REVIEW OF LITERATUR.....

Vol. 18 No. 3 CHINESE JOURNAL OF GEOCHEMISTRY 1999

G e o c h e m i c a l Characteristics and Origin o f Tar M a t s

f r o m the Yaha Field in T a r i m Basin, China

Z H A N G M I N ( ] ~ ~ ) A N D Z H A N G J U N ( ~ {.~) ( Geochemistry Research Center o f Jianghan Petroleum University, Jingzhou 434102, China )

Abstract: Tar mats were firstly discovered and determined accurately in terrestrial oil and gas reservoirs associated with Lower Tertiary sandstone reservoirs in the Yaha field of the Tarim Basin, China, by thin-layer chromatography-flame ionization detector ( T L C - F I D ) and Rock- Eval analysis. The relative content of asphaltene in gross composition of tar mat extracts ac- counts for more than 3 0 % , that in the corresponding oil leg less than 20 %. In the geochemical description profile of oil and gas reservoirs, drastic changes in asphaltene contents between tar mats and oil legs could be discovered. This is an important marker to determine tar mats. Dis- tribution characteristics of saturated and aromatic hydrocarbons from reservoir core extracts and crude oils in the Yaha oil and gas reservoirs in the Tarim Basin are described systematically in this paper, and the results show there are similarities among n-alkane distribution characteris- tics, biomarker distribution characteristics and their combined characteristics of saturated hy- drocarbons, and the geochemical characteristics of aromatic hydrocarbons for tar mats, oil leg, asphaltic sand and crude oil. These characteristics suggest the hydrocarbons in these samples were originated from the common source rocks. However, the geochemical characteristics of tar mats revealed that the mechanism of formation of tar mats is the precipitation of asphaltene from crude oils in petroleum reservoirs caused by increased dissolved gas in oil legs (gas injec- tion).

Key words: tar mat; asphaltene; geochemical characteristic; oil and gas reservoir; Tarim Basin

I n t r o d u c t i o n

T a r m a t s r e p r e s e n t a r e s e r v o i r z o n e c o n t a i n i n g p e t r o l e u m h i g h l y e n r i c h e d in a s p h a l t e n e r e l -

a t i v e t o r e l a t e d oil l e g p e t r o l e u m ( W i l h e l m s e t a l . , 1 9 9 4 a ) , a n d i t is o f t e n s i t u a t e d in g e o l o g i c a l

d i s c o n t i n u i t i e s , g a s - c o n d e n s a t e r e s e r v o i r s a n d l i g h t oil r e s e r v o i r s . T a r m a t s a r e i m p o r t a n t o r -

g a n i c b a r r i e r s for oil a n d g a s r e s e r v o i r s . I n t h e p a s t e i g h t y e a r s , p e t r o l e u m g e o c h e m i s t s h a v e

p u t t h e f o c u s of r e s e a r c h o n oil p r o d u c t i o n a n d r e s e r v o i r - r e l a t e d p r o b l e m s ( E n g l a n d , 1990; L a r t e r e t a l . , 1 9 9 5 ) . E s p e c i a l l y , t h e d e t e r m i n a t i o n o f t a r m a t s h a s b e c o m e p o s s i b l e s i n c e t h e

t h i n - l a y e r c h r o m a t o g r a p h y - f l a m e i o n i z a t i o n d e t e c t i o n ( T L C - F I D , I a t r o s c a n ) t e c h n i q u e is a p -

p l i e d t o o r g a n i c g e o c h e m i c a l d e s c r i p t i o n o f p e t r o l e u m r e s e r v o i r s ( K a r l s e n e t a l . , 1 9 8 9 , 1 9 9 1 ) .

T h e g e o c h e m i c a l c h a r a c t e r i s t i c s a n d f o r m a t i o n m e c h a n i s m s o f t a r m a t s w h i c h w e r e d e r i v e d f r o m

m a r i n e s o u r c e r o c k s a n d t h e i r f o r m a t i o n m e c h a n i s m s h a v e b e e n d o c u m e n t e d ( D a h l e t a l . ,

1 9 8 6 ; W i l h e l m s e t a l . , 1 9 9 4 ) . H o w e v e r , t h e o c c u r r e n c e a n d g e o c h e m i c a l s i g n i f i c a n c e o f t a r

m a t s d e r i v e d f r o m t e r r e s t r i a l s o u r c e r o c k s h a v e n o t y e t b e e n i n v e s t i g a t e d s y s t e m a t i c a l l y ( Z h a n g

M i n , 1 9 9 6 a ) .

