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Maturation sequence of continental crude oils in hydrocarbon basins in China and its significance
Advancesin OrganicGeochemistry1989 Org. Geochem.Vol. 16, Nos 1-3, pp. 521-529, 1990 Printed in Great Britain.All rights reserved
0146-6380/90 $3.00+ 0.00 Copyright© 1990PergamonPress plc
Maturation sequence of continental crude otis in hydrocarbon basins in China and its significance HUANG DIFAN, L~ JINCHAO and ZHANG DAJIANG Research Institute of Petroleum Exploration and Development, 20 College Road, P.O. Box 910, Beijing 100083, China
(Received 19 September 1989; accepted 18 February 1990) Abstract--The continental hydrocarbon-containing basins in China not only ubiquitously possess immature-marginal mature oils as defined by C29 sterane ratios, but a complete oil maturation sequence can also exist. The present paper studies the characteristics of immature oils, their formation and distribution. The study revealed that immature oils were not thermal degradation products of kerogen, since the vitrinite rvflovtance (Re) of source rocks is between 0.4 and 0.6%. The immature oils are derived from the soluble (in organic solvent) organic matter generated during low-temperature hydrocarbon generation at the late stage of diagenesis. The immature oils in China belong to paraffin-base oils which originated from aquatic or terrestrial lipids but not resins. The maturation sequence of oils is composed of immature, low-mature and high-mature members, corresponding to the different levels of hydrocarbon generation. The recognition of this evolution sequence has proved to be of practical significance. When we understand the amounts of oil reservoirs of different maturation, the exploration direction can be guided by the corresponding resource volume comparison. Taking the Qaidam Basin as a good example, most of the oils discovered recently are low-mature, but there are good prospects of finding mature and high-mature oils.
Key words--Chinese hydrocarbon basin, Tertiary, isomerizations of steranes, immature oil, marginal oil, hypersaline lake, maturity sequence of crude oil
INTRODUCTION In recent years, the ubiquity of immature oils in the Tertiary non-marine hydrocarbon-bearing basins in China has been brought to Chinese petroleum geechemists' attention, and their origin has been discussed from various angles. Meanwhile, the existence of a maturation sequence composed of immature, lowmature, mature and high-mature crude oils in many basins is also a conspicuous phenomenon. It appears that during the generation and evolution of hydrocarbons, there are commercial oil and gas accumulations of corresponding hydrocarbon products in each evolutionary stage, and their incorporation and dilution with each do not always occur. Therefore, it is extremely important to study further the maturity of hydrocarbons in individual oil and gas fields. On the basis of such a study, the proven oil and gas reserves at various mature levels can be compared with the corresponding reserves at each hydrocarbongenerating stage. Thereby, the oil and gas prospects would be more scientifically forecast, and the exploration target unequivocally determined.
they arc influenced by more factors besides maturity. In the authors' experience, the most efficient ones for the measurement of crude oil maturity are biomarker parameters dealing with the configuration conversion of polycyclic naphthenes. During diagenesis and catagenesis, biomarkcrs such as steranes and terpanes suffer a series of profound configuration conversions via reduction, rearrangement and isomerization, resulting in a set of new compounds. Thereby, the distinction of oil and source rock maturities can be shown in varying degrevs (Huang et al., 1984a). Based on this principle, the maturity scales of various biomarkers have been established, among which the authors consider the isomerization parameters of C29 steranes as the optimal ones for the measurement of crude oil maturation:
I. 5ct(H),I4~t(H),I7ct(H)-C29steranes
20S/(20R + 20S); 2. 5~(H)-C29 steranes///~/(flfl+ ~t~).
Parameter selection for crude oil maturity
Chinese petroleum geochemists have generally found the above ratios to have values of 0.20--0.25 at the hydrocarbon-generating threshold. From the
There are various parameters for determining crude oil maturity, whose applicable range, efficacy and availability may be different from each other and the applicable range of most parameters is limited, or
results of a correlation study between the above two parameters and vitrinite reflectance by the authors, the corresponding value of Re is around 0.65%, as shown in Fig. 1. Therefore, for both C29 steranes
521
HUANO D ~
522
/,o..
Mature
0.40
et al.
"~II •
0.40
t
Low-mature
[ ~
-p ~0.30
Low-mature -
Mature
/.,
I ~t~ 0 .30 ,
i
0.25
i
o.=
/ I
I
"°°"/i
I
. . . . . . .
I I
0
I =* °'15t I ==
z
E-
I I I 0.10
0.5
0.6
0.7
I
Re e/,~
I
I I
¢
I
I
o.o5~ ;/" at/ I
II I
II I
o.1o
// .
"
1
0.5
I
0.6
i
0.7
I 0.8
Re ~'0)
Fig. 1. Correlation of Ro and two C29steraneisomerization parametersin the southern part of the Dagong oil field [after Cheng K. M. and Jiang C. Q. (1987, unpublished work)]. isomerization parameters in the range 0.20-0.25 are regarded as the P,oundary between immature and low-mature oils herein, and those in the range 0.300.40 as the boundary between low-mature and mature oils. The lower values often occur in those strata which are rich in gypsum, rock salt and carbonate rocks. Besides, a ternary diagram, based on =fir (S + R), =~,~S + ~/~S and ~==R + =flflR configurations of C:9 steranes, can also reveal the maturation sequence of crude oils. In this diagram, the corresponding boundaries between immature and low-mature oils, and between low-mature and mature oils should be 55 and 40% for the original configuration, respectively. The 22S/22R ratios of C3~ or C32 homohopanes can be used as an auxiliary parameter for the oil maturation scale, in which the corresponding value of the hydrocarbon-generating threshold is 1.10-1.25. R E S U L T S AND D I S C U S S I O N
Immature oils As the initial products of hydrocarbon generation from organic matter under lower temperatures during the late diagenesis, immature oils generally exist in the Tertiary sedimentary basins in China (Fu et al., 1985; Shiet al., 1982). Their ubiquity not only theoretically breaks through the limitations of the hydrocarbongenerating theory of kerogen thermal degradation, as propounded by Tissot and Welte (1978), and promotes the development of organic petroleum-generating theory, but also opens up further vast new oil exploration areas. Some immature-marginal mature oils have been reported previously (Tissot and Welte, 1978, 1984; Snowdon and Powell, 1982; Palacas, 1983). These oils occur principally in the Cretaceous and Tertiary sediments, even though they can also be found in the Palaeozoic. However, Chinese immature oils are usually discovered in the Tertiary strata and occur in almost all Tertiary oil- and gas-bearing basins.
Immature oils have been found in the Cretaceous sediment of the Daqing oil field. The related data on immature oils from some Tertiary oil- and gas-bearing basins in China are compiled in Table 1, including the two major maturity parameters. The following features can be drawn from Table 1: 1, The burial depth of immature oils generally does not exceed 2500m with a maximum up to 3000 m, in other words, the complete burial depth is above the depth of the hydrocarbongenerating threshold that is determined according to the data of hydrocarbon generation of the source rocks in related oil fields. 2. Most immature oils show higher specific gravity 0.87-0.96, but light immature crudes and condensates are also observed in hypersaline lake sediments. 3. All the immature oils are referred to paraffin-base crude oil, but their wax contents vary greatly, probably due to the difference in source inputs or the effect of biodvgradation. It is clear that lacustrine lower planktonic algae and terrigenous biological waxes are the main source materials for the oil. The naphthene-base crudes derived from resinite in coal measures, found in Canada and Australia (Snowdon, 1980; Snowdon and Powell, 1982; Shanmugam, 1985; Palacas, 1983), have not been discovered in Chinese non-marine immature oils. 4. The non-hydrocarbon and asphaltene contents are high, generally around 30-50%, even though they are still about 10% in light oils and condensates. All these are apparently higher than that in mature oils. 5. As far as biomarkers are concerned, the immature oils are strongly dominated by the compounds with the original biological configuration, which are the main qualitative indicators used for the immature oils herein.
