Uinta Basin Wurtzilite: a product of natural vulcanization

Pergamon 0146-6380(94)00081-6

Org. Geochem. Vol. 22, No. I, pp. 12%136, 1995 Copyright © 1994 Elsevier Science Ltd

Printed in Great Britain. All fights reserved 0146-6380/95 $7.00 + 0.00

Uinta Basin Wurtzilite: a product of natural vulcanization?

TIM E. RUBLE and R. PAUL PHILP

Department of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A.

Abstract--the macromolecular structure of the pyrobitumen wurtzilite has been examined by a variety of chemical and analytical techniques, including desulfurization with lithium in ethylamine. The results of these analyses provide strong evidence for the presence of intermolecular sulfur linkages in the wurtzilite geopolymer matrix, which are thought to have formed via a process of low temperature "natural vulcanization". The polymerized hydrocarbon components appear to be a complex mixture of n-alkanes, isoprenoids, and heterocompounds, offering further insight toward a bulk structural model for wurtzilite.

Key words--wurtzilite, pyrobitumen, geopolymer, vulcanization, organic sulfur bonds, lithium in ethyl- amine, pyrolysis, Uinta Basin, Utah

INTRODUCTION

Wurtzilite is an asphaltic pyrobitumen found in vari- ous localities in the western portions of the Uinta Basin, Utah (Abraham, 1945). Known popularly by the trade name "elaterite", wurtzilite has also been called "elastic bitumen" or "mineral rubber" because of its sectile and somewhat flexible physical properties (Ladoo, 1920). Previous geochemical investigators have established that wurtzilite is enriched in sulfur in comparison to other native bitumens from the Uinta Basin and it has been proposed that the sulfur acts as a cross-linking agent in the macromolecular structure (Hunt et al., 1954; Bell and Hunt, 1963; Hunt, 1963; Heidberg and Krejci-Graf, 1969; Khavari-Khorasani, 1984; Ruble, 1990; Khavari- Khorasani and Michelsen, 1993). In this investi- gation, wurtzilite has been examined by a variety of chemical and analytical techniques in an attempt to conclusively demonstrate the presence of intermol- ecular sulfur linkages in the wurtzilite geopolymer matrix.

emplacement of this native bitumen are still under investigation, current theories suggest the large verti- cal dikes, now filled with solid hydrocarbons, origi- nated as hydraulic extension fractures from over- pressured, organic-rich source beds during the early stages of post-Laramide regional tectonic extension (Verbeek and Grout, 1992). Extensive geochemical analyses have established the Saline Facies of the upper Green River Formation as the probable source rock of the wurtzilite (Hunt et aL, 1954; Bell and Hunt, 1963; Hunt, 1963). Other geochemical analyses suggest the bitumen originated as an immature product of early hydrocarbon generation and that it may have undergone limited bacterial degradation subsequent to its emplacement (Douglas and Grantham, 1974; Hatcher et al., 1989, 1992; Ruble, 1990; Ruble and Philp, 1991a). For further infor- mation regarding the locations, uses, classification, and physical and chemical characteristics of wurtzilite, see Ladoo (1920), Abraham (1945), Craw- ford (1949), Hunt et al., (1954), Bell and Hunt (1963), and Hunt (1963).

GEOLOGICAL SETTING

The Uinta Basin encompasses an area of approx. 24,000km 2 (Osmond, 1964) in northeastern Utah (Fig. 1). From the Late Cretaceous through the Middle Eocene more than 2200 m of siliciclastic and carbonate lacustrine sediments were deposited in this asymmetrical basin, including the oil shales of the Green River Formation (Picard and High, 1968). These organic-rich source rocks are the origin of a unique assemblage of solid hydrocarbon deposits which include: gilsonite, ozocerite, tabbyite, albertite, ingramite and wurtzilite (Hunt et al., 1954).

Wurtzilite occurs as fracture-filling vein deposits in the area of Indian Canyon southwest of Duchesne, Utah (Fig. 1). Although the exact mechanisms of

EXPERIMENTAL

Sample locations

The wurtzilite sample was provided by Dr J. M. Hunt and is from the Ranger Station Mine located in Sec. 27, T6S, R7W, Duchesne County, Utah.

