Figure
1.
Sketch map showing the geological setting of Lunnan low uplift.
| Citation: | Xiaomei Wang, Shuichang Zhang. Petroleum Characteristics and Controlling Factors in Lunnan Low Uplift, Tarim Basin. Journal of Earth Science, 2010, 21(2): 236-246. doi: 10.1007/s12583-010-0021-4
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The Lunnan low uplift is one of the highest oil and gas reservoir areas and also the key district of Tarim basin. Petroleum distributed in Ordovician, Carboniferous, Triassic and Jurassic, and petroleum types include normal, heavy and high-wax oils, condensates and gas accumulations (Zhou and Zhang, 2000), reflecting the complex evolution of this region. Although numerous studies have been carried out to interpret this phenomenon in the last two decades, such as evaporation fractionation effect (Ma, 2005), movement fractionation effect and gas invasion effect (Zhang et al., 2000), there is still much ongoing debate about the complex oil and gas distributions (Han et al., 2006; Lu et al., 2004, 2003; Chen et al., 2002; Lu, 2002). In this article we try to analyze this phenomena and its reason on the basis of the systemic statistics of the complex oil and gas distribution characteristics and the fine analysis of oil and gas organic geochemistry.
The Lunnan low uplift is part of the Tabei uplift in the Tarim basin, a complex basin situated in northwestern China and it is one of the largest intermontane basins in the world (Graham et al., 1990) (Fig. 1). The Lunnan low uplift is linked to the Manjiaer depression in the south and joined to the Luntai fault uplift in the north. To the west is the Halahatang depression, and to the east side is the Caohu depression. The extent of the basin reaches 7 200 km2, and the Lunnan and Sangtamu fault-horst belt and the JilakeJiefangqudong anticline occur in this region.
This area has undergone a range of tectonic events, which resulted in multiple stages of hydrocarbon generation, accumulation, destruction and re-migration (Yang et al., 2003; Li et al., 2000; Xiao et al., 2000; Huang et al., 1999). The Lunnan area became an uplift at the end of the Early Ordovician due to the late Caledonian orogeny (Zhang, 2000), and then was transformed into a large-scale nose uplift during the Devonian when it was affected by the early Hercynian orogeny. During Carboniferous, northward and northwestward overlapping depositions of sediments led to interbedded sandstones, shales and limestones. The Lunnan and Sangtamu fault-horst belt formed due to Permian east-west faulting, caused by the late Hercynian orogeny. There was marine/ terrestrial transitional development during the Carboniferous and Permian and some small-scale fault activity during the Late Cretaceous. The basement of the basin tilted from south to north and formed the present-day uplifted structure during the Cenozoic (Lu et al., 2004; He et al., 2002).
The Lunnan low uplift is one of the earliest places where petroleum exploration was obtained in the Tarim basin, and it has the most complicated distribution of petroleum phase states. There are heavy oils, common black oils, volatilization oils, condensate oils and natural gas, which are under approximately the same stratum pressure grads and geothermal grads. The heavy oil is mainly distributed in the east of Lunguxi fault, the Sangtamu fault-horst belt and its south belt have volatilization oil, condensate oil, normal oil and natural gas, the Lunnan fault-horst belt and the middle slope is mainly normal oil and high wax oil, and Lungudong slope is dominated by condensate oil and natural gas (Fig. 1).
The original mechanism of various distributions of petroleum phase state has been studied by many researchers from different aspects (Li et al., 2005; Wei et al., 2004; Zhang, 2004; Wang et al., 2002; Hong, 2001; Qiu et al., 1998; Han et al., 1997). Because the Lunnan oil field has multi-hydrocarbon source kitchens, multiple stages of accumulation and several structure movement reconstructions, the explanation will not be symbolic with the actual geological situation, if the distribution of petroleum phase state is explained by single method or aspect. Therefore, the conclusion will be more close to the actual condition if many factors are considered.
The distribution of crude oil is positively complex from the crude oil characteristics of Lunnan low uplift (Fig. 2). The density of crude oil changes from 0.82 to 1.04 g/cm3, the wax value changes from 2 to 25, the colloid and asphaltum value is from 0 to 30 and the sulphur content is from 0 to 3.5. Laterally, the east area such as the Lunnan fault-horst belt (LN), the middle slope (ML), the Sangtamu fault-horst belt (STM) and its south slope have a low oil density, almost lower than or equal to 0.85 g/cm3, while the west area such as Lunguxi (LGX) and Tahe (TH) have high oil density up to 1.05 g/cm3. The colloid and asphaltum value and the sulphur content have a similar change tendency with oil density, while the wax value shows almost contrary change tendency.