ISSN 1000-9426 * This project was supported by the Youth Foundation of China National Petroleum Corporation.

No. 3 CHINESE JOURNAL OF GEOCHEMISTRY 251

The objective of this study is to determine the geochemical characteristics and origin of tar mats in terrestrial petroleum reservoirs using the thin-layer ehromatography-flame ionization de- tection ( T L C - F I D , Iastroscan) rapid screening techniques in combination with routine organic

geochemical methods.

Geological Setting

T h e Yaha field, discovered in 1992, is a larger oil and gas accumulation at the northern margin of the Tarim Basin, the largest sedimentary basin of China. Hydrocarbons derived from Triassic-Jurassic terrestrial source rocks in the Kuche depression were trapped in the Tertiary sandstone reservoirs (Zhang Min et al. 1997), and an individual well has been proven to yield 44 - 225 m 3 condensate oil and 120000 - 370000 m 3 gas per day. The oil and gas reservoirs are composed of front sheet sandstones of alluvial fan facies and braided channel sandstones. The tar mats studied (well YH2) are recognized in the Lower Tertiary Suweiyi sandstones which are compositionally sublithic arkose and subarkosic litharenite with a detrital mode of O60-70 F10-25R10-30. Feldspars are mostly present in unweathering form. Many of the sandstones are

different in grain size. Generally, the Lower Tertiary Suweiyi reservoir sandstones are poorly sorted and medium to coarse in grain size, with higher porosity, ranging from 3 % to 15 % and higher horizontal permeability, 30 x 10-3 ttm 2 to 100 x 10-3 ttm 2"

Experimental

Samples

Reservoir rock and crude oil samples were collected from well YH2 in the Yaha field of the Tarim Basin. Reservoir cores were preserved in order to avoid any variation in hydrocarbon composition. T h e cores were taken from two main reservoir units: the Suweiyi formation

(E2-3s) and the Cretaceous reservoir formation.

Separation

Crude oils were subjected to column chromatography on silica gel with elution by n-hexane to isolate the saturated hydrocarbon fraction and with elution of benzene/n-hexane to isolate the aromatic fraction. Extraction of the crushed reservoir rock samples with dichlormethane : methanol 9 3 : 7 vol%, n-hexane, cyclohexane and toluene were used as mobile phases for the

Chromarods.

Instrumental

T h e saturated and aromatic hydrocarbon fractions were analyzed using a Shimadzu GC-14A with dual F I D / F P D and Finnigan TSQ-45 gas chromatography-mass spectrometry with the Su- per-INCOS data system. The GC column was an elastic silica capillary column, coated with SE- 54 (30m • 0. 259 mm i. d ) . The temperature was kept at 100"(2 for 2 min, programmed from 100 to 300*(2 at 4*(2/min and isothermally at 300*(2 for 20 min. T h e mass spectrometer operat- ed at an electron energy of 70eV, with the injector temperature being 300*(; and helium being used as carrier gas. Samples were analyzed using the selective ion monitoring method. Quan- tifications were based on the peak heights for GC analyses, and the peak areas for G C / M S anal-

yses.

252 C H I N E S E J O U R N A L OF G E O C H E M I S T R Y Vol. 18

T h e gross compositions of

reservoir rock e x t r a c t s and crude

oils w e r e analyzed on an I a t r o s c a n T H - 1 0 , M K I V ( I a t r o n L a b s I N C ,

T o k y o ) , equipped w i t h a flame

ionization d e t e c t o r ( F I D ) and inter- faced w i t h an electronic i n t e g r a t o r ,

for rod scanning and quantification. T y p e C h r o m a r o d - S 111 and C h r o -

m a r o d T y p e A silica rods w e r e used. T h e reservoir rocks w e r e ana-

lyzed on a R o c k - E v a l HI analyzer.