Extraction and fractionation

The powdered sample was extracted in a soxhlet apparatus for 48h using a 1:1 v/v mixture of dichloromethane and methanol. Asphaltenes were removed from the extract by precipitation with n- pentane at 4°C, followed by centrifugation to give the asphaltene and maltene fractions. The maltene frac- tion was then separated into saturate, aromatic, and polar (NSO) fractions by thin-layer chromatography

oG 22/i-. 127

128 TIM E. RUBLE and R. PAUL PHILP

. . . . . . . . . 4 I Wyoming I

Uinta i _ f F basin ~ '/Grand

Junction

[ Utah 7 ] ./ Utah Ooxo~D Colorado

. '~ " x 0 100 ~ t \ I i

k3 \ o r ~" ¢'~ ~ ~ ~ Miles

.q~ • Vernal ,.-b~ II \

~ Roosevelt • ~

I / i . Limit of tertiary sediments ~ ~ . ~ X ~ , i i

Bituminous sanYdSems .%)~ I

Modified from Porter (1963); Hunt et al. (1954)

Fig. I. Base map of the Uinta Basin, showing locations of various native bitumen deposits.

on Silica Gel 60 mesh plates (0.25mm thick, 20 cm x 20 cm), using hexane as developer. Ultra- violet light and rhodamine indicator were used to distinguish the three fractions, which were recovered by removing the silica gel from the plates and exhaus- tively extracting it with dichloromethane.

Hydrous pyrolysis

A 500 mg sample of unextracted wurtzilite was sealed in a pre-conditioned stainless steel vessel (25 ml capacity) with 10 ml of distilled water. The vessel was sealed and heated at 350°C for 72 h. The reaction was

Uinta Basin wurtzilite 129

then quenched by cooling the vessel with dry ice. Upon opening the vessel, a strong smell of liberated H2S gas was detected and the previously insoluble pyrobitumen was now completely soluble in dichloro- methane. The pyrolysate was recovered by rinsing the vessel with dichloromethane and extracting the water with 3 x 15 ml of solvent. Asphaltenes were removed from the pyrolysate and the maltene fraction separ- ated by thin-layer chromatography as described above.

Lithium /ethylamine treatment

The treatment of the wurtzilite with lithium in ethylamine was performed by Dr Wayne N. Harrison at the University of Bristol, Bristol, U.K. Details of the experimental conditions, including the procedures for fractionation and elemental analysis, are given in Hofmann et al. (1992).

Gas chromatography (GC)

Saturated hydrocarbon fractions were analyzed using a Varian 3300 gas chromatograph equipped with a programmable on-column injector (SPI) and a fused silica capillary column (DB-5 phase, 30 m x 0.32 mm i.d., 0.25/~m film thickness, J&W Scientific, Folsom, Calif.). The GC was equipped with an effluent splitter and a flame ionization detec- tor (FID) for the detection of hydrocarbons and a flame photometric detector (FPD) for the detection of organosulfur compounds. Operating conditions were as follows: oven temperature program: 40-300°C at 4°C/min, then a 25 min isothermal at 300°C; SPI temperature program: 0-300°C at 180°C/min, then isothermal at 300°C for the duration of the run; detector temperatures: 300°C; and helium carrier gas with a flow rate of I ml/min. Data were collected and processed in digital format using Nel- son Analytical model 2600 Chromatography Soft- ware (Nelson Analytical, Inc., Cupertino, Calif.).

Gas chromatography-mass spectrometry (GC-MS)

GC-MS analyses were conducted with a Finnigan MAT TSQ 70 spectrometer coupled to a Varian 3400 gas chromatograph inlet system equipped with a fused silica capillary column (DB-5 phase, 30m x 0.32mm i.d., 0.25/~m film thickness, J&W Scientific, Folsom, California). GC operating con- ditions were the same as above, as well as transfer line temperature, 300°C; ion source temperature, 200°C; helium carrier gas, flow rate 1 ml/min. The ion source was operated in the electron impact (El) mode at an electron energy of 70 eV.

