Compressively Matured Bitumen

Geochemical Journal, Vol. 35, pp. 155 to 168, 2001

155

*Corresponding author (e-mail: [email protected])

Compressively matured solid bitumen and its geochemical significance

ZHI-NONG GAO*, YUAN-YIN CHEN and FEI NIU

School of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China

(Received October 21, 1999; Accepted March 15, 2001)

Solid bitumens found in carbonate rocks of the Shiwan Dashan basin, south China, are suggested tohave been matured under high pressure and moderate temperature conditions. These bitumens have differ-ent geochemical characteristics from thermally matured bitumen: very high bitumen reflectance (ROB)values and compact molecular structures (bigger size of crystal nucleus, more layer number of aromaticcycle, lower lamellar distance and so on) in spite of the chemical components reflecting lower thermalmaturity. Therefore these are named as compressively matured bitumen. The geochemical characteristicsof the compressively matured bitumens result from increase in its crystalline degree (grain size) underhigh pressures, not at high temperatures. The compressively matured bitumens retain chemical composi-tion, molecular evolution parameters (associated with their chemical components) and hydrocarbon-pro-ducing potential similar to those of thermally less mature bitumens. High temperature and high pressureexperiments proved that pressure and temperature have similar effect on the maturation indices, such asROB values and molecular structures. The above contrasting geochemical characteristics can be used todistinguish the compressively matured bitumens from thermally matured bitumens.

However, the maturity estimated by the ROB val-ues of primary anisotropic bitumens in the lowerTriassic (T1) carbonate rocks of the ShiwanDashan basin, south China, is much higher thanthat estimated by RO, according to the publishedrelationships between ROB and RO. Gao and Chen(1998) suggested that the difference in maturationpressure could have caused these different typesof bitumens. This study was conducted to under-stand the origin of these bitumens based on thestructural and optical characters and results of highpressure and high temperature experiments.

TWO TYPES OF BITUMENS FROM

BAXI PROFILE

Geological backgroundShiwan Dashan basin locates in south China,

close to Vietnam. It distributes in north-east ori-entation and its area is about 11,600 km2 (Fig. 1).In the basin, the T1 stratum consisting of marine

INTRODUCTION

Reliability of natural bitumen reflectance(ROB) as a maturity index of carbonate rocks isstill a controversial issue (Rogers et al., 1974;Jacob, 1985, 1989; Riediger, 1993; Landis andCastano, 1995; Parnell et al., 1996). Till now sev-eral relationships between ROB and vitrinite re-flectance (RO) have been established (Jacob, 1985;Fu et al., 1989). In nature, the ROB values of pri-mary bitumens sometimes vary considerably inspite of a same stage of maturation. The variationis caused by differences in their origin (e.g., ther-mal alteration, biodegradation and others) andother influence factors (e.g., tectonic activities,etc.), and different types of bitumens show differ-ent optical behaviors (Fu et al., 1989).

In general, primary solid bitumens in carbon-ate rocks in highly mature level have the ROB val-ues similar as or little higher than RO values ofvitrinites in the same stratum (Fu et al., 1989).

156 Z. N. Gao et al.

carbonate rocks is the main source of hydrocar-bon accumulations. It is characterized by a greatthickness, wide distribution and high abundanceof organic matter at a level of moderate maturity.There are many oil seepages and solid bitumensin the stratum at north margin of the basin, be-cause of tectonic activities (Gao and Chen, 1998).

In the Shiwan Dashan basin, the T1 stratumconsists of 61 layers of gray and oolitic limestoneswith 250 m thickness. Above the stratum are themiddle Triassic (T2) rocks consisting of greenshale and shale-arenite with thickness >300 m, orin some part are the Jurassic rocks consisting ofgray-green mudstone and interbeds of purplish-red mudstone and siltstone (>60 m). Below the T1

stratum, there is upper Permian (P2) stratum con-sisting of gray mudstone, bioclastic limestone andsandy mudstone (>300 m).

