geochemical characteristics of oilfield waters from the turpan depression, xinjiang and their...

Vol. 15 No. 4 CHINESE JOURNAL OF GEOCHEMISTRY 1996

Geochemical Characteristics of Oilfield Waters from the Turpan Depression, Xinjiang and Their

Petroleum Geological Significance

LI WEI ( ~ ' ~ ) , LIU JIMIN ( ~ ] ~ : L ~ ) (Research Institute of Petroleum Exploration and Development, CNPC, Beijing, 100083)

AND CI-IEN XIAOHONG ( ~ f l : ) (Research Party of Petroleum Exploration and Development, Turpan-Hami, Xinjiang )

Abstract: This paper, based on the .fundamental inorganic chemical and organic geochemical characteristics of oilfidd waters from the Turpan Depression, presents the contents of organic matter, the distribution of low-carbon fatty acids and the contents of aromatic hydrocarbons as well as their principal ultraviolet absorption spectral and fluorescence spectral characteristics in oilfield waters from different oil/gas-bearing areas. The oil/gas reservoirs in this depression are classified in terms of their conserving conditions. In addition, the paper also discusses the chemical characteristics of oilfiald waters from different types of oil/gas reservoirs with an em- phasis on the characteristics of their localization in the 7NI/TC.-TNa/YC~ correction diagram. On this basis it is attempted to expound the fundamental geochemical characteristics of oilfield waters from the Turpan Depression and their geological significance.

Key words: geochemistry; organic matter; organic acid; ultraviolet spectrum; fluorescence; oil/gas reservoir type; oilfleld water; Turpan Depre~ion

Fundamental Chemical Characteristics of Oilfield Waters

The Turpan Depression is located in the western part of the Turpan-Hami Basin and ex-

tends east-westward with the relief being characterized as being low in the south ( - 154 m be-

low sea level) and high in the north (1300 m above sea level), and low in the west (400 m

above sea level) and high in the east (1000 m above sea level). Within the depression is prevail-

ing an arid continental climate all the year round, and the Middle-Lower Jurassic strata are the

dominant oil-generating and oil-conserving strata, wi th the main oil-accumulating strata buried

at the depths of 2400 - 4000 m.

According to the Su L i n ' s classification, oilfield waters are predominated by the CaCh- type with a mineralization degree of 1 0 0 0 0 - 40000 r a g / L ; coming next is the NaHCO3-type

with a mineralization degree of 2000 - 7000 r a g / L ; the NazSO4 - and MgC12-type are of scarce

occurrence, which are concentrated largely in the strata of the Huoyanshan-Qikt im fold zone.

Present in the oilfield waters are not only large amounts of inorganic salts and trace metal-

lic elements, but also considerable amounts of dissolved organic matter , such as alkane, alkene,

aromatic hydrocarbon, organic acid and organic oxide. The total content of organic matter (ex-

cluding organic acids) in oilfield waters from this depression is estimated at 38 • 10-6 _ 132 x

10- 6, of which the content of alkane is 15 • 10- 6 _ 79 • 10- 6, that of cyclic alkane 0 . 2 5 •

10 -6 - 66 x 10 -6, that of aromatic hydrocarbon 0 . 3 2 x 10 -6 - 5 . 0 5 • 10 -6, that of alkene

0 . 0 2 • 10 -6 _ 0 . 1 8 • 10 -6, the total amount of organic oxides (alcohols, quinones, ketones,

ISSN 1000-9426

No. 4 CHINESE JOURNAL OF GEOCHEMISTRY 375

etc. ) is about 0 .02 x 10-6 _ 0 .42 x 10 -6, and the contents of organic acids range from 329 x

