Tricyclic terpanes in Precambrian bituminous sandstone from the eastern Yanshan region, North China

CHEMICAL GEOLOGY i\,i,!1111 ISOTOPE

ELSEVIER

GEOSCIE.VCE

ChemicalGeology 120 (1995) 155-170

Tricyclic terpanes in Precambrian bituminous sandstone from the eastern Yanshan region, North China T.-G. Wang”, Bernd R.T. Simoneitb ‘Geoscience Department, Universityof Petroleum, Beijing 102200, Peoples Republic of China bPetroleum and Environmental Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA

Received 30 March 1994; revision accepted 7 September 1994

Abstract A bituminous sandstone at the base of the Xiamaling Formation (Precambrian) is a fossil oil pool which was thermally altered into reservoir bitumen N 760 Ma ago. This reservoir bitumen was extracted, fractionated and analyzed for biomarkers. A novel series of extended tricyclic terpanes (C,,&&) with a 13a( n-alkyl)-side chain configuration was identified by co-injection of two standards ( Cr8, CzO) and CC-MS analysis. Some bicyclic sesquiterpanes, C18-norditerpanes, regular extended tricyclic terpanes (C18-C24, no Cz2), and alkylcyclohexanes were also detected. The source of the organic matter, its alteration in the sandstone reservoir and the origin of its novel tricyclic terpanes are all inferred to be microbial.

1. Introduction Organic matter in Precambrian sediments from many countries has been investigated over the past twenty years. Many authors (e.g., McKirdy, 1974; McKirdy and Hahn, 1982; Summons and Walter, 1990; Summons, 1992; Imbus and McKirdy, 1993 ) have provided comprehensive reviews on hydrocarbons as Precambrian biomarkers, dealing with normal and branched alkanes, acyclic isoprenoids, alkylcyclohexanes, alkylbenzenes, triterpanes, steranes, Cmethylsteranes, and mono- and poly-aromatic steroid hydrocarbons. Hoering ( 1976) analyzed a Precambrian oil seep from the Nonesuch shale in the White Pine copper mine, Michigan, U.S.A., and found no detectable sterane and triWA1

terpane hydrocarbons. Later he reported low amounts of triterpenoid hydrocarbons in the hydrous pyrolysate of this kerogen (Hoering and Navale, 1987 ) . Pratt et al. ( 199 1) analyzed various samples from the Nonesuch Formation and confirmed the presence of C2,+& steranes and C2,-C& hopanes. Jackson et al. ( 1986) reported that both hopanes and steranes are absent or occur only in low amounts in the marine Velkerri Formation, while hopanes are abundant and steranes present only in trace levels in the lacustrine Barney Creek Formation. This work emphasized the contribution of prokaryotic precursors and the influence of environmental differences on the geochemistry of Precambrian oils and sediments in the McArthur Basin of northern Australia. However, Wu et al. ( 1986 ) found C2,-CZ9 ( (Y(Y(Y and @/I) steranes as well as C2, and C29-C32 hopanes in an extract of lam-

0009-2541/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZOOO9-2541(94)00113-8

156

T.-G. Wang et al./Chemical Geology 120 (1995) 155-I 70

Lingyuano

Fig. I. Tectonic map of the Yanshan region with distributions of oil and asphalt seeps. Tectonic settings: I= J ibei Depression; II= Liaoxi Depression; III= Jidong Depression; IV= Jingxi Depression; V= Xuanlong Depression; Vi= Mihuai Uplift; VII= Shanhaiguan Uplift. Legend 1, 2= tectonic setting boundary; 3= thrust; 4= normal fault; 5 = inferred boundary; 6= well site; 7=oil seep; 8=asphalt seep.

mated chert from the Precambrian Gaoyuzhuang Formation ( 1400 Ma) of Zhangjiakou, western Yanshan region, China (Fig. 1). Fowler and Douglas ( 1987 ) reported tri- and tetracyclic terpanes, hopanes and steranes for late Precambrian oils from Siberia and interpreted their origin, except for the Cmethylsteranes, to be from prokaryotes. Summons et al. (1988) proposed that the overall abundances of sedimentary C,,+ pentacyclic triter-panes (hopane series ) are strongly maturity dependent, and that the presence of steranes in Precambrian sediments from the McArthur Basin is strong evidence for the existence of eukaryotic organisms as far back as 1690 Ma. Seifert and Moldowan ( 1986 ) considered the absence of tetra- and pentacyclic biomarkers in a crude oil as an indication of extreme maturity, or a specific organism source input of that time. Both of these processes may contribute to the biomarker signature of a Proterozoic sample. Hopanes and steranes are not the only polycyclic biomarkers found in Precambrian samples. Low to moderate abundances of regular tricyclic terpanes (i.e. extended tricyclic terpanes ) are present in the McArthur Basin (Summons et al., 1988) and Siberian (Fowler and Douglas, 1987 ) samples, and two dehydroabietin isomers (i.e. 1% and 19-norabieta-8,11,13-trienes) are