Analytical result of mineral inclusions indi-cates that the organic matter in the T1 stratum hasbeen exposed to the temperature of about 110°Cor higher, in accordance with the geothermal gra-dient in the basin (Gao and Chen, 1998). Sedi-mentation and burial history of the basin also re-

veals that the deepest T1 carbonate hydrocarbonsource rock in the basin reached the depth of 4,300m, where temperature was about 110°C. So themaximum RO value in the basin suggested by time-temperature index (TTI) is less than 1.20 (Gao andChen, 1998).

The T1 stratum in the Shiwan Dashan basincould have regionally experienced a high pressurecondition, because this stratum have reached thedepth of more than 4 km, with good confiningcondition (no fault cutting through the stratum).Soluble organic matter content in the T1 stratumis very high, especially in the middle part of thestratum. In addition, diapiric structures are foundin the shale or mudstone just at the top or bottomof the stratum (Gao and Chen, 1998). These geo-logical fectures are the macroscopic characteris-tics of high pressure in the strata (Hao et al., 1996).

Bitumens from Baxi profileBaxi profile that contains abundant solid

bitumens locates in middle part of the north mar-gin of Shiwan Dashan basin (Fig. 1), where thethickness of T1 carbonate rock stratum is about

Fig. 1. Geographic location map of the Shiwan Dashan basin. � Place names of cities; * locations of outcroppingbitumen.

Compressively matured solid bitumen 157

Tabl

e 1.

T

he t

herm

al e

volu

tion

cha

ract

eris

tics

of

orga

nic

mat

ter

in t

he T

1 st

ratu

m o

f Sh

iwan

Das

han

basi

n

The

lis

ted

num

bers

are

the

av e

rage

d v a

lue s

. As

orga

nic

mat

ter

is c

omm

on a

t al

l th

e st

ratu

m,

the

ave r

aged

val

ues

for

the

stra

tum

are

als

o li

ste d

.C

PI

= [

% C

25–3

3 (o

dd n

umbe

r) +

% C

23–3

1 (o

dd n

umbe

r)]/

2[%

C24

–32

(ev e

n nu

mbe

r)].

MP

I =

me t

hylp

hena

nthr

e ne /

phen

anth

rene

.D

PI

= d

ime t

hylp

hena

nthr

e ne /

phen

anth

rene

.D

NR

= (

2,6-

dim

e thy

lnap

hthe

ne +

2,7

-dim

e thy

lnap

hthe

ne)/

1,5-

dim

e thy

lnap

hthe

ne.

Me t

hyla

dam

anta

ne i

nde x

= [

1-M

D/(

1-M

D +

2-M

D)]

× 1

00,

MD

= m

e thy

lada

man

tane

.M

e thy

ldia

man

tane

ind

e x =

[4-

DM

D/(

1-D

MD

+ 3

-DM

D +

4-D

MD

)] ×

100

, D

MD

= m

e thy

ldia

man

tane

.


100 m. As a complete set of the T1 stratum is avail-able as outcrops in the Baxi profile, we focusedour study on this representative profile.

After comprehensive study (Gao and Chen,1998), we conclude that both bitumen and oil inthe Baxi profile are derived from the same T1 car-bonate stratum. The main evidence is geochemicalsimilarity of the solid bitumen, the oil and the T1

carbonate source rocks as follows:(1) Distribution of terpenoids and steroids in

the solid bitumen, the oil and the source rocks areidentical.

(2) Pyrolysis-gas chromatograms and infraredspectra of their asphaltene fractions are almost thesame.

(3) Content and composition of trace metals inthe solid bitumen, the oil and the source rocks aresimilar.

(4) The δ13CPDB values of their componentsincrease regularly in the expected order (saturatedhydrocarbon → aromatic hydrocarbon → resin →asphaltine → kerogen).

Analytical results of thermal evolution param-eters (Table 1) show that the T1 stratum in theShiwan Dashan basin is in the stage of catagen-esis, not in the stage of metagenesis, except theROB values of anisotropic bitumens (Gao andChen, 1998). All these bitumen samples were col-lected from the north margin of the basin (Fig. 1).The T1 carbonate organic matter in the Baxi pro-file is also in the thermally matured level.