10- 6 tO 629 x 10- 6. From this it can be seen that the contents of organic matter in oilfield wa-

ters tend to decrease in the order of organic acid >2> alkane > cyclic alkane > aromatic hydrocar-

b o n > a lkene> organic oxide. The organic acids are predominated by low-carbon fatty acids. With increasing carbon number, their contents tend to decrease, with acetic acid present in

considerable amounts. From the results of the analysis of low-carbon fatty acids (Cheng Zhong-

di), the content of formic acid in monoacids is estimated at 18 x 10 -6 - 144 x 10 -6, that of acetic acid 58 x 10 -6 - 2 1 1 x 10 -6, that of propionic acid 12 x 10 -6 - 76 x 10 -6, that of bu-

tyric acid 5 .6 x 10 -6 - 14 x 10 -6 and that of valeric acid 4 x 10 -6 - 12 x 10 -6 . In the oilfield

waters the content of binary acids is less than that of monoacids, of which the content of oxalic acid is 58 x 10 .6 - 163 x 10 -6, that of malonic acid 6 .53 x 10-6 _ 137 x 10 -6, that of suecinic

acid 2 .8 x 10- 6 _ 33 x 10- 6, that of glutaric acid 2 .8 x 10- 6 _ 25 x 10- 6, and that of adipic

acid 4 x 10 -6 - 24 x 10 -6. It can be seen clearly that the contents of fatty acids in the oilfield

waters tend to decrease in the order of total monoacid > total binary acid, acetic acid > formic

ac id> propionic acid>>butyric acid>>valeric acid, and oxalic acid > malonic acid>>succinic acid

~g lu t a r i c acid~adipic acid.

Organic Geochemical Characterist ics of Oil f ie ld Waters

Differences in composition and physical and chemical properties of oils and gases contained

in the strata would necessarily lead to the differences in the species of dissolved organic matter

in oilfield waters.

The contents of benzene, phenol and other homoloques are different in oil field waters from merely water-bearing strata and oil-bearing aqueous strata

The content of benzene in oilfield waters from this depression is 0 .05 - 1 .97 m g / L (see

Table 1). The content of benzene in the merely water-bearing strata is lowest (0 .05 m g / L )

while that in water from the oil/gas-bearing strata is 0 . 2 4 - 1 . 9 7 m g / L ; the content of

methylbenzene in the oilfield waters is 0. 021 - 3. 210 m g / L while that in the merely water-

bearing strata is lowest (0. 021 m g / L ) ; and the content of phenol in the oilfield waters is 0.11

- 1. 115 m g / L while that in the merely water-beating strata is 0.11 m g / L and that in the oil-, gas- and water-bearing strata is 0. 245 - 1. 115 m g / L . From the above data it is known that the

contents of benzene, phenol and methylbenzene are lowest in the merely water-bearing strata in

this depression, indicating that benzene and phenol are closely related with oil and gas.

Differences in the properties of oils and gases contained in the strata would lead to significant differences in characteristic ultraviolet absorption spectra of oil field waters

Ultraviolet absorption spectra are a kind of composite spectra, which are a reflection of electron jump in the molecules of a substance. Generally speaking, the more the number of

rings is, the farther the spectra will shift toward the long wave direction. As shown in Fig. 1,

the ultraviolet absorption spectra have the most intensive absorption peak at 220 nm, with the

absorption peaks at 254 - 2 5 8 nm and 280 nm coming next, mainly reflecting the characteris-

tics of ultraviolet absorption spectra of bicyclic aromatic hydrocarbons. Of course, the absorption peaks at 254 - 258 nm and 280 nm are a reflection of the characteristic ultraviolet absorp-

tion spectra of both monocyclic and bicyclic aromatic hydrocarbons, indicating the presence of a

Table 1. Characteristics of water-soluble organic matter In ol l f le ld waters from the Turpan Depression

Stratum d dl~td water

Total water-~l- utde g~/dJL)

Fluore~nce slxxtro~a~e peak value (Ex= 265 rim)

~ i ~ aque~ 4976

strata

6O46 aqueous strata

f,,t~ate all-, gas- and wam-e0exiafing 2612 strata

0il- and water-e0ex- 3281

mata

Me~y wateT-beming 384

su'ata

Wat~r-lm~ al 4107

sllata

M~yl-

(rag/L)

