ARTICLE
Structural factors controlling gas invasion as revealed by physical and geochemical indicators: A case study of Tazhong area, Tarim Basin
2025 Volume 59 Issue 3 Pages 118-128
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2025 Volume 59 Issue 3 Pages 118-128
Significant progress has been made in oil and gas exploration in the Tarim Basin, particularly in the Tazhong area. Exploration practices have shown that gas invasion influences the distribution of petroleum resources. This study uses geochemical analysis methods to investigate the distribution of physical properties, biomarkers, light hydrocarbon compositions, and carbon isotopic ratios in oil and gas reservoirs across different well blocks. The results reveal strong gas invasion in the crude oil in the TZ83 wellblock, while the ZG43 well block experiences weaker gas invasion. The study suggests that the intensity of gas invasion is influenced by the structural positions of the well blocks. The TZ83 well block, located in the high-position structural zone at the intersection of the Tazhong No.I fault slope fold belt and the strike-slip fault, is strongly affected by gas from underlying sources, resulting in condensate oil. In contrast, the ZG43 well block, situated on the platform zone of the Tazhong No.10 fault belt, has fewer deep and large fault systems, leading to weaker gas invasion and waxy oil formation. Additionally, reservoirs with shallow structural positions may contain undiscovered condensate oil accumulations. Future exploration should focus on areas near fault zones to enhance energy reserves.
The continuous process of natural gas filling a reservoir is referred to as gas invasion, which typically alters the composition of crude oil (Dzou and Hughes, 1993; Lomando, 1992; Meulbroek et al., 1998; Thompson et al., 1990). The gas phase flowing in the gas invasion will carry a large amount of light components to form condensate reservoirs in the shallow layer (Curiale and Bromley, 1996; Thompson, 1987). Gas invasion is widespread globally, such as Suez Basin (Khavari-Khorasani et al., 1998), Poland Carpathians Basin (Matyasik et al., 2000), Indonesia Java Basin (Napitupulu et al., 2000), Mexico Basin (Thompson et al., 1990), Qaidam Basin (Zhu et al., 2003), Bohai Bay Basin (Hu et al., 2022), Pearl River Mouth Basin (Chen et al., 2015) and Tarim Basin (Li et al., 2010a; Su et al., 2000; Su et al., 2004). Previous research has established a scientific basis for understanding gas invasion. Thompson (1987) proposed the concept of gas invasion fractionation, noting that gas invasion causes fractionation of light components, such as normal heptane, methylcyclohexane, and dimethylcyclohexane in the reservoir. Kissin (1987) observed that for the crude oil without any fractionation, there is a linear relationship between the logarithmic molar concentration of n-alkanes and their carbon number. Meulbroek (1998) and Losh et al. (2002) used parameters such as slope factor and breakpoint carbon number to identify and evaluate the gas invasion in reservoirs. In addition, Losh et al. (2002) quantitatively evaluated the intensity of gas invasion based on the loss rate of n-alkanes in crude oil. And some researchers suggest that light hydrocarbons and adamantane in crude oil can serve as tracers for late-stage gas invasion (Chakhmakhchev et al., 2017; Kong et al., 2021; Moldowan et al., 2015).
The origin of crude oil diversity in the Tarim Basin is complex and research on this area remains insufficient. However, gas invasion fractionation is widely recognized as a key factor in the adjustment and transformation of oil and gas reservoirs in this region (Yang and Zhu, 2013; Zhang, 2000; Zhang et al., 2021; Li et al., 2022a). Ma and Fan (1995) analyzed the light hydrocarbon characteristics of crude oils in the Lunnan oilfield, and revealed the fractionation in the process of oil and gas reservoir formation. Zhang (2000) confirmed the gas invasion fractionation process by examining the changes in alkane characteristics in crude oils from Lunnan oilfield. Huang et al. (2010) investigated gas invasion in the Tabei area by analyzing the distribution of n-alkane molar concentrations in crude oil. Zhu et al. (2019) demonstrated the transformation mechanism and process of marine oil and gas subjected to secondary geochemical processes such as thermal cracking and gas invasion fractionation, by comparing the geochemical characteristics of different phase types of oil and gas in the Tarim Basin.
Current research on gas invasion in the Tarim Basin mainly provides qualitative descriptions, with limited focus on the factors controlling variations in gas invasion intensity. This study aims to examine the distribution characteristics of physical parameters and biomarkers in the Tazhong area, compare gas invasion intensities across typical reservoirs, and explore the structural factors influencing gas invasion, ultimately providing scientific guidance for condensate oil exploration in the Tazhong area of the Tarim Basin.
