 0.21 0.21 -0.37 -0.40 --

°Distance from dike-shale c o n t a c t as percent o f dike thickness ( 1 3 0 c m ) . bOil immersion, at r a n d o m orientation. q-l/C = a t o m i c H / C ra t i o o f k e r o g e n from acid m a c e r a t e d rock. dOrganic c a r b o n d e t e r m i n a t i o n s by w e t- o xi d a t i on (Bush, 1970) indicated by (*); others by R o c k - E v a l . "HI = H y d r o g e n Index ( S J T O C ) . O ' R = T r a n s f o r m a t i o n R a t i o (SI/S I + $2). t H C = Cis+ hydrocarbons. ~S/A = C~s + s a t u r a t e d - t o - a r o m a t i c h y d r o c a r b o n ratio. '6UC values for C~5+ s a t u r a t e d and a r o m a t i c h y d r o c a r b o n s as %0 relative to P D B . JAuC = 6 ~3C. . . . - ~ UC~t..

% d~ (range)

Sample number

V i tr i ni t e reflectance ~ % Range % Ro HC/TOC ~ (mg/gC) -32.2 25.8 38.0 33.4 33.5 30.5 23.3 34.7 36.1 45.6 38.1 29.4 47.8 57. I

54.7 50.8 58.7 35.9 38.7 15.3 7.1 9.3 3.5 11.8 2.7 .

EOM/TOC (mg/gC) 81.6 57.8 50.0 72.2 67.6 72.6 66.4 49.9 71.9 85.6 95.2 72.0 71.6 92.4 128.3

121.9 81.0 120.2 78.2 72.5 32.6 12.3 20.6 8.3 27.4 8.3 11.9

T a b l e I. A n a l y t i c a l d a t a for the W o l c o t t d i k e site

.

4.5 5.3 4.5 4.0 2.1 . 1.6 0.7 1.9 0.5

3.8

-2.8 1.5 2.5 2.9 2.7 2.8 1.7 2.5 2.7 3. I 1.7 3. l 4.3 3.0

S/A h

.

. 0.3 0.4 0.4 0.3

0.8 0.6 0.7 0.3 0.1

0.9

--1.5 1.0 1.3 1.3 1.3 1.4 1.2 1.2 0.9 1.3 I. 1 1.0 1.0

nCl7

Pr/

.

. 2.4 4.3 2. I 2.9 .

3.0 3.2 3.6 2.4 2.4

3.2

3.7 -4.0 3.5 3.3 3.5 3.5 3.8 3.3 3.1 3.0 4.3 3.0 3.0 3.3

Pr/Ph

.

--28.6 -- 28.3 -- 28.0 -- 27.7 --26.5 . 26.8 -27.7 - 26.9 -.

-- 28.6

28.8 - 28.7 - 29.3 - 28.7 - 28.8 - 28.9 -- 28.8 29.4 -- 29.0 -- 28.8 -- 28.8 -- 29.3 -- 28.7 -- 28.8 -- 28.8

Sat.

613Ci

- 25.4 25.6 -- 26.0 --24.6

26.9 26.6 26.7 26. I -25.5

26.9

-26.9 - 27.0 - 26.8 - 27.1 - 26.9 - 27.1 27.2 - 26.8 - 27.2 - 27.0 27.0 26.9 27.0 -- 27.0 26.9

A ro m .

1.4 2.1 1.9

2.4 1.7 1.8 1.9 1.7 1.7 1.7 1.3 1.6 1.0

1.8

1.8

1.9 1.7 2.5 1.6 1.9 1.8 1.6 2.6 1.8

Ar~C:

t::r. Z

Z

~

.t-

Organic matter near an igneous dike were performed on a HP 5880 GC equipped with a 50-m x 0.32-mm SE-54 fused silica capillary column. The GC was programmed from 50 to 340°C min a with hydrogen carrier gas flow rate at 25cmsec-L Stable carbon isotope ratios were determined using a Finnigan MAT 251 isotope mass spectrometer. Results are reported in the usual 6-notation relative to the PDB marine carbonate standard. Precision for 6 ~3C values is +0.1%o. Vitrinite reflectances are averages of 40-80 small grains (mostly <20 m) measured at random orientation in grain concentrates made by acid maceration. In each sample, the lowest-reflectingpopulation was selected except for samples with average reflectance greater than 2.0% R0.