Yuejin (Guang 33 Well) Kaitemilike
Qaidam
E3S ] E3S2 E3d-E3S t E3d E3S t F_.2K2 EO-E3S I ES t E3b-E2n F-.3dI E3f E3d I Shale Eht +2 Eht + 2 Ehl-s E2ql +2
N,-E 3 N2
Liaohe: West
South Jiyang: North Dongpu: Guangchang Baise: East Subei: Dongtai
Qaidam: Yuejin 8 Mangya
N2
3.3-137.8
Biyang Jianghan: Qilmjiang
Nanxiang: Nanyan8
! .85 1.38 1.90 1.10 0.5-1.6 1.8-2.0 1.3-2.2 1.1 0.85-1.45 0.5-1.3 2.3 0.6 1.3 3.5 0.65
Con~ (%)
0.4 0.3
Reservoir
Sandstone
Sandstone, Oolite lime Sandstone Sandstone Sandstone Sandstone Dolomite Dolomite Gypsum-salt bearing sandy shale Sandstone 0.7711 condensate
0.8155
0.87-0.94 0.8728 0.9042 0.8694 0.9416 0.9420 0.8822
0.9018--0.9586
Specific gravity
1.8
7. !
1446.9 2769.5 43.6
300-2400 33.9
300-2000
Viscosity (cP)
Dew
32-36 37 40
4-10
point (°C)
0.1200 0.0450
0.1113 0. I ! 25 -0.02-0.08 0.041 0.16-0.22 <0.13 0.0379 0.025-0.082 0.03-0.15 0.083 0.0215 0.0257 0.2654 0.1
Extract " A " (%)
450 258
358 472 320-380 52-463 260-350 850 <250 300 50-350 100-400 348 ! 16 235 1312 100-300
Hydrocarbon (ppm)
18.3 12.2
4.1 6-12 8.3 --
5.3 5.9 ----10.0 -7.0 6.0
"A"/ C~r
4.0 4.5
3.0 2.0 4.0 4.0
i.7 2.4 2.0-3.0 2.0-4.0 3.2--4.8 2.6-4.2 3.0-5.8 <3 I-3 I-3
Hydrocarbon/C~
0.07
60-75 43
50 50-70 -80-90
-46 40-80 46 70-80 -60-80 70-85
0.7-0.8
1.6-2. I 1.2-2.3 1.2-2.6 0.4--0.7 1.6-3.0
-1.2-2.4 1.3-2.1 1.1-2.4 2.0-3.0 1.5-2.0 1.2-1.8 1.5
1.5-3.5
OEP
1.21 1.10
1.35 1.3 I 1.44 !.30
(141, 35)
(216, 74)
700 500
l m m a t u l ~ oil-stained
400
Hypersaline lake facies Hylmrmline lake facies Hypersaline lake facies
M a wanyi (1984) M a wanyi (1984) W a n g Qijun (1980)
(7)
(352, 234-183)
(45)
0.25
0.27
0.20
0.20 0.10 0.13 0.30 0.19
Note (sample No.)
0.23
0.18
0.24
0.32 0.18 0.15 0.26 0.18
0.25
1000 700 1500
20O 650 650
300-500 4O0 200 60O
300
Mudstonc thickness (m)
8.6
14.2
31-48 32.8 35.7 16.0 52.6 42.9 54.5
39.4-49.0
Non-hydrocarbons Cz9 steranes + asphaltenes (%) s i s + R ##1/~# + o~
1.3 -1.3 1.3-1.5 1.2 1.25 1.0-1.2 1.0-1.3
1.3
H/C
17.5
17.2 27.9 8.4 14.2 22.0
0.97 0.21 0.74 0.55 2.93
0.03
6.4-20.9
Non-hydrocarbon + asphaltene (%) 60-90
(%)
4.0~.4
Wax
0.11-0.31
0.42-0.85
Sulphur (%)
Table 2. Geochemical parameters o f immature source rocks in parts o f oil-bearing basins
NI
E2hs F-.~qI
E2h 3
~b-Exn
E3f3
F-.2k E3S I
E3S3+ 4
Formation
2254.0-2447.2
1400-2300 2855-2928 2932-2941 600-1600 1967-1962 1970-1975 1829.3-1829.6
Formation
Liaodong Bay: East Huanghua: N o r t h
Depth (m)
1400-1800
Basin
(Kaiq~n 67)
Cangdong-Nanpi Yihezhuang An 2 Well Tiandong Bi 103 Well An I Well Guanghua (Guang 33 Well)
Huanghua Jiyang Subei Baise Biyang
Jianghan
Gaosheng
Liaohe
Basin
Well No. or Oil field
Table 1. Physical properties and maturation imrameters of immature crude oils
¢3
t=e
.=
O
o
g
o
g~
524
HUANGDIFANet al.
Table 2 shows related data on the abundance and conversion rate of organic matter in immature source rocks from some of the immature-oil-bearing basins. These data indicate that common lacustrine immature source rocks belong to the mixed type (type II) hydrocarbon-generating material, mostly with organic carbon content > 1.0%, extract amount around 0.1% and a total hydrocarbon content of about 200-500ppm, suggesting a better oilgenerating potential, but showing a low conversion level in comparison with mature source rocks. However, the hypersaline lake sediments are characterized by a low content of organic carbon and high conversion rate of soluble organic matter, which may be attributed to the batter preserving conditions for organic matter in source rocks, and higher carbonate content. In brief, the abundance of organic matter in immature source rocks from immature-oil-occurring regions is higher except for that in hypersaline lake sediments; also, the conversion rate to hydrocarbons in some regions exceeds the previously determined threshold value, i.e. 3%, especially in those regions with significant reserves of immature-marginal mature oils, e.g. the Upper Kongdian Formation in the southern part of Huanghua Depression (Dagong oil field) is rich in organic matter, and intercalated with poor oil shales. A statistical treatment of the data reveals that this suite of immature source rocks is, on average, 2400 m in burial depth, with organic carbon up to 1.8-2.0% (5.11% in maximum), extract amount from 0.16 to 1.22% and a total hydrocarbon content of 850 ppm. It is interesting to note that early conversion of organic matter to immature petroleum at low temperatures during late diagenesis has been reported from the Huabei and Liaohe oil fields (Huang et al., 1987). It is suggested that there is a simultaneous generation path for immature dispersed bitumen and hydrocarbon, both of which are not derived from kerogen, during the generation and maturation of kerogen in late diagenesis. Compared with that in early diagenesis, the dispersed bitumen is more hydrogen-rich, in which the hydrocarbon content increases and the nonhydrocarbon and asphaltene contents decrease from 80-90% to 60-70%. It seems that decarboxylation is an important reaction not only for kerogen maturation, but also for the generation of the hydrogen-rich bitumen (Tissot and Welte, 1978, 1984; Huang et al., 1984b). Our previous investigations have confirmed that under natural conditions, hydrocarbons are not formed from kerogen before the kerogen becomes mature and fossilized (i.e. young kerogen turns into fossil kerogen). Therefore, as a general rule, the generation of bitumen and hydrocarbon in late diagenesis must be related with a direct generation and conversion of hydrocarbon from water- and aliphatics-soluble organic matters. Hydrocarbon generation results from decarboxylation and breaking of ester linkages of the soluble precursors derived from hydrogen-rich maceral and biochemical
components. In short, it is assumed that the soluble (in organic solvent) organic matter is the source material for immature oils. The soluble and insoluble organic matters in sediments or sedimentary rocks are an associate entity and related to each other. Along with the variation of physico-chemical conditions, organic matters are in a series of dynamic equilibria during the hydrocarbon generation and evolution. Therefore, whoever wants to establish a hydrocarbon-generation mode, which accords with the objective reality, should consider the organic matters in rocks as an entity. As far as the formation of immature oil pools or oil fields are concerned, early primary migration of hydrocarbon during the late diagenesis has to be involved. If we assume that the later primary migration of thermal degraded hydrocarbons from kerogen in source rock is dependent on the occurrence of an abnormal pressure zone, the early primary migration would be much more favourable. This is an inevitable consequence of hydrocarbonand liquid-expeUing phenomenon as the source bed is compacting. According to a hydrocarbon migration study in two wells located in the Norwegian permafrost zone, Leythaeuser et al. (1984) documented that "An early stage of expulsion, controlled by shale compaction and associated with chromatographic separation effects, appears to precede the main phase of primary migration . . . . Thus, the composition of the hydrocarbon mixture expelled from the source beds at this stage appears to be more controlled by physical processes rather than by the composition and structure of the generating source rock itself." This result shows that the hydrocarbons in immature source rocks are not sourced from kerogen, their expelling process appears as a geochromatographic fractionation effect under compaction conditions. At this time, the porosity of mudstone generally decreases by 5-6% along with the expulsion of a vast amount of pore water, which provides very favourable conditions for petroleum primary migration. According to Baker's (1967) calculation, if only 1.8 ppm hydrocarbon is involved in the expelled liquids from mudstones, the known oil reserves could be explained under suitable geological settings. We assume that so long as the appropriate conditions of hydrocarbon accumulation and preservation are available in late diagenesis, immature oil expelled from their source rocks can certainly form commercial oil and gas accumulations. In a correlation study of source rocks and mature oils from the Liaoxi Depression, and of source rocks and immature oils from the Gaosheng oil field, the compositional differentiation of soluble organic matter caused by the commercial accumulation of this kind of crude oil was shown (see Fig. 2). In the immature oil composition, the non-hydrocarbon and asphaltene contents have decreased by 30-35% in comparison with their source rocks.
