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Thermal regime and petroleum systems in Junggar basin, northwest China
Physics of the Earth and Planetary Interiors 126 (2001) 237–248
Thermal regime and petroleum systems in Junggar basin, northwest China Shejiao Wang a,b,∗ , Lijuan He a , Jiyang Wang b a b
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China Received 1 March 2000; received in revised form 9 January 2001; accepted 1 June 2001
Abstract The conversion of organic matter into oil and gas depends upon temperature and time, and the study of thermal maturation of oil-source rocks in a petroleum system is of importance for formation of a petroleum system. Based on the temperature data from 196 wells and thermal conductivity measurements of 90 core samples, altogether 35 heat flow data are obtained in the Junggar basin. The results show that the Junggar basin is a relatively “cold” basin at present, with a mean temperature gradient and heat flow of 21◦ C/km and 42 mW/m2 , respectively. Thermal history reconstructed from vitrinite reflectance data indicates that the Paleozoic formations experienced their maximum paleotemperature during the Permian and Triassic at higher paleoheat flow of about 85 mW/m2 and that the basin then cooled down to the present low heat flow. The high paleoheat flow can be attributed to the Carboniferous to Permian rifting. The thermal evolution has a quite important effect on the formation and evolution of the petroleum systems in the Junggar basin, i.e. the Permian and the Jurassic systems. The Jurassic petroleum system is quite limited in space for the cooled thermal regime during the Meso-Cenozoic and the source rocks of the Middle–Lower Jurassic entered the oil window only along the North Tianshan foreland region, where the Jurassic is buried to the depth of 5–7 km at present. In contrast, the Middle–Lower Permian source rocks have experienced oil and gas generation in late Permian to Triassic, and the Permian petroleum system was formed prior to the Triassic when the upper Paleozoic formations reached their maximum paleotemperature due to higher paleoheat flow. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Heat flow; Thermal history; Hydrocarbon generation; Petroleum system; Junggar basin
1. Introduction A ‘petroleum system’ encompasses a pod of active source and all related oil and gas and includes all the essential elements and processes needed for oil and gas accumulations to exist. The essential elements are the source rock, reservoir rock, seal rock and overburden rock and the processes include trap formation and the generation–migration–accumulation ∗ Corresponding author. E-mail address: [email protected] (S. Wang).
of petroleum (Magoon and Dow, 1994). This concept has been widely and successfully used by geologists in petroleum exploration as a comprehensive method. One important aspect of petroleum system analysis is thermal maturation of oil-source rocks in the petroleum system. Temperature and time are main factors for hydrocarbon generation (e.g. Bodner and Sharp, 1988; Deming and Chapman, 1989; Brigaud et al., 1992; Funnell et al., 1996; Uysal et al., 2000). An effective oil-source rock in a petroleum system must be heated enough to enter the oil generation window before the petroleum system formed. Data of
0031-9201/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 ( 0 1 ) 0 0 2 5 8 - 8
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thermal history reconstruction can be used to identify, quantify and characterize the major thermal episodes (Green et al., 1997). Thermal history reconstruction plays, therefore, an important part in petroleum system analysis. Two petroleum systems were recognized in the Junggar basin, i.e. the Permian and the Jurassic systems (Zhao and He, 1996), each system has its own source, reservoir and cover formations. The Permian petroleum system located in the Mahu and West Pen 1 Well depressions, and the Jurassic system located in the North Tianshan foreland depression. The thermal maturation of the Permian and the Jurassic source rocks for the two systems are evaluated based on the present-day heat flow and the reconstructed thermal history from vitrinite reflectance data in this paper, and the relationship between thermal regime and the formation of petroleum system in the Junggar basin is discussed.
2. Geologic setting The Junggar basin with an area of about 130,000 km2 is located in Northwest China. It is bounded by
Kelameili Mountains to the East and by Tianshan Mountain to the South and by the Zhayier Mountains to the Northwest (Fig. 1). It is an asymmetric foreland basin at present with its deepest part to the South at the northern edge of the Tianshan and becomes gradually shallow toward the north (Fig. 2). Two types of basement exist in Junggar basin, i.e. the Precambrian basement and a younger basement deformed during Hercynian movement. The tectonic evolution of the basin can be divided into six phases: (1) backarc basin in the early Carboniferous and backarc foreland basin in the late Carboniferous; (2) extensional basin in the early Permian and intracratonic subsidence in the Middle–Late Permian; (3) stable subsidence in the Triassic; (4) extensional basin in the early Jurassic; (5) stable subsidence in the Cretaceous; (6) foreland basin in the Tertiary (Zhao, 1993). The sedimentary rock sequence is 15,000 m thick along the North Tianshan foreland region and consists of Carboniferous to Neogene rocks that overlie a pre-Carboniferous metamorphic basement (Lee, 1985). The lower Carboniferous is 4 km thick, composed mainly of marine volcaniclastics, and coals along the eastern margin of the basin near the Kelameili mountains region (Lee, 1985; Carroll et al.,
Fig. 1. Tectonic setting of the Junggar basin. Shown also the heat flow sites and lines of cross-sections A–A and B–B shown in Figs. 2 and 5. Inset map shows the locality of the Junggar basin.
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Fig. 2. Simplified cross-section of the Junggar basin. Locality of the cross-section is marked in Fig. 1.
1990). Thick mudstones were deposited in the Permian and are the main source rock in the Junggar basin (Jiang and Fan, 1983; Ulmishek, 1984; Jiang et al., 1990). The Mesozoic stratigraphy in the Junggar basin has a thickness of >7000 m in some places, consisting mainly of nonmarine clastic rocks. The Triassic, about 100–800 m thick, thins toward the basin margins and consists of sandstone, conglomerate, siltstone, mudstone and shale (Chiyi, 1981; Lee, 1985). The Jurassic sedimentary section is >4000 m thick (Clayton et al., 1997). It has been proved that the oil and gas in some fields, such as the Cainan, the Qigu, and the Dushanzi oil fields and the Hutubi gas field, originated from the mature Jurassic source rocks distributed along the North Tianshan foreland region (e.g. Clayton et al., 1997). The stratigraphy of the Junggar basin is summarized in Table 1.
3. Present-day thermal regime A logical starting point for a thermal history analysis of a sedimentary basin is determining its present thermal state. The present is the only time when the thermal state of a sedimentary basin may be determined by directly measuring temperature, and the heat flow may be estimated by combining temperature data with laboratory measurements of thermal conductivity.
The data are presented here to illustrate the database for the thermal history reconstruction. 3.1. Temperature gradient and thermal conductivity The downhole temperature data used to study temperature gradient and heat flow mostly come from continuous temperature logs, partially from single corrected BHT measurements. The BHT data are corrected using the method of Shen and Beck (1986). Some of the subsurface temperatures in the Junggar basin are obviously affected by groundwater movement (Fig. 3a). In order to obtain conductive thermal gradients representing the background thermal regime, 28 well logs are selected out of a total of 196 well logs for thermal gradient calculation in addition to BHT data from eight wells. Temperature gradients in the Junggar basin range from 11.6 to 27.6◦ C/km, whereby higher values are observed in the Luliang uplift and the East uplift, and lower values in the Mahu depression (see Fig. 1) and the North Tianshan foreland depression, respectively (Fig. 1). In the latter areas, the geothermal gradient is as low as 11.6◦ C/km. Thermal conductivity was measured on 96 core samples (Table 2). The thermal conductivity values vary from 0.8 to 3.6 W/mK except a low value (0.17 W/mK) for a coal sample. Sandstone shows a larger scatter of thermal conductivity (0.7–3.3 W/mK)
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Table 1 Summary of the Junggar basin stratigraphy (modified from Clayton et al., 1997) Era
System
Series
South
Northwest
Cenozoic
Quaternary
Q
Eolian, Lacustrine, Alluvial, and alluvial deposit of silt, clay, and gravel, Eolian losses, and paleosoils
Eolian, Lacustrine, Alluvial, and alluvial deposit of silt, clay, and gravel, Eolian losses, and paleosoils
Tertiary
Upper (N)
Dushanzi Fm. Taxihe Fm. Shawan Fm. Anjihaihe Fm. Ziniquanzi Fm.