The Tarim Basin is a large superimposed basin with an area of 56 × 104 km2, showing a structural pattern of three uplifts and four depressions, with a maximum thickness of more than 10 km (Wang, 2023; Wang et al., 2024; Zhao et al., 2023). The study area is located in the Tazhong uplift belt of the basin, as shown in Fig. 1. The uplift belt was formed during the late Caledonian period, finalized in the Hercynian period, and weakly reworked in the Yanshan-Himalayan period. It displays characteristics of strong early tectonic activity, multiple associated faults, and stable late tectonic activity, primarily marked by overall uplift. From north to south, it is divided into three secondary tectonic units: Tazhong north slope, Tazhong low uplift, and Tazhong south slope (Li et al., 2020; Zhang et al., 2015). The TZ83 well block is situated in the southern part of the Tazhong north slope and the middle of the Tazhong No.I fault zone, while the ZG43 well block is located in the central-western part of the Tazhong north slope, in the middle section of the Tazhong No.10 fault zone. The study of source-reservoir-cap assemblage shows that the hydrocarbon source of Ordovician oil and gas reservoirs mainly originates from the shale of the Lower Cambrian Yuertusi Formation. The primary reservoirs are the Yingshan Formation and Yijianfang Formation of the Middle-Lower Ordovician, as well as the Lianglitage Formation of the Upper Ordovician, with the cap being the Sangtamu Formation of the Upper Ordovician (Guo et al., 2023; Li et al., 2023).
Structural position (a–b) and stratigraphic column (c) of the study area
The oil samples in this study were collected from the Lianglitage Formation of the Upper Ordovician in the Tarim Basin. The ZG43 wellblock includes ZG43, ZG431, ZG44, ZG441, ZG45, ZG46, ZG461 and ZG462. Besides, TZ62-3, TZ82, TZ83, TZ72, TZ721, TZ722, TZ724 and TZ73 belong to the TZ83 wellblock. The oil samples were precipitated with n-hexane and filtered, followed by fractionation into compound groups using a packed solid-phase chromatography column (silica gel : alumina = 2:3). Sequentially, n-hexane, a mixed solvent of n-hexane and dichloromethane (n-hexane : dichloromethane = 7:3), and a mixed solvent of dichloromethane and methanol (dichloromethane : methanol = 95:5) were added for elution, yielding saturated hydrocarbons, aromatic hydrocarbons, and non-hydrocarbon fractions.
Density analysisDensity analysis was performed according to the Chinese Petroleum Industry Standard (SY/T 5154-2012) using the U-tube vibration method, the main instrument is SYP-0604A U-tube vibration densimeter from Shanghai Mitong Electromechanical Technology Co., Ltd. The specific procedure is as follows: turn on the instrument to preheat for 30 min, then slowly inject the oil into the test tube, measure for several times until the reading is stable, record the data and take the average value.
Viscosity analysisViscosity analysis was performed by rotary viscosity balance method according to the Chinese Petroleum Industry Standard (SY/T 0520-2008), the main instrument is NDJ-1F Brinell rotational viscometer from Shanghai Changji Geological Instrument Co., Ltd. The specific procedure is as follows: turn on the power supply, the air compressor, and the water bath. When the pressure rises to 5 bar, turn on the RHEOPLUS software, input the required test temperature, and then add the sample to the scale line for testing.
Wax content analysisThe wax content analysis was performed by DSC204F1 differential scanning calorimeter from NETZSCH according to the Chinese Petroleum Industry Standard (GB/T 26982-2011). The specific procedure is as follows: turn on the instrument power, air source, and compressor power, then preheat the instrument for 30 min. Load the sample into the DSC spool, open the measurement software, set the purge gas and protection gas flow rate, and then run the automatic measurement program until completion.
Gas chromatography analysisThe light hydrocarbons were analyzed by Agilent 7890B GC System from Agilent Technologies, Inc. according to the Chinese Petroleum Industry Standard (GB/T 13610-2014). The column was HP-5MS (30 m × 0.25 mm × 0.25 μm). The heating procedure was as follows: held at 30°C for 15 minutes, then heated to 300°C at 4°C/min for 50 minutes. Helium was used as the carrier gas with a flow rate of 1.0 mL/min. The standard sample for quantitative compound concentration is 1-hexene.