Pyrolysis-gas chromatography (Py-GC)

Py-GC was performed using a Chemical Data Systems pyroprobe coupled to a Varian 3300 GC. A standard dilution of the asphaltene fraction in tolu- ene was quantitatively transferred in a 200 g g aliquot into 1 cm quartz tubes filled with glass wool plugs. In addition, a 0 .6gg of an internal standard (poly p-t-butylstyrene, Polyscience, Inc.) was added to the quartz tube as a standard solution dissolved in a I : I : 1 mixture of toluene, dichloromethane, and acetone. The quartz tube was heated at 150°C for 45 min to evaporate the solvent and then placed in the pyroprobe and subsequently pyrolyzed at 800°C for 20 s. The pyrolysis products were transferred from the heated interface block under a stream of helium directly onto a fused silica capillary column (DB-5 phase, 30 m x 0.25 mm i.d., 0.25/~m film thickness). The GC was equipped with an effluent splitter and FID and FPD detectors. Conditions of operation were as follows: injector temperature, 300°C; oven temperature program, -25°C for 4 min then - 2 5 to 300°C at 4°C/min, followed by 35 min isothermal at 300°C; detector temperatures, 300°C; and the inter- face temperature, 300°C.

RESULTS AND DISCUSSION

Solvent extract

Using the generic classification scheme developed by Abraham (1945) and later modified by Hunt et al. (1954), wurtzilite is classified as a pyrobitumen based primarily on its insolubility in carbon disulfide. Our results support this classification in that the extract accounted for only 6.5 wt% of the original bitumen (Table 1). The majority of the components in the extract were high molecular weight asphaltene and polar constituents, with the saturated hydrocarbon fraction accounting for only 14.5% of the extract (Table 1). The primary components in the saturated hydrocarbon extract were isoprenoids, steranes, and hopanes (Fig. 2). The conspicuous absence of n-al- kanes may be related to the immature expulsion of this bitumen or the initial effects of biodegradation. The presence of low molecular weight isoprenoids, which are typically removed at moderate levels of biodegradation, suggests that any biological alter- ation has been mild (Palmer, 1993).

The distribution of the sterane and hopane bio- markers are illustrated in the mass chromatograms shown in Figs 3 and 4. The steranes are dominated by the C27 components and exhibit distributions

Table I. Bulk chemical composition data for wurtzilite solvent extract and hydrous pyrolysate

%EOM* %SATt %AROMt %POLARt %ASPHt

Wurtzilite 6.5 14.5 13.4 41.0 3 I. 1 Wurtzilite 100 12.3 13.6 35.7 38.3 Hydrous pyrolysate

*Expressed as % of weight of original bitumen. tExpressed as % of total EOM.

130 TIM E. RUBLE and R. PAUL PHILP

r 1 WURTZILITE SATURATE FRACTION

St¢l'ane$ G anlmacgran~ Phytane , ,

WURTZILITE HYDROUS PYROLYSIS SATURATION FRACTION n-C28

WURTZILITE Li/EtNH2 HEXANE FRACTION . } I [

l i . 2 21'.t 28'.9 31.7 14'.5 52'.3 60.1 68 75.8 Minutes

8;.6 91.,

Fig. 2. Gas chromatograms of the various aliphatic fractions analyzed with the FID. Note the absence of n-alkanes in the solvent extract.

typical of immature hydrocarbons (Mackenzie et al., 1980) with low 20S/(20S+20R) and 5ot,14fl,17fl/ (5~,14~,17~ +5~,14fl,17fl) ratios and include the presence of 5fl-steranes. The terpanes are dominated by gammacerane, derived by reduction of tetrahy- manol (Venkatesan, 1989; ten Haven et al., 1989), and a source specific indicator of aerobic protozoa which appear to have flourished during the later hypersaline depositional stage of ancient Lake Uinta. For further discussion of the paleoenvironmental significance of the biomarker distributions see Ruble and Philp (1991a, b) and Ruble et al. (1994).

Hydrous pyrolysate

In contrast to the mainly insoluble nature of the original wurtzilite, the wurtzilite hydrous pyrolysate was completely soluble in dichloromethane. In spite of this difference, the distribution of components in the various fractions was very similar to that found in the solvent extract and was again dominated by the polar and asphaltene components (Table 1). The distribution of saturated hydrocarbons in the py- rolysate differed significantly from the bitumen ex- tract, with the hydrous pyrolysis releasing a series of n-alkanes extending from C~3 to C40 (Fig. 2). A series of low molecular weight C4 benzothiophenes was also detected in the saturated hydrocarbon fraction (Fig. 5), this series represents only a small portion of the volatile organosulfur compounds and H2 S produced

during hydrous pyrolysis as detected by the pungent odor released when the reaction vessel was initially opened.