RESULTS

AND

137

values, to what is generally observed for burial heating of source rocks in many sedimentary basins (Albrecht et al., 1976; Tissot and Welte, 1978). The transformation ratio (TR, Rock-Eval) increases from about 0.1-0.4, corresponding to 0.5-2.5% R0 (Fig. 2). The C15+ hydrocarbon (and EOM) generation zone occurs over TR values of 0.1 to about 0.25. The onset of generation occurs at the same TR value that Espitali6 et al. (1977) reported for burial heating, but the end of generation occurs at a lower TR value than they found. The C~5÷ hydrocarbon content (Fig. 2) parallels the EOM content. Onset of the main hydrocarbon generation phase between 0.5 and 0.6% R0 is accompanied by an increase in the ratio of C~5+ saturated-toaromatic hydrocarbons (S/A) (Fig. 3). A decrease of hydrocarbon content (Fig. 2) and S/A ratio (Fig. 3) occurs at alteration levels above 0.9 and 1.1% R0, respectively. The very low S/A ratio ( < 1) at high R0 values is interpreted to result from the relatively greater thermal stability of aromatic compounds. This situation is similar to the effect reported by Albrecht et al. (1976) for burial heating in the Douala Basin. Organic carbon content (TOC) varies between about 1.0 and 1.6 wt%, with one value of 1.85 (Table 1, Fig. 3). The sample set indicated by triangles in Fig. 3 has consistently lower TOC values than the later samples (open circles). As discussed previously, these two sample sets were collected during two separate field seasons. Although an attempt was made to sample approximately the same bed, the two sample sets were possibly taken from slightly different

DISCUSSION

Extract yields and organic carbon content

Analytical results are given in Table 1. Total nonvolatile extractable organic matter (EOM) versus distance from the dike (as percent of dike thickness), vitrinite reflectance, and estimated maximum temperature are shown in Fig. 1. Approaching the dike, an increase in the EOM content is evident starting at a distance of about 65% of the dike thickness. This corresponds to a vitrinite reflectance of about 0.55% Ro. The EOM increase indicates the onset of thermal generation of liquid organic matter in response to the dike heating event. A peak in the EOM content occurs at about 0.9% R0. The amount of EOM then decreases rapidly over the 45-21% distance interval (1.1-2.2% R0) corresponding to thermal degradation of the EOM to lower molecular weight molecules not detected in the EOM gravimetric analysis. This curve of EOM content is remarkably similar, in terms of R0

160 50~crn

lOOcm

150cm

140-

\

/.

•

'C

•

LU

/Ix

20-

500 o C 3.0

0 ~--Dike

,

,---

f

/

2.5

10 Siltstone-.~

20

I

A0 f ~0o f ~C 4

2.0

"

~3t0 ~ f0~ o fC,

I

1.5 30

1.0 40

0.8 50

i

.

.

.

Regional r e f l e c t a n c e leve

.

~ 0 0 o C E s t i' m a t e d m a x i'm u m 2

0.6 60

0.5 70

temperature

% R r Vitrinite S0

90

100

110

120

30

D I S T A N C E FROM DIKE C O N T A C T (% of d i k e t h i c k n e s s )

. . _ = ._. . . . . .

Fig. 1. Amount of extractable organic matter in thermally altered siltstone (EOM in mgtg organic carbon) vs distance from the dike contact expressed as % of dike thickness. Actual distance from dike contact is given in centimeters at top. Shown also are vitrinite reflectance values (R0) in %. Cirlces and triangles represent two different sample sets collected in the same trench during two different field seasons.