reported in the Zhangjiakou chert (Wu et al., 1986 ) . Since it has been proposed that the regular tricyclic terpanes could originate from a suitable precursor constituent (e.g., tricyclohexaprenol) in the membranes of prokaryotic microorganisms (Aquino Neto et al., 1982, 1983 ), their occurrence in Precambrian samples should be expected. However, the presence of the dehydroabietin isomers in a Precambrian chert is indeed unexpected, because diterpanes with the abietane skeleton are common biomarkers of resins from higher plants that evolved since the Carboniferous (Simoneit, 1977, 1986; Simoneit et al., 1986). These compounds have not been identified in bitumen from microbial organic matter, although related skeletons (i.e. with 1% methyl,ll-alkyl substitution) have been characterized in tasmanite (Simoneit et al., 1990). Here we report features of the biomarker geochemistry of the Longtangou bituminous sandstone, especially a novel tricyclic terpane series. To our knowledge this constitutes the first conclusive identification of these compounds in a Precambrian geological sample. 2. Geological setting The Yanshan region (Fig. 1) contains an unmetamorphosed Precambrian sequence of pre-

T.-G. Wang et al./Chemical Geology 120 (1995) 155-I 70

dominantly carbonate rocks up to 9000 m in thickness and ranging in age from 800 to 1850 Ma. The sequence includes the Upper Proterozoic Qingbaikou System and the Middle Proterozoic Jixian and Changchen Systems (cf. Fig. 2 ) , which are the oldest sedimentary strata in China. At least 66 oil and asphalt seeps have been discovered in this region. Most of them (43 seeps or 65% of the total) are located within the Tieling and Wumishan Formations (Table 1). Tectonic mapping has delineated five depressions and two uplifts in the Yanshan region (Fig. 1). It is evident that all the seeps are distributed

Chonglongshan

Fm. 850

/=I

Slate

5

Y G B

only within the depressions where the Precambrian sequence is well developed, particularly in the Jibei and Xuanlong Depressions (Fig. 1; Table 2). There, the seep distributions are closely correlated with the depocenters of the Precambrian strata. The Jibei Depression is the main depocenter of the Jixian System where the oil and asphalt seeps are more abundant in the Tieling and Wumishan Formations, while the major depocenter for Qingbakou System is located in the Xuanlong Depression where the seeps are concentrated in the Xiamaling Formation (Fig. 1; Table 3). The distribution of the oil and asphalt

Quortzose

Ma

rrr rr

5

sandstone

and

shale

Gabbro-diabose

Slate

Block Slate

r-l 763 MO rrr

;r” W

0

Hongshuizhuong

Fm.

5 x

sill -

2

shale and slotey shale

Gobbro-diobase

Wumishon

sill -

3

Slate Black shale Bituminous sandstone Limestone,

? o-J

I

and shale

Gobbro-diabase

Y t-

I-

sil

rrr

rrr rr

Block

dolomite

and shale

shale

Dolomite

Fm.

5

E Yangzhuong P_----

Fm. 1400 Ma Fm.

Gooyuzhuang Dahongyu

Fm.

Tuanshonzi Chuanlinggou

Changzhougou

Muddy

dolomite

Dolomite and dolomitic Sandstone

Fm. Fm. Fm.

151

and

limestone

dolomite

Dolomite Sandstone,

shale and siltstone

Sandstone

and

conglomerate

1850 MO

Fig. 2. Stratigraphic section in Longtangou, Lingyuan County, eastern Yanshan region.

158

T.-G. Wang et al./Chemical Geo1og.v120 (1995) 155-I 70

Table 1 Stratigraphic distribution No.