Changjihe Gp. Suosuoquan Fm.
Cretaceous
Upper (K2) Lower (K1)
Lower (E) Mesozoic
300–3000
1300–3300
Ulungurha Fm. Honglishan Fm.
50–1600
Donggou Fm. (Tugulu Gp.) Lianmuqin Fm. Shengjinkou Fm. Hutubihe Fm. Quingshuihe Fm.
Ailikehu Fm. (Tugulu Gp.) Lianmuqin Fm. Shengjinkou Fm. Hutubihe Fm.
60–1000
Kalaza Fm. Qigu Fm. Toutunhe Fm Xishanyao Fm. Sangonghe Fm. Badaowan Fm.
Kalaza Fm. Qigu Fm. Toutunhe Fm Xishanyao Fm. Sangonghe Fm. Badaowan Fm.
Upper (T3) Middle (T2) Lower (T1)
Haojiagou Fm Huangshanjie Fm. Kalamay Shaofanggouy Fm. Jiucaiyuan Fm.
Baijiantan Fm. Kalamay Baikouquan Fm.
120–1400
Permian
Upper (P2)
Wutonggou Fm. Quanzijie Fm. Hongyanchi Fm. Lucaogou F. Jingjingzigou Fm. Wulapo Fm.Ss. Xiajijicaozi Gp.
Wuerhe4 Fm Xiazijie Fm.
500–3000
Fengcheng + Jiamuhe Fms.
100–2000
Carboniferous
Upper (C3)
Jurassic
Upper (J3) Middle (J2) Lower (J1)
Triassic
Paleozoic
Thickness (m)
Lower(P1)
C1 + C2
Devonian
Aotuer Fm. Qijiagou Fm. Liushugou Fm. Nanmingshui Fm. Jiangbasitao Fm. Donggulubasitao Fm
800–1300
50–700 500–3000
130–1100
200–7000 1500–5700
Undifferentiated marine + continental sedimentary, volcanic, and sedimentary rocks
than any other lithology due to its variable mineral composition and porosity. Low conductivity is observed for mudstone and siltstone due to appreciable amount of clay minerals in these kinds of rocks. The highest value represents a dolomite sample from Feng 1 Well. The average thermal conductivity values corresponding to the depth intervals calculating the temperature gradients are obtained by a thickness–
weighted method based on the mean of different lithologies as listed in Table 2. 3.2. Heat flow The temperature logs, without obvious disturbance by groundwater movement and showing stable gradients with depth, were selected for heat flow
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Fig. 3. Borehole temperature profiles in the Junggar basin (a) and the selected profiles for heat flow calculation (b). Table 2 The measured thermal conductivity values in the Junggar basin Lithology
Formation
Thermal conductivity (W/mK)
No. of samples
Average (S.D.) (W/mK)
0.87–2.66 1.72–1.87 0.87–1.35 1.76–2.00 1.15–2.38 1.28–2.66 1.25–2.34 1.85–2.23
44 3 2 5 12 8 12 2
1.88 (0.38)
Late Tertiary Early Tertiary Cretaceous Jurassic Triassic Permian Carboniferous
1.23–2.89 1.84 1.23 1.30–1.57 1.37–2.89 2.11–2.41
11 1 1 3 3 3
1.84 (0.54)
Late Tertiary Early Tertiary Cretaceous Jurassic Permian
0.69–3.27 0.86 0.80–1.91 1.11–1.19 0.97–3.27 2.81–2.91 0.69–2.84
41 1 2 2 22 3 11
2.17 (0.69)
Late Tertiary Early Tertiary Cretaceous Jurassic Triassic Permian
2.06–3.35 2.35 2.61 2.06–3.35
5 1 1 3
2.59 (0.53)
Jurassic Triassic Permian
1.67–2.56 1.67–2.07 2.05–2.18 1.69–2.33 1.29–2.56
14 2 2 2 8
1.98 (0.37)
Jurassic Triassic Permian Carboniferous
Mudstone
Siltstone
Sandstone
Conglomerate
Volcanic
Coal
Jurassic
0.17
1
0.17
Limestone
Carboniferous
1.85
1
1.85
Dolomite
Permian
3.64
1
3.64
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calculation. The average thermal conductivity is calculated using a thickness–weighted method. The calculated heat flow in the Junggar basin ranges from 23 to 54 mW/m2 (Table 3). Laterally, the distribution of heat flow in the Junggar basin is higher on the uplifts than in the depressions as already indicated by the temperature gradients. The highest heat flow is observed in the Luliang uplift with an average value of 45 mW/m2 . The heat flow in the Northwest thrust-fault belt is 44 mW/m2 and close to the value of 43 mW/m2 in the East uplift. The heat flow is lower
in the Wulungu depression (Lun 5 well, 43 mW/m2 ) and the Mahu depression (Ma 2 well, 36 mW/m2 and Aican 1 well, 38 mW/m2 ). The lowest heat flow occurs in the North Tianshan foreland depression, with an average of 34 mW/m2 . This pattern indicates that heat flow is obviously controlled by burial depth. The thicker is the sedimentary sequence, the lower is the heat flow. This can be attributed to the redistribution of heat within the basin due to the difference of thermal conductivity between sediments and basement. In addition, the low heat flow characteristics in the
Table 3 Heat flow data in the Junggar basin No.
Well code
Longitude
Latitude
Depth range (m)
Temperature gradient (G ± S.D., 䊐/km)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Cong 43 Bai 57 Bai 6 Hong 31 Hong 35 Hong 1 Lun 5 Madong 1 Shinan 4 Sancan 1 Quan 3 Shixi 2a Lu nan 1a Shinan 2 Dixi 2 Ma 2a Aican 1a Mobei 2a Guai 4 Pen 4a Pencan 2a Cai 31 Caican 2 Caican 1 Che 30 Che 17 Che 2025 Che 2037 Xican 2 Sican 1 Hu 2 Bei 74 Xiao 1 Sha’nan 1 Bei 21
85◦ 48 28 85◦ 30 17 85◦ 29 14 85◦ 02 51 85◦ 01 30 84◦ 56 12 87◦ 53 46 86◦ 27 14 86◦ 44 04 87◦ 55 06 88◦ 06 44 86◦ 53 12 87◦ 08 52 87◦ 33 49 87◦ 41 24 85◦ 57 18 85◦ 39 25 86◦ 44 23 85◦ 09 54 86◦ 18 32 86◦ 31 26 88◦ 12 51 88◦ 22 05 88◦ 48 16 84◦ 52 59 84◦ 57 42 84◦ 57 30 84◦ 56 22 84◦ 41 15 84◦ 09 00 86◦ 59 10 88◦ 23 51 87◦ 18 20 88◦ 49 26 88◦ 48 08
46◦ 08 20 45◦ 59 20 45◦ 51 25 45◦ 23 44 45◦ 20 48 45◦ 12 14 46◦ 20 00 46◦ 07 00 45◦ 37 32 45◦ 35 13 45◦ 37 11 45◦ 26 04 45◦ 18 47 45◦ 24 57 45◦ 11 47 45◦ 57 51 45◦ 46 43 45◦ 13 40 45◦ 14 35 45◦ 03 02 44◦ 54 55 44◦ 56 03 45◦ 51 25 45◦ 07 50 44◦ 58 47 44◦ 44 42 45◦ 51 25 44◦ 46 16 44◦ 23 01 44◦ 37 17 44◦ 10 40 44◦ 14 28 43◦ 37 18 44◦ 45 54 44◦ 22 25
100–495 1350–1830 260–2150 320–2550 200–2150 1560–2120 0–3300 3248–4548 2566–3302 100–2400 50–3450 0–4578.5 0–4349.9 2506–4230 3203–3835 0–2632.5 0–5300 0–4438 1908–3472 0–4265.6 0–5180 1800–3390 1600–2200 700–3154 30–2970 3040–3650 2410–3230 2400–3170 500–4000 35–4300 100–3500 2190–3177 2600–3180 200–2066 100–2430
20.1 18.5 20.3 27.6 24.7 18.7 19.5 26.5 16.7 23.7 22.9 26.2 26.2 18.1 26.0 17.9 18.7 23.2 19.3 20.4 20.9 26.9 25.7 26.0 20.3 17.2 17.2 15.8 20.5 16.5 21.4 18.5 11.6 27.1 21.5
a
Thermal gradient calculated from BHT.