1.97 3.210 L115

O. 24 O. 252 O. 28

0.50 0.194 1.05

0.344 1,316 0.81

0.05 0.021 0.11

1.50 0.398 0.245

Ultraviolet aix~- tion specnnm a~ 230 m (1031gE)

2.1

3.1

O. 5 262

2.25

2.0

2.15

Em= 325 nm % Em= 365nm %

112.5

52

378

85

225

376 CHINESE JOURNAL OF GEOCHEMISTRY Vol. 15

Total organic ~t- ter (exdu~ 0~-

(Xl0 -6) gmc~id~) (Xl0 -6)

0.187 37.89

0.068 40.49

1.653 20,11

0. O72 106.84

0.020 47.57

0.024 132.

3.5

30[

.~ 1.5 / 2

0L___ t I , __ 200 220 240 260 280 300 320 340

Wavelength (nm)

Fig. 1. Characteristics of the ultraviolet absorption spectra of oilfield waters. 1. Oilfield water in which the density of oil

contained is 0. 867g/cm3; 2. oilfield water containing oxi- dized crude oil; 3. oiifield water containing condensate oil and gas; 4, oilfield water in which the density of oil contained is

0. 835g/cm 3 ; 5. oilfield water containing no oil and gas; 6.

oilfield water in which the density of oil is 0.82g/cm 3 .

cer ta in amount of monocycl ic aromatic

hydrocarbons . W h a t is most obviously

shown in Fig . 1 is that the grea ter the

dens i ty of crude oil contained in the stra-

ta, the higher the absorbance of the ul-

t raviolet absorpt ion spec t ra of aromatic

hydrocarbons and thei r homoloques will

be. In case the densi ty of oil contained in

the s t ra ta is 0. 8 6 7 g / c m 3, the m a x i m u m

absorbance of the ul traviolet absorpt ion

spect ra of oilfield waters will be expect-

ed, as indicated by the obvious double

peak. However , the absorbance of the

ul t raviolet absorpt ion spect ra of forma-

t ion wa te r samples f rom the s t ra ta con-

ta ining condensate oil and gas is ex t reme-

ly low, below 0. 5 • 1031gE, w h i c h

seems to be related wi th the fact that the

content of aromatic hydrocarbons is low

in the condensate oil (2 % - 3 . 2 % ) .

The characteristics of 3-dimensional fluorescence spectra are different for oil field zoaters from different types of oil/gas reservoirs

Fluorescence is given off by molecules of organic m a t t e r when they are exci ted by a given

wavelength l ight and it is caused mainly by aromat ic compounds . General ly , the longer the


wave lengths of excitation light and emission light are, the greater the number of rings of aro-

matic hydrocarbons will be. The 3-dimensional fluorescence peak values of oilfield waters from

the Turpan Depression are mostly within the range where the wave-length of excitation light is

at 220 - 300 nm and that of emission light at 300 - 400 nm (Figs. 2 and 3) . This implies that

low-ring aromatic hydrocarbons are more abundant, which are predominated by the naphtha-

lene series compounds. The peak values characterized by a wave-length of excitation light of

265 nm and a wave-length of emission light of 365 nm in Fig. 3 are indicative of the presence of

a certain amount of tricyclic aromatic hydrocarbons. Meanwhile, it can be seen that there are

no peak values characterized by Ex = 265 nm and E m = 365 nm in Fig. 2 as in Fig. 3. It also

can be seen from Fig. 4 that oilfield waters from the condensate oil/gas-containing strata show

no peak values at Em = 365 nm, but at E m = 325 nm. This is the difference in 3-D fluorescence

spectra of oilfield waters between the condensate oil /gas reservoirs and the light oil reservoirs.