In the T1 carbonate hydrocarbon source rocksof the Baxi profile, however, there exist two typesof primary solid bitumens, i.e., isotropic bitumenand anisotropic crystalline bitumen. These twokinds of bitumens have distinct geochemical char-acteristics, such as the disagreement in ROB val-ues, although they are derived from the same T1

stratum (i.e., the same primary organic matter) andexposed to the similar temperature conditions dur-ing maturation (Gao and Chen, 1998). Isotropicbitumens are distributed only in narrow zones ofabout 15 m near the top and bottom of the T1 stra-tum (i.e., coexistence of two bitumens), while inthe middle of the stratum (about 70 m thick) onlyanisotropic crystalline bitumens are found. The

ROB values progressively increase from the twosides to the middle of the stratum (Fig. 2; Gaoand Chen, 1998).

Analytical procedureThe solid natural bitumens were peeled off

from rock samples and washed with dilute hydro-chloric acid. Then the bitumens were analyzedwith the following analytical instruments: RigakuD/max-RA X-ray diffractometer, Leitz MPV-IIImicrospectrophotometer, Nicolet 170SX infraredspectrometer, Perkin Elmer 240B elementalanalyzer, Shimadzu DT-30B differential thermalanalyzer, and Rock-Eval III (measuring Tmax val-ues). For pyrolysis gas chromatography, a COS820 pyrolysis analytical system was used, and tem-perature was increased to 550°C for pyrolysis.Pyrolysis products were collected in a cold trapand then transferred to capillary gas chromato-graph to identify with the mass spectrograph andtheir retention time (Zhou and Cao, 1991).

Chloroform and acetone-methanol-benzene(1.5:1.5:7) also extracted natural bitumen samplessuccessively. The solvent-extracted fractions wereseparated on a silica gel/alumina column chroma-tograph, and the saturated and aromatic hydrocar-bons were eluted with normal heptane and ben-zene, respectively. Each fraction was analyzed byShimadzu GC-9A gas chromatograph (27 m × 0.23mm i.d., OV-101 capillary column), and both hy-

Fig. 2. Variation of bitumen reflectance in differentlayers of T1 stratum in the Baxi profile (stratum layersare numbered from bottom to top).


drocarbon fractions were analyzed by FinniganTSQ-70 GC/MS/MS combined equipment (30 min length, 0.25 mm i.d., DB-5 quartz capillarycolumn). In addition, carbon-13 NMR aromaticitymeasurements of bitumen using CP-MAS tech-niques were also performed to examine the resultsobtained by X-ray diffractometer, and the datawere consistent.

ResultsThe analytical results of natural isotropic and

anistropic bitumens from the Baxi profile areshown in Tables 1 and 2. When difference betweenmaximum and minimum ROB values for bitumensis less than 0.1%, we classified the natural bitu-men as isotropic bitumen. When the difference islarger than 0.2–0.3%, we classified it as the ani-sotropic. Crystals of the anisotropic bitumens fill-ing cracks, vugs and cavities in microfossils areobserved under microscope (Fig. 3). The crystal-line bitumens show extinction phenomenon, andtheir values of ROB are much higher than that ofisotropic bitumens and directly proportional tocrystal grain size (Table 3). Therefore, the

Tabl

e 2.

G

e oc h

e mic

al c

hara

c te r

s of

iso

trop

ic a

nd a

niso

trop

ic b

itum

en s

ampl

e s i

n th

e B

axi

prof

ile

Dat

a fo

r 2

anis

otro

pic

bitu

men

sam

ple s

and

1 i

sotr

opic

bit

umen

sam

ple

are

disp

lay e

d.T

he l

ast

5 pa

ram

e te r

s w

e re

me a

sure

d by

Rig

aku

D/m

ax X

-ray

dif

frac

tom

e te r

.d 0

02—

the

lam

e lla

r di

stan

c e o

f bi

tum

ens.

f a,

Nc

are

for

the

arom

atic

ity ,

lay

e r n

umbe

r of

aro

mat

ic c

y cle

re s

pec t

ive l

y .14

60 c

m–1

/160

0 c m

–1 i

s IR

int

e nsi

ty r

atio

at

1460

, 16

00 c

m–1

.