Gas chromatography-mass spectrometry analysisThe saturated hydrocarbons and aromatic hydrocarbons were analyzed by GC-MS with Agilent 7890/5975 GC-MS System from Agilent Technologies, Inc. according to the Chinese Petroleum Industry Standard (GB/T 18340-2010). The column was HP-5MS (30 m × 0.25 mm × 0.25 μm). The heating procedure was as follows: the temperature was held at 50°C for 5 minutes, then increased to 250°C at 2.5°C/min, and further raised to 290°C at 2°C/min for 15 minutes. The total experimental time was 120 min.
Quantitative analysis of n-alkanesAs described by Kissin (1987) and Losh et al. (2002), the quantitative analysis of n-alkanes briefly determined the relationship between molar concentration and carbon number, with the formula expressed as follows:
where Mc(n) is the molar concentration of n-alkanes, n is the carbon number of n-alkanes, a is the slope, and A is the normalization factor.
Secondly, Q was used as the standard to evaluate the intensity of gas invasion. The formula is expressed as follows:
where Q is the loss of n-alkanes after gas invasion, and MnCi is the mass percentage of n-alkanes with a carbon number of n.
Gas chromatography-isotope ratio mass spectrometry analysisThe whole oil carbon isotope analysis was performed by GC-IRMS with Agilent HP6890-Isoprime gas chromatography-stable isotope mass spectrometry from Agilent Technologies, Inc. according to the Chinese Petroleum Industry Standard (SY/T 5238-2019). The column was HP-5MS (30 m × 0.25 mm × 0.25 μm). The heating procedure was as follows: the temperature was held at 80°C for 15 minutes, then increased to 130°C at 20°C/min, followed by heating to 310°C, with a stabilization time of 15 minutes.
Quantitative analysis of adamantaneThe adamantane component was analyzed by Shimadzu GC-MS-QP2010 PLUS desktop mass spectrometer according to the Chinese Petroleum Industry Standard (SY/T 7470-2020). The chromatographic column was HP-5ms quartz elastic column (30 m × 0.2 mm × 0.25 μm). The heating procedure was as follows: an initial stabilization at 80°C for 1 minute, then heating at 20°C/min from 80°C to 100°C and at 3°C/min from 100°C to 310°C, followed by a 15-minute hold at 310°C. The injector temperature was 300°C, and the carrier gas was helium with a flow rate of 1.04 mL/min. The standard sample for quantitative compound concentration was deuterated adamantane (C10D16).
The distribution of oil physical parameters in the study area has certain regularity, especially in the horizontal plane. As shown in Fig. 2 and Table 1, density, viscosity, wax content, and gas-oil ratio exhibit clear zonal characteristics, with notable differences between TZ83 wellblock and ZG43 wellblock. In the TZ83 well block, the density, viscosity, wax content, and gas-oil ratio of crude oil range from 0.7968 to 0.8113 g/cm3, 2.28 to 3.96 Pa·s, 8.56 to 12.41% and 3912 to 9977 m3/m3, respectively. The average values are 0.8052 g/cm3, 3.20 Pa·s, 11.15%, and 5593 m3/m3. Overall, it has the characteristics of low density, high viscosity, high wax content and high gas-oil ratio. In the ZG43 wellblock, the density ranges from 0.8052 to 0.8540 g/cm3, with an average of 0.8297 g/cm3. The viscosity ranges from 1.02 to 1.88 Pa·s, with an average of 1.52 Pa·s. The wax content ranges from 6.40% to 8.11%, with an average of 7.33%. The gas-oil ratio ranges from 759 to 1628 m3/m3, with an average of 1095 m3/m3.