± ± ± ± ± ± ± ± ± ± ±
0.06 0.12 0.06 0.07 0.09 0.10 0.17 0.35 0.01 0.11 0.12
± 0.15 ± 0.56
± 0.23 ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.10 0.11 0.14 0.05 0.10 0.10 0.09 0.22 0.16 0.29 0.08 0.10 0.28 0.11
Thermal conductivity (W/mK)
Heat flow (mW/m2 )
2.17 2.08 2.09 1.89 1.98 2.04 2.21 1.97 2.12 1.94 1.94 2.05 2.02 1.99 1.96 2.00 2.02 1.98 2.07 2.01 2.01 1.95 2.02 2.04 1.93 1.93 1.90 1.91 1.90 2.00 1.98 1.94 2.02 1.95 1.98
43.5 38.5 42.3 52.3 48.9 38.1 43.2 52.1 35.4 46.1 44.4 53.7 52.9 35.9 51.1 35.7 37.8 45.9 39.9 41.0 42.0 52.2 52.0 52.8 39.2 33.2 32.7 30.2 38.9 33.0 42.4 35.8 23.4 52.8 42.6
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North Tianshan foreland area is likely controlled by the rapid subsidence and deposition during the Late Tertiary and the Quaternary as the collision between Indian and Eurasian plates. The collision caused the Tianshan rapidly rising and formed the present tectonic pattern in the Junggar basin as an asymmetric foreland basin.
4. Thermal history The thermal history of a sedimentary sequence is crucial to the thermal maturation of hydrocarbon source rocks, and in close relation to the dynamic evolution of a basin. The method of thermal history reconstruction used in this study is a heat-flow-based approach (Lerche et al., 1984; Hu et al., 1998, 1999), which couples the geohistory/burial history with the inversion of thermal indicators. It is a more complex method in relation to the temperature-gradient-based method (Duddy et al., 1991) because of accommodating both, the burial history reconstruction and the heat transfer modeling of the sediment pile, including the variation of thermal conductivity for different formations evaluated with time. For the burial history reconstruction (Sclater and Christic, 1980), the in situ porosity was modeled using a generalized porosity-depth function φ = φ0 e−z/λ where z is depth in km, φ0 the porosity at zero depth and λ the compaction factor. Table 4 lists these parameters for major rocks. The paleothermal conductivity is calculated as φ
(1−φ)
K = Kw Kr
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where Kw and Kr are thermal conductivity of water and matrix conductivity of rock, respectively. The temperature dependence of thermal conductivity was taken into account in thermal history modeling assuming that matrix conductivity is proportional to the reciprocal of the absolute temperature (Deming and Chapman, 1989), thus, 293 KmT = Km20 , T + 273 where KmT is matrix conductivity at temperature T (◦ C) and Km20 is the harmonic-mean matrix conductivity, measured in the laboratory at 20◦ C. The entire sediment pile is divided into separate structural layers or subsections separated by unconformities. Each subsection represents an episode, in which the basal heat flow is assumed to vary linearly with time. The thermal history corresponding to the top subsection is reconstructed first by simultaneously varying both, the removed thickness at a unconformity on the top (if any) and the heat flow at the beginning of deposition for this subsection, until a minimum in the mean-square-residual fit between the measured and the predicted thermal indicator values is reached. The calculated vitrinite reflectance values at the sampled depths are calculated using the kinetic model of Burnham and Sweeney (1990) based on the temperature–time paths. If the lower subsection experienced its maximum paleotemperature at an earlier time than the upper, one can apply the above method to each section downwards to reveal the entire thermal history. This approach puts constraints on the evolution from the upper subsection to the lower ones. Through the thermal indicator inversion, the thermal history from the onset of cooling after maximum paleotemperatures to the present-day heat flow and the amounts of the eroded section at an unconformity separating each
Table 4 Related parameters for burial and thermal history reconstruction in the Junggar basin Lithology
Initial porosity
Compaction factor (1/m)
Density (kg/m3 × 103 )
Matrix conductivity (W/mK)
Heat capacity (kJ/m3 )
Heat production (W/m3 )
Sandstone Mudstone Siltstone Dolomite Basalt
0.6 0.5 0.45 0.47 0.0
0.000515 0.00038 0.00044 0.00022 0.0
2.55 2.2 2.6 2.7 2.67
2.2 1.92 1.88 3.68 2.1
2700 2200 2400 2610 2500
0.8 1.6 0.5 0.6 0.21
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subsection can be determined if the subsections were hotter in the past. A schematic explanation was given by Hu et al. (1998) and the computer program called “Thermodel” is employed in this study. Using the present-day heat flow, the measured vitrinite reflectance data, and the measured thermophysical data as input parameters (Table 4), the thermal history in the Junggar basin has been reconstructed. The results indicate that the paleoheat flow during the Permian and Triassic in the Junggar basin was about 85 mW/m2 and the corresponding paleothermal gradient about 36–47◦ C/km in the Permian. Fig. 4 shows as an example the reconstructed thermal history from vitrinite reflectance data through integration of burial and paleotemperature paths and the kinetic model of organic maturity (Burnham and Sweeney, 1990). The example well is the Aican 1 well (see Fig. 6 for its location) in the Mahu depression
where erosion at the unconformity is not significant that heat flow is the main variable to be adjusted to match the measured and the calculated or predicated vitrinite reflectance (Fig. 4d). The result from the best-fit shows that heat flow gradually decreased from Permian to the present (Fig. 4b). The decrease in heat flow is consistent with the tectonic evolution of the basin. The Junggar basin experienced an extensional stage during the Permian to Triassic and later on a relatively stable development. Thinning of the crust may have caused upwelling of hot mantle during the extensional stage resulting in much higher paleoheat flow than observed today. Drilling has proved that many volcanic activities took place in the Permian deposition center of the basin, such as the Mahu depression. The consequent temperature paths of different formations indicated that the Permian formation reached its maximum paleotemperature (120–180◦ C) at early Jurassic, and that the Jurassic experienced
Fig. 4. The reconstructed burial (a) and thermal history (b, c) as well as the calculated (dots) and the measured (line) vitrinite reflectance (d) for Aican 1 well.
S. Wang et al. / Physics of the Earth and Planetary Interiors 126 (2001) 237–248
its maximum temperature (60–90◦ C) at the end of Tertiary (Fig. 4c).
5. Discussion The Junggar basin is characterized by low heat flow (23–54 mW/m2 and a mean of 42 ± 8 mW/m2 , n = 35) similar to the Tarim basin (31–77 mW/m2 and a mean of 44 ± 10 mW/m2 , n = 22) to the south of Tianshan (Wang et al., 1995). It is lower than the average value of 61 ± 16 mW/m2 in the continental area of China (Hu et al., 2000). In terms of the reconstructed paleoheat flow derived from vitrinite reflectance data and the present-day heat flow, a general cooling from the Permian to the present was revealed for the Junggar basin. The Permian high paleoheat flow can be attributed to the rifting in the deep depressions, such as the Mahu and North Tianshan depressions, during the pre-Mesozoic syn-rift phase. The Meso-Cenozoic cooling can be explained by the regional post-rift thermal subsidence during the Mesozoic and the intensified subsidence as a foreland basin during the Cenozoic. The sediments during the syn-rift, post-rift and foreland phases can be well seen in the SSE–NNE and the S–N cross-sections as shown in Figs. 2 and 5. The assumed linear cooling can be considered as a reasonable approximation in view of the tectonic evolution. Comparing with the heat flow and thermal history of the Utah–Wyoming Thrust Belt (Deming and Chapman, 1989), the Junggar basin has little lower present-day heat flow due to the continuously cooling since the Permian and the rapid
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sedimentation during the Cenozoic thrusting along the Tianshan belt. In contrast to the “hot” Cenozoic rifted basins in the East China (e.g. Hu et al., 1999), the Junggar basin is a “cold” basin as a Paleozoic rift basin. The largest petroleum system in the Junggar basin is the Permian petroleum system located in the Mahu and the West Pen 1 Well depressions. The source rocks are Permian mudstones deposited in a lacustrine environment, and the reservoirs are the Permian and the Triassic clastic rocks and the Permian and Carboniferous volcanic rocks near the deposition centers. The overburden is the Triassic formation, and the seal rocks are mudstones of Baijiantan group of the Upper Triassic (a regional seal in the Junggar basin) (Fig. 5). Many oil fields, for example, the largest Kalamay field, belong to this system. Contrast of oil and source rock shows that the oil and gas came from deep buried mature Permian mudstone in the Mahu and West Pen 1 Well depressions, where the organic maturity of the Permian source rock is high as indicated by vitrinite reflectance (1.2–2.1% for borehole samples). K–Ar dating indicates that most reservoirs in the Northwest Thrust Fault belt, located on the West of Mahu depression, formed between the Permian and the Triassic when the Junggar basin experienced a period of higher heat flow (∼85 mW/m2 ). The Permian oil-source rocks (Jiamuhe and Fengcheng groups) reached their maximum paleotemperatures at the Triassic (Fig. 4) and entered the oil or gas generation window due to the high thermal gradient and then cooled down (Fig. 6). It is, therefore, the high paleoheat flow and paleotemperature that played
Fig. 5. Cross-section across the basin from the West to the East at the end of the Triassic.