1 2 3 4 5 6 7 8 9 10

of oil and asphalt seeps in the Yanshan region of China

Horizons

Number of Type of seeps seep sites

erathem

system

formation

Cenozoic

Quaternary Neogene Lower Cretaceous Jurassic

Q N K,

Mesozoic

Paleozoic

Ordovician Cambrian

I2

Mantou and Maozhuang Fms. E,, +lrl Fujunshan Fm. E,. Jing’eryu and Xiamaling Fms. Pt,+,, Xiamaling Fm. Pzqi

Proterozoic

15

Upper

16 17

Qingbaikou

Jixian Middle

18 19

Changcheng unknown

I II III IV V

Jibei Depression Liaoxi Depression Jidong Depression Jingxi Depression Xuanlong Depression

VI VII

Mihuai Uplift Shanhaiguan Uplift

1 5 5

19

Wumishan Fm. Pt

12 1

Pt,

seeps

1 D

I.5

3.3

15 3.0 3.U 1.3 4.5 is IS 4.5

9.n t?.a ; _

1.5 7.6 1.5

‘1.6 7.6 ?R.P

‘\

”1

1.5 i.5

66

in various tectonic settings ~--

Tectonic setting

5

Tieling Fm. Pt,,

Total number of seep sites

Table 2 Oil and asphalt seep distribution in the Yanshan region

oil film gas anomaly oii stain oil stain oil stain oil stain oil and asphai: asphalt asphalt oil stain, visccuc oil and asphalt oil stain, viscous oil and asphalt oil stain, viscoiis oil and asphizlt viscous oil viscous oil and asphalt oil stain, viscous oil and asphalt oil stain, visro:rs oil and asphait oil stain, viscous cnl and asphalt aspi:alt gas anomaly

Upper Series J3 Yudaishan Fm. Jzy Xiahuayuan Fm. J,, Majiakou Fm. OLm Yeli Fm. O,, Changshan Fm. E,, Zhangxia Fm Ezr Xuzhuang Fm. E,,,

13 14

total 1

11

Percentage in ____-

Oil and asphalt seeps sites

percentage

46 1 2 12 6

68.2 1.S 3.0 18.2 9.1

0 0

0 0

Table 3 Correlation between depocenters and seep dis,!li~&i;~n ir ib<~ Yanshan region -.--~~ ----.-. _._.._ Tectonic setting

Xuanlong Depression _.

Qingbaikou System thickness (m) seep (sites) Jixian System

thickness (0,~

3cep /,zitzs i --_______~.-.

606 7 :;;;q

Jhel

Deprcssiola

. - .__ ?f,f,

J,klY 29

seeps implies their origin from rrecambrian strata (Wang, 1Y80, 1984 j The Precambrian source of these 011 and asphalt seeps is also supponed by stable carbon isotope data. The 6’%ZpDr,-values of Precambrian and Lower Cambrian 011s and rock ex-

T.-G. Wang et al./ChemicaI Geology 120 (1995) 15.5-l 70

tracts are very similar, ranging from - 30 to - 27.5%0, and obviously different from Carboniferous and Jurassic shale and coal extracts ( - 26 to -2l%o) in both the Jibei and Xuanlong Depressions (Fig. 3 ) . The Jurassic and Carboniferous formations in this region include additional potential oil source rocks (Wang, 1980, 1984). In the Jibei Depression, the Wumishan and Tieling Formations of the Jixian System are algal-rich carbonate sediments with a better oilthan gas-generating potential. The source of Precambrian oil seeps and generation of oil and gas in the Jibei Depression have been documented elsewhere (Wang et al., 1979, 1988; Wang, 1980, 1984,199l). The Longtangou bituminous sandstone is located at the southeastern side of the Jibei Depression (Fig. 1) , and occurs as a basal quart-

+

I

A2 03 04 +5

-20

-24 a3

-28 CPD,

-32

Pool

Fig. 3. B’3CPDB-values of crude oils and rock extracts from the Jibei and Xuanlong Depressions, Yanshan region (Wang, 1980, 1984). Legend: 1 =crude oil; 2=solid asphalt; jl=darkcolored shale and carbonate extracts; I=carbonaceous shale extract; 5 = coal extract.