138

J.L. CLAYTONand N. H. BOSTICK -.4

106~,'n

50Pcrn

')

o~

\

~SEom

TR

70-

60-

° ~

e r

CO 0

50

c-g <

< 0 o O "E 40 t'~ O) >"1+ 30 ¸ t/J

>

~

..o--

..... TR

10 3i0•, 2iS

o

10 Dike /

2;0

2o

1;51 30

Siltstone---~-

~l\l~.l~ li I i~

1;0 40

0i8

0i6

50

r • . . % Ro Wtrmde , r ---r~ 80 90 1 oo

0.5 i 7~0

60

m

DISTANCE FROM DIKE C O N T A C T (% o f d i k e t h i c k n e s s )

:':-.:-= ~'~

Fig. 2. Amount of 0~5+ hydrocarbons (mill organic carbon) vs distance from dike contact. but parallel beds. Further, as noted in Table 1, two different methods were used for TOC analysis. The samples indicated by open circles in Fig. 3 were analyzed for TOC using Rock-Eval pyrolysis, whereas the earlier sample set (triangles) was analyzed by wet-chemical oxidation. The earlier sample

set has slightly lower EOM, Cls+ hydrocarbon content, and S/A ratios (Figs 1, 2 and 3), but higher pristane/phytane ratios. Desl~ite some variability in TOC for a given distance from the dike contact, a trend of decreasing values with increased heating is evident starting at m

i

-1,S

.

Z~

o

O

I

,','~

/

\

o

\

~'"

/

1.4 ~ "~

15()cm

lOOcm~

50'cm

j

f ~ f

~

o

o

0

/

\o \

t

o

\ \ \

\ ~o%

0

0 <

•

3

S

LU

3.0

2.5

lo 20 l S i l t s t o n e - ~ - -\-q.-.-Oike / \//.\ /~L--~.~:..~.~_~.~_..

2.0

li5

3'o

li0

io

0i8

50

oi6 60

% Rr V i t r i n ) t e

oi5 r

zo

~0

9'o

,oo

,io

1~o

13o

DISTANCE FROM DIKE CONTACT (% of d i k e t h i c k n e s s )

\ " / I \.oF-5- ":-= ' - - ~__.:-~Fig, 3. Ratio of saturated to aromatic (CI5+) hydrocarbons and organic carbon ( w t % ) vs distance from

dike contact.

Organic matter near an igneous dike about 90%d. This decrease may be due to primary variation in TOC content along the bed, but more likely it represents loss of carbon mainly as CO2 during initial thermal alteration. The TOC decrease through the hydrocarbon generation zone is about 0.1-0.2%, or 1000-2000ppm. The total decrease from 1.5 to 1.1 represents approximately 25% conversion of kerogen to liquid or gaseous products. Rock grain density was measured on chips (< 1 mm) of rock in water for selected samples over the range of decreasing TOC below 90%d to determine whether changes in dry rock density occur. TOC values, expressed as weight percent, are implicitly assumed to reflect variation in carbon content normalized to a constant rock density. Measured densities varied only slightly, ranging between 2.68 and 2.72 g cm -3 and showed no systematic change with increased heat. Therefore, the TOC decrease noted above is due to loss of organic mass and not to changes in dry rock properties.

139

to nC~8 decreases from an average of 0.4 in the unaltered zone to about 0.2 at R0 = 2.1%. The decreasing ratios of pristane and phytane relative to n-alkanes is interpreted to indicate preferential generation of n-alkanes during the main phase of saturated hydrocarbon generation. This observation is in general accordance with trends in saturated hydrocarbon composition reported for source rock maturation during burial heating (Albrecht et al., 1976).

ip

+

[

d : 119% of Chke th+ckness [155cm)

C[RO:05~ d : 73% o1 dike thickness [95cm) <

i

Fluorescence Microscopy of autofluorescence from blue and UV excitation of whole-rock polished samples shows changes that mirror the chemical changes. At levels of thermal alteration below 0.6% R0, the organomineral groundmass fluoresces yellowish green and the particulate liptinite bright orange. At maturity levels of 0.8-1.5% R o (where secondary bitumen has already been produced) the primary groundmass is much less fluorescent under UV excitation and the particulate liptinite fluoresces only dull gray orange. At this level of maturity, the mounting plastic has taken on bright green "extract" halos of secondary bitumen around rock grains. At high levels of maturity (1.5-1.7% R0), the organic groundmass has lost visible fluorescence even to blue excitation, though some liptinite grains still fluoresce dull gray orange. The plastic mount has no bitumen halos. At 1.7-2.3% R 0 a faintly fluorescent fine liptinite (?) chaff can be seen, visible only because all other fluorescence in the rock is extinguished. In summary, fluorescent parts (absorbed or bonded) of the primary groundmass and the lipitinite have become part of the secondary organic matter as a result of moderate heating in the zone of hydrocarbon generation. The secondary organic matter has then been driven from the rock or broken to non-fluorescent molecules at still higher temperatures.