R e s u l t s and D i s c u s s i o n

T h e gross composition o f p e t r o l e u m

c o l u m n :

T w o t a r m a t s m e a s u r i n g 0 . 9 and 1. 5 m in thickness ( d e p t h

ranges: 5 1 0 5 . 2 0 - 5 1 0 6 . 1 0 m and

5 1 2 5 . 0 0 - 5 1 2 6 . 5 0 m , respective- ly) w e r e found in well Y H 2 of the

Y a h a field in the T a r i m Basin ( F i g .

1 ) . T h e t a r m a t s located in oil and gas reservoirs, w h i c h can yield 225 m 3 con-

densate oil and 308728 m 3 gas p e r day.

T h e p r e s e n t o i l - w a t e r contact is a p p r o x i -

m a t e l y 50 m d o w n d i p p i n g t h e t a r m a t s . T h e t a r m a t s can be recognized using

T L C - F I D analysis. T h e asphaltene con- t e n t of t a r m a t s is higher t h a n t h a t of oil

legs, so t h e t a r m a t s are s h a r p l y s e p a r a t e d

f r o m oil legs in the geochemical descrip- tion profile of p e t r o l e u m reservoirs. T h e absolute c o n t e n t of total h y d r o c a r b o n s

( 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 ,

m g / g r o c k ) and asphaltene of the t a r m a t s in the Y a h a field are m o s t l y a b u n d a n t

( F i g . 2, T a b l e 1 ) . T h e total h y d r o c a r b o n

a m o u n t s r a n g e f r o m 0 . 4 4 m g / g r o c k to 2 . 2 9 m g / g rock, a v e r a g i n g 0 . 7 3 - 0 . 9 4

m g / g rock, asphaltene 0 . 1 5 - 1 . 2 2 m g / g rock, a v e r a g i n g 0 . 3 5 - 0 . 4 5 m g / g rock. I n contrast, t h e total h y d r o c a r b o n c o n t e n t

in t h e oil legs r a n g e s generally f r o m 0 . 3 5

Fig. 1. Rock properties and organic geochemical signatures of the tar mats (Yaha field, Tarim Basin).

Fig. 2. Plot of gross composition and extract yield versus depth (Yaha field, Tarim Basin).

N o . 3 C H I N E S E J O U R N A L OF G E O C H E M I S T R Y 253

SAT(%) 100 0

9 0 ~ 0 ~ Oil leg

8 ~ 0 ~ 13 Tea" mat 7 0 / V ~r \ 3 0 +Oil

5 0 ~ 5 0

30/VV /VV ,TO

0 ~ 100 1 0 0 90 80 70 60 50 40 30 20 10 0

ARO(%) POL(%)

Fig. 3. Triangular diagram of the gross composition of core extracts (as determined by Iatroscan) from well YH2, Yaha field, Tarim Basin.

m g / g rock to 0. 60 m g / g rock,

averaging 0 . 0 7 - 0 . 0 9 m g / g rock, b u t

the asphaltene content ranges f r o m 0 . 0 4 m g / g rock to 0 . 1 2 m g / g rock w i t h an

a v e r a g e of 0 . 0 7 - 0 . 0 9 m g / g rock. I n

addition, t h e total h y d r o c a r b o n c o n t e n t

in the barriers r a n g e s f r o m 0 . 3 0 m g / g r o c k to 0 . 4 3 m g / g rock, w i t h a n aver-

age of 0 . 4 0 m g / g rock, b u t the asphal-

tene content is close to t h a t of t h e oil legs. H o w e v e r , the highest s a t u r a t e d

h y d r o c a r b o n content is produced up to 0. 35 m g / g rock, followed b y lower

c o n t e n t s of t a r m a t s and the lowest con-

t e n t s of barriers. I n contrast, the high- est a r o m a t i c h y d r o c a r b o n c o n t e n t s

( 0 . 1 1 m g / g r o c k ) are noticed in t a r m a t s , a b o u t 0 . 0 5 m g / g r o c k in oil legs

and lower t h a n 0 . 0 3 m g / g rock in barri-

ers. I n t a r m a t s , the relative c o n t e n t s of asphaltene are higher t h a n 30 % , w i t h an average of

4 2 . 2 % - 4 4 . 5 % ( T a b l e 1 ) . S a t u r a t e d h y d r o c a r b o n content is lower t h a n 30 % , a r o m a t i c h y -

drocarbon content is a b o u t 10 % and resin content is lower t h a n 20 % . I n contrast, in oil leg ex- t r a c t s the relative c o n t e n t of asphaltene is lower t h a n 20 % , b u t s a t u r a t e d h y d r o c a r b o n c o n t e n t is h i g h e r ( o v e r 45 % ) .