159

zose sandstone lens in the Xiamaling Formation (Qingbaikou System, Upper Proterozoic) (Fig. 2) which has an isotopic age of - 1050 Ma. The asphaltic bitumen is distributed only within intergranular voids of the black bituminous sandstone and has a partial solvent-insoluble residue ranging from 8% to 15% in the sandstone sample as measured under the microscope. Thus, this bituminous sandstone is undoubtedly an example of a fossil oil pool, and the asphalt can be referred to as a reservoir bitumen (Wang et al., 1988). It is interesting that the regional organic metamorphic level of the Middle to Upper Proterozoic sequence is quite low in the main part of Jibei Depression. For example, most of the seeps are liquid oils and the associated bitumens are solvent-extractable. Also, the spores of micropaleoflora have a pale yellow to yellowish-gray color even in the carbonates of the Wumishan Formation and show absolutely no sign of high-temperature carbonization. In addition, the measured geothermal gradient through the Xiamaling Formation in the Shuang-l well is only lS”C/lOO m. Therefore, the regional thermal history of the organic matter has not been high enough to cause an ancient oil pool to be metamorphosed within the sandstone of the Xiamaling Formation. However, all the aforementioned liquid oil seeps have been heavily biodegraded. Only the bitumen in the Longtangou bituminous sandstone is not severely affected by this process. Overlying the basal sandstone, there are three gabbro-diabase sills interbedded within the Xiamaling Formation (Fig. 2 ), which have produced obvious alteration phenomena in the adjacent shales, notably growth of andalusite and development of slaty cleavage. The stratigraphitally lowest sill (No. 3 in Fig. 2) is only 33.7 m above the top of the basal sandstone. Beneath this sill is a zone of thermally-altered slate just 16 m thick. However, the zone of thermal alteration extends beyond the bottom of the Xiamaling Formation so that not only the ancient oil pool in the basal sandstone, but also the organic matter in the underlying Tieling Formation, has been altered to asphalt. Thus there is no liquid oil show associated with the fossil oil pool. According to

160

T.-G. Wang et a//Chemical

Geology 120 (199s) 155-l 70

an isotopic age determination of the No. 3 gabbro-diabase sill in the Shuang-I well, this magmatic intrusion occurred - 760 Ma ago, which means that the oil pool in the sandstone was thermally altered during the Late Proterozoic Erathem.

at 4°C min-‘, then 220” to 300°C at 2°C min-’ or 100” to 300°C at 4°C min-‘; using He as carrier gas. The MS was operated at 70-eV electron energy. More detailed analytical procedures and operating conditions have been reported previously (Kawka and Simoneit, 1987; Wang, 199 1).

3. Sampling and experimental

4. Results

For the present study, > 70 m3 of overburden were removed by trenching to expose the basal sandstone outcrop of the Xiamaling Formation at Longtangou, Lingyuan County, in the eastern Yanshan region of North China. This enabled fresh, unweathered, black, bituminous quartzose sandstone samples to be obtained for analysis. In these samples black solid asphalt was preserved as reservoir bitumen in the sandstone, so that we define the rock as a bituminous sandstone. The supervised sampling and careful preparation of duplicate samples for analysis precluded the possibility of variability and contamination. The sample was pulverized, extracted with chloroform, and then the extract was further fractionated by column chromatography. An aliquot of the total extract was also analyzed on an isotope ratio mass spectrometer ( AEI Scientific Apparatus, Ltd., MS-20, precision ? 0.2%) for 613C. The saturated hydrocarbon fraction was subjected to gas chromatographic (GC ) analysis and gas chromatography-mass spectrometric ( GC-MS ) analysis. GC analyses were carried out on Shimadzu@ Model GC-9A and Hewlett-Packard@ Model 584OA instruments. The gas chromatographs were equipped with fused silica capillary columns (SE-54 or DB-I, 25 m x 0.25mm i.d.); temperature programmed from 100” to 300” C at 4°C min- ’ using He as carrier gas. GC-MS analyses were carried out on a Finnigan@ Model TSQ-45 GC-MS/MS-DS system (using only the GC-MS mode) and a Finnigana Model 4021 quadrupole GC-mass spectrometer, respectively. The gas chromatographs of the MS systems were fitted with fused silica capillary columns (DB-5 or DB-I, 30 m x 0.25mm i.d.); temperature programmed from 100” to 220°C

4.1. ndlkanes n-Alkanes are the major resolvable hydrocarbons of the saturated fraction from the bituminous sandstone. They range from C,, to C35, and the n-C,,+&,, alkanes are predominant. The two most abundant homologs are n-C,, ( 13.4%) and n-C,, (11.5%). The n-C,,_/n-C,,, ratio is 2.0 which indicates a higher concentration of lighter components, and the CPI-value of 1.10 indicates no odd-even preference (Table 4; Fig. 4). 4.2. Acyclic isoprenoid alkanes Significant concentrations of C,6 and C,&&, regular acyclic isoprenoid alkanes are common in Precambrian hydrocarbons, and the Longtangou bituminous sandstone is no exception. For this sample, phytane is predominant over pristane with a Pr/Ph ratio of 0.75 (Table 4; Fig. 4), and there are no detectable C,,, long-chain isoprenoids. 4.3. Sesquiterpanes and diterpanes A series of bicyclic sesquiterpanes (m/z 123 fragmentogram ) , consisting of a C 14 component, 4,4,8,9,9’-pentamethyldecalin, drimane and homodrimane, has been detected in the sample (Fig. 5 ). These compounds were identified based on matching published mass spectra (Kagramanova et al., 1976; Alexander et al., 1984) and comparison with their presence in a biodegraded Carboniferous bitumen from the U.K. (Didyk et al., 1983). We were not able to identify the two dehydroabietin isomers, i.e. 18- and 19-norabieta-8,11,13-