Maturation sequence of oils in hydrocarbon basins in China Saturated hydrocarbon 4o0
60
80 100 Non-hydrocarbon + osphottene
Aromatic hydrocarbon • OiLs ~ Mature Rocks ) o Rocks - Immature
(~) (~
+
OILs ~Irnmature R°cks~GO°sheng °lLfieLd
Fig. 2. Triangular diagram of Tertiary oil and rock extract composition in the Liaohe Basin [after Piao M. Z. (1984, unpublished work); modified].
Maturation sequence of crude oils During the whole course of hydrocarbon generation and evolution, from late diagenesis through taragenesis to metagenesis, varying quantities of oils and gases with diverse maturities will inevitably be generated (Tissot and WeRe, 1984), and an oil and gas maturation sequence formed. This is a ubiquitous
7F
e018
610its
• Tr
u r / / e / /o"
to E , + - o
sh,,.n 20, . ..--'7 • o/ /
_
/
¢~
/
.I,
?ooo
S;'~ueli n =-~ c~,~'l"shlzIQou --~7-o," 0 . 7 o /
,
• o
o
Immature I 0
4
Ig I 2 3 C29 Sterones ~ / ( # ~
4
I 5 + == )
I 6
Fig. 3. Maturation and evolution of Tertiary oils and rocks in the Qaidam Basin illustrated by two C~ sterane isomerization parameters.
525
and significant phenomenon in Chinese Tertiary oiland gas-beating basins. Based on the evolution of stcrane configurations, Figs 3 and 4 show the maturity evolution of oils and rocks in the Qaidam Basin. Figure 3 is constructed using the two C29 sterane maturity parameters. The triangular diagram (Fig. 4) is based on the normalized abundances of the biological (20R) and geological (20S and ~flfl) configurations. Its three apex angles are assigned as R, S and t, respectively. The samples were selected from various oil fields in the Qaidam Basin (Fig. 5), detailed in Table 3. GC-MS analysis was carried out on TSQ-45 GC-MS--MS-DS (Finnigan Mat). The column was fused-silica capillary column, coated with SE-54. It can be seen from Figs 3 and 4 that the groups of points belonging to source rock extracts and crude oils are regularly distributed. Both constitute a complete evolution sequence from immaturity through low-maturity and maturity up to high-maturity (Nos 08 and 018 are condensates, at the end of isomerization). The distributions of oils and rocks share the same trend, suggesting a genetic correlation between them, and also indicating that these oils, which show a range of maturities, are the products of organic matter in different stages of hydrocarbon generation and evolution. It is interesting to note that the data from a systematic analysis from Well Shishen 20 represented by $5, $7 and $9 in the two figures ( - - - ) deviate to the left from the normal maturation pathway. It is suggested that this is because the isomerization of ~ = to ~,flfl steranes is lagging behind the true maturity. The formation of/so-sterancs means a conversion from the 5~(H), 14~(H), 17~(H) to the 5~(H), 14fl(H), 17fl(H) configurations, involving a transformation from trans- to c/s-configurations between rings C and D, and requiring a higher activation energy than the C-20 cpimerization from R to S configurations on the side chain. The three samples ($5, $7 and $9) with highest deviations are located in a gypsum-salt- and carbonate-enriched interval (3000-4000 m in depth) where these minerals have the lowest efficiency in catalysing the isomerization of steranvs in this well. This is the reason that the conversion from "20R" to "20S" is faster than that from " ~ " to "~flfl" configurations, the detected isomerization pathway of C29 steranes is not along the bisector or centreline of apex angle C29 ( = ~ R + • ~flR), but deviates to the left angle C29 ( ~ = S + ~/~/~S) before reaching the maturation threshold, then turns towards the ~ f l ( S + R ) axis, and finally returns to the centreline. In this triangular diagram, there are four circles which are composed of the point groups of crude oils with different maturities, and the centres of the four circles just fall on the abovementioned evolution pathway of the source rocks, which clearly illustrates that the four groups of oils with different maturities originated at different stages
526
HUANODIFANeta/. C29
rma a R+ (I,8.8R 100/~0
/ ............... f~'~I 7~0
o
80
20
40
~£ SlI c~
30 70
30
V
V
60
50
V D s( 40
60 08
01,1
C29
V
V
V
70
30
20
I0
0
C29
Fig. 4. Triangular diagram of relative abundance distribution of three C~ sterane configurations in oils and rocks from the Qaidam Basin.
Fig. 5. The distribution of oil fields and oil samples in the western part of the Qaidam Basin.
of the hydrocarbon generation and evolution from the organic matter. According to the maturity boundaries based on the parameters determined above, crude oils from the Qaidam Basin can be classified into immature, low-mature, mature and high-mature oils. The characteristics of each type arc as follows. 1. Immature oils. The immature oils, including the condensate from Kaitemilike (No. 016, sp. gr. 0.7711) and light oil from Yuecan 1 Well (No. 03, sp. gr. 0.8155), were generated in the diagenvsis stage of organic matter evolution, and reservoired in the Pliocen¢ and the Miocene strata. Even though the saturated hydrocarbon contents in these oils are as high as 83 and 88.9%, respectively, the odd-to-oven predominance of n-alkanes is not evident and the moretanc/hopane ratio is not very high, there is a predominance of steranes with the original biological configuration R, and the isomerization level of ==~ to ~/~/~ steranes is very low. It is clear from the two maturity parameters in Tables 1 and 3 that the maturity of No. 016 condensate approximates the hydrocarbon-generating
527
M a t u r a t i o n sequence o f oils in h y d r o c a r b o n b a s i n s in C h i n a Table 3. Geologic data of grade oils in the Qaidam Basin
No.
Oil field
01 02 03
Hongiiuquan Yuejin Yuejin
04 05 06 07 08
Yuejin Yuejin Yuejin
09 010 011 012
Shizigou Shizigou
Huaiugou Hualugou Hualugou Youshashan
Age'
Depth (m)
No.
Oil field
E3 Nj Ni E3 N2 N2--N I NI E3 N2 N1 N~ Ni
2565.1 1366-1368 2255-2447 3300-3323 1658-1661 1447 1187-1188 4131.62 639.6 808-1209 1039-1268 658-859
013 014 015 016 017 018 019 022 023 024 025
Ganchaigou Xianshuiquan Youdunzi Kaitemilike N,nyishan Nanyishan Jiandingshan Lenghu Lenghu Lenghu Yuka
Ag@
Depth (in)
N2 N2 N2 N2 N3 E3 N2 J2 E3 Nt J3
2332-2382 Un~laed 0-530.38 3.3-137.8 2981 2981 106-61 I 478-639 610-760 396-68 ! .4 200
"The age of the pay bed.