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Fig. 6. Events chart for the Permian petroleum system.
an important role in the formation of the Permian petroleum system at the end of Triassic in the basin. Another major petroleum system in the Junggar basin is the Jurassic petroleum system. Its source rocks are mudstone and coal of the Jurassic, and the reservoir rocks are siltstone, sandstone, and conglomerate of the Jurassic and the Tertiary. The overburden
rocks consist of the Upper Jurassic, Cretaceous, and Cenozoic formations. Mudstone in the Jurassic Sangonghe group, the Cretaceous Tugulu group, and the Tertiary Anjihaihe group served as the seal rocks of the system. The essential elements of Jurassic petroleum system are presented in Figs. 2 and 7. The cooling of the basin resulted in the lower thermal
Fig. 7. Events chart for the Jurassic petroleum system.
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maturity for most of the Jurassic organic matter (vitrinite reflectance 0.5–0.6%). The Jurassic system is limited in space and only in the southern part of the basin; the Lower Jurassic formation has been buried to 5–7 km depth entering the oil and gas generation window. It provided as a source rock the oil and gas for some oil fields, such as the Cainan, the Qigu oil fields and the Hutubi gas field, along the North Tianshan foreland region. If an exponential decrease in heat flow with time to be assumed instead, the Jurassic system would be more limited as the cooling would be intensified at early Mesozoic time relative to the linear model. Summarizing, the effect of thermal regime on the formation and distribution of petroleum systems has been quite obvious. The early higher paleoheat flow and thermal gradients during Permian and Triassic played a important role for both the formation of the Permian system at the Late Triassic and its wide distribution in the basin. The later lower heat flow and thermal gradients are not in favor of the Lower Jurassic source rocks to get mature. Hydrocarbon generation in the Jurassic system is restricted to the North Tianshan foreland region where it is overlain by huge Late Tertiary sediments. It is the rapid subsidence in Late Tertiary that played a more important role for the formation of the Jurassic system rather than the thermal regime from the basement (lithosphere). 6. Conclusions Based on heat flow measurements, thermal history reconstruction and analysis of relationship between the thermal regime and the petroleum systems, the following conclusions can be made: 1. The Junggar basin is characterized by low presentday heat flow, ranging from 23 to 54 mW/m2 with a mean value of 42 mW/m2 . The heat flow pattern is somewhat higher on the basement uplifts and lower in the depressions. 2. The basin experienced a higher heat flow (∼85 mW/m2 ) period from the Permian to the Triassic and cooled down from the Triassic to present (∼42 mW/m2 ). The high paleoheat flow resulted likely from the rifting process during the pre-Mesozoic. 3. The Permian and the Jurassic oil-source rocks reached their maximum paleotemperatures at the late Triassic and the Tertiary (except for the North
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Tianshan foreland region), respectively. The higher paleothermal regime was a quite important factor on the formation of the Permian petroleum system at the Late Triassic in the Junggar basin. The lower heat flow and thermal gradient were not in favor of the formation of the Jurassic systems and most of the Jurassic source-rocks are immature outside of the North Tianshan foreland region, where heating process was continued to the present due to thick deposition during the Cenozoic. Acknowledgements This research was supported by Grant No. 49672169 from the National Natural Science Foundation of China (NSFC) and a project from China National Petroleum Corporation (CNPC). We appreciate Ilmo T. Kukkonen and two anonymous reviewers for their comments and suggestions. Thanks to S. Hu for his kind help and discussion. References Bodner, D.P., Sharp Jr., J.M., 1988. Temperature variations in South Texas subsurface. AAPG Bull. 72, 21–32. Brigaud, F., Vasseur, G., Caillet, G., 1992. Thermal state in the north Viking graben (North Sea) determined from oil exploration well data. Geophysics 57, 69–88. Burnham, A.K., Sweeney, J.J., 1990. A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53, 2649–2657. Carroll, A.R., Lang, Y., Graham, S.A., Xiao, X., Hendrix, M.S., Chu, J., Mcknight, C.L., 1990. Junggar basin, Northwest China, trapped late Paleozoic ocean. Tectonophysicis 181, 1–14. Chiyi, C., 1981. Alluvial-fan corse clastic reservoirs in Kalamay. In: Mason, J.F. (Ed.), Petroleum, Geology in China. Tulsa, Oklahoma, Pennwell, pp. 154–170. Clayton, J.L., Yang, J., King, J.D., Lillis, P.G., Warden, A., 1997. Geochemistry of oils from the Junggar basin, Northwest China. AAPG Bull. 81 (11), 1926–1944. Deming, D., Chapman, D., 1989. Thermal history amd hydrocarbon generation: example from Utah–Wyoming thrust belt. AAPG Bull. 37 (12), 1455–1471. Duddy, I.R., Green, P.F., Hegarty, K.A., Bray, R.J., 1991. Reconstruction of thermal history in basin modelling using apatite fission track analysis: what is really possible? Offshore Australia Conference Proceedings. Melbourne 1, 1149–1161. Funnell, R., Chapman, D., Allis, R., Armstrong, P., 1996. Thermal state of the Taranaki basin, New Zealand. J. Geophys. Res. 101, 25197–25215. Green, P.F., Hegarty, K.A., Duddy, I.R., 1997. Timing as a vital element of the petroleum system: use of thermal history
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reconstruction using AFTA and VR as a route to more efficient exploration. In: Proceedings of Southeast Asia and Australia Conference on Indones. Petrol. Assoc. Petrol. Syst. Jakarta, Indones, 21–23 May 1997, pp. 899–912. Hu, S., Zhang, R., Zhou, L., 1998. Methods of thermal history reconstruction for oil-gas basin. Petrol. Explorationist 4, 52–54 (in Chinese with English abstract). Hu, S., Zhang, R., Luo, Y., Cai, D., 1999. Thermal history and tectono-thermal evolution of Bohai basin, East China. Chinese J. Geophys. 42 (6), 748–754. Hu, S., He, L., Wang, J., 2000. Heat flow in the continental area of China: a new data set. Earth Planet. Sci. Lett. 179, 407–419. Jiang, Z.S., Fan, G.H., 1983. The study of the Carboniferous source rock of the Kelamayi oilfield. Xinjiang Petrol. Geol. 2, 80–83 (in Chinese). Jiang, Z.S., Fowler, M.G., Lewis, C.A., Philp, R.P., 1990. Polycyclic alkanes in a biodegraded oil from the Lelamayi oilfield, northwestern China. Org. Geochem. 15, 35–46. Lee, K.Y., 1985. Geology of the Petroleum and Deposites in the Junggar (Zhungaer) Basin, Xinjiang Uygur Zizhiqu, Northwest China. US Geological Survey Open-File Report, Vol. 85–230, 53 pp. Lerche, I., Yarzab, R.F., Kendall, C.G.S.C., 1984. Determination of paleoheat flux from vitrinite reflectance data. AAPG Bull. 68, 1704–1717.