D (Ro: 057) d : 58% Of dike Ihickness (75cm)

I

E (R e : 1 3 4 ) d : 42% o t d i ~ e Ih+cknes$ ( 5 5 ¢ m )

+

+

",_-.~

H (RO : 2 7)

Composition of saturated hydrocarbons

th,ckness [18cm~

Change in the composition of saturated hydrocarbons is evident in the capillary gas chromatograms (Fig. 4). A slight odd carbon number preference is present in the C2s423~ n-alkanes in the relatively immature samples not heated by the dike. Closer to the dike, at R0 values of about 1.0% and greater, the odd carbon number preference is diminished. The ratio of pristane to n CI7 decreases from about 1.3-0.3 over the range 0.4%-1.3% R0. The ratio of phytane

o-__

Fig. 4. Capillary gas chromatograms of saturated hydrocarbons at various distances (thermal maturities) from the dike contact.

140

J.L. CLAYTONand N. H. BOSTICK i 50cm

100crn

150cm

Z o n e of Hydrocarbon 5-

uJ Z l.>"r" n

Evolution

4-

,~,

3-

Z t-~

2.

1

0

3.0 iI 10

2.5 I

210

-.w--Dikl, I Siltstone.-~-

\, \.. / -/~/F- ----- ._--.

-

-

2.0 i

1.5 i

3fO

1.0 i

4qO

0.8 i

5~0

0.6 --~

60

L0 . 5

i

70

% R~

Vitrinite i

80

i

9tO

10 0

i 110

-7 120

i 130

DISTANCE FROM DIKE CONTACT (% of dike thickness)

Fig. 5. Pristane/phytane ratio vs distance from the dike. The ratio is relatively constant through the zone of hydrocarbon evolution. The ratio of pristane-to-phytane (pr/ph) (Fig. 5) does not show any significant change over the thermal alteration range represented by 0.4-1.1% R0. At alteration levels greater than 1.1% R 0 the pr/ph ratio decreases slightly (from about 3.1-2.4) for the closely-spaced set of samples (675 series, Table 1). With the exception of a sample with pr/ph of over 4 (sample 485G, Table 1), the preliminary 485 sample series shows a similar pr/ph decrease. This decrease agrees with data reported by Brooks et al. (1969) for Australian coals. Thermal alteration levels are somewhat higher in the present study than in Brooks' coal samples. The constant pr/ph values at lower maturity levels (up to 1% R0) is in contrast with Brooks' coal samples which had increasing pr/ph ratios. The present data are, however, in general agreement with studies of hydrocarbon generation in shales in deep basins. Sajgo (1980) reported that pr/ph ratios were "barely sensitive" to thermal stress in Tertiary rocks from the Pannonian Basin ranging from about 0.35 to 1.5% R0. For Sajgo's data, with the exception of three points, the ratio is fairly constant at about 1 up to 0.9-1.0% R0, then decreases slightly between 1.0 and 1.5% R0. Rashid (1979) observed increasing pr/ph ratios with increasing thermal alteration index (visual kerogen analysis), although the correlation between the two measurements was low (r = 0.48). Schiener and Perregaard (1981) found constant pr/ph ratios in Upper Cretaceous shales of Greenland adjacent to a sill for 0.6-1.0% R0. Stable carbon isotope ratios Figure 6 shows stable carbon isotope ratios (613C in %0 vs PDB) for the Ct5 + saturated and aromatic hydrocarbons fractions. The c~13C values of these fractions are relatively constant through the alteration range where the increase of hydrocarbon content occurs (0.6-0.9% R0, Fig. 2). No significant isotopic shift occurs, at least in these dike contact