threshold, while that of the No. 03 light crude oil is lower. 2. Low-mature oils.The crude oil samples in Figs 3 and 4 are representativeof oilsin the Qaidam Basin. Therefore, most of the crude oils in the known oil fields of the Qaidam Basin are classifiedas lowmature oils.These occur in the Miocene and Oligocene sediments at burial depths of 3000-4000m (the diversity of their m a x i m u m depths is due to the differencesin geothermal gradient and in the extent of structural uplift).Their epimerization parameter value is 0.3-0.4 (cyclicisomcrizationparameter value 0.25-0.41), far from the equilibrium values of these isomerization ends, and thus, the oils are in the low-mature evolution stage. The specific gravities of these oils vary between 0.8297-0.8780, and the total recovery in 310°C fraction is only about 18-32% in general. Saturates account for 65-70%, aromatics 15.5-25.5% and non-hydrocarbons plus asphaltenc up to 10-20%, showing in those oils with low-mature features. As judged by the level of hydrocarbon generation and evolution of organic matter in their source rocks, these low-mature oils were generated at vitrinite reflectance values of 0.6-0.8%. The oils abound with resin-degraded compounds (C19--C20 with m/z 123 base peak in mass spectra), which is also a marker for low maturity. It has been suggested that resinite can generate hydrocarbons at relatively low maturities (Snowdon, 1980). The distribution of low-mature oils in Figs 3 and 4 shows that the maturity of crude oils from the Shizigou structural zone and Chaishen 1 Well (No. 013) plus three shallow layer oils (Nos 015, 017 and 019) from Youquanzi, Nanyishan and Jiandingshan are relatively higher than that in the Yuejin oil field. These oils belong in the products of low-mature stages. Meanwhile, it also indicates that a significant amount of the shallow oils in the Qaidam Basin originated from low-mature organic matter in more deeply buried source rocks and reached the shallow reservoirs via verticalmigration. 3. Mature oils and high-mature condensates. From Figs 3 and 4 it is apparent that the No. 8 oil from Shishen 20 Well (4131.6m in depth) and No. 18 condensate from Nan 2 Well (2982 m in depth) in
Nanyishan, both recentlydiscovered,and No. 014 oil from the older Xiancan I Well in Xianshuiquan, as well as the oils sourced from the Jurassic in Lcnghu region, are all in the same maturation stage. Their epimerization parameter is 0.48-0.55, approximating or reaching the end of epimerization,and thus, these oilsare realmature crude oils.The Oligocene oilsNos 08 and 018, are of the highest maturity. The oil in the deep part of the Shishen 20 Well was produced beneath the gypsum-salt sectionwhich belongs to the bottom of the Oligocene according to palynological study. These mature oilsare characterizedby their lower specific gravities and asphaltene contents, as well as by higher saturate contents (> 80%). The most strikingcharacteristicof these mature oilsis that the predominance of biomarker compounds with the original biologicalconfigurationis slightor has even completely disappeared, being replaced by the predominant /so-stcranes and analogous compounds of lower carbon number. Moreover, the clear predominance of tricyclictcrpanes over hopanes in the Tertiary condensates is conspicuous. The same phenomenon has been observed in rock extracts of Han 2 Well where the depth is over 4450 m and the corresponding temperature is up to > 160°C (Huang et aI., 1984b). This also proves that the source rocks of mature and high-mature crude oils have been buried over 3000-4000 m in depth, and have undergone a considerable hydrocarbon generation and conversion. This maturation sequence of crude oils is a ubiquitous phenomenon in Chinese Tertiary oil-and gas-beating basins. W e have previously reported a similar maturation sequence in the Baisc Basin and the Jiyang Depression (Huang and Li, 1987). The maturation sequence of crude oils in the Huanghua Depression (Dagang Oilfield) is another example (Fig. 6). Basins with the above characteristicsare mostly observed in East China where they arc referredto as the Tertiary extension fault depression basins. It is often seen that there are severaloil-generatingdepressions that contain good source rocks in an oil-bearing basin, controlling the associated oil and gas occurrences, respectively.Figure 7 shows the distribution
528
HUANODW,4J~et al. o o
o Ouan. Zoo areas •
Congdong area et at.
~oo.5
O,D-
,/ /
,/ ./ / ÷ ÷
Ma,u
/
0.4
Mature
.
.
÷ /"
.
---7.
"
•
7 / ~
o~" o.2
o
. Rooyongcorbonote r~ervolr Rooyan 0 sandetone ruervolr + St)enxlan
0
0") 0= 0.1
0.2 -
o
0.1
Immature
I I I I I
I
i
o.1
0.2
x Shutu 0 dinxion
I o.a
C2, Sterones
Immature
o.1
0.4
i
I
o.e
o.6
I o.T
.8,B/t,~ + aa )
Fig. 8. Grading of crude oil maturities in the Jizhong Basin.
I l I I
I
I
I
0,2
0.3
0.4
0,5
Sterones Cze/9# / ( #19 + a '! )
Fig. 6. Grading of crude oil maturities in the Huanghua Depression [after Cheng K. M. and Jiang C. Q. (1987, unpublished work)].
of source area and oil and gas fields in the Jizhong Basin. There are six oil-generating depressions, i.e. Langgu, Baxian, Raoyang, Shenxian, Shulu and Jinxian Depressions, which have developed in this basin that control the distribution patterns of oil and gas. The age of their source beds and reservoirs is mainly Lower Tertiary. However, the lazgest and most well-known oil field in the basin, Renqiu, is a buried-hill oil field associated with oil-generating Raoyang Depression. The major reservoirs are Cambrian-Ordovisian and Upper Proterozoic carbonates. The distribution of crude oil maturities is illustrated Beijinq
I~
.
/:!/.
o. 4 • (jr)
J[ ~
.
OJt generating deprNlion/~
anO oi~ Qenerotl~ ~t~r d
~
,~
~
.-
in Fig. 8, from which it can be seen that the maturation sequence of crude oils is more complete in the Langgu and Raoyang Depressions, while other hydrocarbon-generating depressions are short of the products of certain hydrocarbon-generation and evolution stages. This has been confirmed by further detailed investigation. Therefore, it is not difficult to scientifically predict the explorational prospects in this wellexplored basin by means of a comparison of resource extents in each hydrocarbon-generating stage. In addition, Fig. 8 also shows the lower isomerization phenomenon of ~t~tusteranes in crude oils in carbonate reservoirs. CONCLUSIONS
The differentiation and definition of the four maturity levels in crude oils mentioned above is significant both for petroleum geochemical studies and for petroleum exploration. Taking the Qaidam Basin as an example, it indicates that the oil and gas resources so far discovered are principally at the low-mature stage, and thus, not only are there prospects of discovering further shallow immature oils, but also the exploration and development of deeper mature crude oils is just beginning and has bright prospects. Acknowledgement--This work was financially supported by
the Laboratory of Biogeochemistry and Gas-geochemistry, Lanzhou, People's Republic of China. U
REFERENCF~ Baker E. G. (1967) A geochemical evaluation of petroleum migration and accumulation. In F u n d a m e n t a l A s p e c t s of Petroleum Geochemistry, pp. 299-329. Elsevier, New ffc~lF~
~
0 '
25 '
50 Km '
Fig. 7. The distribution of Tertiary oil-generating depressions and oil fields in the Jizhong Basin.
York. Fu J., Sheng G. and Jiang J. (1985) Immature oil originated from a saline deposit-bearing basin. Oil Gas Geol. 6(2), 150-158 (in Chinese). Hnang D. and Li J. (1987) Immature petroleum in continental deposits and its significance. Acta Pet. Sin. 8(I), I-9 (in Chinese).
Maturation sequence of oils in hydrocarbon basins in China Huang D., Shang H. and Li J. (1984a) Advances in theoretical research on continental oil generation in China. In Proceedings of Beijing Petroleum Symposium, pp. 90-109. Petroleum Publishing, Beijing (in Chinese). Huang D., Li J., Zhou Z., Gu X. and Zhang D. (1984b) Evolution and Hydrocarbon-generating Mechanism of Nanmarine Organic Matter. Petroleum Publishing, Beijing (in Chinese). Huang D., Liao Q. and Xu Y. 0987) A Prel~r,linary Study on the Genesis of Immature Petroleum, pp. 1-19. Annual Research Report 1987, Biogeochem. and Gasgeochem. Lab., Lanzhou Inst. of Geol., Acad. Sin. Gansu, Beijing (in Chinese). Leythaeuser D., Mackenzie A., Schaefer R. G. and Bjoroy M. (1984) A novel approach for recognition and quantification of hydrocarbon migration effects in shale-sandstone sequences. Bull. Am. Assoc. Pet. Geol. 68, 196-219. Palacas J. G. (1983) Carbonate rocks as source of petroleum: geological and chemical characteristics and oil-source correlations. In Proceedings of the Eleventh World Pet-
529
roleum Congress, Vol. 2, pp. 31-43. Wiley, New York. Shanmugam G. (1985) Significance of coniferous rain forests and related organic matter in generating commercial quantities of oil, Gippsland Basin, Australia. Bull. Am. Assoc. Pet. Geol. 69, 1241-1254, Shi J., Mackenzie A. S., Eglinton G., Gowar A. P. and Maxwell J. R. (1982) Biological markers for petroleum and shales in Shengii Oilfield and their geochemical implications. Geochimica 1, 1-20 (in Chinese). Snowdon L. R. (1980) Resinite---a potential source on the Upper Cretaceous/Tertiary of the Beaufort-Mackenzie Basin. Can. Soe. Pet. Geol. Mere. 6, 421-446. Snowdon L. R. and Powell T. G. (1982) Immature oil and condensate---modification of hydrocarbon generation model for terrestrial organic matter. Bull. Am. Assoc. Pet. Geol. 66, 775-788. Tissot B. P. and Welte D. H. (1978) Petroleum Formation and Occurrences, 1st edn. Springer, Berlin. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, 2nd edn. Springer, Berlin.