161

T-G. Wang et al./Chemical Geology 120 (1995) 155-I 70 Table 4 Analytical data for the Longtangou bituminous

sandstone

Rock-Eval” pyrolysis

o/c

TOC (%)

T ( ?)

R;(a)

R (b)

(o/o)”

(%)b

0.55

0.13

5.0

451

2.1-3.1

1.7-2.3

Bulk composition

of the extract (Oh)

H/C

MPI,“’

2.02

HC

Saturates, (ST)

CPIcd)

n-r& _ n-c,, +

Pr/Ph

Aromatics, (AR)

ST/AR

N&G compounds

Asphaltenes

48.2

35.1

1.10

2.0

0.75

13.2

2.7

46.3

5.5

a Rb = bitumen reflectance measured by microphotometry. b R, is calculated according to the regression equation: R,= 0.6 18Rb”+ 0.40 (Jacob, 1985). c MPI*= [ 3 ( 2 - MP ) ] / [P + (9 - MP ) + ( 1 - MP ) 1, where P= phenanthrene and MP = methylphenanthrene (Radke and Welte, 1983). d Carbon preference index calculatd as (2Zodd C,,-C,,)/ [ (Ceven C14-C34) + (Ceven C16-&) ] (Cooper and Bray, 1963).

Reservoir bitumen Xiamaling Fm. (ca.700-IOOOMa) Longtangou, N. Chino

~--________________________’ Time -

Fig. 4. Gas chromatogram of the saturated fraction from the Longtangou bituminous sandstone, Xiamaling Formation, Qingbaikou System, Upper Proterozoic Erathem in eastern Yanshan region. Pr= pristane; Ph = phytane; T. T = novel tricyclic terpane; AT= alkyl toluene.

trienes (Wu et al., 1986), in the Longtangou bituminous sandstone. However, two C I ,-norditerpane isomers have been detected in this sample (Fig. 6), based on their mass spectra and relative retention times which are similar to those reported by Richardson and Miiller ( 1982).

4.4. Extended tricyclic terpanes There are two different series of extended tricyclic terpanes in the Longtangou bituminous sandstone (Fig. 7 ). One is the regular series with m/z 191 as base peak in the mass spectra, prob-

162

T.-G. Wang et aLlChemical Geology 120 (1995) 155-l 70

m/z

123

300

350

400

450

Scan Fig. 5. Mass fragmentogram (m/z 123) of bicyclic sesquiterpanes. calin; 3 = drimane; 4= homodrimane.

I =CId bicyclic sesquiterpane;

2=4,4,8,9,9’-pentamethylde-

m/z 248 II

650

”

”

I

700

”

”

I

750

’

”

7

800

Scan 100

50

0 50

100

150

100 -

200

@

135 _

250

300

AC‘

6961 163 123

50

‘,,

233

248

177

50

100

150

200 m/z

250

300

-

Fig. 6. Mass fragmentograms [m/z 123,248 and 262: (a), (b) and (c), respectively] and mass spectra [(d) and (e)] of Clsnorditerpanes. 1,2=C,,-norditerpanes; 3= tricyclic terpane; 4=C19 13a( methyl)-tricyclic terpane.

T-G.