samples, during C~5+ hydrocarbon generation. Above 0.9% R0 the &13C values rapidly become less negative by about 2%0 for both the saturated and aromatic hydrocarbon fractions. Apparently, at the very high alteration levels, thermal cracking of the C~5+ hydrocarbons in the dike contact has been accompanied by preferential breakage of carbon-12 bonds, making the residual C~5+ hydrocarbons isotopically heavier than the original starting material. As Fig. 6 and Table 1 show, the 6 ~3C values for saturated hydrocarbons increase more than for the aromatic hydrocarbons. The greater isotope shift for saturated hydrocarbons may be related to the decreasing S/A ratio observed over the higher thermal alteration levels ( > I. 1% R0). The C~5+ saturated-hydrocarbons are apparently thermally degraded more rapidly than the C~5+ aromatics causing both a decrease in the S/A ratio and a more pronounced shift to heavier isotopic values (kinetic effect). Atomic" H / C and hydrogen index Both the H/C atomic ratio and the hydrogen index (HI, Rock-Eval) show a progressive loss of hydrogen in the kerogen between 0.7 and 2.0% R0 (Fig. 7). This hydrogen loss continues to much higher reflectance than the main zone of C]5+ hydrocarbon occurrence (Fig. 2). The "missing" hydrogen may be attributable to rapid cracking of C~5+ hydrocarbons being generated from the kerogen or to direct generation of hydrocarbons lighter than C~5+, especially gas-range hydrocarbons, from the kerogen at this high level of thermal alteration. H/C and HI are not strictly comparable because different sample splits were used and because the H/C ratio can be affected by loss of some of the kerogen during recovery procedures following demineralization of the rock. This loss may be selective for certain kerogen constituents depending upon morphology, particle size, and chemical composition

Organic matter near an igneous dike to

50'cm

OO

100'cm

150 cm

Zone of Hydrocarbon Evolution {

-30 -

o

141

-29-

-28t.) 12.

o =

~-~,-

~z ~

-26 -

-25-

03

-24-

O ¢'3 cO

3.0 iI 10

-23 - 0 -.=-Dike

2.5 ~ i 20

2.0 I

1.5 i

Siltstone.-~

1.0 i

i 40

3'0

DISTANCE

0.8 i 50

FROM

0.6 { I 60

0 . 5 %R r I i 70

Vitrinite i 80

t 90

(% of d i k e

DIKE CONTACT

i 100

, 110

i 120

I 130

thickness)

Fig. 6. Stable carbon isotopic composition of C~s+ aromatic and saturated hydrocarbons vs distance from the

dike

(Pearson, 1981). The organic matter in the siltstones at the Wolcott site is mostly like that of usual humic coals. Nonetheless, the H/C ratio for our unaltered samples is slightly lower than that of most coals at comparable maturities (0.5% R0). Approaching the dike contact, the H/C ratio decreases regularly to the same level as in anthracites. The very low HI values of the unaltered rocks (Fig. 7) do not result simply from a high inertinite content,

contact.

because, for example, bitumenous coals with inertinite as high as 30% by volume still may have HI values in the 100-200 range. In any case, the maturity where the main HI decrease occurs in our samples (0.7-1.7% R0) is about the same maturity where the main decrease occurs in coals.

Estimated temperatures Based on vitrinite reflectance of samples of clay-

50'cm

.8 -~-80

100cm JL

150cm H/C &..,~

'

0 ~-.

.6--60

1

=

,,',

_o

-~

< O

170%

HI

~ C o r g . = 1 . 8 % (far above range ' of other samples) i

.4 - - 4 0 r., ~

"1Z LU

X LU

o

O or" LLI

'

_z Z UJ .2--20

,~ n>"1-

3.0 0 I0 ~.-- D i k e I S i l t s t o n e ~ Fig.

7. Kerogen

A 2.5" 2'0

2.0 i

3'0

1.5 I i

4'0

1.0 i

0.8 i

5'0

0.6 I

60

0.5 i

% Rr

7'0

DISTANCE FROM DIKE CONTACT (% atomic

H/C

ratio

and

Hydrogen

Index

Vitrinite

8'0

9'0

,io

I (~0

I~o

of dike t h i c k n e s s )

(Rock-Eval)

vs distance

from

dike

contact.