Table 1. Gross composition data of reservoir core solvent extracts from

well YH2 in the Yaha field, Tarim Basin

Absolute yield (mg/g rock) Relative composition ( % ) Sample type

SAT ARO RES ASP SAT ARO RES ASP

Oilleg(6) 0 . 2 1 - 0 . 3 5 0 . 0 3 - 0 . 0 7 0 . 0 8 - 0 . 1 1 0 . 0 6 - 0 . 1 2 4 8 . 4 - 6 1 . 3 9 . 3 - 1 4 . 4 1 5 . 1 - 2 2 . 4 1 1 . 7 - 2 4 . 2 (0.27) (0.05) (0.10) (0.09) (52.5) (10.4) (19.0) (18.1)

Tarmatl (5) 0 . 1 3 - 0 . 4 5 0 . 0 4 - 0 . 3 4 0 . 1 3 - 0 . 2 8 0 . 1 6 - 1 . 2 2 1 9 . 6 - 3 4 . 4 8 . 7 - 1 4 . 7 1 1 . 9 - 2 5 . 6 3 6 . 5 - 5 3 . 6 (0.23) (0.11) (0.15) (0.45) (27.7) (10.8) (19.3) (42.2)

Lean zone (2) 0.13-0.18 0.03-0.03 0.08-0.13 0.06-0.08 43.0-43.7 7 . 4 - 8 . 5 26.2-29.6 20.0-21.6 (0.16) (0.03) (0.11) (0,07) (43,4) (8.0) (27.8) (20.8)

Oilleg (17) 0.15-0.31 0.03-0.10 0.08-0.27 0.04-0.14 35.6-56.2 7.4-23.5 22.5-41.1 6.1-23.5 (0.21) (0.06) (0.19) (0.07) (46.1) (12.4) (26.4) (15.1)

Tar mat2 (3) 0.09-0.21 0.04-0.13 0.13-0.15 0.15-0.66 17.6-39.5 8.1-11.6 12.9-23.7 27.8-57.9 (0.17) (0.07) (0.14) (0.35) (25.1) (9.6) (20.8) (44.5)

Oil 84.1 11.3 3.0 1.6 Note: The numbers in the parentheses a r e t h e numbers of samples.

T h e r e is a significant difference b e t w e e n asphaltene and s a t u r a t e d h y d r o c a r b o n contents in b o t h t a r m a t s and oil legs, b u t the relative c o n t e n t s of a r o m a t i c h y d r o c a r b o n and resin in t a r

m a t s and oil legs are of no difference. Fig. 3 s h o w s the r o u g h compositional variations of reservoir core e x t r a c t s in t a r m a t s and

oil legs. T h e t a r m a t e x t r a c t s s h o w high contents of polar compounds ( a s p h a l t e n e and r e s i n )

2 5 4 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 18

Depth (m)

ASP (mg/grock)

0.00 0.~8 5102-

5107- ("

5113"

5118.

5124. ! ~....

i I I ASP PG $4 TPI (%) (rag HC/mg rock)(mg CO2/g rock)

0.0 35.0 ]0.0 ?i0 10.0 7? 0.000.50

Fig. 4 . Identification of the tar mats using Rock-Eval analysis in well Y H 2 , Yaha field, Tarim Basin.

( 6 0 % ) , but those of saturated and aromatic hydrocarbons are relatively l o w . Saturated hydrocarbon c o n t e n t of crude oils is higher than 80 % ; aromat- ic hydrocarbon content, about 1 0 % ; and asphaltene and resin contents, o n l y 4 % . However, asphaltene and resin contents in oil legs range from 30 % to 40 % ; saturated hydrocarbon contents, from 50 % to 60 % ; and aromatic h y - drocarbon contents, 10 %. It should be noticed that the reservoir core extracts from tar mats and oil legs are very sim- ilar in gross composition, falling in a small range as in Fig. 3. So the results imply that the extracts from tar mats and oil legs have genetic relationship.