Magoon, L.B., Dow, W.G., 1994. The petroleum system-from source to trap. AAPG Memoir 60, 1–49. Sclater, J.G., Christic, P.A.F., 1980. Continental stretching, an explanation of the post-mid-cretaceous subsidence of the central North Sea basin. J. Geophys. Res. 85, 3711–3739. Shen, P.Y., Beck, A.E., 1986. Stabilization of bottom hole temperature with finete circulation time and fluid flow. Geophys. J. R. Astron. Soc. 86, 63–90. Ulmishek, G., 1984. Geology and Petroleum Resources of Basins in Western China. Argonne Natinal Laboratory, ANL/ES-146, 131 pp. Uysal, I.T., Glikson, M., Golding, S.D., Audsley, F., 2000. The thermal history of the Bowen basin, Queensland, Australia: vitrinite reflevtance and clay mineralogy of Late Permian coal measures. Tectonophysics 323, 100–129. Wang, L., Li, C., Shi, Y., 1995. Distribution of terrestrial heat flow dnsity in Tarim basin, Western China. Acta Geophys. Sinica 38 (6), 855–856. Zhao, B., 1993. The partition of structure and its characteristics in Junggar basin. Xin Jiang Petrol. Geol. (in Chinese) 14 (3), 209–216. Zhao, W., He, D., 1996. Principle of petroleum system and its application in oil-gas exploration. Petrol. Explorationist 1 (2), 1–10 (in Chinese with English abstract).
Thermal regime and petroleum systems in Junggar basin, northwest China Shejiao Wang a,b,∗ , Lijuan He a , Jiyang Wang b a b
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China Received 1 March 2000; received in revised form 9 January 2001; accepted 1 June 2001
Abstract The conversion of organic matter into oil and gas depends upon temperature and time, and the study of thermal maturation of oil-source rocks in a petroleum system is of importance for formation of a petroleum system. Based on the temperature data from 196 wells and thermal conductivity measurements of 90 core samples, altogether 35 heat flow data are obtained in the Junggar basin. The results show that the Junggar basin is a relatively “cold” basin at present, with a mean temperature gradient and heat flow of 21◦ C/km and 42 mW/m2 , respectively. Thermal history reconstructed from vitrinite reflectance data indicates that the Paleozoic formations experienced their maximum paleotemperature during the Permian and Triassic at higher paleoheat flow of about 85 mW/m2 and that the basin then cooled down to the present low heat flow. The high paleoheat flow can be attributed to the Carboniferous to Permian rifting. The thermal evolution has a quite important effect on the formation and evolution of the petroleum systems in the Junggar basin, i.e. the Permian and the Jurassic systems. The Jurassic petroleum system is quite limited in space for the cooled thermal regime during the Meso-Cenozoic and the source rocks of the Middle–Lower Jurassic entered the oil window only along the North Tianshan foreland region, where the Jurassic is buried to the depth of 5–7 km at present. In contrast, the Middle–Lower Permian source rocks have experienced oil and gas generation in late Permian to Triassic, and the Permian petroleum system was formed prior to the Triassic when the upper Paleozoic formations reached their maximum paleotemperature due to higher paleoheat flow. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Heat flow; Thermal history; Hydrocarbon generation; Petroleum system; Junggar basin
1. Introduction A ‘petroleum system’ encompasses a pod of active source and all related oil and gas and includes all the essential elements and processes needed for oil and gas accumulations to exist. The essential elements are the source rock, reservoir rock, seal rock and overburden rock and the processes include trap formation and the generation–migration–accumulation ∗ Corresponding author. E-mail address: [email protected] (S. Wang).
of petroleum (Magoon and Dow, 1994). This concept has been widely and successfully used by geologists in petroleum exploration as a comprehensive method. One important aspect of petroleum system analysis is thermal maturation of oil-source rocks in the petroleum system. Temperature and time are main factors for hydrocarbon generation (e.g. Bodner and Sharp, 1988; Deming and Chapman, 1989; Brigaud et al., 1992; Funnell et al., 1996; Uysal et al., 2000). An effective oil-source rock in a petroleum system must be heated enough to enter the oil generation window before the petroleum system formed. Data of
0031-9201/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 ( 0 1 ) 0 0 2 5 8 - 8
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thermal history reconstruction can be used to identify, quantify and characterize the major thermal episodes (Green et al., 1997). Thermal history reconstruction plays, therefore, an important part in petroleum system analysis. Two petroleum systems were recognized in the Junggar basin, i.e. the Permian and the Jurassic systems (Zhao and He, 1996), each system has its own source, reservoir and cover formations. The Permian petroleum system located in the Mahu and West Pen 1 Well depressions, and the Jurassic system located in the North Tianshan foreland depression. The thermal maturation of the Permian and the Jurassic source rocks for the two systems are evaluated based on the present-day heat flow and the reconstructed thermal history from vitrinite reflectance data in this paper, and the relationship between thermal regime and the formation of petroleum system in the Junggar basin is discussed.
2. Geologic setting The Junggar basin with an area of about 130,000 km2 is located in Northwest China. It is bounded by
Kelameili Mountains to the East and by Tianshan Mountain to the South and by the Zhayier Mountains to the Northwest (Fig. 1). It is an asymmetric foreland basin at present with its deepest part to the South at the northern edge of the Tianshan and becomes gradually shallow toward the north (Fig. 2). Two types of basement exist in Junggar basin, i.e. the Precambrian basement and a younger basement deformed during Hercynian movement. The tectonic evolution of the basin can be divided into six phases: (1) backarc basin in the early Carboniferous and backarc foreland basin in the late Carboniferous; (2) extensional basin in the early Permian and intracratonic subsidence in the Middle–Late Permian; (3) stable subsidence in the Triassic; (4) extensional basin in the early Jurassic; (5) stable subsidence in the Cretaceous; (6) foreland basin in the Tertiary (Zhao, 1993). The sedimentary rock sequence is 15,000 m thick along the North Tianshan foreland region and consists of Carboniferous to Neogene rocks that overlie a pre-Carboniferous metamorphic basement (Lee, 1985). The lower Carboniferous is 4 km thick, composed mainly of marine volcaniclastics, and coals along the eastern margin of the basin near the Kelameili mountains region (Lee, 1985; Carroll et al.,
Fig. 1. Tectonic setting of the Junggar basin. Shown also the heat flow sites and lines of cross-sections A–A and B–B shown in Figs. 2 and 5. Inset map shows the locality of the Junggar basin.
S. Wang et al. / Physics of the Earth and Planetary Interiors 126 (2001) 237–248
239
Fig. 2. Simplified cross-section of the Junggar basin. Locality of the cross-section is marked in Fig. 1.
1990). Thick mudstones were deposited in the Permian and are the main source rock in the Junggar basin (Jiang and Fan, 1983; Ulmishek, 1984; Jiang et al., 1990). The Mesozoic stratigraphy in the Junggar basin has a thickness of >7000 m in some places, consisting mainly of nonmarine clastic rocks. The Triassic, about 100–800 m thick, thins toward the basin margins and consists of sandstone, conglomerate, siltstone, mudstone and shale (Chiyi, 1981; Lee, 1985). The Jurassic sedimentary section is >4000 m thick (Clayton et al., 1997). It has been proved that the oil and gas in some fields, such as the Cainan, the Qigu, and the Dushanzi oil fields and the Hutubi gas field, originated from the mature Jurassic source rocks distributed along the North Tianshan foreland region (e.g. Clayton et al., 1997). The stratigraphy of the Junggar basin is summarized in Table 1.
3. Present-day thermal regime A logical starting point for a thermal history analysis of a sedimentary basin is determining its present thermal state. The present is the only time when the thermal state of a sedimentary basin may be determined by directly measuring temperature, and the heat flow may be estimated by combining temperature data with laboratory measurements of thermal conductivity.