The sterane and hopane distributions in the hy- drous pyrolysate are very similar to those in the solvent extract (Figs 3 and 4). This suggests that under the conditions used in this experiment, the effects of the artificial maturation on the distributions was slight• A slight decrease was observed in the abundance of the 5ct,14ct,17~t-(20R) steranes in the pyrolysate relative to their distribution in the solvent extract (Fig. 3). However, no dramatic shifts were observed in the sterane maturity parameters that would be expected if these parameters were solely dependent upon thermally mediated isomerization and epimerization reactions (Mackenzie and McKen- zie, 1983). This observation is consistent with the conclusions of Abbott et al. (1990) that direct chiral isomerization at C-20 in the non-rearranged steranes appears to be relatively unimportant during hydrous pyrolysis.

Pyrolysis-gas chromatography

The Py-GC (Fig. 6; FID) of the wurtzilite asphalt- enes is very similar to a previously reported flash pyrogram of wurtzilite analyzed on a comparable system (Khavari-Khorasani and Michelsen, 1993). The hydrocarbon components are dominated by a homologous series o f n-alkene/n-aikane doublets

Uinta Basin wurtzilite 131

% 100

80

60

40

20

0

% 100

80

60

2;

40

20

0

% 100

80

60

40

20

WURTZILITE SATURATE FRACTION

4

1

WURTZILITE HYDROUS PYROLY, q 4

1

12

10 /

;IS SATURATE FRACTION

12

7 8 910

' ' 3

• . . ! • • . . . . . . . , . . . . o . . . . . . . . . . . . .

WURTZILITE Li/EtNII z HEXANE FRACTION

1

12

8 9 10

. , .-.- . " . . ;. . ,: : ,

*E+05 2.341

*E+05 1.028

*E+04

5.352

TIME

Fig. 3. m/z 217 mass chromatograms showing the distribution of the steranes in the various aliphatic fractions. Peak identifications are given in Table 3.

132 TIM E. RUBLE and R. PAUL PHILP

% 100

80

60

40

20

%

>, t--

Z [- z

0

100 ;

80

6 0

WURTZILITE SATURATE FRACTION 7

% 100 ]

8o] 6 0

4 18 10 sll

I • ~ I ! I J I " i • I " w • 1 " ~ " ! " i •

WURTZILITE HYDROUS PYROLYSIS SATURATE FRACTION 7

4

1 I i ! i

40'

WURTZILITE Li/EtNII2 ItExANI~ FR/kcTI()N

7

4 0

20

A4 4 1 2 3J~A 5 6

! i I i v

8 9 ,10,~-~.,/%X-, ~ • I ! I t

*E+05 8.625

*E+05 5.075

*E+04 1.967

TIME >

Fig. 4. m/z 191 mass chromatograms showing the distribution of the hopanes in the various aliphatic fractions. Peak identifications are given in Table 4.

Uinta Basin wurtzilite 133

WURTZILITE HYDROUS PYROLYSIS SAT (FID)

n-C 15 Pristane n-C20

C4 Benzothio )henes

n-C24

15 19.3 23.6

WURTZILITE HYDROUS PYROLYSIS SAT (FPD)

27.9 32.2 36.5 40.8 45.1 49.5 53.8 58.1

Minutes

Fig. 5. Gas chromatograms of the aliphatic fraction of the hydrous pyrolysate. The top chromatogram shows the hydrocarbon distribution analyzed with the FID, while the bottom chromatogram shows the

distribution of organosulfur compounds analyzed with the FPD.

WURTZILITE ASPHALTENE (FID)

2

4 .3

0 10 20

WURTZILITE ASPHALTENE (FPD)

10

8 11

I 12 13 I . . . d l . i . . . . .