F ig. 3. A polarizing microphotograph of a“compressively matured solid bitumen” from theShiwan Dashan basin (in this photograph, both thewhite and black bars are crystalline bitumens of ran-dom orientation).


anistoropic crystalline bitumens are likely be acluster of small crystalline bitumens. The analy-sis of X-ray diffractometer also proves the result.

In spite of different ROB values, Tmax valuesand molecular structure (d002 value, size of crys-tal nucleus, aromaticity and layer number of aro-matic cycle), the isotropic and anisotropic crys-talline bitumens have similar contents of hydro-carbons, aromatics, resin and asphaltine (about3:1:1:1), and similar ratios of 1-methyl and

Table 3. Relationship between bitumen reflectance (ROB) and grain size

The grain size is measured in JEOL JSM-35CF scanning electron microscope (2500×), and more than 300 points are deter-mined for one sample; ROB values are determined with Leitz MPV-III microspectrophotometer, and more than 30 points aremeasured.

Fig. 4. Gas chromatograms of saturated hydrocarbons in bitumens. a: isotropic bitumen (ROB = 1.14%), nC14–nC37, the highest peak: nC20; b: anisotropic bitumen (ROB = 2.41%), nC13–nC36, the highest peak: nC19.

Degree of crystallization ROB mean value (%) Mean grain size (mm) Number of samples

Fine-grained crystalline bitumen 1.52 0.0045 10Medium-grained crystalline bitumen 2.29 0.007 12Coarse-grained crystalline bitumen 2.55 0.010 8

2-methyl diamondoid alkanes (two isomers relatedto thermal maturity; Table 1). Their chloroformand acetone-methanol-benzene solvent-extractedfractions also have similar moretane/hopane ra-tios (0.31–0.35), ratios of sterane isomers, C27

hopane Ts/Tm ratios, carbon preference index(CPI) values, methylphenanthrene index (MPI),dimethylphenanthrene index (DPI),dimethylnaphthene ratio (DNR),methyladamantane index and methyldiamantane


index (Table 1). The last two parameters are novelmaturity indices for crude oils (Chen et al., 1996).Gas chromatograms of the saturated hydrocarbonsin the two bitumen types are very similar to eachother (Fig. 4). Although the infrared spectra of twobitumens are also similar, the spectrum of the ani-sotropic bitumens shows less intense absorptionof some hydrocarbons (e.g., 1380 cm–1, 1460cm–1 and 720 cm–1) and heteroatomic functionalgroups (e.g., 1700 cm–1, 1000–1200 cm–1, etc.)compared with that of the isotropic bitumens (Fig.5). Pyrolysis-gas chromatography analysis (Py-GC) shows that the anisotropic crystallinebitumens contain slightly smaller amount ofheteroatomic compounds and saturated and cyclichydrocarbon components in the pyrolytic productsthan those in the isotropic bitumens do (Fig. 6).In Fig. 6, many alkane peaks of isotropic bitumenis higher than those of anisotropic bitumen, such

Fig. 5. Infrared spectra of two bitumen samples.a: anisotropic bitumen (ROB = 2.41%); b: isotropic bi-tumen (ROB = 1.14%). Note: The intense absorption ofA) 1700 cm–1 (C=O), B) 1460 cm–1, 1380 cm–1 (>CH2,–CH3), C) 1000–1200 cm–1 (C–O), D) 720 cm–1

[(–CH2–)n] of anisotropic bitumen is less than those ofisotropic bitumen.

Fig. 6. Pyrograms of bitumens in the Shiwan Dashan basin (first and second peak of the paired peaks with thesame carbon number in nC8–nC16 are assigned to alkenes and alkanes, respectively). a: anisotropic bitumen(ROB = 2.41%); b: isotropic bitumen (ROB = 1.14%). Note: Many alkane peaks of isotropic bitumen is higher thanthose of anisotropic bitumen, such as nC10, nC12, nC17, nC20, nC22, nC23, nC25 and so on. Peaks of heteroatomicand cyclic compounds between those of normal alkanes are also higher in isotropic bitumen.