The data are presented here to illustrate the database for the thermal history reconstruction. 3.1. Temperature gradient and thermal conductivity The downhole temperature data used to study temperature gradient and heat flow mostly come from continuous temperature logs, partially from single corrected BHT measurements. The BHT data are corrected using the method of Shen and Beck (1986). Some of the subsurface temperatures in the Junggar basin are obviously affected by groundwater movement (Fig. 3a). In order to obtain conductive thermal gradients representing the background thermal regime, 28 well logs are selected out of a total of 196 well logs for thermal gradient calculation in addition to BHT data from eight wells. Temperature gradients in the Junggar basin range from 11.6 to 27.6◦ C/km, whereby higher values are observed in the Luliang uplift and the East uplift, and lower values in the Mahu depression (see Fig. 1) and the North Tianshan foreland depression, respectively (Fig. 1). In the latter areas, the geothermal gradient is as low as 11.6◦ C/km. Thermal conductivity was measured on 96 core samples (Table 2). The thermal conductivity values vary from 0.8 to 3.6 W/mK except a low value (0.17 W/mK) for a coal sample. Sandstone shows a larger scatter of thermal conductivity (0.7–3.3 W/mK)
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Table 1 Summary of the Junggar basin stratigraphy (modified from Clayton et al., 1997) Era
System
Series
South
Northwest
Cenozoic
Quaternary
Q
Eolian, Lacustrine, Alluvial, and alluvial deposit of silt, clay, and gravel, Eolian losses, and paleosoils
Eolian, Lacustrine, Alluvial, and alluvial deposit of silt, clay, and gravel, Eolian losses, and paleosoils
Tertiary
Upper (N)
Dushanzi Fm. Taxihe Fm. Shawan Fm. Anjihaihe Fm. Ziniquanzi Fm.
Changjihe Gp. Suosuoquan Fm.
Cretaceous
Upper (K2) Lower (K1)
Lower (E) Mesozoic
300–3000
1300–3300
Ulungurha Fm. Honglishan Fm.
50–1600
Donggou Fm. (Tugulu Gp.) Lianmuqin Fm. Shengjinkou Fm. Hutubihe Fm. Quingshuihe Fm.
Ailikehu Fm. (Tugulu Gp.) Lianmuqin Fm. Shengjinkou Fm. Hutubihe Fm.
60–1000
Kalaza Fm. Qigu Fm. Toutunhe Fm Xishanyao Fm. Sangonghe Fm. Badaowan Fm.
Kalaza Fm. Qigu Fm. Toutunhe Fm Xishanyao Fm. Sangonghe Fm. Badaowan Fm.
Upper (T3) Middle (T2) Lower (T1)
Haojiagou Fm Huangshanjie Fm. Kalamay Shaofanggouy Fm. Jiucaiyuan Fm.
Baijiantan Fm. Kalamay Baikouquan Fm.
120–1400
Permian
Upper (P2)
Wutonggou Fm. Quanzijie Fm. Hongyanchi Fm. Lucaogou F. Jingjingzigou Fm. Wulapo Fm.Ss. Xiajijicaozi Gp.
Wuerhe4 Fm Xiazijie Fm.
500–3000
Fengcheng + Jiamuhe Fms.
100–2000
Carboniferous
Upper (C3)
Jurassic
Upper (J3) Middle (J2) Lower (J1)
Triassic
Paleozoic
Thickness (m)
Lower(P1)
C1 + C2
Devonian
Aotuer Fm. Qijiagou Fm. Liushugou Fm. Nanmingshui Fm. Jiangbasitao Fm. Donggulubasitao Fm
800–1300
50–700 500–3000
130–1100
200–7000 1500–5700
Undifferentiated marine + continental sedimentary, volcanic, and sedimentary rocks
than any other lithology due to its variable mineral composition and porosity. Low conductivity is observed for mudstone and siltstone due to appreciable amount of clay minerals in these kinds of rocks. The highest value represents a dolomite sample from Feng 1 Well. The average thermal conductivity values corresponding to the depth intervals calculating the temperature gradients are obtained by a thickness–
weighted method based on the mean of different lithologies as listed in Table 2. 3.2. Heat flow The temperature logs, without obvious disturbance by groundwater movement and showing stable gradients with depth, were selected for heat flow
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Fig. 3. Borehole temperature profiles in the Junggar basin (a) and the selected profiles for heat flow calculation (b). Table 2 The measured thermal conductivity values in the Junggar basin Lithology
Formation
Thermal conductivity (W/mK)
No. of samples
Average (S.D.) (W/mK)
0.87–2.66 1.72–1.87 0.87–1.35 1.76–2.00 1.15–2.38 1.28–2.66 1.25–2.34 1.85–2.23
44 3 2 5 12 8 12 2
1.88 (0.38)
Late Tertiary Early Tertiary Cretaceous Jurassic Triassic Permian Carboniferous
1.23–2.89 1.84 1.23 1.30–1.57 1.37–2.89 2.11–2.41
11 1 1 3 3 3
1.84 (0.54)
Late Tertiary Early Tertiary Cretaceous Jurassic Permian
0.69–3.27 0.86 0.80–1.91 1.11–1.19 0.97–3.27 2.81–2.91 0.69–2.84
41 1 2 2 22 3 11
2.17 (0.69)
Late Tertiary Early Tertiary Cretaceous Jurassic Triassic Permian
2.06–3.35 2.35 2.61 2.06–3.35
5 1 1 3
2.59 (0.53)
Jurassic Triassic Permian
1.67–2.56 1.67–2.07 2.05–2.18 1.69–2.33 1.29–2.56
14 2 2 2 8
1.98 (0.37)
Jurassic Triassic Permian Carboniferous
Mudstone
Siltstone
Sandstone
Conglomerate
Volcanic
Coal
Jurassic
0.17
1
0.17
Limestone
Carboniferous
1.85
1
1.85
Dolomite
Permian
3.64
1
3.64
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calculation. The average thermal conductivity is calculated using a thickness–weighted method. The calculated heat flow in the Junggar basin ranges from 23 to 54 mW/m2 (Table 3). Laterally, the distribution of heat flow in the Junggar basin is higher on the uplifts than in the depressions as already indicated by the temperature gradients. The highest heat flow is observed in the Luliang uplift with an average value of 45 mW/m2 . The heat flow in the Northwest thrust-fault belt is 44 mW/m2 and close to the value of 43 mW/m2 in the East uplift. The heat flow is lower
in the Wulungu depression (Lun 5 well, 43 mW/m2 ) and the Mahu depression (Ma 2 well, 36 mW/m2 and Aican 1 well, 38 mW/m2 ). The lowest heat flow occurs in the North Tianshan foreland depression, with an average of 34 mW/m2 . This pattern indicates that heat flow is obviously controlled by burial depth. The thicker is the sedimentary sequence, the lower is the heat flow. This can be attributed to the redistribution of heat within the basin due to the difference of thermal conductivity between sediments and basement. In addition, the low heat flow characteristics in the
Table 3 Heat flow data in the Junggar basin No.