Cross plot of density versus gas-oil ratio (a) and wax content versus gas-oil ratio (b) in different wellblocks of Tazhong area
Geochemical characteristics of oils in different wellblocks of the Tarim Basin
| Area | Well | Depth (m) | Density (g/cm3) | Viscosity (Pa·s) | Wax content (%) | Gas/oil Ratio (m3/m3) | δ13C (‰) | Ts/(Ts + Tm) | C27dia/C27reg | MPI-1 | nC7/MCC6 | Tol/nC7 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TZ83 | TZ62-3 | 4915–4925 | 0.8230 | 3.57 | 8.56 | 3912 | –30.1 | 0.77 | 0.71 | 1.41 | 2.05 | 0.27 |
| TZ82 | 5666–5684 | 0.7968 | 3.39 | 9.78 | 5487 | –30.9 | 0.82 | 0.81 | 1.36 | 2.07 | 0.33 | |
| TZ83 | 5433–5441 | 0.8020 | 3.96 | 12.37 | 6502 | –30.3 | 0.79 | 0.68 | 1.34 | 2.14 | 0.23 | |
| TZ72 | 5325–5330 | 0.8011 | 2.85 | 11.84 | 9977 | –31.1 | 0.71 | 0.69 | 1.31 | 2.21 | 0.41 | |
| TZ721 | 5355–5405 | 0.8024 | 2.28 | 12.06 | 4827 | –30.4 | 0.77 | 0.72 | 1.33 | 2.19 | 0.29 | |
| TZ722 | 5375–5395 | 0.8019 | 3.01 | 11.77 | 4477 | –30.5 | 0.74 | 0.77 | 1.39 | 2.17 | 0.31 | |
| TZ724 | 5301–5321 | 0.8113 | 3.49 | 10.39 | 4591 | –31.0 | 0.81 | 0.75 | 1.35 | 2.11 | 0.30 | |
| TZ73 | 5295–5305 | 0.8031 | 3.03 | 12.41 | 4974 | –30.7 | 0.85 | 0.83 | 1.37 | 2.03 | 0.35 | |
| ZG43 | ZG43 | 4357–4374 | 0.8052 | 1.02 | 7.19 | 1628 | –30.7 | 0.71 | 0.49 | 1.29 | 2.99 | 0.14 |
| ZG431 | 4412–4433 | 0.8110 | 1.64 | 6.40 | 1021 | –31.7 | 0.70 | 0.58 | 1.27 | 2.91 | 0.19 | |
| ZG44 | 4432–4446 | 0.8144 | 1.10 | 6.55 | 1325 | –31.5 | 0.67 | 0.44 | 1.21 | 2.94 | 0.13 | |
| ZG441 | 4457–4471 | 0.8201 | 1.21 | 7.18 | 1411 | –31.2 | 0.62 | 0.51 | 1.19 | 2.91 | 0.11 | |
| ZG45 | 4327–4344 | 0.8330 | 1.81 | 7.40 | 759 | –31.6 | 0.65 | 0.57 | 1.31 | 3.07 | 0.15 | |
| ZG46 | 4367–4381 | 0.8540 | 1.79 | 7.75 | 827 | –31.3 | 0.67 | 0.47 | 1.29 | 3.19 | 0.11 | |
| ZG461 | 4388–4392 | 0.8491 | 1.88 | 8.11 | 913 | –31.7 | 0.65 | 0.46 | 1.27 | 3.11 | 0.15 | |
| ZG462 | 4402–4421 | 0.8504 | 1.74 | 8.03 | 875 | –31.5 | 0.61 | 0.50 | 1.22 | 3.00 | 0.13 |
Note: Ts/(Ts + Tm): 18–22, 29, 30-trisnorneohopane/(18–22, 29, 30-trisnorneohopane + 17–22, 29, 30-trisnorhopane), C27dia/C27reg: C27 diasteranes/C27 regular steranes, MPI-1 = 1.5 × (3-Methyl phenanthrene + 2-Methyl phenanthrene)/(Phenanthrene + 9-Methyl phenanthrene + 1-Methyl phenanthrene), nC7/MCC6: normal heptane/methylcyclohexane, Tol/nC7: toluene/normal heptane.
Gas chromatography experiments revealed abundant amounts of n-heptane (nC7), toluene (Tol), and methylcyclohexane (MCC6). In the TZ83 wellblock, the nC7/MCC6 and Tol/nC7 ratios range from 2.03 to 2.21 and 0.23 to 0.41, respectively. In contrast, the nC7/MCC6 and Tol/nC7 ratios in the ZG43 wellblock are distinctly different, ranging from 2.91 to 3.19 and 0.11 to 0.19, with average values of 3.02 and 0.14, respectively, indicating significant variation (Fig. 3).