0146-6380/90 $3.00+ 0.00 Copyright© 1990PergamonPress plc
Maturation sequence of continental crude otis in hydrocarbon basins in China and its significance HUANG DIFAN, L~ JINCHAO and ZHANG DAJIANG Research Institute of Petroleum Exploration and Development, 20 College Road, P.O. Box 910, Beijing 100083, China
(Received 19 September 1989; accepted 18 February 1990) Abstract--The continental hydrocarbon-containing basins in China not only ubiquitously possess immature-marginal mature oils as defined by C29 sterane ratios, but a complete oil maturation sequence can also exist. The present paper studies the characteristics of immature oils, their formation and distribution. The study revealed that immature oils were not thermal degradation products of kerogen, since the vitrinite rvflovtance (Re) of source rocks is between 0.4 and 0.6%. The immature oils are derived from the soluble (in organic solvent) organic matter generated during low-temperature hydrocarbon generation at the late stage of diagenesis. The immature oils in China belong to paraffin-base oils which originated from aquatic or terrestrial lipids but not resins. The maturation sequence of oils is composed of immature, low-mature and high-mature members, corresponding to the different levels of hydrocarbon generation. The recognition of this evolution sequence has proved to be of practical significance. When we understand the amounts of oil reservoirs of different maturation, the exploration direction can be guided by the corresponding resource volume comparison. Taking the Qaidam Basin as a good example, most of the oils discovered recently are low-mature, but there are good prospects of finding mature and high-mature oils.
Key words--Chinese hydrocarbon basin, Tertiary, isomerizations of steranes, immature oil, marginal oil, hypersaline lake, maturity sequence of crude oil
INTRODUCTION In recent years, the ubiquity of immature oils in the Tertiary non-marine hydrocarbon-bearing basins in China has been brought to Chinese petroleum geechemists' attention, and their origin has been discussed from various angles. Meanwhile, the existence of a maturation sequence composed of immature, lowmature, mature and high-mature crude oils in many basins is also a conspicuous phenomenon. It appears that during the generation and evolution of hydrocarbons, there are commercial oil and gas accumulations of corresponding hydrocarbon products in each evolutionary stage, and their incorporation and dilution with each do not always occur. Therefore, it is extremely important to study further the maturity of hydrocarbons in individual oil and gas fields. On the basis of such a study, the proven oil and gas reserves at various mature levels can be compared with the corresponding reserves at each hydrocarbongenerating stage. Thereby, the oil and gas prospects would be more scientifically forecast, and the exploration target unequivocally determined.
they arc influenced by more factors besides maturity. In the authors' experience, the most efficient ones for the measurement of crude oil maturity are biomarker parameters dealing with the configuration conversion of polycyclic naphthenes. During diagenesis and catagenesis, biomarkcrs such as steranes and terpanes suffer a series of profound configuration conversions via reduction, rearrangement and isomerization, resulting in a set of new compounds. Thereby, the distinction of oil and source rock maturities can be shown in varying degrevs (Huang et al., 1984a). Based on this principle, the maturity scales of various biomarkers have been established, among which the authors consider the isomerization parameters of C29 steranes as the optimal ones for the measurement of crude oil maturation:
I. 5ct(H),I4~t(H),I7ct(H)-C29steranes
20S/(20R + 20S); 2. 5~(H)-C29 steranes///~/(flfl+ ~t~).
Parameter selection for crude oil maturity
Chinese petroleum geochemists have generally found the above ratios to have values of 0.20--0.25 at the hydrocarbon-generating threshold. From the
There are various parameters for determining crude oil maturity, whose applicable range, efficacy and availability may be different from each other and the applicable range of most parameters is limited, or
results of a correlation study between the above two parameters and vitrinite reflectance by the authors, the corresponding value of Re is around 0.65%, as shown in Fig. 1. Therefore, for both C29 steranes
521
HUANO D ~
522
/,o..
Mature
0.40
et al.
"~II •
0.40
t
Low-mature
[ ~
-p ~0.30
Low-mature -
Mature
/.,
I ~t~ 0 .30 ,
i
0.25
i
o.=
/ I
I
"°°"/i
I
. . . . . . .
I I
0
I =* °'15t I ==
z
E-
I I I 0.10
0.5
0.6
0.7
I
Re e/,~
I
I I
¢
I
I
o.o5~ ;/" at/ I
II I
II I
o.1o
// .
"
1
0.5
I
0.6
i
0.7
I 0.8
Re ~'0)
Fig. 1. Correlation of Ro and two C29steraneisomerization parametersin the southern part of the Dagong oil field [after Cheng K. M. and Jiang C. Q. (1987, unpublished work)]. isomerization parameters in the range 0.20-0.25 are regarded as the P,oundary between immature and low-mature oils herein, and those in the range 0.300.40 as the boundary between low-mature and mature oils. The lower values often occur in those strata which are rich in gypsum, rock salt and carbonate rocks. Besides, a ternary diagram, based on =fir (S + R), =~,~S + ~/~S and ~==R + =flflR configurations of C:9 steranes, can also reveal the maturation sequence of crude oils. In this diagram, the corresponding boundaries between immature and low-mature oils, and between low-mature and mature oils should be 55 and 40% for the original configuration, respectively. The 22S/22R ratios of C3~ or C32 homohopanes can be used as an auxiliary parameter for the oil maturation scale, in which the corresponding value of the hydrocarbon-generating threshold is 1.10-1.25. R E S U L T S AND D I S C U S S I O N
Immature oils As the initial products of hydrocarbon generation from organic matter under lower temperatures during the late diagenesis, immature oils generally exist in the Tertiary sedimentary basins in China (Fu et al., 1985; Shiet al., 1982). Their ubiquity not only theoretically breaks through the limitations of the hydrocarbongenerating theory of kerogen thermal degradation, as propounded by Tissot and Welte (1978), and promotes the development of organic petroleum-generating theory, but also opens up further vast new oil exploration areas. Some immature-marginal mature oils have been reported previously (Tissot and Welte, 1978, 1984; Snowdon and Powell, 1982; Palacas, 1983). These oils occur principally in the Cretaceous and Tertiary sediments, even though they can also be found in the Palaeozoic. However, Chinese immature oils are usually discovered in the Tertiary strata and occur in almost all Tertiary oil- and gas-bearing basins.
Immature oils have been found in the Cretaceous sediment of the Daqing oil field. The related data on immature oils from some Tertiary oil- and gas-bearing basins in China are compiled in Table 1, including the two major maturity parameters. The following features can be drawn from Table 1: 1, The burial depth of immature oils generally does not exceed 2500m with a maximum up to 3000 m, in other words, the complete burial depth is above the depth of the hydrocarbongenerating threshold that is determined according to the data of hydrocarbon generation of the source rocks in related oil fields. 2. Most immature oils show higher specific gravity 0.87-0.96, but light immature crudes and condensates are also observed in hypersaline lake sediments. 3. All the immature oils are referred to paraffin-base crude oil, but their wax contents vary greatly, probably due to the difference in source inputs or the effect of biodvgradation. It is clear that lacustrine lower planktonic algae and terrigenous biological waxes are the main source materials for the oil. The naphthene-base crudes derived from resinite in coal measures, found in Canada and Australia (Snowdon, 1980; Snowdon and Powell, 1982; Shanmugam, 1985; Palacas, 1983), have not been discovered in Chinese non-marine immature oils. 4. The non-hydrocarbon and asphaltene contents are high, generally around 30-50%, even though they are still about 10% in light oils and condensates. All these are apparently higher than that in mature oils. 5. As far as biomarkers are concerned, the immature oils are strongly dominated by the compounds with the original biological configuration, which are the main qualitative indicators used for the immature oils herein.