Wanget

ul./Chemicul Geoiogy IL0 (IYYS) 155-l 70

,,,’

H

163

Intensily 11120

@

m/z 123

m/z 191

m/z 248 m/z 262 m/z 276 m/z 290 m/z 304 m/z 318 600

800

Scan Fig. 7. Mass fragmentagrams of two different series of tricyclic terpanes. m/z 123 for the 1301(n-alkyl)-tricyclic terpanes (novel series); m/z 191 for the 13j?(H),I4a(H)-tricyclic terpanes (regular series); m/z 248+ 14n for the molecular ions ofboth series.

ablywiththe 13B(H),14a(H)-configurationand ranging from Cl9 to C14, but without Cz2. This series is present only at a trace concentration level. The other is a novel series of tricyclic terpanes, with m/z 123 as base peak in the mass spectra (Figs. 7 and 8 ) , and carbon number range from Cl8 to Cz3. The relative concentration of this series is N 10 times greater than that of the regular

tricyclic terpanes, based on the comparison of the m/z 123 and 19 1 mass chromatograms (Fig. 7 ). This novel series is conclusively identified as tricyclic terpanes with the 13a( n-alkyl)-configuration by coinjection of synthetic standards (two homologs: Cl8 and CzO), and comparison with the mass spectrum of another standard (Cl9 homolog; Aquino Neto et al., 1983). Fig. 9 is a plot of the natural log of the retention times (scan num-

T.-G. Wang et al./Chemical Geology 120 (1995) 155-I 70

164

loo

a)

IGo b)

,.

123

> 95

109,

@ “;’

I

log

55

1

233 I

50

i ,I 0I _ 50

1

135 ,

,

,i l35

I

Ill

150

250

200

150

200

250

m/z-

m/z -

i d)

I 23

EII

23

85 69 95 l;illl

109

ci5’

Standard Mass SpenRlm

247

247 149

149

135

I

I35

163 177

I91

I91 1, I50

,,,.,

1; 1

I/ ‘,’

,21g,

100

250

200

e)

L

250

09

1

I63 I

loo

i

1;

loo1 f)

1;

261

50

200

I50

219

m/z -

m/z -

“‘1

~ 262

163 177

262

150

0

250

200

m/z lo0

I

I63

I

200

150 m/z

250

-

I: !3

67 50

150

200

250

300

m/z Fig. 8. Mass spectra of some members of the 13a(alkyl)-tricyclic terpane series. (a), (c), (e) and (g): compounds detected from Longtangou bituminous sandstone; (b), (f ): standards synthesized by Dr. Edmund0 A. Ruveda; (d): standard published by Aquino Neto et al. ( 1983).

T.-G. Wang et al./Chemical

Geology 120 (1995) 155-I 70

165

trast to many Precambrian oils or bitumens which have abundant n-alkylcyclohexanes, e.g. McArthur Basin crudes (Jackson et al., 1986; Summons et al., 1988) and the Nonesuch seep oil (Johns et al., 1966; Hoering, 1976). A dodecyltoluene ( Fig. 4 ) , C 15X2 1 alkylxylenes and alkyl-C,-benzenes are also detectable in this sample because monoaromatics carry over into the saturates fraction during liquid chromatographic (LC ) separation.

5. Discussion 6.7

-

5.1. Source ofprecursor organic matter 6.6

’ 18

I 19

MOLECULAR

I 20

I 21

CARBON

I 22

I 23

NUMBERS

Fig. 9. Correlation between the natural log of retention times and carbon numbers for the novel 13a( n-alkyl)-tricyclic terpane series.

bers) vs. carbon numbers of the novel tricyclic terpane series. The linearity of the plot confirms that these compounds are a homologous series, with an extended side-chain at the C-13 position, and not with methyl or extended alkyl substitution at the C-14 position. 4.5. Other biomarkers It is notable that both steranes and hopanes are totally absent in this sample. However, traces of 25-norhopanes, as found for example in Coalport bitumen (Didyk et al., 1983), are detectable. Hoering ( 1976) was not able to detect these biomarkers in the oil seep from the Nonesuch shale but Pratt et al. ( 1991) did characterize trace levels. Jackson et al. ( 1986) reported the same feature in oil and marginally mature source rocks from the McArthur Basin, although traces of C29-C30 hopanes and CZ8-CZ9 norhopanes were detectable. Only traces of C, 3-C20 n-alkylcyclohexanes and C14-C20 n-alkylmethylcyclohexanes are detectable in this bitumen, and both series have the Cr, homologs predominant (Fig. 10). This is in con-