In addition, it also can be seen that the more oil and gas the strata contain, the greater the in-

tensity of fluorescence given off by oilfield waters will be. For example, the intensities of 3-D

fluorescence spectra of oilfield waters from both water-bearing oil strata and the strata where oil

and water are coexisting are all greater than 200 % while those of oilfield waters from the mere-

ly water-bearing strata are all less than 100 %, as observed in the section with Ex = 265 nm

(Table 1) . Therefore, we can identify the oil/gas-bearing potentialities of the strata on the ba-

sis of the above characteristics.

4 3 5 . 6

>, 3 1 6 . 3

�9 ~- 197

e~

~ "/?.6

- 4 1 . 7 300 350 400 450

Emission wavelength (nm) 5 0 0 ~ w

Fig. 2. Characteristics of the 3-D fluorescence spectra of oilfield waters from a condensate oil/gas reservoir.

The better the oil-bearing potentiality o f the strata is, the more organic matter the oilfield waters ~aill have, but the less organic matter the oil f ield waters f rom condensate oil~gas

reservoirs wi l l contain

Generally, the content of organic matter in oilfield water ranges from 20 • 10-6 to 50 •

10-6. But oilfield water samples from the Turpan Depression contain as much organic matter as

64 .23 • 10 -6 on average. It is known from Table 1 that formation waters from oil/water-coex-


i 26o.2 273.5 : //249

Emission wavelength (nrn)

Fig. 3. Characteristics of the 3-D fluorescence spectra of oilfield waters from a light oil reservoir.

isting strata with oil flows of commercial importance and water-bearing oil strata are the oilfield

waters which have the highest content of organic matter . Coming next are those from oil-bear-

ing aqueous strata. The lowest content of organic matter is produced in oilfield waters from

those condensate oil/gas-bearing strata. From this it can be seen that when the total amount of

organic matter in the formation water exceeds 100 x 10-6, there would most probably exist oil/

gas flows of commercial importance in the strata. Moreover, it is found that the content of wa-

ter-soluble gas in the merely water-bearing strata is lowest against a highest value for the oil-

bearing aqueous strata and water-bearing oil strata. Water-soluble gases contained in marginal

water from the condensate oil /gas strata or in interbedded water are present in moderate

amounts. Therefore, we can identify the oil/gas-bearing potentialities of the strata in terms of

the contents of water-soluble gases in formation waters. As also can be seen from Table 1, the

highest content of alkene is produced in oilfield waters from condensate oil/gas reservoirs while

the lowest content of it is recognized in formation waters from merely water-bearing strata.

This characteristic feature is also helpful for the identification of oil/gas-bearing potentialities of

the strata.

The contents of lo~-carbon fatty acids in oil field ~oaters from condensate oil/gas reservoirs are higher than those from oil reservoirs

The contents of low-carbon fatty acids in oilfield waters from the oil reservoirs in this depression are within the range of 329 • 10 -6 - 580 • 10 -6 with an average of 443 .9 x 10 -6 a-

gainst a higher value of 629 • 10 -6 for those from the condensate oil/gas reservoirs. This fea-

ture is comparable with that encountered in oilfield waters from western Siberia. In oilfield waters from the oil reservoirs in western Siberia are contained 260 • 10- 6 _ 570 x 10- 6 low-car-

bon fatty acids, with an average of 370 x 10-6 while in those from the condensate oil/gas reservoirs there are contained 480 x 10 -6 - 1800 • 10 -6 low-carbon fatty acids, with an average of

1260 x 10-6. On this basis we can determine the phase-state characteristics of underground hy-

drocarbons.


.--= 2o0[-

~176 150

~. I00

0 270 300

~4 2

380 420 460 500 ErnL,~ion wavelength (nm)

Fig. 4. Characteristics of the fluorescence transmission spectra of oilfield waters under the excitation of light whose wave-length is 265 nm. 1. Oil/gas-bearing aqueous strata; 2. oil-bubble-bearing aqueous strata; 3. condensate oil/gas-bearing edge aquifer; 4. oil/water-coexisting strata; 5. aqueous strata; 6. water-bearing oil strata.