as nC10, nC12, nC17, nC20, nC22, nC23, nC25 and soon. Peaks of heteroatomic and cyclic compoundsbetween those of normal alkanes are also higherin isotropic bitumen. The pyrograms of twobitumens are apparently similar and only aftercareful calculations the above difference can beobtained. In addition, their hydrocarbon-produc-ing values in pyrolysis chromatography, totalweight loss values in 700°C, hydrogen indexS1/(S1 + S2) are almost the same, and their H/Catomic ratios, IR intensity ratio 1460 cm–1/1600cm–1 are also similar (Tables 1 and 2). These char-acteristics reveal that two bitumens possess simi-lar hydrocarbon-producing potential.

DiscussionIn general, sedimentary organic matter of

ROB = 1.2% (corresponding to RO = 1.17%) is justin mature phase, while those of ROB = 1.9% and2.4% (corresponding to RO = 1.65%, 2.0%) are athighly matured and overmature stages respectively(Fu et al., 1989). In this sense, the isotropicbitumens in Table 2 are in mature stage and mostof the anisotropic crystalline bitumens are inovermatured phase. However, the differences inH/C atomic ratio, 1460 cm–1/1600 cm–1 specificvalue and weight loss amount at 700°C are smallbetween the two types of bitumens, compared withthe large difference in ROB values and molecularstructures (Tables 1 and 2). Rock-Eval Tmax valueis another maturity index most in use and has acertain difference between the isotropic and ani-sotropic bitumens. As both bitumens are in thecatagenesis stage and are derived from the sameprimary organic matter as stated above, these dif-ferences in the geochemical characteristics can-not be explained simply by differences in the de-gree of thermal maturation.

Stasiuk (1997) reported that the differences inoptical textures (isotropic vs. mosaic pyrobitumen)and ROB values can be related to differences inchemical compositions of precursor oil (non-graphitizing, NSO-rich petroleum vs. graphitizing,low-S petroleum). However, as was shown before,two of the bitumens in Baxi profile are derivedfrom the same primary petroleum, and their con-

tent of S, O, and N elements is also the same (Gaoand Chen, 1998). Barker and Bone (1995) discov-ered that high ROB (2.4–2.7%) zone of 6 m thick-ness distributes in contact aureole while ROB val-ues are lower at out sides of this zone, and con-cluded that thermal stress caused the high ROB

zone. However, there is not heat source igneousrocks in the Baxi profile. Curiale (1986) proposedthat solid bitumens associated with oil sourcerocks can be classified as either pre-oil or post-oil, and that the former (early-generation imma-ture product) is much less mature than the latter(generated from a mature source rock). Follow-ing generation and expulsion, both pre-oil andpost-oil solid bitumens are subject to the samemodification processes, such as thermal alteration.Therefore, although there can be a large differ-ence in their ROB values, neither pre-oil nor past-oil bitumens cannot have high ROB values if bothbitumens and their source rocks experience onlylow thermal stress as those in T1 carbonate rocksof Baxi profile. So the following question arises.Why did the carbonate rocks exposed to the samelevel of thermal stress form two types of thebitumens with different values of ROB?

A sedimentary basin is actually athermochemical reactor at low temperatures. Be-sides temperature, pressure is another importantfactor controlling maturation processes of organicmatter (McTavish, 1978; Braun and Burnham,1990; Hunt, 1990; Jiang et al., 1998). As men-tioned above, the T1 stratum in Shiwan Dashanbasin (including the Baxi profile) could have ex-perienced the high pressure of 1,000 bars (4 kmdepth). Therefore, it is plausible that fluid pres-sures inside the rocks increased to a peak in theoil-generating widow and decreased below it(Hunt, 1995). As the source rocks and thermalmaturation levels of these two bitumens are simi-lar, Gao and Chen (1998) suggested that pressuredifference between the middle and the margin ofthe stratum could be the only possible origin oftwo different bitumens. If formation pressure ishigher at the middle than at the margins of thestrata, that can cause the progressive ROB increasefrom its side to the middle of the strata (Fig. 2).


In order to test the hypothesis of “compressivelymatured solid bitumen”, high pressure and hightemperature simulation experiment was conductedto evaluate the effect of pressure on maturationprocesses.