Rock-Eval parameters have great differences from oil and gas reservoirs (well Y H 2 ) in both the tar mats and

Yaha-field tar mat saturated hydrocarbon C,-C

Produced oil Oil leg

a Pr/Ph=2.82 b Pr/Ph=l.52 nCi7 Pr/nCtT=O.18 | Pr/nC]7---O.19 I I I l l Ph/nCis=0.07 ~ ~ ] Ph/nCts=0.09

//rnC2,

_: i[ L_ 3

Tar mat Asphaltic sand

i c Pr/Ph=l.50 net7 d pr/Ph=l.54

P~nC.=010 / I l l P~nC,,~012 I 510S.5m I / ~ / / / | 5137.0m

nC2s nC2s

i RCI34 31

Fig. 5. Characteristic saturated hydrocarbon fraction gas chromatograms of D S T oil, tar mat and oil leg extracts from well Y H 2 , Y a h a field, Tarim B a s i n .

No. 3 CHINESE JOURNAL OF GEOCHEMISTRY 255

t h e oil legs ( F i g . 4 ) . T a r m a t s have higher 81 and $2 yields. P G ( P G = $1 + $2, m g H C / g rock) c o n t e n t s of t a r m a t s are h i g h e r t h a n 3 m g H C / g rock. I n addition, t h e r e is a close relationship

b e t w e e n $4 content ( m g C O 2 / g r o c k ) and asphaltene c o n t e n t . T h e $4 c o n t e n t of oil legs is

a b o u t 1 m g C O 2 / g rock, and t h e $4 content of t a r m a t s is h i g h e r t h a n 4 m g C O z / g rock, w i t h

t h e highest value up to 13 m g C O 2 / g rock. H o w e v e r , the production indices [ S 1 / ( S 1 + $ 2 ) ]

have significant differences b e t w e e n the oil legs and the t a r m a t s . T h e indices of t a r m a t s are

higher, relative to those of oil legs. T h i s p r o b a b l y reflects a high p r o p o r t i o n of non-volatile

c o m p o u n d s in t a r m a t s as c o m p a r e d w i t h oil legs.

Yaha-field tar mat GC-MS m/z 191

" 53~20 '

Produced oil

a Ts/Tm=l.15 s/(S+R)=0.58

1 1 afl(afl+fla)=0"87

Ts Tm fla S R

Tar mat

c Tsfrm=l.32 S/(S+R)=0.59 up(ap+pa)=0.88

5105.5m

I I I I I

60.00 66.40 73.20 Retention time(rain)

Oil leg b Ts/Tm =1.19

S/(S+R)=0.57 afl Carl +fla)=0.87

53.20

Asphaltic sand

d Ts/Tm=1.45 S/(S+R)=0.58 afl ( afl + fla)=0.87

5137.0m

;0.00 66.40 73.20 Retention time(rain)

Fig. 6. Characteristic mass chromatograms ( m / z 191) of saturated hydrocarbon fractions in DST oil, tar mat and oil leg extracts from well YH2, Yaha field, Tarim Basin.

T h e s a t u r a t e d h y d r o c a r b o n gas c h r o m a t o g r a m s of t h e t a r m a t and oil leg e x t r a c t s s h o w a

similar n-alkane distribution w i t h carbon n u m b e r s r a n g i n g f r o m C15 to C35. N o n e of these h y - d r o c a r b o n e x t r a c t s s h o w s a n y evidence of biodegradation ( F i g . 5 ) . P r i s t a n e / p h y t a n e ratios r a n g e f r o m 1 . 5 2 to 1 . 5 0 for the t a r m a t a n d oil leg e x t r a c t s , and those of the produced oil are 2 . 8 2 . Similar t r e n d s can be seen in pristane/n-C17 and p h y t a n e / n - C l 8 ratios, which range f r o m

0 . 2 0 to 0 . 1 0 for oil legs and 0 . 1 9 to 0 . 1 0 for t a r m a t s .