Yuejin (Guang 33 Well) Kaitemilike
Qaidam
E3S ] E3S2 E3d-E3S t E3d E3S t F_.2K2 EO-E3S I ES t E3b-E2n F-.3dI E3f E3d I Shale Eht +2 Eht + 2 Ehl-s E2ql +2
N,-E 3 N2
Liaohe: West
South Jiyang: North Dongpu: Guangchang Baise: East Subei: Dongtai
Qaidam: Yuejin 8 Mangya
N2
3.3-137.8
Biyang Jianghan: Qilmjiang
Nanxiang: Nanyan8
! .85 1.38 1.90 1.10 0.5-1.6 1.8-2.0 1.3-2.2 1.1 0.85-1.45 0.5-1.3 2.3 0.6 1.3 3.5 0.65
Con~ (%)
0.4 0.3
Reservoir
Sandstone
Sandstone, Oolite lime Sandstone Sandstone Sandstone Sandstone Dolomite Dolomite Gypsum-salt bearing sandy shale Sandstone 0.7711 condensate
0.8155
0.87-0.94 0.8728 0.9042 0.8694 0.9416 0.9420 0.8822
0.9018--0.9586
Specific gravity
1.8
7. !
1446.9 2769.5 43.6
300-2400 33.9
300-2000
Viscosity (cP)
Dew
32-36 37 40
4-10
point (°C)
0.1200 0.0450
0.1113 0. I ! 25 -0.02-0.08 0.041 0.16-0.22 <0.13 0.0379 0.025-0.082 0.03-0.15 0.083 0.0215 0.0257 0.2654 0.1
Extract " A " (%)
450 258
358 472 320-380 52-463 260-350 850 <250 300 50-350 100-400 348 ! 16 235 1312 100-300
Hydrocarbon (ppm)
18.3 12.2
4.1 6-12 8.3 --
5.3 5.9 ----10.0 -7.0 6.0
"A"/ C~r
4.0 4.5
3.0 2.0 4.0 4.0
i.7 2.4 2.0-3.0 2.0-4.0 3.2--4.8 2.6-4.2 3.0-5.8 <3 I-3 I-3
Hydrocarbon/C~
0.07
60-75 43
50 50-70 -80-90
-46 40-80 46 70-80 -60-80 70-85
0.7-0.8
1.6-2. I 1.2-2.3 1.2-2.6 0.4--0.7 1.6-3.0
-1.2-2.4 1.3-2.1 1.1-2.4 2.0-3.0 1.5-2.0 1.2-1.8 1.5
1.5-3.5
OEP
1.21 1.10
1.35 1.3 I 1.44 !.30
(141, 35)
(216, 74)
700 500
l m m a t u l ~ oil-stained
400
Hypersaline lake facies Hylmrmline lake facies Hypersaline lake facies
M a wanyi (1984) M a wanyi (1984) W a n g Qijun (1980)
(7)
(352, 234-183)
(45)
0.25
0.27
0.20
0.20 0.10 0.13 0.30 0.19
Note (sample No.)
0.23
0.18
0.24
0.32 0.18 0.15 0.26 0.18
0.25
1000 700 1500
20O 650 650
300-500 4O0 200 60O
300
Mudstonc thickness (m)
8.6
14.2
31-48 32.8 35.7 16.0 52.6 42.9 54.5
39.4-49.0
Non-hydrocarbons Cz9 steranes + asphaltenes (%) s i s + R ##1/~# + o~
1.3 -1.3 1.3-1.5 1.2 1.25 1.0-1.2 1.0-1.3
1.3
H/C
17.5
17.2 27.9 8.4 14.2 22.0
0.97 0.21 0.74 0.55 2.93
0.03
6.4-20.9
Non-hydrocarbon + asphaltene (%) 60-90
(%)
4.0~.4
Wax
0.11-0.31
0.42-0.85
Sulphur (%)
Table 2. Geochemical parameters o f immature source rocks in parts o f oil-bearing basins
NI
E2hs F-.~qI
E2h 3
~b-Exn
E3f3
F-.2k E3S I
E3S3+ 4
Formation
2254.0-2447.2
1400-2300 2855-2928 2932-2941 600-1600 1967-1962 1970-1975 1829.3-1829.6
Formation
Liaodong Bay: East Huanghua: N o r t h
Depth (m)
1400-1800
Basin
(Kaiq~n 67)
Cangdong-Nanpi Yihezhuang An 2 Well Tiandong Bi 103 Well An I Well Guanghua (Guang 33 Well)
Huanghua Jiyang Subei Baise Biyang
Jianghan
Gaosheng
Liaohe
Basin
Well No. or Oil field
Table 1. Physical properties and maturation imrameters of immature crude oils
¢3
t=e
.=
O
o
g
o
g~
524
HUANGDIFANet al.
Table 2 shows related data on the abundance and conversion rate of organic matter in immature source rocks from some of the immature-oil-bearing basins. These data indicate that common lacustrine immature source rocks belong to the mixed type (type II) hydrocarbon-generating material, mostly with organic carbon content > 1.0%, extract amount around 0.1% and a total hydrocarbon content of about 200-500ppm, suggesting a better oilgenerating potential, but showing a low conversion level in comparison with mature source rocks. However, the hypersaline lake sediments are characterized by a low content of organic carbon and high conversion rate of soluble organic matter, which may be attributed to the batter preserving conditions for organic matter in source rocks, and higher carbonate content. In brief, the abundance of organic matter in immature source rocks from immature-oil-occurring regions is higher except for that in hypersaline lake sediments; also, the conversion rate to hydrocarbons in some regions exceeds the previously determined threshold value, i.e. 3%, especially in those regions with significant reserves of immature-marginal mature oils, e.g. the Upper Kongdian Formation in the southern part of Huanghua Depression (Dagong oil field) is rich in organic matter, and intercalated with poor oil shales. A statistical treatment of the data reveals that this suite of immature source rocks is, on average, 2400 m in burial depth, with organic carbon up to 1.8-2.0% (5.11% in maximum), extract amount from 0.16 to 1.22% and a total hydrocarbon content of 850 ppm. It is interesting to note that early conversion of organic matter to immature petroleum at low temperatures during late diagenesis has been reported from the Huabei and Liaohe oil fields (Huang et al., 1987). It is suggested that there is a simultaneous generation path for immature dispersed bitumen and hydrocarbon, both of which are not derived from kerogen, during the generation and maturation of kerogen in late diagenesis. Compared with that in early diagenesis, the dispersed bitumen is more hydrogen-rich, in which the hydrocarbon content increases and the nonhydrocarbon and asphaltene contents decrease from 80-90% to 60-70%. It seems that decarboxylation is an important reaction not only for kerogen maturation, but also for the generation of the hydrogen-rich bitumen (Tissot and Welte, 1978, 1984; Huang et al., 1984b). Our previous investigations have confirmed that under natural conditions, hydrocarbons are not formed from kerogen before the kerogen becomes mature and fossilized (i.e. young kerogen turns into fossil kerogen). Therefore, as a general rule, the generation of bitumen and hydrocarbon in late diagenesis must be related with a direct generation and conversion of hydrocarbon from water- and aliphatics-soluble organic matters. Hydrocarbon generation results from decarboxylation and breaking of ester linkages of the soluble precursors derived from hydrogen-rich maceral and biochemical
components. In short, it is assumed that the soluble (in organic solvent) organic matter is the source material for immature oils. The soluble and insoluble organic matters in sediments or sedimentary rocks are an associate entity and related to each other. Along with the variation of physico-chemical conditions, organic matters are in a series of dynamic equilibria during the hydrocarbon generation and evolution. Therefore, whoever wants to establish a hydrocarbon-generation mode, which accords with the objective reality, should consider the organic matters in rocks as an entity. As far as the formation of immature oil pools or oil fields are concerned, early primary migration of hydrocarbon during the late diagenesis has to be involved. If we assume that the later primary migration of thermal degraded hydrocarbons from kerogen in source rock is dependent on the occurrence of an abnormal pressure zone, the early primary migration would be much more favourable. This is an inevitable consequence of hydrocarbonand liquid-expeUing phenomenon as the source bed is compacting. According to a hydrocarbon migration study in two wells located in the Norwegian permafrost zone, Leythaeuser et al. (1984) documented that "An early stage of expulsion, controlled by shale compaction and associated with chromatographic separation effects, appears to precede the main phase of primary migration . . . . Thus, the composition of the hydrocarbon mixture expelled from the source beds at this stage appears to be more controlled by physical processes rather than by the composition and structure of the generating source rock itself." This result shows that the hydrocarbons in immature source rocks are not sourced from kerogen, their expelling process appears as a geochromatographic fractionation effect under compaction conditions. At this time, the porosity of mudstone generally decreases by 5-6% along with the expulsion of a vast amount of pore water, which provides very favourable conditions for petroleum primary migration. According to Baker's (1967) calculation, if only 1.8 ppm hydrocarbon is involved in the expelled liquids from mudstones, the known oil reserves could be explained under suitable geological settings. We assume that so long as the appropriate conditions of hydrocarbon accumulation and preservation are available in late diagenesis, immature oil expelled from their source rocks can certainly form commercial oil and gas accumulations. In a correlation study of source rocks and mature oils from the Liaoxi Depression, and of source rocks and immature oils from the Gaosheng oil field, the compositional differentiation of soluble organic matter caused by the commercial accumulation of this kind of crude oil was shown (see Fig. 2). In the immature oil composition, the non-hydrocarbon and asphaltene contents have decreased by 30-35% in comparison with their source rocks.