LABORATORY MATURATION EXPERIMENT

Introduction for the experimentBecause of the lack of practical geobarometers

compared with many useful geothermometers, theinfluence of pressure to maturation is difficult todistinguish in the nature. Therefore, studies in thisarea are few and the results are not consistent(McTavish, 1978; Braun and Burnham, 1990).Most organic geochemists believe in pressure re-tardation of organic matter maturation (Hunt,1990; Price and Wenger, 1992). High pressure,however, causes structural changes of organicmatter, such as increase in the size of crystal nu-cleus and aromaticity and decrease in lamellardistance (Jiang et al., 1998). As more compact themolecular structures of bitumens are and thehigher their ROB values will be, these changesshould result in the increase in ROB values. To testthe effect of pressure on the formation of bitumenof this type, “compressively matured bitumen”, aseries of laboratory experiments was performed.

Samples and experimental procedureSimulation experiments are conducted with the

T1 isotropic bitumens in Baxi profile, ShiwanDashan basin, and their basic geochemical char-acteristics are shown in Table 2. A rock samplerich in isotropic bitumens was divided into 11aliquots, and reacted under different experimentconditions as listed in Table 4. The high pressureexperiments were performed with a specially de-signed piston cylinder pressure vessel, which canreach 1200 atm at 700°C. The container materialin the piston cylinder (19.05 mm i.d.) is platinumand medium of pressure transmission ispyrophyllite. Pressure was increased to a requiredvalue by piston-in method, and then temperaturewas increased. Pressure values were calibratedusing lead melting point and temperatures were

Tabl

e 4.

R

un c

ondi

tion

s an

d an

aly t

ical

re s

ults

of

sim

ulat

ion

e xpe

rim

e nts

of

bitu

men

mat

urat

ion

The

ana

lyti

c al

e rro

r of

RO

B m

easu

rem

e nts

is

abou

t ±0

.01(

%).

f a,

Nc

are

f or

t he

arom

ati c

i ty,

lay

er n

umbe

r of

aro

mat

i c c

ycl e

res

pect

i vel

y.


measured with a WRe 5-WRe 26 thermocouple.The calibrated pressure values were also comparedwith those produced by high-pressure liquids. Thehigh-pressure liquids (e.g., oils) were producedand transmitted by a high-pressure pump, and thepressure values can be watched through a manom-eter. The pressure error of two methods is less than2%. The temperature was controlled within ±1°C,and the error induced by high pressure was ig-nored. When the simulation experiments finished,the samples were quenched to room temperaturebefore pressure release. During the ambient pres-sure experiments, the samples were held in quartztubes filled with nitrogen. As both ends of thetubes are capillaries open to the atmosphere, theinside pressure was kept at one atmosphere dur-ing the experiment. All simulation samples were

subjected to the required temperatures and pres-sures for 96 hrs.

After the experiments, the solid bitumens werepeeled off from the rock samples and washed withdilute hydrochloric acid. Then, the bitumen sam-ples were analyzed mainly with Rigaku D/max-RA X-ray diffractometer, Py-GC and Leitz MPV-III microspectrophotometer in addition to the or-dinary analyses (e.g., elemental analysis and ther-mal analysis). In addition, GC, GC/MSchromatograms were also obtained for the solventextracts from samples 10 and 11 for specific mo-lecular distributions.

Results and discussionResults of laboratory simulation are listed in

Table 4. In sample 1, which reacted at 200°C and

Fig. 7. Variation of a) H/C atomic ratio, b) d002, c) ROB and d) aromacity with temperature and pressure.


The

sam

ple

num

bers

are

con

sist

e nt

wit

h Ta

ble

5.C

y cli

c hy

droc

arbo

n, p

oly c

y cli

c ar

omat

ic h

y dro

c arb

on a

nd p

heno

l ar

e al

so c

ount

e d i

n sa

tura

ted

hydr

ocar

bon,

aro

mat

ic h

y dro

c arb

on a

nd n

on-h

ydro

carb

on,

resp

e cti

v ely

.L

ight

-we i

ght

hydr

ocar

bon

rati

o =

lig

ht h

y dro

c arb

on (

�C

10)/

we i

ght

hydr

ocar

bon

( �C

11).