11o

142

J.L. CLAYTONand N, H. BOSTICK

rich lignite heated for one month in water in pressure bombs to known temperatures (Bostick, 1970), we estimate 200°C for initiation of hydrocarbon generation, 300°C for the peak of Cjs+ occurrence, and 350°C for the end of the main decline in H/C and HI values for the dike-heated samples (Fig. 1). The temperatures are slightly lower (with respect to %d) than those adjacent to some other dikes, but the temperature-distance profile shows the characteristic parallelism to the profile plotted from heat-flow theory (Bostick and Pawlewicz, 1984). These tempratures are about 150°C higher than those at which corresponding geochemical changes occur in sedimentary basins, yet the duration of heating to near maximum adjacent to the dike was probably only about one month. The geochemical features are remarkably similar to those of burial catagenesis. The virtrinite in samples close to the dike does not show features of coke, probably because it is very fine grained and was below "coking-quality" vitrinite at the time of intrusion. Some solid bitumen with features characteristic of high temperatures occurs in samples close to the dike as mineral grain coatings with fine mesophase structure and as rare spherulites with extinction-cross anisotropy.

REFERENCES

Akiyama M. (1979) Thermal alteration of kerogen by basalt dikes intruded in the Oligocene Poronai Formation, Hokkaido, Japan. J. Fac. Sci. Hokkaido Univ. 19, 149-156. Albrecht P., Vandenbroucke M. and Mandengue M. (1976) Geochemical studies on the organic matter from the Douala Basin (Cameroon)--I. Evolution of the extractable organic matter and the formation of petroleum. Geochim. Cosmochim. Acta 40, 791-799. Alteb~iumer F. J., Leythaeuser D. and Schaefer R. G, (1983) Effect of geochemically rapid heating on maturation and hydrocarbon generation in Lower Jurassic shales from NW-Germany. In Advances in Organic Geochemistry 1981 (Edited by Bjoray M. et al.), pp. 80-86. Wiley, New York. Baker E. E., Huang W. Y., Rankin J. G., Castano J. R., Guinn J. R. and Fuex A. N. (1977) Electron paramagnetic resonance study of thermal alteration of kerogen in deep-sea sediments by basaltic sill intrusion. In Initial Reports o f the Deep Sea Drilling Project (Edited by Lancelot Y., Siebold E. et al.), Vol. 41, pp. 839-847. U.S. Govt Printing Office. Bostick N. H. (1970) Thermal alteration of clastic organic particles (phytoclasts) as an indicator of contact and burial metamorphism in sedimentary rocks. Dissertation, Stanford University, 220 pp. Bostick N. H. and Pawlewicz M. J. (1984) Paleotemperatures based on vitrinite reflectance of shales and limestones in igneous dike aureoles in the Upper Cretaceous Pierre Shale, Walsenburg, Colorado. In Hydrocarbon source rocks in the greater Rocky Mountain region

SUMMARY AND CONCLUSIONS

Organic geochemical studies of siltstone at the Wolcott dike contact provide an opportunity to observe effects of heating on the organic components of the rock without significant variation in the original depositional environment and/or organic source input. Despite the rapid heating rate compared to burial heating, the organic geochemical measurements show systematic changes approaching the dike that are generally analogous to those often observed with increasing burial in a sedimentary basin. The main increase in C~5+ hydrocarbon and total extractable organic matter occurs between vitrinite reflectance values of 0.6-1.0%, The increasing S/A and n-alkane/isoprenoid ratios suggest preferential generation of n-alkanes, similar to catagenesis in many sedimentary basins. The pristane/phytane ratio is relatively constant over the C~5÷ hydrocarbon generation zone, indicating that this ratio is insensitive to thermal stress associated with petroleum generation, and, as previous studies have suggested, this ratio is an indicator of original source input or depositional conditions. At high maturities (greater than about 1.0% R0) the pristane/phytane ratio decreases slightly. The ~5~3C values for C15+ saturated and aromatic hydrocarbons are constant to an R 0 of about 1%, then become isotopically heavier (i.e less negative) at higher maturities. Acknowledgements--A. Cook, I. Koncz, and M. Pawlewicz

helped with sample collection. A. Love did the HCN analyses. T. Daws did the Rock-Eval analyses. Sr. C. Lubeck did the extractions and gas chromatography.