Cross plot of nC7/MCC6 versus Tol/nC7 (a) and Tol/nC7 versus δ13C in different wellblocks of Tazhong area (b)
The Ts/(Ts + Tm), C27dia/C27reg, and MPI-1 of crude oil in the TZ83 wellblock range from 0.71 to 0.85, 0.68 to 0.83, and 1.31 to 1.41, respectively. In the ZG43 wellblock, the Ts/(Ts + Tm) value ranges from 0.61 to 0.71, with an average of 0.66. The C27dia/C27reg ranges from 0.44 to 0.58, with an average of 0.50. The MPI-1 ranges from 1.19 to 1.31, with an average of 1.26. According to the thermal evolution equilibrium, both ZG43 and TZ83 wellbocks are in the mature stage, with the crude oil maturity in TZ83 wellbock is higher than that in ZG43 wellbock (Fig. 4).
Cross plot of C27dia/C27reg versus Ts/(Ts + Tm) (a) and MPI-1 versus δ13C in different wellblocks of Tazhong area (b)
The distribution of n-alkanes in crude oil from the ZG43 wellblock is relatively complete, suggesting it has undergone minimal gas invasion, as shown in Fig. 5. In contrast, the crude oil in the TZ83 wellblock has experienced significant gas invasion.
Cross plot of carbon number versus the log of mole fraction for oils in different wellblocks of Tazhong area
Quantitative calculations of gas invasion (Table 2) show that the average Q value for crude oil in the TZ83 wellblock is 73.0%, with the carbon number breakpoint concentrated between 24–26. The average Q value for the ZG43 wellblock is only 51.6%, with the carbon number breakpoint concentrated between 16–21 (Fig. 5).
Calculation results of loss of n-alkanes of oils in the Tarim Basin
| Area | Well | Depth (m) | Loss of n-alkanes (Q/%) | Break point of carbon number |
|---|---|---|---|---|
| TZ83 | TZ62-3 | 4915–4925 | 75.1 | 25 |
| TZ82 | 5666–5684 | 75.7 | 26 | |
| TZ83 | 5433–5441 | 82.1 | 26 | |
| TZ72 | 5325–5330 | 68.9 | 24 | |
| TZ721 | 5355–5405 | 71.2 | 25 | |
| TZ722 | 5375–5395 | 69.4 | 26 | |
| TZ724 | 5301–5321 | 71.3 | 24 | |
| TZ73 | 5295–5305 | 70.7 | 25 | |
| ZG43 | ZG43 | 4357–4374 | 53.7 | 20 |
| ZG431 | 4412–4433 | 50.4 | 19 | |
| ZG44 | 4432–4446 | 49.3 | 17 | |
| ZG441 | 4457–4471 | 49.1 | 18 | |
| ZG45 | 4327–4344 | 55.7 | 19 | |
| ZG46 | 4367–4381 | 57.1 | 21 | |
| ZG461 | 4388–4392 | 48.6 | 16 | |
| ZG462 | 4402–4421 | 48.5 | 17 |
As shown in Table 3, there is a significant difference in the total amount of adamantane compounds and related parameters among the distinct wellblocks. In the TZ83 wellblock, the ratios of 1-MA + 1,3-MDA, 4-MD + 3-MD, As, Ds, and As + Ds are relatively high, in the range of 863.95–1245.94 × 10–6, 74.29–198.91 × 10–6, 2615.36–4630.94 × 10–6, 260.23–586.85 × 10–6, and 2970.41–4997.56 × 10–6, with average values of 1028.72 × 10–6, 105.94 × 10–6, 3103.46 × 10–6, 360.42 × 10–6, and 3530.59 × 10–6, respectively. The ratios of 1-MA + 1,3-MDA, 4-MD + 3-MD, As, Ds and As + Ds in ZG43 wellbolck are 181.82–604.41 × 10–6, 6.98–41.16 × 10–6, 494.51–1677.43 × 10–6, 15.96–65.01 × 10–6 and 510.47–1732.78 × 10–6, respectively. The average values are 362.85 × 10–6, 25.91 × 10–6, 962.75 × 10–6, 42.92 × 10–6 and 1058.41 × 10–6, respectively (Fig. 6).