Chemical Characteristics of Oilfield

Waters from Oil/Gas Reservoirs

Conserved under Different

Conditions

Due to the influence of the Himalayan

60

50

40

c3

30

20

0.7

/ /

/

t " A ~

t l t 0.9 1. 1.3 ! .5 7NJTCt

Fig. 5. 7s,/Tc~-)'~,/'/ca correlation diagram of oilfield waters from the Turpan Depression. I . Shanshan oil reservoir and Wenjisang oil/gas reservoir; I1 . Wenxi No. 1 oil reservoir; HI. Oiuling oil reservoir; IV. Shengjinkou oil reservoir; V. Qik-

tectonic movement, the strata in the Turpan De- tim oil reservoir.

pression have experienced strong compressional destruction, leading to the exposure of some deeply buried strata in local places. As a result, there are significant differences in the degree of destruction brought about by the tectonic movement to the oiL/gas reservoirs in the Turpan De- pression. In accordance with the petroleum geological characteristics of the oiL/gas reservoirs and the hydrogeological conditions of the oilfields, the oil/gas reservoirs in this depression can be roughly divided into three categories and five types with respect to the conditions under which they have been conserved (Table 2 ) , i . e . , non-destructive, weakly destructive and strongly destructive oil/gas reservoirs. The weakly destructive category can be further divided into the slightly weakly destructive type and the unevenly destructive type; the strongly destructive category into the oil bed-exposed type and the tectonically strongly destructive type. The characteristics of oilfield waters from the three categories of oil/gas reservoirs are presented as follows.

Oil field waters from the non-destructive-category oil~gas reservoirs

Oilfield waters from this category of oiL/gas reservoirs are characterized by high mineralization degree, low Na/CI equivalent concentration ratio, SO4/C1 equivalent concentration ratio, and low Na/Ca equivalent concentration. They are designated to the CaC12 type. In the Turpan Depression the Shanshan oil reservoir and the Wenjisaug oil/gas reservoir can be taken as typi-


cal examples. As shown in Fig. 5, oilfield waters fall within the I area, with "/Na/TC~ being 2 .5

- 10 and 7Na/Tct being 0 .70 - 0 .85 . The SO4/C1 equivalent concentration ratios are so small as

to be 0 .02 • 10 -2 - 2 . 0 0 • 10 -2 . The ehemieal parameters for these oilfield waters are indiea-

tive respectively of oilfield water without surface water involved, highly metamorphic oilfield

water and oilfield water from well closed oil/gas reservoirs.

T a b l e 2 . C l a s s i f i c a t i o n a n d c h a r a c t e r i s t i c s o f the c o n s e r v a t i o n c o n d i t i o n s o f

o i l / g a s r e s e r v o i r s in the T u r p a n D e p r e s s i o n

Oil]gas reserv~ ehs~ea+ 6m

_ TD$ (rag/L) i)

o "~ ~ )'SO2 • 100

Water-type

Degree ol nap de-

o water cO

I~ I.,

; Other aspects

Example d dl]gas raser~r

Nm-destmcove( I )

15000- 70000

0.70-0.85

0.02-2.00

2.5-10.00

c~c~

Not d e ~ l

Oilfidd water tel~li~ m be stagnam

Ca3tnt~,ete cover su'~ta

Shamhan dl resea~r, Wenjisang ~]e~ate

oiL/gas re~vdr

Weakly ~ tru~ ( II ) Sfightly weakly de~mm- tire (K i)

4500 - 12000

1.00-1.15

0.0-2.25

8-12

NaHCO3

Local faults interlinked with formation water at shllow depth

Nal'ICOs-ty~ water

p~em locally, surface water permeating locally

lnoxapletdy closed cov- et !Rl-ata

Wenxi No. 1 ~ reser~r

Unevenly destructive

(112)

2000 - 6000

1.00 - 1,50

0.20-20.00

10- 50

NaHffh

Faults rdafivdy devd- oped, locally interlinked with the Earth' s mtrhee

Surface water ~ t -

thtm~ fauh

Oil and gm km in ~nall ~ p ~ migrat. ing m other bcauom

r al r~er~r

O~ bed-expmed we (mr)