Well code
Longitude
Latitude
Depth range (m)
Temperature gradient (G ± S.D., 䊐/km)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
Cong 43 Bai 57 Bai 6 Hong 31 Hong 35 Hong 1 Lun 5 Madong 1 Shinan 4 Sancan 1 Quan 3 Shixi 2a Lu nan 1a Shinan 2 Dixi 2 Ma 2a Aican 1a Mobei 2a Guai 4 Pen 4a Pencan 2a Cai 31 Caican 2 Caican 1 Che 30 Che 17 Che 2025 Che 2037 Xican 2 Sican 1 Hu 2 Bei 74 Xiao 1 Sha’nan 1 Bei 21
85◦ 48 28 85◦ 30 17 85◦ 29 14 85◦ 02 51 85◦ 01 30 84◦ 56 12 87◦ 53 46 86◦ 27 14 86◦ 44 04 87◦ 55 06 88◦ 06 44 86◦ 53 12 87◦ 08 52 87◦ 33 49 87◦ 41 24 85◦ 57 18 85◦ 39 25 86◦ 44 23 85◦ 09 54 86◦ 18 32 86◦ 31 26 88◦ 12 51 88◦ 22 05 88◦ 48 16 84◦ 52 59 84◦ 57 42 84◦ 57 30 84◦ 56 22 84◦ 41 15 84◦ 09 00 86◦ 59 10 88◦ 23 51 87◦ 18 20 88◦ 49 26 88◦ 48 08
46◦ 08 20 45◦ 59 20 45◦ 51 25 45◦ 23 44 45◦ 20 48 45◦ 12 14 46◦ 20 00 46◦ 07 00 45◦ 37 32 45◦ 35 13 45◦ 37 11 45◦ 26 04 45◦ 18 47 45◦ 24 57 45◦ 11 47 45◦ 57 51 45◦ 46 43 45◦ 13 40 45◦ 14 35 45◦ 03 02 44◦ 54 55 44◦ 56 03 45◦ 51 25 45◦ 07 50 44◦ 58 47 44◦ 44 42 45◦ 51 25 44◦ 46 16 44◦ 23 01 44◦ 37 17 44◦ 10 40 44◦ 14 28 43◦ 37 18 44◦ 45 54 44◦ 22 25
100–495 1350–1830 260–2150 320–2550 200–2150 1560–2120 0–3300 3248–4548 2566–3302 100–2400 50–3450 0–4578.5 0–4349.9 2506–4230 3203–3835 0–2632.5 0–5300 0–4438 1908–3472 0–4265.6 0–5180 1800–3390 1600–2200 700–3154 30–2970 3040–3650 2410–3230 2400–3170 500–4000 35–4300 100–3500 2190–3177 2600–3180 200–2066 100–2430
20.1 18.5 20.3 27.6 24.7 18.7 19.5 26.5 16.7 23.7 22.9 26.2 26.2 18.1 26.0 17.9 18.7 23.2 19.3 20.4 20.9 26.9 25.7 26.0 20.3 17.2 17.2 15.8 20.5 16.5 21.4 18.5 11.6 27.1 21.5
a
Thermal gradient calculated from BHT.
± ± ± ± ± ± ± ± ± ± ±
0.06 0.12 0.06 0.07 0.09 0.10 0.17 0.35 0.01 0.11 0.12
± 0.15 ± 0.56
± 0.23 ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.10 0.11 0.14 0.05 0.10 0.10 0.09 0.22 0.16 0.29 0.08 0.10 0.28 0.11
Thermal conductivity (W/mK)
Heat flow (mW/m2 )
2.17 2.08 2.09 1.89 1.98 2.04 2.21 1.97 2.12 1.94 1.94 2.05 2.02 1.99 1.96 2.00 2.02 1.98 2.07 2.01 2.01 1.95 2.02 2.04 1.93 1.93 1.90 1.91 1.90 2.00 1.98 1.94 2.02 1.95 1.98
43.5 38.5 42.3 52.3 48.9 38.1 43.2 52.1 35.4 46.1 44.4 53.7 52.9 35.9 51.1 35.7 37.8 45.9 39.9 41.0 42.0 52.2 52.0 52.8 39.2 33.2 32.7 30.2 38.9 33.0 42.4 35.8 23.4 52.8 42.6
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North Tianshan foreland area is likely controlled by the rapid subsidence and deposition during the Late Tertiary and the Quaternary as the collision between Indian and Eurasian plates. The collision caused the Tianshan rapidly rising and formed the present tectonic pattern in the Junggar basin as an asymmetric foreland basin.
4. Thermal history The thermal history of a sedimentary sequence is crucial to the thermal maturation of hydrocarbon source rocks, and in close relation to the dynamic evolution of a basin. The method of thermal history reconstruction used in this study is a heat-flow-based approach (Lerche et al., 1984; Hu et al., 1998, 1999), which couples the geohistory/burial history with the inversion of thermal indicators. It is a more complex method in relation to the temperature-gradient-based method (Duddy et al., 1991) because of accommodating both, the burial history reconstruction and the heat transfer modeling of the sediment pile, including the variation of thermal conductivity for different formations evaluated with time. For the burial history reconstruction (Sclater and Christic, 1980), the in situ porosity was modeled using a generalized porosity-depth function φ = φ0 e−z/λ where z is depth in km, φ0 the porosity at zero depth and λ the compaction factor. Table 4 lists these parameters for major rocks. The paleothermal conductivity is calculated as φ
(1−φ)
K = Kw Kr
243
where Kw and Kr are thermal conductivity of water and matrix conductivity of rock, respectively. The temperature dependence of thermal conductivity was taken into account in thermal history modeling assuming that matrix conductivity is proportional to the reciprocal of the absolute temperature (Deming and Chapman, 1989), thus, 293 KmT = Km20 , T + 273 where KmT is matrix conductivity at temperature T (◦ C) and Km20 is the harmonic-mean matrix conductivity, measured in the laboratory at 20◦ C. The entire sediment pile is divided into separate structural layers or subsections separated by unconformities. Each subsection represents an episode, in which the basal heat flow is assumed to vary linearly with time. The thermal history corresponding to the top subsection is reconstructed first by simultaneously varying both, the removed thickness at a unconformity on the top (if any) and the heat flow at the beginning of deposition for this subsection, until a minimum in the mean-square-residual fit between the measured and the predicted thermal indicator values is reached. The calculated vitrinite reflectance values at the sampled depths are calculated using the kinetic model of Burnham and Sweeney (1990) based on the temperature–time paths. If the lower subsection experienced its maximum paleotemperature at an earlier time than the upper, one can apply the above method to each section downwards to reveal the entire thermal history. This approach puts constraints on the evolution from the upper subsection to the lower ones. Through the thermal indicator inversion, the thermal history from the onset of cooling after maximum paleotemperatures to the present-day heat flow and the amounts of the eroded section at an unconformity separating each
Table 4 Related parameters for burial and thermal history reconstruction in the Junggar basin Lithology
Initial porosity
Compaction factor (1/m)
Density (kg/m3 × 103 )
Matrix conductivity (W/mK)
Heat capacity (kJ/m3 )
Heat production (W/m3 )
Sandstone Mudstone Siltstone Dolomite Basalt
0.6 0.5 0.45 0.47 0.0
0.000515 0.00038 0.00044 0.00022 0.0
2.55 2.2 2.6 2.7 2.67
2.2 1.92 1.88 3.68 2.1
2700 2200 2400 2610 2500
0.8 1.6 0.5 0.6 0.21
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subsection can be determined if the subsections were hotter in the past. A schematic explanation was given by Hu et al. (1998) and the computer program called “Thermodel” is employed in this study. Using the present-day heat flow, the measured vitrinite reflectance data, and the measured thermophysical data as input parameters (Table 4), the thermal history in the Junggar basin has been reconstructed. The results indicate that the paleoheat flow during the Permian and Triassic in the Junggar basin was about 85 mW/m2 and the corresponding paleothermal gradient about 36–47◦ C/km in the Permian. Fig. 4 shows as an example the reconstructed thermal history from vitrinite reflectance data through integration of burial and paleotemperature paths and the kinetic model of organic maturity (Burnham and Sweeney, 1990). The example well is the Aican 1 well (see Fig. 6 for its location) in the Mahu depression
where erosion at the unconformity is not significant that heat flow is the main variable to be adjusted to match the measured and the calculated or predicated vitrinite reflectance (Fig. 4d). The result from the best-fit shows that heat flow gradually decreased from Permian to the present (Fig. 4b). The decrease in heat flow is consistent with the tectonic evolution of the basin. The Junggar basin experienced an extensional stage during the Permian to Triassic and later on a relatively stable development. Thinning of the crust may have caused upwelling of hot mantle during the extensional stage resulting in much higher paleoheat flow than observed today. Drilling has proved that many volcanic activities took place in the Permian deposition center of the basin, such as the Mahu depression. The consequent temperature paths of different formations indicated that the Permian formation reached its maximum paleotemperature (120–180◦ C) at early Jurassic, and that the Jurassic experienced
Fig. 4. The reconstructed burial (a) and thermal history (b, c) as well as the calculated (dots) and the measured (line) vitrinite reflectance (d) for Aican 1 well.
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its maximum temperature (60–90◦ C) at the end of Tertiary (Fig. 4c).