Adamantane content and parameters of oils in the Tarim Basin
| Area | Well | Depth (m) | (1-MA + 1,3DMA)/10–6 | (4-MD + 3-MD)/10–6 | As/10–6 | Ds/10–6 | (As + Ds)/10–6 |
|---|---|---|---|---|---|---|---|
| TZ83 | TZ62-3 | 4915–4925 | 917.54 | 88.21 | 2774.64 | 264.79 | 3011.21 |
| TZ82 | 5666–5684 | 965.63 | 84.84 | 2710.19 | 260.23 | 2970.41 | |
| TZ83 | 5433–5441 | 1245.94 | 104.02 | 4630.94 | 366.62 | 4997.56 | |
| TZ72 | 5325–5330 | 863.95 | 74.29 | 2862.51 | 230.59 | 3093.10 | |
| TZ721 | 5355–5405 | 1187.66 | 198.91 | 3472.99 | 586.85 | 4059.83 | |
| TZ722 | 5375–5395 | 1025.63 | 90.16 | 2615.36 | 443.25 | 3125.64 | |
| TZ724 | 5301–5321 | 998.26 | 101.35 | 2634.54 | 333.54 | 3111.28 | |
| TZ73 | 5295–5305 | 1025.21 | 105.77 | 3126.47 | 397.45 | 3875.69 | |
| ZG43 | ZG43 | 4357–4374 | 181.82 | 6.98 | 494.51 | 15.96 | 510.47 |
| ZG431 | 4412–4433 | 322.51 | 10.55 | 724.00 | 23.05 | 747.04 | |
| ZG44 | 4432–4446 | 312.45 | 22.59 | 884.25 | 33.11 | 864.15 | |
| ZG441 | 4457–4471 | 304.11 | 31.15 | 714.11 | 35.47 | 1011.23 | |
| ZG45 | 4327–4344 | 359.13 | 21.44 | 1073.88 | 64.43 | 1138.31 | |
| ZG46 | 4367–4381 | 604.41 | 41.16 | 1677.43 | 55.36 | 1732.78 | |
| ZG461 | 4388–4392 | 412.36 | 33.95 | 1021.36 | 50.97 | 1114.67 | |
| ZG462 | 4402–4421 | 405.97 | 39.48 | 1112.44 | 65.01 | 1348.57 |
Note: (1-MA + 1,3DMA): 1-methyladamantane + 1,3-dimethylmonadamantane, (4-MD + 3-MD): 4-methyldiadamantane + 3-methyldiadamantane, As: adamantane, Ds: diadamantane, As + Ds: adamantane + diadamantane.
Adamantane chromatographic characteristics (m/z = 135 + 149 + 187) in different wellblocks of Tazhong area
As shown in Fig. 3 and Table 1, the carbon isotope values of oil in the TZ83 and ZG43 wellblocks of the Tazhong area exhibit significant differences. The carbon isotopes in the TZ83 wellblock are relatively heavier, ranging from –31.1‰ to –30.1‰, with an average value of –30.6‰. In contrast, the carbon isotopes in the ZG43 wellblock are relatively lighter, ranging from –31.7‰ to –30.7‰, with an average value of 31.4‰, which is approximately 0.5 to 1.0‰ lighter than those in the TZ83 wellblock.
Gas invasion may take away some of the lighter components from crude oil, altering fluid properties. A significant amount of gas dissolves in the oil, leading to asphaltene precipitation (Wu et al., 2009; Yang et al., 2009). Evidence of late-stage gas invasion in the study area was found through analysis of physical properties, light hydrocarbon compositions, biomarkers, and stable carbon isotopes.
In terms of physical properties, the TZ83 wellblock exhibits higher viscosity, wax content, and gas-oil ratios compared to the lower values observed in the ZG43 wellblock. It indicates a trend of decreasing values from TZ83 to ZG43. Gas invasion affects the physical properties of crude oil near fault zones, with stronger gas invasion occurring closer to the fault zones, promoting natural gas accumulation. With the increase of the degree of gas invasion, the ability of gas to dissolve higher carbon number hydrocarbons also improves, resulting in higher quality and wax content in the migrating oil and gas phase.
In terms of light hydrocarbon components, gas invasion leads to the migration of hydrocarbon fluids, and pressure and temperature changes in oil and gas reservoirs cause fluid separation into gas and liquid phases. The n-heptane compounds gradually decrease, while methylcyclohexane and toluene compounds become more enriched (Kong et al., 2021; Zhu et al., 2021). This led to the increase of Tol/nC7 and the decrease of nC7/MCC6 in the TZ83 wellblock, whereas the reverse occurs in the ZG43 wellblock.
Additionally, the loss of n-alkanes (Q value) and the carbon number breakpoint are sensitive indicators for identification of gas invasion. A strong gas invasion results in a higher Q value and a higher carbon number breakpoint. N-alkanes confirm that the gas invasion in TZ83 wellblock is stronger than that in ZG43 wellblock.