2000 - 25000

1.04)- 1.55

>3.50

30 - 175

N~:O3

al beds sdfenm+ de-

nudation

l+ead~ by rain water,

surface water peme~t-

m+

NO c~er strata, oil and g~ k~ in L~ m~otmts

Qik~im oil re~rvdr

T e c ~ y destructive ~e (12)

5000- 500~

0.85-1.00

0.00-3.50

15:-55

Strong teethe uplift- ing, faults leading to the surface

Oilf~d water rdafi~y

sunngly active, surface

water permea~i~

l l l ~ e t e cover ~ l a ~

oil and gas lost in la~e mnounl3

Sher~iinl~u oil reservoir

2. Oil field waters from the slightly weakly destructive-type oil~gas reservoirs

Oi l f i e ld w a t e r s f rom th is t ype of o i l / g a s rese rvo i r s a re la rge ly d e s i g n a t e d to the NaHCO3

type , w h i c h are charac te r i zed by h igh mine ra l i za t ion deg ree ( 4 5 0 0 - 12000 r a g / L ) , r e l a t ive ly

low N a / C 1 equiva len t co n c e n t r a t i o n ra t io ( 1 . 0 0 - 1 . 1 5 ) , low N a / C a equ iva len t concen t r a t i on

ra t io ( 8 - 12) and low SO4/C1 equ iva len t concen t r a t i on ra t io ( 0 . 0 0 • 10 -2 - 2 . 2 5 • 1 0 - z ) .

T h e typ i ca l e x a m p l e is the W e n x i N o . 1 oil reservoi r , w h o s e oi l f ie ld wa te r s , as is s h o w n in F i g .

5, fall w i t h i n the II area . T h i s ind ica t e s t ha t the oi l f ie ld w a t e r s are f rom those r e l a t ive ly closed

o i l / g a s rese rvo i r s , w i t h a t race a m o u n t of e x t r a n e o u s w a t e r invo lved o w i n g to the local p e r m e -

a t ion of w a t e r f rom sha l low s t r a t a t h r o u g h local i n t e r l i n k e d faul t s .

3. Oil field waters'from the unevenly destructive-type oil/gas reservoirs

Oi l f i e ld w a t e r s f rom th i s t ype of o i l / g a s r e se rvo i r s are m o s t l y a s s igned to the NaHCO3

type , w h i c h are possessed of l ow mine ra l i za t i on degree ( 2 0 0 0 - 6000 r a g / L ) , and h igh ly var i -

able N a / C I equiva len t eo n e e n t r a t i o n ra t io , N a / C a equ iva len t concen t r a t i on r a t i o and SO4/C1 e-

qu iva len t concen t r a t i o n ra t io . T h e l a t t e r th ree aqueous ly ehernieal p a r a m e t e r s are w i t h i n the


ranges o f l . 0 0 x 1 0 - 2 - 1 . 5 0 x 1 0 -2, 1 0 x 1 0 - 2 - 5 0 • -2 , a n d 0 . 2 0 • - 2 - 2 0 . 0 0 • 10-2, respectively. The Qiuling oil/gas reservoir can be taken as a typical example, whose oilfield waters fall, as shown in Fig. 5, within the Ill area, indicating significant differences in metamorphic extent of the oilfield waters, the extent of involvement of surface water and the closeness of the strata. Those differences may be attributed to the heterogeneous physical properties of the reservoir strata through which surface water permeated into the oil/gas reservoir through the SN-extending faults which are interlinked with the Ea r th ' s surface in the middle part of the oil/gas reservoir.