Vertically, the oil density of Lungudong slope and Sangtamu fault-horst belt in Ordovician is lighter than Carboniferous and Triassic, which means the oil density gets heavier from lower to upper strata, while the oil density of Tahe in Ordovician is lighter than that in Carboniferous and Triassic (Fig. 3). The colloid and asphalt content has similar changing trend with oil density, while the wax content shows opposite trend, which means the wax content decreased from lower to upper sections.
Natural gas is mainly distributed in Lungudong (LGD), Sangtamu fault-horst belt, east of Lunnan fault-horst belt (LN) such as Well Lunnan10, middle slope (MS) such as wells Lunnan18 and Lunnan30 (Fig. 4), and it is all dry gas, with the dry coefficient being over 0.95 (C1/C2+). Especially, the dry coefficients of Lungudong, east Sangtamu fault-horst belt and middle slope are over 0.98, and overall, the dry coefficient in east Lunnan low uplift is high, and west is low. Tahe oil field (TH), west of Lunnan low uplift, has a low dry coefficient with an average of 0.85, a kind of typical wet natural gas. The content of CO2 is low in Lungudong, Sangtamu and Lunnan fault-horst belt, middle slope and high in Lunguxi. The content of N2 is low at Lungudsong and Sangtamu fault-horst belt, while other areas have a high content of N2.
The natural gas characteristics in Carboniferous is very similar to that of Ordovician, but Triassic is different from that of Ordovician, usually less than 0.95, belonging to wet gas. The content of CO2 in Triassic is low in Lungudong and Sangtamu fault-horst belt, and high in Lunnan fault-horst belt and Tahe, but lower than those of Ordovician and Carboniferous. The content of N2 in Triassic is high in Lungudong compared with the overlying Ordovician and Carboniferous, and increase from east to west (Fig. 5).
The gas/oil ratio is at the range from 10 000 to 20 000 m3/m3 in Lungudong at Ordovician, and drops on average to 5 000 m3/m3 in the Sangtamu fault-horst belt (STM) (Fig. 6). However, the gas/oil ratio is high at the east of Lunnan low uplift, such as east Sangtamu and east Lunnan fault-horst belt (LND) and the middle slope, especially, well Lunnan10 and well Lunnan30, still as high as over 10 000 m3/m3. In the west of Lunnan low uplift, such as Lunguxi, the gas/oil ratio dropped significantly to just 25.92 m3/m3. The Tahe oil field also has higher gas and oil ratio in the east (well Tahe3 500–6 000 m3/m3) than in the west (well Tahe4, low as just 13–27 m3/m3). The gas/oil ratio in Carboniferous is very similar to that of Ordovician, while it is greatly decreased in Triassic, usually below 1 000 m3/m3, and mostly it is just about 100 m3/m3.
The great differences of the petroleum characteristics between the west and east of Lunnan low uplift are not only lateral, but also vertical. In this article, we discuss the differences from the aspect of petroleum geochemistry.
Usually, most heavy oils are formed from biodegradation and the geochemical evidence includes increased oil gravity, decreased ratio of saturated hydrocarbon and aromatic hydrocarbon, increased colloid and asphaltum value and the existence of 25-norhopanes. The oil gravity in west Lunnan low uplift is very high, such as the Lunguxi slope, the Tahe oil field 4 and the block of the well Lunnan1, usually over 0.95 g/cm3. A typical heavy oil, and its colloid and asphaltum value is higher than other areas in Lunnan low uplift, and also the ratio of saturated hydrocarbon to aromatic hydrocarbon is the lowest (Fig. 8). The baseline of saturated hydrocarbon has some drifts, usually called bulge (Fig. 9), which shows that there occurred crude oil biodegradation (Wei et al., 2004). There are also abundant 25-norhopanes in Lunnan slope and Tahe oil field (Fig. 10), not only in the light oil well TK305, but in the viscous oil well TK402. The existence of relative whole normal paraffin hydrocarbons together with the chromatogram baseline bulge show the characteristics of multi-stage intermingling of crude oil. The baseline bulge shows the characteristics of early oil charging and the whole normal paraffin hydrocarbons indicating the late oil filling up, which shows that the early oil is damaged and late oil is filling up again in the Lunnan low uplift.