Ts/(Ts + Tm), C27dia/C27reg and MPI-1 are maturity parameters that apply to a range from immature to mature and overmature stages (Radke et al., 1982; Peters and Moldowan,1992; Li et al., 2010b, 2022b). The impact of gas invasion on maturity is closely related to the filling of high-maturity natural gas. The stronger the gas invasion, the greater the maturity of crude oil. This explains why the ratios of Ts/(Ts + Tm), C27dia/C27reg, and MPI-1 in TZ83 wellblock are higher than that in ZG43 wellblock. Additionally, the carbon isotope indicates that the maturity of crude oil in the TZ83 wellblock is higher than that of crude oil in the ZG43 wellblock, consistent with the above parameters.
The content of adamantane compounds is a clear indicator of gas invasion. After gas invasion, the adamantane components in liquid crude oil become enriched, increasing in content due to solubility and saturated vapor pressure factors (Chakhmakhchev et al., 2017; Moldowan et al., 2015). This directly proves that the TZ83 wellblock has a higher degree of gas invasion than the ZG43 wellblock.
The main controlling factors of gas invasionThe most important geological conditions for the occurrence of gas invasion include three aspects: (1) the original oil and gas reservoir; (2) the generation of large amounts of natural gas in the late stage; (3) the migration pathways and enrichment sites for natural gas (Gussow, 1954; Silverman, 1963).
The Tazhong area experienced multiple periods of oil and gas filling: crude oil predominated during the Caledonian and Late Hercynian periods, while natural gas dominated during the Himalayan period. The different phases of oil and gas filling, particularly the intense natural gas filling during the Himalayan period, provided the material foundation for gas invasion in the study area (Li et al., 2013; Lu et al., 2004). At the same time, multiple fault sets developed in the Tazhong area, especially the Tazhong No.I and the Tazhong No.10 fault zones. These faults cut through several strata, linking Lower Paleozoic reservoirs and source kitchens, thereby providing migration pathways for natural gas (Zhang et al., 2009). Furthermore, a well-developed unconformity surface exists at the top of the Ordovician Yingshan Formation. This surface is in conjunction with the faults, and has transformed the Lower Paleozoic carbonate strata into numerous lithologic traps, creating new enrichment sites for gas invasion and evaporation fractionation (Yu et al., 2011). Consequently, both the ZG43 and TZ83 wellblocks in the Tazhong area have favorable geological conditions for large-scale gas invasion.
There are differences in the degree of gas invasion between both sides of the Tazhong No.I and the Tazhong No.10 fault zones. In the study area, the ZG43 wellblock on the north side of the Tazhong No.10 fault zone has different physical properties, maturity, GOR, adamantane content, and n-alkanes loss compared to the TZ83 wellblock on the south of the Tazhong No.I fault zone, with a lower degree of gas invasion. This difference is due to the structural variation in the fault zones. The TZ83 wellblock is located in an upward slope from northwest to southeast. In the process of gas invasion, highly mature natural gas from the Himalayan period preferentially migrates to the structural high part. So the TZ83 wellblock, located at the intersection of the TZ82 strike-slip fault and the Tazhong No.I fault zone, experiences higher gas invasion.
In contrast, as shown in Fig. 7, the ZG43 wellblock has a less developed Tazhong No.10 fault zone with a smaller fault opening, resulting in a smaller scale of continuous natural gas filling and a relatively lower degree of gas invasion.
Oil and gas migration profile in Tazhong area (a) ZG43 wellblock; (b) TZ83 wellblock
(1) The crude oils from typical wellblocks in the Tazhong area of the Tarim Basin exhibit significant differences in physical properties, light hydrocarbon composition, biomarkers, and carbon isotope distribution, primarily due to varying degrees of gas invasion.
(2) The variations in gas invasion intensity are mainly controlled by differences in fault zones, natural gas migration pathways, and accumulation sites associated with each wellblock.
(3) Regions with strong gas invasion, such as the TZ83 wellblock, are generally associated with condensate oil reservoirs, while regions with weak gas invasion, such as the ZG43 wellblock, predominantly develop waxy oil reservoirs. Additionally, shallow layers in these areas may host light oil reservoirs, suggesting that future exploration should focus on areas near fault zones.
This work was supported by PetroChina’s forward-looking and basic major scientific and technological project “Research on the accumulation law and key exploration technologies of large gas fields (areas)”, Sub-project 3: Research on the formation mechanism, accumulation conditions, and resource potential of condensate oil and light oil (2021DJ0603).