4. Oil field waters from the oil bed-exposed-type oil reservoirs

Oilfield waters from this type are largely of the NaHC03 type, characterized by relatively high mineralization degree (2000- 25000 mg / L) , Na/C1 equivalent concentration ratio (1.00 - 1 . 5 5 ) , Na/Ca equivalent concentration ratio ( > 3 5 ) and SO4/C1 equivalent concentration

ratio ( > 3.5 x 10-2). The Qiktim oil reservoir can be taken as a typical example. As is shown in Fig. 5, its oilfield waters all fall within the V area, indicating that water from the oil beds has experienced a low-grade metamorphism and has been affected seriously by surface water. Characteristics of these parameters are the result of direct leaching of the exposed-on-the surface oil reservoir by rain water under current arid-hot climate conditions.

5. Oil field waters from the strongly tectonically destructive-type oil/gas reserzoirs

Oilfield waters of this type mostly belong to the CaCI2 type, although some of them are designated to the NaHCO3, MgC12 and Na2SO4 types. Their mineralization-degree values are variable over a large range (5000 - 50000 rag/L) , the Na/C1 equivalent concentration ratios are mostly within the range of 0.85 - 1.00, the SOJC1 equivalent concentration ratios are mostly

within the range of 0 • 10 -2 _ 3 .5 • 10 -2, and the Na/Ca equivalent concentration ratios are high, ranging from 15 - 55. Oilfield waters from the Shengjinkou oil reservoir in this depression are possessed of these characteristics, which are clustered in the IV area as is shown in Fig. 5. It is reflected that the oilfield waters have experienced metamorphism to a certain extent, but the closeness of the strata is poor, thus leading to the extensive involvement of surface water. The reasonable explanation is" the structures were strongly compressed and hence the deeply buried oil reservoirs have been uplifted to shallow levels with numerous faults leading to the Ear th ' s surface; under such circumstances oilfield waters became very active with the de- pressurization of the strata and later surface water also found its way into the oil reservoirs through the fault system.

From what is described above, it may be concluded that oilfield waters from various types of oil/gas reservoirs conserved under different conditions possess their own aqueously chemical characteristics and, as shown in the 7Na/"/Ca-]tNa/)'CI diagram, they fall within their respective areas. So, we can analyze and ascertain the differences in the conservation conditions of various oil/gas reservoirs and identify their oil/gas-bearing potentialities (or those of the regions and zones where the oil/gas reservoirs occur) in terms of this diagram.

Conclusions

1. Formation water from the merely water-bearing strata is characterized by low contents of water-soluble gas, benzene, methylbenzene and phenol; oilfield waters from the condensate oil/gas reservoirs are low in total organic matter but high in alkene and low-carbon fatty acid,


and low in the absorbance of their ultraviolet spectra, as indicated by the appearance of peaks at

325 nm in the case of excitation by a 265-nm-wavelength light source.

2. The content of total organic matter in oilfield waters from the strata where oil flows are

of commercial importance is greater than 100 x 10- 6 ; the peak value at 365 nm is greater than

200 % in the case of excitation by a 265-nm-wavelength light source. 3. Five types of oil/gas reservoirs conserved under different conditions are recognized in

the Turpan Depression. Oilfield waters from these types of oil/gas reservoirs fall within their

respective areas as shown in Fig. 5 and also have their own aqueously chemical properties. Type

I reflects the aqueously chemical conditions in favor of the conservation of oil and gas, types

II 1 and II 2 reflect the aqueously chemical conditions relatively favorable to the conservation of

oil and gas, and type Ill I and m2 reflect the conditions unfavorable to the accumulation and

conservation of oil and gas.

References

Liu Chongxi and Sun Shixiong, 1988, Hydrogeochemical theories and methods for oil prospecting: Beijing, Geol- ogy Press, p. 82 - 87 (in Chinese)

Liu Jimin, 1982, Comprehensive application of aqueous chemistry and its relevant characteristic indices in oilfield hydrogeological exploration: Petroleum Exploration and Development, v. 9, p. 49 - 55 (in Chinese).

Yong Kelan, 1992, 3-dimensional fluorescence finger-print technique and its application in petroleum geochemical exploration: Petroleum Exploration and Development, v. 14, p. 432- 442 (in Chinese).

geochemical characteristics of oilfield waters from the turpan depression, xinjiang and their...

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