Tabl

e 5.

T

he a

naly

tica

l r e

sult

s of

sat

urat

ed h

ydr o

carb

ons

in n

atur

al a

nd s

imul

atio

n sa

mpl

es

Num

bers

1 a

nd 2

are

for

the

sam

ple s

of

outc

rop

isot

ropi

c an

d an

isot

ropi

c bi

tum

e ns

resp

e cti

v ely

, whi

le n

umbe

rs 3

and

4 r

e pre

sent

sim

ulat

ion

sam

ple s

10

and

11 r

e spe

c tiv

e ly .

Pr

and

Ph

repr

e se n

t pr

ista

ne,

phy t

ane

resp

e cti

v ely

.

Tabl

e 6.

P

y rol

y tic

pro

duc t

s of

nat

ural

and

sim

ulat

ion

sam

ple s


1 atm, no change was observed from the startingmaterial. While in sample 2, which reacted at200°C and 1000 atm, changes in its structure andROB value were detected. As the temperature rises,the effect of pressure on the changes in the mo-lecular structure (e.g., d002 value, aromaticity, sizeof crystal nucleus, layer number of aromatic cy-cle and H/C atomic ratio) and ROB value becomesmore clear (Table 4, Fig. 7). The results also indi-cate that, at a same temperature, the change be-comes more significant with increasing pressure.For example, decrease in d002 and increase in ROB

values and aromaticities with pressure becomemore distinct at higher pressure (Fig. 7). Samples3, 7, 10 and 1 were reacted at 1 atm with variabletemperature, and the variations in the results aresmaller than those observed at higher pressures.Comparing samples 9 and 10 with sample 7 or 1respectively, we can observe that pressure mainlyaffects the compact degree of bitumen’s molecu-lar structure, in contrast with the change in thecarbon content caused by the temperature differ-ence. Although most parameters are affected bythe pressure and temperature changes, the changesof H/C atomic ratio, IR intensity ratio 1460 cm–1/1600 cm–1 and total weight loss are small com-pared with those of the ROB value and molecularstructure (Table 4).

The experimental results clearly show that notonly increasing temperature but also increasingpressure can enhance solid bitumen evolution.Moreover, it seems that temperature and pressurecompensate each other in bitumen evolution, butlow pressures (such as 300 atm) may have littleeffect (comparing sample 3 with sample 4). Incontrast, significant difference was observed in theresults at 800 and 1000 bars, indicating that thesmall pressure difference at higher pressure canhave the important effect on the maturation. Thesaturated hydrocarbon fractions extracted fromsamples 10 and 11 were analyzed using GC andGC/MS. The two samples reacted at the same tem-perature have similar values of molecular evolu-tion parameters (Table 5), although ROB values ofthe bitumens increase with pressure, that is con-sistent with the observation in the natural bitu-

men samples (isotropic bitumen vs. anisortopiccrystalline bitumen). Pyrolytic products of thesimulation samples reacted at the same tempera-ture have similar composition and the two naturalbitumens in the Baxi profile also have similar com-position of the pyrolytic products each other (Ta-ble 6).

Although the increase of ROB values due toincrease of pressure is apparently small, in par-ticular at lower temperatures (Table 4), the pres-sure effect could be underestimated due to theshort time duration of the laboratory experimentcompared with the geological time scale. Theabove similarity of simulation products and natu-ral bitumens in geochemical characteristics andchemical composition proved formation of thecompressively matured bitumen in nature.