It is still very difficult for the quantification of biodegradation, not only because of multi-stage charging, the different mixing ratios of biodegradation oil and non-biodegradation oil, but also the strong selectivity of biodegradation due to the temperature-pressure condition, nutrient delivery and oxidation-reduction condition. From the petroleum feature of the Lunnan low uplift, it is deduced that Lunguxi slope first experienced biodegradation and the modern industrial heavy oil is the mixed product of early biodegradation oil and late non-biodegradation oil.
Light oil (condensate oil) and normal crude oil coexist in Lungudong slope, which may be the result of the mixing of two stages or several stages of oil charge, because the condensate oil has the typical characteristics as follows. (1) Natural gas has dry coefficient, which is greatly different from the characteristics of the original single condensate gas reservoir, which should be a kind of wet gas. (2) Condensate gas mainly exists in the traps relevant with fault-horst belt, that is relevant with fault, which maybe caused by the invasion of late natural gas transporting along a fault or erosion surface at late Himalayas stage. (3) According to the sapropel type parent material generating scheme, the condensate gas should coexist with light oil, not normal crude oil, the features of the petroleum in the Lunnan low uplift, so the condensate gas in the Lunnan low uplift is mixed by oil and gas. (4) The existence of waxy oil besides the condensate gas reservoir and large quantities of asphaltum are also proofs of mixing of crude oil and natural gas. Thus, the condensate gas reservoir is the mixing result of crude oil and natural gas.
One special characteristic is that condensate oil is very heavy, and the reason should be that the gas reservoir is formed on the basis of heavy oil reservoirs, and although the early oil is dissolved in natural gas, the heavy components still exist, and after the condensate gas is extracted, the previous condensate oil returns to its earlier component condition, so the heavy condensate oil exists.
Because the dry coefficient decreased from the lower to upper sections, and the dry coefficient in Ordovician and Carboniferous is higher than that in Triassic, it is deduced that the natural gas migrated from the lower to upper strata. Also, the high dry coefficient in the east is caused by the gas invasion effect. The phenomenon of the gas invasion effect only happens in a special geographical position. The Lungudong slope is adjacent to the Manjiaer depression and the Caohu depresssion hydrocarbon area, so it is a favorable position for oil and gas accumulation, and it has a sufficient mother source for oil and gas charging, dissolving and accumulation. The new born oil and gas dissolve and accumulate, and at the same time migrate towards northeast direction. It is very light and the dissolved velocity is greater than dissipation velocity, then the excessive gas and oil reverse to dissolve to the previous gas to form condensate gas accumulation. Therefore, the Lungudong Ordovician petroleum reservoir is controlled by later accumulated dynamic petroleum reservoir. With the increasing gas charging, the strata pressure gets higher and higher, the gas phase components separate along the unlocked fault channel to form condensate gas reservoir (Zhang et al., 2000). Thus, the petroleum shows regular characteristics from the east to the west, the density, wax value, the colloid and asphalt content increase from the Lungudong slope to the Lunnan fault-horse belt. The mixing effect and gas invasion effect also induced the high gas/oil ratio in the east Lunnan low uplift and a low one in the west Lunnan low uplift.
The Lunnan fault-horst belt has an abundant petroleum supplement of Ordovician hydrocarbon, while it has poor conservation condition at the early time, so only gas condensate reservoir is formed in this area. The Ordovician crude oil in the Sangtamu fault-horst belt has high content of saturated hydrocarbon, nonhydrocarbon and asphaltine. All of the above discussion means that the petroleum in Lunnan low uplift has been destroyed previously and later experienced oil and gas filling up, that is the gas poured into the previously formed oil to carry out gas intrusion effect, so the top gas condensate reservoir and oil control ring gas condensate reservoir are formed.
There is an abundant amount of 25-norhopanes, a special biomarker of biodegradation, and the saturated hydrocarbon has a bulge, which is also a typical characteristic of biodegradation. However, the existence of comparatively whole normal paraffin shows the characteristics of later crude oil. Also, from the burial history, we can also find the crude oil in Caledonion, when the reservoir depth was less than 2 000 m, and reservoir temperature was less than 70 ℃, so the crude oil at that time extensively suffered biodegradation to form viscous oil. Later, the late Hercynian viscous oil filled into the previous viscous oil to form the modern large quantities of flowing payable heavy oil. It is deduced that the now found heavy oil is the mixture of early biodegradation oil and later normal oil.