The biomarker composition of the Longtangou reservoir bitumen indicates a microbial contribution, principally from prokaryota, to its source material, although a primitive algal input cannot be excluded. The distribution of the Cr4C35 n-alkanes, with no odd-even carbon number predominance, is characteristic of microbial organic matter. In particular, the predominance of the C 15-C,7 n-alkanes is evidence for a microbial source, since bacterial production of n-alkanes in the C I&ZZ2 range with a strong predominance of one or two homologs is relatively common (Grimalt et al., 1986; Simoneit, 1978), n-C,, alkane has been attributed to cyanobacteria (McKirdy, 1974), and fungi and yeast have also been reported to biosynthesize such n-alkanes (Grimalt et al., 1986). Thus, in Precambrian samples nalkanes with chain lengths < CZOmay be indicators of input from bacterial and/or algal sources, and n-alkanes > CZ1 may be derived from fungal spores, sulfate-reducing bacteria or some algae (Albro, 1976; McKirdy, 1974). This n-alkane distribution may also be generated from kerogen by long-term maturation and cracking. The n-alkylcyclohexanes and n-alkylmethylcyclohexanes, both with a predominance of the Ci, homolog, could possibly have the same biogenic precursors as the n-alkanes (Fowler et al., 1986). The precursors for the regular acyclic isoprenoid alkanes such as phytane and pristane might be photosynthetic bacteria or perhaps archaebacteria. Furthermore, it has been proposed that bi-

166

T.-G. Wanget

600

al./Chemical

Geology Iii’0 (1995) 155-I 70

800

1000

I200

1400

Scan Fig. 10. Massfragmentograms of C13-Cz4 n-alkylcyclohexanes wide peaks reflect multiple isomers).

cyclic sesquiterpanes based on the drimane skeleton are most likely of a prokaryotic origin (Alexander et al., 1983), and the regular tricyclic terpanes could have a similar origin (Aquino Neto et al., 1982, 1983; Simoneit et al., 1990). The absence of steroid biomarkers seems to be a common feature of many Precambrian hydrocarbon compositions, implying a lack of eukaryotic biota. However, the presence of trace amounts of norhopanoids indicates that certain prokaryotes, not enriched in hopanoids such as diplopterol and bacteriohopanepolyol, were involved in the precursor organic matter for this reservoir bitumen; or perhaps the overall low hopane content is due to thermal degradation caused by a magmatic intrusion. Naturally, both processes discussed above may have affected the biomarker distribution. 5.2. Alteration in bituminous sandstone

All the Precambrian oil seeps examined in the Yanshan region have been heavily biodegraded due to their prolonged exposure to meteoric weathering. Only the Longtangou reservoir bitumen has escaped destruction of its biomarker information. The carbonates of the Wumishan

(m/z 83) and C,,-C,,

n-alkylmethylcyclohexanes

(m/z

97, the

and Tieling Formations may be considered as potential source rocks, the basal lenticular sandstone itself as an ideal reservoir, and the shale of the Xiamaling Formation as a thick caprock (Wang et al., 1979). The Longtangou bituminous sandstone is then a Late Proterozoic oil pool, which formed more than 1000 Ma ago and subsequently was metamorphosed by a magmatic intrusion -760 Ma ago (Wang et al., 1988). Therefore, the oil in the reservoir sandstone was thermally degraded pyrobitumen before biodegradation of the other oil seeps occurred in the Yanshan region. Under the same regional conditions, the pyrobitumen was less susceptible to biodegradation than were the other oil seeps which escaped the effects of contact metamorphism. This is the reason why some biomarker information is still preserved in the bituminous sandstone but not in the other oil seeps of the region. As evidence for thermal degradation, the reservoir bitumen was found to have a high maturity, i.e. the bitumen has a measured reflectance, Rho, of 2.1-3.1%, from which a computed vitrinite reflectance, R,, was calculated and ranges from 1.7% to 2.3% based on Jacob’s (1985, table 4) empirical equation. Hence, its thermal maturity is generally equivalent to a low volatile bitumi-

T.-G. Wung et ai./i.hemicai Geologv I20 (1995) 155-l 70

nous coal to semi-anthracite coal rank I ‘fkirhmiller and Tev_-zhmiiller, 1982 j and it is not surprising that no liquid oil seepage is associated with the Longtangou bituminous sandstone. Other indications of thennodegradation can he found in the organic composition of the bltumlnous sandstone. lhese include a hlgher propartion of branched and cyclic hydrocarbons in the saturated fraction, i.e. the maJor “hump” ofunresolved complex mixture (I-ICM ) (Fig. 4 J. Almost all the thermally less stable components, e.g. C 20+ acyclic isoprenoid alkanes and pentacyclic terpanes, are absent or at trace levels, and n-alkylcyclohexanes and n-alkylmethylcyclohexanes are present in low concentrations. Only n-alkanes, acyclic isoprenoid alkanes (C , 6-C20, without C,,), and bi- and tricyclic terpanes are relatively concentrated as more stable components. Even in the aromatic fraction, the 160 polycyclic aromatic hydrocarbons (PAH ) detected are predominantly tri- and tetracyclic compounds and lack pentacyclic PAH (Wang et al., 1988).