(Edited by Woodward J., Meissner F. F. and Clayton J. L.), pp. 387-392. Rocky Mountain Association of Geologists, Denver. Brooks J. D., Gould K. and Smith J. W. (1969) Isoprenoid hydrocarbons in coal and petroleum. Science 222, 256-259. Bush P. R. (1970) A rapid method for determination of carbonate carbon and organic carbon. Chem. Geol. 6, pp. 5%62. Clayton J. L. and Swetland P. J. (1978) Subaerial weathering of sedimentary organic matter. Geochim. Cosmochim. Acta 42, 305-312. Clayton J. L. and Swetland P. J. (1980) Petroleum generation and migration in Denver Basin. Am. Assoc. Pet. Geol. Bull. 64~ 1613-1633. Dow W. G. (1977) Contact metamorphism of kerogen in sediments from Leg 41, pp. 1113-1118. U.S. Govt Printing Office. Durand B. and Espitalie J. (1976) Geochemical studies on the organic matter from the Douala Basin (Cameroon)--II. Evolution of kerogen. Geochim. Cosmochim. Acta 40, 801-808. Espitali6 J., Madec M., Tissot B., Menning J. J. and Leplat P. (1977) Source rock characterization method for petroleum exploration. Proc. Offshore Technol. Conf., pp. 439-444. LaPlante R. E. (1974) Hydrocarbon generation in Gulf Coast Tertiary sedments. Am. Assoc. Pet. Geol. Bull. 58, 1281-1289. Pearson D. B. (1981) Experimental simulation of thermal maturation in sedimentary organic matter. Dissertation, Rice University, 563 pp. Perregaard J. and Schiener E. J. (1979a) Thermal alteration of sedimentary organic matter by a basalt intrusive (Kimmeridgian Shales, Milne Land, East Greenland). Chem. Geol. 26, 331-343. Perregaard J. and Schiener E. J. (1979b) Organic geochemical and microscopical study of sedimentary organic matter, central West Greenland. Report o f Activities, 1978, Greenland, Geol. Unders, Repp., No. 95, pp. 55-60. Philippi G. T. (1965) On the depth, time, and mechanism of

Organic matter near an igneous dike petroleum generation. Geochim. Cosmochim. Acta 29, 1021-1049. Pittion J.-L. and Bostick N. H. (1981) MOD 20-24 ring analysis, preliminary report. International Committee for Coal Petrology, Commission 2, Internal Document, 39 PP. Rashid M. A. (1979) Pristane-phytane ratios in relation to source and diagenesis of ancient sediments from the Labrador Shelf. Chem. Geol. 25, 109-122. Sajgo Cs. (1980) Hydrocarbon generation in a super-thick neogene sequence in South-east Hungary. A study of the extractable organic matter. In Advances in Organic Geochemistry 1979 (Edited by Maxwell J. R.), pp. 103-113. Pergamon Press, Oxford. Schiener E. J. and Perregaard J. (1981) Thermal maturation of organic matter by a thick basic sill in Upper Cretaceous Shales, Svartenhuk Halvo, Central West Greenland. Gr~nlands Geol. Unders., Rapp. No. 102, 16 pp. Simoneit B. R. T., Brenner S., Peters K. E. and Kaplan I. R. (1981) Thermal alteration of Cretaceous black shale by diabase intrusions in the Eastern Atlantic--II. Effects on bitumen and kerogen. Geochim. Cosmochim. Acta 45, 1581-1602.

143

Spiro B. (1984) Effect of the mineral matrix on the distribution of geochemical markers in thermally affected sedimentary sequences. In Advances in Organic Geochemistry 1983 (Edited by Schenck P. A., de Leeuw J. W. and Lijmbach G. W. M.). Org. Geochem. 6, 543-559. Pergamon Press, Oxford. Tissot B. and Welte D. (1978) Petroleum Formation'and Occurrence, 538 pp. Springer, New York. Tissot B., Califet-Debyser Y., Deroo G. and Oudin J. L. (1971) Origin and evolution of hydrocarbons in early Toarcian shales, Paris basin, France. Am. Assoc. Pet. Geol. Bull. 55, 2177-2193. Tissot B., Durand B., Espitali6 J. and Combaz A. (1974) Influence of nature and diagenesis of organic matter in formation of petroleum. Am. Assoc. Pet. Geol. Bull. 58, 499-506. Ujii6 Y. (1984) Thermal alteration of kerogen as an indicator of contact metamorphism to sedimentary rocks: IH-NMRTI and elemental compositon. Geochem. J. 18, 163-166.