5. Discussion The Junggar basin is characterized by low heat flow (23–54 mW/m2 and a mean of 42 ± 8 mW/m2 , n = 35) similar to the Tarim basin (31–77 mW/m2 and a mean of 44 ± 10 mW/m2 , n = 22) to the south of Tianshan (Wang et al., 1995). It is lower than the average value of 61 ± 16 mW/m2 in the continental area of China (Hu et al., 2000). In terms of the reconstructed paleoheat flow derived from vitrinite reflectance data and the present-day heat flow, a general cooling from the Permian to the present was revealed for the Junggar basin. The Permian high paleoheat flow can be attributed to the rifting in the deep depressions, such as the Mahu and North Tianshan depressions, during the pre-Mesozoic syn-rift phase. The Meso-Cenozoic cooling can be explained by the regional post-rift thermal subsidence during the Mesozoic and the intensified subsidence as a foreland basin during the Cenozoic. The sediments during the syn-rift, post-rift and foreland phases can be well seen in the SSE–NNE and the S–N cross-sections as shown in Figs. 2 and 5. The assumed linear cooling can be considered as a reasonable approximation in view of the tectonic evolution. Comparing with the heat flow and thermal history of the Utah–Wyoming Thrust Belt (Deming and Chapman, 1989), the Junggar basin has little lower present-day heat flow due to the continuously cooling since the Permian and the rapid
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sedimentation during the Cenozoic thrusting along the Tianshan belt. In contrast to the “hot” Cenozoic rifted basins in the East China (e.g. Hu et al., 1999), the Junggar basin is a “cold” basin as a Paleozoic rift basin. The largest petroleum system in the Junggar basin is the Permian petroleum system located in the Mahu and the West Pen 1 Well depressions. The source rocks are Permian mudstones deposited in a lacustrine environment, and the reservoirs are the Permian and the Triassic clastic rocks and the Permian and Carboniferous volcanic rocks near the deposition centers. The overburden is the Triassic formation, and the seal rocks are mudstones of Baijiantan group of the Upper Triassic (a regional seal in the Junggar basin) (Fig. 5). Many oil fields, for example, the largest Kalamay field, belong to this system. Contrast of oil and source rock shows that the oil and gas came from deep buried mature Permian mudstone in the Mahu and West Pen 1 Well depressions, where the organic maturity of the Permian source rock is high as indicated by vitrinite reflectance (1.2–2.1% for borehole samples). K–Ar dating indicates that most reservoirs in the Northwest Thrust Fault belt, located on the West of Mahu depression, formed between the Permian and the Triassic when the Junggar basin experienced a period of higher heat flow (∼85 mW/m2 ). The Permian oil-source rocks (Jiamuhe and Fengcheng groups) reached their maximum paleotemperatures at the Triassic (Fig. 4) and entered the oil or gas generation window due to the high thermal gradient and then cooled down (Fig. 6). It is, therefore, the high paleoheat flow and paleotemperature that played
Fig. 5. Cross-section across the basin from the West to the East at the end of the Triassic.
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Fig. 6. Events chart for the Permian petroleum system.
an important role in the formation of the Permian petroleum system at the end of Triassic in the basin. Another major petroleum system in the Junggar basin is the Jurassic petroleum system. Its source rocks are mudstone and coal of the Jurassic, and the reservoir rocks are siltstone, sandstone, and conglomerate of the Jurassic and the Tertiary. The overburden
rocks consist of the Upper Jurassic, Cretaceous, and Cenozoic formations. Mudstone in the Jurassic Sangonghe group, the Cretaceous Tugulu group, and the Tertiary Anjihaihe group served as the seal rocks of the system. The essential elements of Jurassic petroleum system are presented in Figs. 2 and 7. The cooling of the basin resulted in the lower thermal
Fig. 7. Events chart for the Jurassic petroleum system.
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maturity for most of the Jurassic organic matter (vitrinite reflectance 0.5–0.6%). The Jurassic system is limited in space and only in the southern part of the basin; the Lower Jurassic formation has been buried to 5–7 km depth entering the oil and gas generation window. It provided as a source rock the oil and gas for some oil fields, such as the Cainan, the Qigu oil fields and the Hutubi gas field, along the North Tianshan foreland region. If an exponential decrease in heat flow with time to be assumed instead, the Jurassic system would be more limited as the cooling would be intensified at early Mesozoic time relative to the linear model. Summarizing, the effect of thermal regime on the formation and distribution of petroleum systems has been quite obvious. The early higher paleoheat flow and thermal gradients during Permian and Triassic played a important role for both the formation of the Permian system at the Late Triassic and its wide distribution in the basin. The later lower heat flow and thermal gradients are not in favor of the Lower Jurassic source rocks to get mature. Hydrocarbon generation in the Jurassic system is restricted to the North Tianshan foreland region where it is overlain by huge Late Tertiary sediments. It is the rapid subsidence in Late Tertiary that played a more important role for the formation of the Jurassic system rather than the thermal regime from the basement (lithosphere). 6. Conclusions Based on heat flow measurements, thermal history reconstruction and analysis of relationship between the thermal regime and the petroleum systems, the following conclusions can be made: 1. The Junggar basin is characterized by low presentday heat flow, ranging from 23 to 54 mW/m2 with a mean value of 42 mW/m2 . The heat flow pattern is somewhat higher on the basement uplifts and lower in the depressions. 2. The basin experienced a higher heat flow (∼85 mW/m2 ) period from the Permian to the Triassic and cooled down from the Triassic to present (∼42 mW/m2 ). The high paleoheat flow resulted likely from the rifting process during the pre-Mesozoic. 3. The Permian and the Jurassic oil-source rocks reached their maximum paleotemperatures at the late Triassic and the Tertiary (except for the North
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Tianshan foreland region), respectively. The higher paleothermal regime was a quite important factor on the formation of the Permian petroleum system at the Late Triassic in the Junggar basin. The lower heat flow and thermal gradient were not in favor of the formation of the Jurassic systems and most of the Jurassic source-rocks are immature outside of the North Tianshan foreland region, where heating process was continued to the present due to thick deposition during the Cenozoic. Acknowledgements This research was supported by Grant No. 49672169 from the National Natural Science Foundation of China (NSFC) and a project from China National Petroleum Corporation (CNPC). We appreciate Ilmo T. Kukkonen and two anonymous reviewers for their comments and suggestions. Thanks to S. Hu for his kind help and discussion. References Bodner, D.P., Sharp Jr., J.M., 1988. Temperature variations in South Texas subsurface. AAPG Bull. 72, 21–32. Brigaud, F., Vasseur, G., Caillet, G., 1992. Thermal state in the north Viking graben (North Sea) determined from oil exploration well data. Geophysics 57, 69–88. Burnham, A.K., Sweeney, J.J., 1990. A chemical kinetic model of vitrinite maturation and reflectance. Geochim. Cosmochim. Acta 53, 2649–2657. Carroll, A.R., Lang, Y., Graham, S.A., Xiao, X., Hendrix, M.S., Chu, J., Mcknight, C.L., 1990. Junggar basin, Northwest China, trapped late Paleozoic ocean. Tectonophysicis 181, 1–14. Chiyi, C., 1981. Alluvial-fan corse clastic reservoirs in Kalamay. In: Mason, J.F. (Ed.), Petroleum, Geology in China. Tulsa, Oklahoma, Pennwell, pp. 154–170. Clayton, J.L., Yang, J., King, J.D., Lillis, P.G., Warden, A., 1997. Geochemistry of oils from the Junggar basin, Northwest China. AAPG Bull. 81 (11), 1926–1944. Deming, D., Chapman, D., 1989. Thermal history amd hydrocarbon generation: example from Utah–Wyoming thrust belt. AAPG Bull. 37 (12), 1455–1471. Duddy, I.R., Green, P.F., Hegarty, K.A., Bray, R.J., 1991. Reconstruction of thermal history in basin modelling using apatite fission track analysis: what is really possible? Offshore Australia Conference Proceedings. Melbourne 1, 1149–1161. Funnell, R., Chapman, D., Allis, R., Armstrong, P., 1996. Thermal state of the Taranaki basin, New Zealand. J. Geophys. Res. 101, 25197–25215. Green, P.F., Hegarty, K.A., Duddy, I.R., 1997. Timing as a vital element of the petroleum system: use of thermal history
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