5.3. Origin

ofthe novel tricyclic terpanes

By means of a comparison with synthetic standards, Aquino Neto et al. ( 1983) determined the structures of the regular molecular 23#I(H),14c~(H)-tricyclic terpane series, and of an isomer, identified as the Cl9 13a(CH,)-tricyclic terpane. This Cl9 isomer is a homolog of the 13a (n-alkyl)-tricyclic terpane series reported here. Aquino Neto et al. ( 1983) suggested the regular tricyclic terpane series may have originated from the cyclization of regular hexaprenol under natural conditions (see reactions I and 2 in Fig. 11). The ubiquitous occurrence of this regular series in numerous geological samples clearly implies that it has a microbial source [possibly algal, e.g., tasmanites (Simoneit et al., 1990) 1. Proterozoic microbes, such as cyanobacteria, archaebacteria, and possibly certain fungi and primitive algae, are probably the source of the biomarkers in the Longtangou reservoir bitu-

167

men. This conclusion is consistent with the accepted genetic interpretation of the regular tricyclic terpane series. Since the novel 23a( nalkyl )-tricyclic terpanes are structurally similar to the regular series they could have a similar source and ori+n, i.e , an analogous tvclization of regular hexaprenol (reactions 2 and 2 in Fig. ! 1). The difference in their cyclization would be that the methyl at the C-11 position of hexaprenol is involved directly in the cyclization so that the longer isoprenoid substituent remains at the C-13 position as a main side-chain of the novel tricyclic terpanes (see reactions 3 and 4 in Fig. 11). This may indicate a specific group of microorganisms as the source of these compounds. As reported previously, certain bacteria (e.g., some species of Pseudomonas) are able to utilize the methyl side-chain of isoprenoids as their sole carbon source (Cantwell et al., 1978) and thus could eliminate the methyl groups from an isoprenoidal chain on a terpenoid skeleton, resulting in a straight chain substituent structure. In other words, these species have two kinds of specific enzymes, i.e., carboxylase and lysase, where the former activates the P-methyl group of the isoprenoids via carboxylation and the latter catalyzes the removal of the activated &carboxymethy1 group. Hence, these bacteria could transform the isoprenoid substituent of a tricyclic precursor into a normal alkyl substituent by Boxidation, decarboxymethylation and subsequent hydrogenation (reaction 4 in Fig. 11). Simple cleavage at the C-22 branch carbon would yield the C23 homolog. Since the 13a (n-alkyl) tricyclic terpanes are more abundant than the regular tricyclic terpanes in the thermally degraded reservoir bitumen, this novel series may also be thermodynamically more stable than the regular series. However, because tricyclic terpanes and aromatic hydrocarbons are relatively enriched in this sandstone, and other biomarkers (e.g., steranes) as well as the pentacyclic PAH in the aromatic fraction (Wang et al., 1988) are absent, a thermogenic origin for these novel tricyclic terpanes cannot be completely ruled out.

T.-G. Wang et al./Chemical Geology I20 (1995) 155-l 70

168

2

Hydrogenation

Regular trlcycllc terpanes

,

73

4

Hydrogenation, microbial \ demethylation or cleavage at C-22

@ \\ 13u&Alkyl)-tncyclic

terpanes, C,s

&

Fig. 11. A possible genetic scheme of two kinds of tricyclic terpane series. Reactions 1 and 2 are according to Aquino Neto et (1983).

6. Conclusions The Longtangou bituminous sandstone is a Precambrian oil pool which was altered by intrusions of gabbro-diabase sills - 760 Ma ago. The presence of n-alkanes, acyclic isoprenoid alkanes, methylcyclohexanes, n-alkylmethylcyclohexanes, bicyclic sesquiterpanes, and two series of tricyclic terpanes in this reservoir bitumen indicate a major input from prokaryotic biota to the source rock of the parent oil. However, a primitive algal contribution cannot be excluded.

al.

The absence of hopanes, 25norhopanes and steranes may reflect the high maturity of the bitumen which altered the less stable biomarkers over time. A novel series of tricyclic terpanes with a 13o (n-alkyl)-side-chain configuration has been confirmed. Only its C,9 homolog was reported previously. This new biomarker series may have a specific prokaryotic precursor which remains to be elucidated, although a thermogenic origin from preexisting triterpenoids cannot be discounted.

T.-G. Wang et al./Chemical Geology 120 (1995) 155-170

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