ORIGIN OF THE COMPRESSIVELY

MATURED BITUMEN

As the anisotropic crystalline bitumen in Baxiprofile, Shiwan Dashan basin, forms under highpressures and moderate temperatures, we call it“compressively matured solid bitumen”. This typeof pressure-induced bitumen differs ingeochemical characteristics from thermally ma-tured bitumen: high ROB values, high Tmax valuesand compact molecular structures vs. the chemi-cal components reflecting lower thermal maturity.Especially, its hydrocarbon-producing potential issimilar to isotropic bitumens with much lower ROB

values.Sajgo et al. (1986) discovered that pressure

could not only retard but also accelerate the or-ganic matter maturation, depending on the experi-mental conditions. Costa Neto (1991) approvedthis conclusion based on theoretical considera-tions. Pressures can accelerate mature reactionsof organic matter, as distance among reactants issmaller at higher pressure. On the other hand,McTavish (1998) discussed that high pressurecauses decrease in porosity of mudstone strataresulting in decrease in heat transfer, so the cu-mulative heat flow in high-pressure strata is lowerthan that in low-pressure strata. As the result, ap-


parent retardation of organic matter maturation canbe observed due to the high pressure.

As porosity is large and a number of continu-ous fractures exist in carbonate rocks, high pres-sure might not affect temperature distribution inthe Baxi profile as McTavish (1998) discussed,and therefore affect only reaction processes of theorganic matter as Jiang et al. (1998) suggested.Similar to thermally matured bitumens,compressively matured bitumens also increasetheir directional structural arrangement from nocrystalline structure to crystalline-like structure,forming anisotropic bitumens with high ROB. Incontrast, chemical composition of thecompressively matured bitumens is relatively un-changed and their hydrocarbon-producing poten-tial does not decrease obviously. The anisotropiccrystalline bitumen in the Baxi profile, ShiwanDashan basin, is an actual example ofcompressively matured bitumens. The simulationexperiments also prove the pressure-induced matu-ration of solid bitumens.

GEOCHEMICAL SIGNIFICANCE

Compressively matured bitumen can be formedat moderate temperatures and high pressures innature. It has high ROB values without having beenexposed to a high level of thermal stress, there-fore does not follow the usual relationships be-tween ROB and RO which have been reported forthermally matured bitumens. As the compressivelymatured bitumens yield apparently higher valuesof ROB which overestimate the true thermal matu-rity of carbonate rocks, it is essential to distin-guish compressively matured bitumens from ther-mally matured bitumens in oil and gas explora-tion.

Geochemists classified natural bitumens basedon their geochemical characteristics (Curiale,1986; Georgy, 1992). The compressively maturedsolid bitumens have self-contradictory evolutionparameters and geochemical characteristics. Incontrast to the abnormally high ROB values, highTmax values and compact molecular structures, thecompressively matured bitumen preserves the

chemical components (including biomarkers)formed at low thermal maturity. By combining thegeochemical information such as biomarkers, wemight be able to use the ROB as a new maturityindex to identify the compressively matured bitu-men.

Compressively matured solid bitumens crys-tallize mainly because of pressure effect. Theircondensation polymerization (i.e., at the time offormation of polymers in bitumens, small mol-ecules are released) might not be as complete asthat of thermally matured bitumens, and light hy-drocarbons and hydrocarbon groups might betrapped in crystal-like structures of the bitumen.As a result, hydrocarbon content and hydrocar-bon-producing potential of bitumens of this typeare high (e.g., high hydrogen index, high weightloss value, high H/C atomic ratio and so on), de-spite their high ROB values (Tables 1, 2 and 4).This phenomenon is consistent with their lowerdegrees of thermal evolution.

CONCLUSION

Field study and laboratory experiments haveelucidated that pressure can accelerate evolutionof natural solid bitumens. Under a high pressureand moderate temperature, solid bitumens crys-tallize and their reflectance increase with progressof crystallization. This pressure-induced solid bi-tumen (“compressively matured bitumen”) hasself-contradictory evolution characteristics. It hashigh bitumen reflectance (ROB) values and com-pact molecular structures (lower d002 values, big-ger sizes of crystal nucleus, etc.), whereas it alsohas a similar chemical composition and hydrocar-bon-production potential to the thermally maturedbitumens with lower ROB values. Thesegeochemical characteristics can be used to distin-guish the compressively matured bitumens fromthermally matured bitumens.

Acknowledgments—We thank Professor Ming-de LU(Department of Petroleum Geology, China Universityof Geosciences) for his technical assistance duringstudying on optical characteristics of natural bitumens.


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Compressively Matured Bitumen