Biomarker compounds o f saturated hydrocarbons

N o n d r i m a n e , d r i m a n e and h o m o d r i m a n e series c o m p o u n d s in the t a r m a t e x t r a c t s are a b u n d a n t , t h e i r distribution p a t t e r n s are consistent w i t h those of oil leg e x t r a c t s , and h o m o d r i - m a n e c o m p o u n d s s h o w a highest p e a k . L o w e r c a r b o n - n u m b e r c o m p o u n d s in tricyclic t e r p a n e s in the t a r m a t e x t r a c t s are higher, and C2t c o m p o u n d s are highest, characterized b y the p r e - dominance of C19-C23 t e r p a n e s . T h e p a t t e r n s have a good m a t c h w i t h those of tricyclic t e r p a n e s of terrestrial crude oils in this basin ( Z h a n g M i n et a l . , 1 9 9 6 b ) . C27 to C35 hopane series corn-

256 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 18

Yaha-field tar mat G-C-MS m/z 217

Produced oil 20S/(20S+20R)=0"48 / 1 flfl (flfl + aa)=.O.3 7 ,,./

/

! i I t I

Oil leg b 20S/(20S+20R)=.0.44 / /

tiff(tiff+eta)=0.43 / 5104.2m /

/ /

/

.,h, !

Tar m a t

e 20S/(20S+20R)=0.42 tiff (tiff + tra)=0.42

43120 ' 46140 ' 50:00 Retention time(rain)

A~phaltie sand / / 20S/(20S+20R)=0.41 / tiff (tiff +aa)=0.39 / .:.7% /

l i I i I. 43.20 46.40 50 00

Retention time(rain)

Fig. 7. Characteristic mass ehromatograms (m/z 217) of saturated hydrocarbon fractions in DST oil, tar mat and oil leg extracts from well YH2, Yaha field, Tarim Basin.

pounds and gammacerane are i m p o r t a n t biomarkers of pentacyclic triterpanes in the t ar m a t ex- tract, oil leg e x t r a c t and produced oil, w i t h similar distribution characteristics ( F i g . 6 ) . T h e T s / T m ratios in the tar mat and oil leg extract s are in t h e range 1 . 1 5 to 1 . 4 5 , w i t h t h e 2 2 S / ( 2 2 S + 2 2 R ) ratio of C22 isomerization in 1 7 ~ ( H ) , 2113( H ) C32-hopane in the m_nge 0 . 5 7 to 0 . 5 9 . T h e results indicated these extracts might be similar in m a t u r i t y . Similar distribution of C27 to

C29 regular steranes and diasteranes can be seen in t ar mat and oil leg extracts and produced oil,

and the 2 0 S / ( 2 0 S + 2 0 R ) ratios in 5a, 14a, 17a-ethylcholestanes are more t h a n 0 . 4 0 ( r a n g i n g 0 . 4 1 from 0 . 4 8 ) ( F i g . 7 ) . T h e results suggest these hydrocarbons were derived mainly f r o m the same source rock and are similar in m a t u r i t y .

D i s t r i b u t i o n o f p o l y c y c l i c a r o m a t i c h y d r o c a r b o n s ( P H A )

Polycyclic aromatic hydrocarbons such as naphthalenes, phenanthranes, biphenyals, dibenzofuranes, dibenzothiophenes, fluorenes, benzofluorenes, naphthobenzothiophenes, fluo- ranthenes, pyrenes, chrysenes, perylenes and t h ei r derivatives of a wide molecular w e i g h t range in the aromatic fractions of reservoir rocks and crude oils have been identified on t h e basis of G C / M S and MS. Tricyclic and tetracyclic compounds in t h e aromatic fractions of t ar mats are i m p o r t a n t compounds, and the relative abundance of t h e p h e n a n t h r a n e and chrysene series is

N o . 3 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 2 5 7

m o r e t h a n 50 % o f t h e a b u n d a n c e o f P H A . T h e b i p h e n y a l series c o m p o u n d s are h i g h in a b u n -

d a n c e . I n g e n e r a l , t h e c o m p o s i t i o n a l c h a r a c t e r i s t i c s o f P H A in t h e t a r m a t e x t r a c t s h a v e a g o o d

m a t c h w i t h t h o s e o f P H A in oil leg e x t r a c t s a n d p r o d u c e d oil.

C o n c l u s i o n s

I n t e r r e s t r i a l oil a n d g a s reservoirs, t h e relative c o n t e n t s o f a s p h a l t e n e in t h e t a r m a t e x -

t r a c t s r a n g e f r o m 30 % t o 60 % , b u t t h o s e of t h e c o r r e s p o n d i n g oil legs are less t h a n 2 0 % .