The previous study shows that the content of 25-norhopanes is different after the different ratios of heavy oil and normal oil mixed together, so the mixing ratio of heavy oil and normal oil can be calculated according to the content of 25-norhopanes (Fig. 12) (Yang et al., 2003). Because LG9 represents the early oil filling-up, which is very heavy, rich nonhydrocarbon and bitumen, high content of 25-norhopanes and triaromatic steroid, while HD401 and TK303 show the characteristics of late oil fillingup, light, no 25-norhopanes, low content of triaromatic steroid, so the experiment calculated the contribution of the two stages of crude oil filling-up by LG9 and TK303 by the content of 25-norhopanes, and then set up the mixing ratio plate.
The recognized characteristics of organic origin gas is δ13C1 < δ13C2 < δ13C3 < δ13C4 (Xia et al., 1998), while it is inorganic gas when δ13C1 > δ13C2 > δ13C3 > δ13C4 (Nivin et al., 2005; Potter et al., 2004). However, the carbon isotope of methane and its homologous compound have inversion in some organic origin natural gas and this phenomena is very common in our sedimentary basins (Dai et al., 2004), such as well Ma 8 in Hetianhe in the Tarim basin (1 790–1 800 m, O1). The carbon isotope is respectively -34.39‰, -37.96‰, -35.52‰, and -31.34‰, that is δ13C1 > δ13C2 < δ13C3 < δ13C4, which is believed to be caused by the fractionation effect in the process of gas dissolution and the different solubilities of natural gas. The carbon isotope in Ordovician in Lunnan low uplift has some inversion (Fig. 13), and according to the study of Dai et al. (2004, 2001), the carbon isotope of natural gas is caused by the following reasons: (1) the mixing of organic hydrocarbon gas and inorganic hydrocarbon gas; (2) the mixing of coal-derived gas and petroliferous gas; (3) the mixing of the different sources and the same source with different charging stage natural gas; (4) some or certain components are oxidized by bacteria. According to the characteristics of the natural gas, the carbon isotope inversion in Ordovician in Lunnan low uplift is caused by the mixing of the natural gas from the same source and different stage natural gas, that is the result of multistage reservoir formation, while there is no carbon isotope inversion in Triassic in Lunnan low uplift, so the natural gas in Triassic has no mixing phenomenon and it is a one stage reservoir formation.
Along with the rising depth and temperature, the crude oil is cracked into gas, and because the oil and heavy hydrocarbon is cracked into smaller molecules to form gas hydrocarbon and solid bitumen at the temperature of 150–200, which is an important ℃ reason for the formation of the deep stratum gas. The natural gas in Lungudong might come from crude oil cracking gas from the proof of reservoir bitumen, biomarkers such as adamantane and sterane/terpane and stratum burial history.
(1) There are multi-production zones and reservoir types in the Lunnan low uplift. It is a typical duplex type petroleum accumulation area and its east part is mainly composed of condensate oil and natural gas, the middle part is multi-petroleum and the west part is heavy oil, that is, the crude oil density is increased from east to west. Natural gas is mostly distributed in the east area, and the natural gas in Ordovician and Carboniferous is dry, while in Triassic it is wet. The oil/gas ratio in the east is high and low in the west.
(2) The petroleum geochemistry is the principal reason for the great difference of oil and gas characteristics. The large quantities of productive heavy oil in the west part are established by the biodegradation and mixing effect. The mixing effect in the middle part produced the complex petroleum distribution. The oil cracking effect and mixing effect, or air cutting effect in the east part is the proof of the existence of the industrial condensate gas reservoir and waxy oil.
(3) Knowledge of the complex petroleum reservoir can further supervise the development of petroleum exploration, which is to seek natural gas in the east and heavy oil in the west. And the wells Lundong 1 and Aiding 4 have already confirmed the correctness of the exploration guidance.
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Xiaomei Wang, Shuichang Zhang. Petroleum Characteristics and Controlling Factors in Lunnan Low Uplift, Tarim Basin. Journal of Earth Science, 2010, 21(2): 236-246. doi: 10.1007/s12583-010-0021-4
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