Maturation sequence of oils in hydrocarbon basins in China Saturated hydrocarbon 4o0
60
80 100 Non-hydrocarbon + osphottene
Aromatic hydrocarbon • OiLs ~ Mature Rocks ) o Rocks - Immature
(~) (~
+
OILs ~Irnmature R°cks~GO°sheng °lLfieLd
Fig. 2. Triangular diagram of Tertiary oil and rock extract composition in the Liaohe Basin [after Piao M. Z. (1984, unpublished work); modified].
Maturation sequence of crude oils During the whole course of hydrocarbon generation and evolution, from late diagenesis through taragenesis to metagenesis, varying quantities of oils and gases with diverse maturities will inevitably be generated (Tissot and WeRe, 1984), and an oil and gas maturation sequence formed. This is a ubiquitous
7F
e018
610its
• Tr
u r / / e / /o"
to E , + - o
sh,,.n 20, . ..--'7 • o/ /
_
/
¢~
/
.I,
?ooo
S;'~ueli n =-~ c~,~'l"shlzIQou --~7-o," 0 . 7 o /
,
• o
o
Immature I 0
4
Ig I 2 3 C29 Sterones ~ / ( # ~
4
I 5 + == )
I 6
Fig. 3. Maturation and evolution of Tertiary oils and rocks in the Qaidam Basin illustrated by two C~ sterane isomerization parameters.
525
and significant phenomenon in Chinese Tertiary oiland gas-beating basins. Based on the evolution of stcrane configurations, Figs 3 and 4 show the maturity evolution of oils and rocks in the Qaidam Basin. Figure 3 is constructed using the two C29 sterane maturity parameters. The triangular diagram (Fig. 4) is based on the normalized abundances of the biological (20R) and geological (20S and ~flfl) configurations. Its three apex angles are assigned as R, S and t, respectively. The samples were selected from various oil fields in the Qaidam Basin (Fig. 5), detailed in Table 3. GC-MS analysis was carried out on TSQ-45 GC-MS--MS-DS (Finnigan Mat). The column was fused-silica capillary column, coated with SE-54. It can be seen from Figs 3 and 4 that the groups of points belonging to source rock extracts and crude oils are regularly distributed. Both constitute a complete evolution sequence from immaturity through low-maturity and maturity up to high-maturity (Nos 08 and 018 are condensates, at the end of isomerization). The distributions of oils and rocks share the same trend, suggesting a genetic correlation between them, and also indicating that these oils, which show a range of maturities, are the products of organic matter in different stages of hydrocarbon generation and evolution. It is interesting to note that the data from a systematic analysis from Well Shishen 20 represented by $5, $7 and $9 in the two figures ( - - - ) deviate to the left from the normal maturation pathway. It is suggested that this is because the isomerization of ~ = to ~,flfl steranes is lagging behind the true maturity. The formation of/so-sterancs means a conversion from the 5~(H), 14~(H), 17~(H) to the 5~(H), 14fl(H), 17fl(H) configurations, involving a transformation from trans- to c/s-configurations between rings C and D, and requiring a higher activation energy than the C-20 cpimerization from R to S configurations on the side chain. The three samples ($5, $7 and $9) with highest deviations are located in a gypsum-salt- and carbonate-enriched interval (3000-4000 m in depth) where these minerals have the lowest efficiency in catalysing the isomerization of steranvs in this well. This is the reason that the conversion from "20R" to "20S" is faster than that from " ~ " to "~flfl" configurations, the detected isomerization pathway of C29 steranes is not along the bisector or centreline of apex angle C29 ( = ~ R + • ~flR), but deviates to the left angle C29 ( ~ = S + ~/~/~S) before reaching the maturation threshold, then turns towards the ~ f l ( S + R ) axis, and finally returns to the centreline. In this triangular diagram, there are four circles which are composed of the point groups of crude oils with different maturities, and the centres of the four circles just fall on the abovementioned evolution pathway of the source rocks, which clearly illustrates that the four groups of oils with different maturities originated at different stages
526
HUANODIFANeta/. C29
rma a R+ (I,8.8R 100/~0
/ ............... f~'~I 7~0
o
80
20
40
~£ SlI c~
30 70
30
V
V
60
50
V D s( 40
60 08
01,1
C29
V
V
V
70
30
20
I0
0
C29
Fig. 4. Triangular diagram of relative abundance distribution of three C~ sterane configurations in oils and rocks from the Qaidam Basin.
Fig. 5. The distribution of oil fields and oil samples in the western part of the Qaidam Basin.
of the hydrocarbon generation and evolution from the organic matter. According to the maturity boundaries based on the parameters determined above, crude oils from the Qaidam Basin can be classified into immature, low-mature, mature and high-mature oils. The characteristics of each type arc as follows. 1. Immature oils. The immature oils, including the condensate from Kaitemilike (No. 016, sp. gr. 0.7711) and light oil from Yuecan 1 Well (No. 03, sp. gr. 0.8155), were generated in the diagenvsis stage of organic matter evolution, and reservoired in the Pliocen¢ and the Miocene strata. Even though the saturated hydrocarbon contents in these oils are as high as 83 and 88.9%, respectively, the odd-to-oven predominance of n-alkanes is not evident and the moretanc/hopane ratio is not very high, there is a predominance of steranes with the original biological configuration R, and the isomerization level of ==~ to ~/~/~ steranes is very low. It is clear from the two maturity parameters in Tables 1 and 3 that the maturity of No. 016 condensate approximates the hydrocarbon-generating
527
M a t u r a t i o n sequence o f oils in h y d r o c a r b o n b a s i n s in C h i n a Table 3. Geologic data of grade oils in the Qaidam Basin
No.
Oil field
01 02 03
Hongiiuquan Yuejin Yuejin
04 05 06 07 08
Yuejin Yuejin Yuejin
09 010 011 012
Shizigou Shizigou
Huaiugou Hualugou Hualugou Youshashan
Age'
Depth (m)
No.