3o 4o

Minutes

60 70 80 96 100

Fig. 6. Pyrolysis-gas chromatograms of the wurtzilite asphaltene fraction. The top chromatogram shows the hydrocarbon distribution analyzed with the FID, while the bottom chromatogram shows the distribution of organosulfur compounds analyzed with the FPD. Peak identifications are given in Table 5.

134 TIM E. RUBLE and R, PAUL PH1LP

Table 2. Bulk chemical composition data for treatment of wurtzilite with lithium in ethylamine

Frac I Frac 2 Frac 3 Frac 4 Product yield';- yield't yield+ yield+

TOC TOC* Stol Sto t * yield';- (hexane) (Hex/Tol) (Tol/MeOH) (MeOH)

Wurtzilite 78.5 57.5 4.3 2.9 117 1.4 0 t 12.4 3.2

*After Li/EtNH 2 reduction. +Expressed as mg/g TOC.

which extend past C30. Aromatic and isoprenoid hydrocarbon components were relatively insignificant compared to the alkenes and alkanes in this sample. Another important series of peaks detected in the PY-GC was that of H2S and the organosulfur com- pounds (Fig. 6; FPD). These tow molecular weight organosulfur compounds are dominated by thio- phene and various methyl substituted thiophenic compounds.

Lithium /ethylamine extract

The results of the pyrolysis experiments suggest the presence of significant sulfur cross-linkages within the wurtzilite geopolymer matrix. To examine this hy- pothesis further, wurtzilite was reacted with lithium in ethylamine (Li/EtNH2), a powerful reducing agent which will reductively cleave sulfides, and diaryl, alkyl-aryl and alkyl-allyl ethers, as well as esters (Hofmann et al., 1992; and references therein). This treatment has also been previously used effectively to reduce sulfur linkages in macromolecular kerogens and asphaltene fractions (Hofmann et al., 1992).

The exhaustively extracted wurtzilite was reacted with Li EtNH 2 for 10 h, giving a total product yield of 117 mg/g TOC and resulting in a 33% reduction in S,o, content (Table 2). The reaction products were fractionated, with the polar components accounting for most of the eluent (Table 2). The saturated hydrocarbon fraction (1.15%; Fig. 2) is dominated by n-alkanes from C~6 to C~0 and isoprenoids, typical of the compounds released by pyrolysis. Sterane and hopane distributions are very similar to those in the solvent extract, with the exception of an enhanced relative abundance of gammacerane in the Li/EtNH 2 extract in comparison to its relative abundance in the solvent extract (Fig. 3). The results of this experiment provide strong evidence for the presence of inter- molecular sulfur linkages in the wurtzilite geopolymer matrix.

Table 3. Peak identification of steranes shown in Fig. 3

Peak No. Compound

I 14~,17~-Cholestane (20S) + 5fl-cholestane 2 14//,17/¢-Cholestane (20R) 3 14t3,17fl-Cholestane (20S) 4 14~,17~-Cholestane (20R) 5 24-Methyl- 14~,17~-cholestane (20S) 6 24-Met hyl- 14fl, 17/Lcholestane (20R) 7 24-Methyl- 14fl, I 7fl-cholestane (20S) S 24- Methyl- 14c~, 17cc-cholestane (20R) 9 24-Ethyl- 14c~,17~-cholestane (20S)

10 24-Ethyl-14fl,17//-cholestane (20R) + 5fl-ethylcholestane II 24-Ethyl- 14fl, 17fl -cholestane (20S) 12 24-Ethyl- 14~,17~-cholestane (20R)

Wurtzili te structural model

Khavari-Khorasani and Michelsen (1993) have re- cently proposed a bulk structural model for the wurtzilite geopolymer based on pyrolysis and X-ray diffraction data. In this model, the wurtzilite structure is proposed to consist of aliphatic and non-condensed aromatic components connected by intermolecular sulfur linkages (Fig. 7) and is very poor in other hetero-atomic compounds. Our results clearly sup- port the incorporation of aliphatic hydrocarbons released by pyrolysis and treatment with lithium in ethylamine as major cross-linked components in the wurtzilite geopolymer. Our results also suggest that some portions of the Khavari-Khorasani and Michelsen model need to be revised.