B a s e d o n t h e geological a n d g e o c h e m i c a l c h a r a c t e r i s t i c s , t h e a u t h o r h a s p r o p o s e d a m o d e l o f t a r

m a t s in t h e Y a h a field as f o l l o w s : t h e f o r m i n g m e c h a n i s m of t a r m a t s is p r e c i p i t a t i o n o f a s p h a l -

t e n e f r o m c r u d e oils in p e t r o l e u m r e s e r v o i r s c a u s e d b y t h e i n c r e a s e d dissolved g a s c o n t e n t o f a n

oil leg ( g a s i n j e c t i o n ) . T a r m a t s are o f i m p o r t a n t g e o c h e m i c a l significance in b o t h p e t r o l e u m

r e s e r v o i r e x p l o r a t i o n a n d e x p l o i t a t i o n . T a r m a t is a n o r g a n i c b a r r i e r in oil a n d g a s r e s e r v o i r s ,

a n d it is a n o n - p r o d u c t i o n r e s e r v o i r zone. S o t h e g e o c h e m i c a l m o d e l o f t a r m a t s in oil a n d g a s

fields is e s t a b l i s h e d t o p r o v i d e useful i n f o r m a t i o n o n b o t h p r o d u t a b l e r e s e r v e s a n d l o c a t i o n s o f

i n j e c t i n g w a t e r wells. G e o c h e m i c a l c h a r a c t e r i s t i c s o f t h e t a r m a t s c a n help u s t o d e t e c t t h e di-

r e c t i o n a n d t i m e o f oil a n d g a s filling i n t o t h e reservoirs. Based o n t h e g e o c h e m i c a l c h a r a c t e r i s -

tics a n d f o r m a t i o n m e c h a n i s m o f t a r m a t s , t h e f o r m i n g m e c h a n i s m a n d e v o l u t i o n h i s t o r y o f oil

a n d g a s reservoirs can be revealed.

R e f e r e n c e s

Dahl, B. and G.C. Speers, 1986, The geochemical characterization of a tar mat in the Oseberg field Norwegian sector, North Sea: Organic Geochemistry, v. 10, p. 547 - 558.

England, W . A . , 1990, The organic geochemistry of petroleum reservoirs: Organic Geochemistry, v. 16, p. 415 - 426.

Karlsen, D. and S.R. Larter, 1989, A rapid correlation method for petroleum population mapping within indi- vidual petroleum reservoirs-application to petroleum reservoir description, in J. Collins, eds., Correlation in hydrocarbon exploration: Norwegian Petroleum Society, p. 7 7 - 85.

Karlsen, D. and S.R. Larter, 1991, Analysis of petroleum fraction by TLC-FID: applications to petroleum reser- voir description: Organic Geochemistry, v. 17, p. 603 - 617.

Latter, S . R . and A.C. Aplin, 1995, Reservoir geochemistry: methods, applications and opportunities, in J. M. Cubitt and W. A. E~fgland , eds., The geochemistry of reservoirs= v. 86, p. 5 - 32.

Wilhelms, A. and S.R. Latter, 1994a, Origin of tar mats in petroleum reservoirs, Part I: introdution and case studies: Marine Petroleum Geology, v. l l , p . 4 1 8 - 4 4 1 .

Wilhelms, A. and S . R . Latter, 1994b, Origin of tar mats in petroleum reservoirs, Part I I : formation mecha- nisms for tar mats: Marine Petroleum Geology, v. 11, p.442 - 4 5 6 .

Zhang Min, 1996a, Discovery and its geological significances of the tar mats in terrestrial gas-condensate reser- voirs: Chinese Science Bulletin: v. 41, p. 1 9 6 7 - 1969.

Zhang Min, Lin Renzi, and Mei Bowen, 1997, Reservoir geochemistry---approach to the Kuche petroleum sys- tem of the Tarim Basin, China: Chongqing, Chongqing University Press, 25p.

Zhang Min and Zhu Yangming, 1996b, Geochemical characteristics of crude oils of the Kuehe petroleum system: Geological Review, v. 42, p. 229 - 235.