Oil field
E3 Nj Ni E3 N2 N2--N I NI E3 N2 N1 N~ Ni
2565.1 1366-1368 2255-2447 3300-3323 1658-1661 1447 1187-1188 4131.62 639.6 808-1209 1039-1268 658-859
013 014 015 016 017 018 019 022 023 024 025
Ganchaigou Xianshuiquan Youdunzi Kaitemilike N,nyishan Nanyishan Jiandingshan Lenghu Lenghu Lenghu Yuka
Ag@
Depth (in)
N2 N2 N2 N2 N3 E3 N2 J2 E3 Nt J3
2332-2382 Un~laed 0-530.38 3.3-137.8 2981 2981 106-61 I 478-639 610-760 396-68 ! .4 200
"The age of the pay bed.
threshold, while that of the No. 03 light crude oil is lower. 2. Low-mature oils.The crude oil samples in Figs 3 and 4 are representativeof oilsin the Qaidam Basin. Therefore, most of the crude oils in the known oil fields of the Qaidam Basin are classifiedas lowmature oils.These occur in the Miocene and Oligocene sediments at burial depths of 3000-4000m (the diversity of their m a x i m u m depths is due to the differencesin geothermal gradient and in the extent of structural uplift).Their epimerization parameter value is 0.3-0.4 (cyclicisomcrizationparameter value 0.25-0.41), far from the equilibrium values of these isomerization ends, and thus, the oils are in the low-mature evolution stage. The specific gravities of these oils vary between 0.8297-0.8780, and the total recovery in 310°C fraction is only about 18-32% in general. Saturates account for 65-70%, aromatics 15.5-25.5% and non-hydrocarbons plus asphaltenc up to 10-20%, showing in those oils with low-mature features. As judged by the level of hydrocarbon generation and evolution of organic matter in their source rocks, these low-mature oils were generated at vitrinite reflectance values of 0.6-0.8%. The oils abound with resin-degraded compounds (C19--C20 with m/z 123 base peak in mass spectra), which is also a marker for low maturity. It has been suggested that resinite can generate hydrocarbons at relatively low maturities (Snowdon, 1980). The distribution of low-mature oils in Figs 3 and 4 shows that the maturity of crude oils from the Shizigou structural zone and Chaishen 1 Well (No. 013) plus three shallow layer oils (Nos 015, 017 and 019) from Youquanzi, Nanyishan and Jiandingshan are relatively higher than that in the Yuejin oil field. These oils belong in the products of low-mature stages. Meanwhile, it also indicates that a significant amount of the shallow oils in the Qaidam Basin originated from low-mature organic matter in more deeply buried source rocks and reached the shallow reservoirs via verticalmigration. 3. Mature oils and high-mature condensates. From Figs 3 and 4 it is apparent that the No. 8 oil from Shishen 20 Well (4131.6m in depth) and No. 18 condensate from Nan 2 Well (2982 m in depth) in
Nanyishan, both recentlydiscovered,and No. 014 oil from the older Xiancan I Well in Xianshuiquan, as well as the oils sourced from the Jurassic in Lcnghu region, are all in the same maturation stage. Their epimerization parameter is 0.48-0.55, approximating or reaching the end of epimerization,and thus, these oilsare realmature crude oils.The Oligocene oilsNos 08 and 018, are of the highest maturity. The oil in the deep part of the Shishen 20 Well was produced beneath the gypsum-salt sectionwhich belongs to the bottom of the Oligocene according to palynological study. These mature oilsare characterizedby their lower specific gravities and asphaltene contents, as well as by higher saturate contents (> 80%). The most strikingcharacteristicof these mature oilsis that the predominance of biomarker compounds with the original biologicalconfigurationis slightor has even completely disappeared, being replaced by the predominant /so-stcranes and analogous compounds of lower carbon number. Moreover, the clear predominance of tricyclictcrpanes over hopanes in the Tertiary condensates is conspicuous. The same phenomenon has been observed in rock extracts of Han 2 Well where the depth is over 4450 m and the corresponding temperature is up to > 160°C (Huang et aI., 1984b). This also proves that the source rocks of mature and high-mature crude oils have been buried over 3000-4000 m in depth, and have undergone a considerable hydrocarbon generation and conversion. This maturation sequence of crude oils is a ubiquitous phenomenon in Chinese Tertiary oil-and gas-beating basins. W e have previously reported a similar maturation sequence in the Baisc Basin and the Jiyang Depression (Huang and Li, 1987). The maturation sequence of crude oils in the Huanghua Depression (Dagang Oilfield) is another example (Fig. 6). Basins with the above characteristicsare mostly observed in East China where they arc referredto as the Tertiary extension fault depression basins. It is often seen that there are severaloil-generatingdepressions that contain good source rocks in an oil-bearing basin, controlling the associated oil and gas occurrences, respectively.Figure 7 shows the distribution
528
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I l I I
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of source area and oil and gas fields in the Jizhong Basin. There are six oil-generating depressions, i.e. Langgu, Baxian, Raoyang, Shenxian, Shulu and Jinxian Depressions, which have developed in this basin that control the distribution patterns of oil and gas. The age of their source beds and reservoirs is mainly Lower Tertiary. However, the lazgest and most well-known oil field in the basin, Renqiu, is a buried-hill oil field associated with oil-generating Raoyang Depression. The major reservoirs are Cambrian-Ordovisian and Upper Proterozoic carbonates. The distribution of crude oil maturities is illustrated Beijinq
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in Fig. 8, from which it can be seen that the maturation sequence of crude oils is more complete in the Langgu and Raoyang Depressions, while other hydrocarbon-generating depressions are short of the products of certain hydrocarbon-generation and evolution stages. This has been confirmed by further detailed investigation. Therefore, it is not difficult to scientifically predict the explorational prospects in this wellexplored basin by means of a comparison of resource extents in each hydrocarbon-generating stage. In addition, Fig. 8 also shows the lower isomerization phenomenon of ~t~tusteranes in crude oils in carbonate reservoirs. CONCLUSIONS
The differentiation and definition of the four maturity levels in crude oils mentioned above is significant both for petroleum geochemical studies and for petroleum exploration. Taking the Qaidam Basin as an example, it indicates that the oil and gas resources so far discovered are principally at the low-mature stage, and thus, not only are there prospects of discovering further shallow immature oils, but also the exploration and development of deeper mature crude oils is just beginning and has bright prospects. Acknowledgement--This work was financially supported by
the Laboratory of Biogeochemistry and Gas-geochemistry, Lanzhou, People's Republic of China. U
REFERENCF~ Baker E. G. (1967) A geochemical evaluation of petroleum migration and accumulation. In F u n d a m e n t a l A s p e c t s of Petroleum Geochemistry, pp. 299-329. Elsevier, New ffc~lF~
~
0 '
25 '
50 Km '
Fig. 7. The distribution of Tertiary oil-generating depressions and oil fields in the Jizhong Basin.
York. Fu J., Sheng G. and Jiang J. (1985) Immature oil originated from a saline deposit-bearing basin. Oil Gas Geol. 6(2), 150-158 (in Chinese). Hnang D. and Li J. (1987) Immature petroleum in continental deposits and its significance. Acta Pet. Sin. 8(I), I-9 (in Chinese).
Maturation sequence of oils in hydrocarbon basins in China Huang D., Shang H. and Li J. (1984a) Advances in theoretical research on continental oil generation in China. In Proceedings of Beijing Petroleum Symposium, pp. 90-109. Petroleum Publishing, Beijing (in Chinese). Huang D., Li J., Zhou Z., Gu X. and Zhang D. (1984b) Evolution and Hydrocarbon-generating Mechanism of Nanmarine Organic Matter. Petroleum Publishing, Beijing (in Chinese). Huang D., Liao Q. and Xu Y. 0987) A Prel~r,linary Study on the Genesis of Immature Petroleum, pp. 1-19. Annual Research Report 1987, Biogeochem. and Gasgeochem. Lab., Lanzhou Inst. of Geol., Acad. Sin. Gansu, Beijing (in Chinese). Leythaeuser D., Mackenzie A., Schaefer R. G. and Bjoroy M. (1984) A novel approach for recognition and quantification of hydrocarbon migration effects in shale-sandstone sequences. Bull. Am. Assoc. Pet. Geol. 68, 196-219. Palacas J. G. (1983) Carbonate rocks as source of petroleum: geological and chemical characteristics and oil-source correlations. In Proceedings of the Eleventh World Pet-
529
roleum Congress, Vol. 2, pp. 31-43. Wiley, New York. Shanmugam G. (1985) Significance of coniferous rain forests and related organic matter in generating commercial quantities of oil, Gippsland Basin, Australia. Bull. Am. Assoc. Pet. Geol. 69, 1241-1254, Shi J., Mackenzie A. S., Eglinton G., Gowar A. P. and Maxwell J. R. (1982) Biological markers for petroleum and shales in Shengii Oilfield and their geochemical implications. Geochimica 1, 1-20 (in Chinese). Snowdon L. R. (1980) Resinite---a potential source on the Upper Cretaceous/Tertiary of the Beaufort-Mackenzie Basin. Can. Soe. Pet. Geol. Mere. 6, 421-446. Snowdon L. R. and Powell T. G. (1982) Immature oil and condensate---modification of hydrocarbon generation model for terrestrial organic matter. Bull. Am. Assoc. Pet. Geol. 66, 775-788. Tissot B. P. and Welte D. H. (1978) Petroleum Formation and Occurrences, 1st edn. Springer, Berlin. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, 2nd edn. Springer, Berlin.