The absence of aromatic hydrocarbon components in the products released by the treatment with lithium in ethylamine casts some doubt as to their significant incorporation into the wurtzilite geopolymer. The minor occurrence of these components in the py- rolysates may represent trapped hydrocarbons rather than bound moieties. Additionally, our analyses suggest that hetero-atomic compounds do constitute a significant proportion of the cross-linked species in the wurtzilite geopolymer matrix. The large pro- portion of this class of compounds in the products released by desulfurization imply that heteroatomic species other than organosulfur compounds may be present. This is further suggested by elemental analy- ses which have shown a relatively high average weight percentage of nitrogen in wurtzilite (2.2%; Hunt et al., 1954) and by preliminary GC and Py-GC results which indicate that heterocyclic organonitrogen com- pounds may be very important polar constituents in the wurtzilite geopolymer (Ruble, 1990).

CONCLUSIONS

Vulcanization is the chemical reaction in which a polymer is heated with a few percent by weight of

Table 4. Peak identification of hopanes shown in Fig. 4

Peak No. Compound

t 17~ (H)-22,29,30-Trisnorhopane (T m ) 2 17a (H),21//(H)-30-Norhopane 3 17fl (H),21~ (H)-Normoretane 4 17~t (H),2 lfl (H)-Hopane 5 17fl (HL21~ (H)-Moretane 6 22S-17~ (H),21fl (H)-30-Homohopane 7 Gammacerane + 22R- 17x (H),2 lfl (H)-30-Homohopane 8 22S and 22R-17~ (H),21fl(H)-30,3t-Bishomohopane 9 22S and 22R-17~ (H),21fl(H)-30,31,32-Trishomohopane

10 22S and 22R-17~ (H),21//(H)-Tetrakishomohopane I 1 22S and 22R- 17~ (H),21 fl (H)-Pentakishomohopane

Uinta Basin wurtzilite 135

Table 5. Pyrolysate peak identifications shown in Fig. 6

Peak No. Compound

1 C 6 n -Alkene 2 Toluene 3 Internal standard 4 Ct4 n-Alkene 5 Prist- 1 -ene 6 C2z n-Alkene + n-alkane 7 "'gas" peak (H2S, COS, SO2) 8 Thiophene 9 2-Methylthiophene

10 3-Methylthiophene 11 2,3-Dimethylthiophene 12 Ethylmethylthiophene 13 Trimethylthiophene

sulfur to bring about the formation of cross-linkages between the long polymer chains (Petrucci, 1982). In the case of vulcanized natural rubber, disulfide cross- linkages connect cis-polyisoprene polymer chains, forming a substance which is both stronger and more elastic (Petrucci, 1982). Our results indicate that wurtzilite is a geopolymer which has undergone a process of "natural vulcanization" resulting in the formation of intermolecular sulfur linkages connect- ing various hydrocarbon and heterocyclic com- ponents. In contrast to the vulcanization of natural rubber, wurtzilite vulcanization apparently occurred at relatively low geothermal temperatures ( ~ 100°C; Anders et al., 1992). Also, unlike the relatively simple polymerized structure of natural rubber, the polymer-

WURTZILITE

/Vk/N

®

Aliphatic structures

Non-condensed aromatic structures

Poly-condensed aromatic layers

Sulphur

Modified from Khavari Khorasani and Michelsen (1993)

Fig. 7. Bulk structural model for wurtzilite proposed by Khavari-Khorasani and Michelsen (1993).

ized hydrocarbon components in wurtzilite appear to be more complex, consisting of n-alkanes, iso- prenoids, and various unidentified heterocompounds which require further investigation. We suggest that the formation of wurtzilite via a process of "natural vulcanization" is consistent with the previously pro- posed origin of sulfur-rich geomacromolecules (Sin- ninghe Damst6 et al., 1988; Kohnen et al., 1991) and involved the intermolecular reaction of reduced inor- ganic sulfur species with functionalized biolipids during early diagenetic emplacement of the wurtzilite veins.

Acknowledgements--The authors would like to acknowl- edge the helpful assistance of J. M. Hunt for collecting and providing the samples used in this study, W. N. Harrison and J. R. Maxwell for performing the lithium in ethylamine treatment, and A. Galvez-Sinibaldi for assistance in per- forming the hydrous pyrolysis experiments.

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