The measured inclusions are mainly inclusions in the expanding edge and secondary cracks, and some inclusions in siliceous cements. Because this kind of inclusions has high transparency, clear phase boundary and phase transition, and is easy to observe and test, gas-liquid two-phase salt solution inclusions are selected to measure uniform temperature, freezing point and salinity in the experiment.
I. Formation period of oil and gas
In the whole process of diagenesis, oil generation and gas generation, as long as authigenic minerals are formed or recrystallized, oil, gas and water fluids will be wrapped to form inclusions. As long as these inclusions are not rebalanced or destroyed in the later stage, they represent the characteristics of fluid media at that time, and important information of fluid properties in different oil-generating (gas-generating) stages can be obtained through inclusions.
The test results show that the formation temperatures of inclusions in different authigenic minerals or secondary fractures in the same sample are quite different, indicating that they were formed in different diagenetic stages. Generally speaking, the homogenization temperature of inclusions at the edge of the timely expansion is generally low, while the formation temperature of inclusions in cracks is high (Table 6- 1). The depth of most samples measured this time is about 2000 m, and no pressure correction is made for uniform temperature. According to the difference of uniform temperature and salinity and the characteristics of organic inclusions, the inclusions in the samples can be divided into different stages: early formation of inclusions with low uniform temperature and salinity, organic inclusions with light color and small gas-liquid ratio; Inclusions with higher homogenization temperature and salinity, darker color of organic inclusions, larger gas-liquid ratio and higher contents of gaseous hydrocarbon inclusions and asphalt inclusions were formed in the later stage.
The types and characteristics of organic inclusions in authigenic minerals or diagenetic fractures in this area show regular changes. At the early stage, the number of organic inclusions expanding the edge is relatively small; However, the abundance of organic inclusions in secondary fractures is high, mostly gas-liquid two-phase organic inclusions, and the gas-liquid ratio is 10% ~ 30% (Table 6-2). There are many gaseous hydrocarbon inclusions and asphalt inclusions in late fractures or siliceous cements.
There are some differences in diagenetic evolution, oil and gas filling and migration between the eastern and western parts of the northern margin of Qaidam Basin. According to the development of authigenic minerals and the changing characteristics of organic inclusions, there are mainly two stages of oil generation and migration in Lenghu area (only three stages appear in Lengsi 1 well). In the early stage, the organic inclusions mainly reflected by the edge of the timely beetle are mostly gas-liquid hydrocarbon inclusions, and some are liquid hydrocarbon inclusions. According to the phase composition and physical characteristics of organic inclusions, organic matter is in the mature stage. However, the diagenetic temperature in this period is low (the average temperature is 6 1 ~ 87℃), which may be the result of vertical migration after oil formation and the influence of the upper shallow temperature. The migration and filling of oil in the second phase can be reflected by the organic inclusions in the synchronous fracture and secondary synchronous. In this period, the color of organic inclusions is dark, and there are many gas-liquid inclusions with high gas-liquid ratio, and some gaseous hydrocarbon inclusions (Table 6-2). Judging from the salinity and homogenization temperature of the salt solution inclusions containing organic inclusions, the salinity and homogenization temperature of the inclusions in the edge cracks of submersible 1 sample are close to those in the timely cracks of Leng 4 1, the secondary response time of Well 86 and Leng 7 1 (the average range is 9.8% ~1respectively. However, no organic inclusions were found in the first phase inclusions of Lengqi 1 well, indicating that oil and gas did not migrate to this layer. Only organic inclusions appeared in the late-stage timely fractures, that is, the formation of Lengqi 1 well was filled with oil and gas only in the first stage. The remaining wells 1, Lengsi 1 and Shen 86 all have two oil charges. According to the history of tectonic sedimentation and thermal evolution in this area, these two oil and gas filling periods occurred in E3 and E3 respectively.
Table 6- 1 Test Data Sheet of Fluid Inclusions in Northern Qaidam Basin
Table 6-2 Characteristics of Organic Inclusions in the East-North of Northern Qaidam Basin
There are two phases of inclusions in Xian 3 and Xian 4 samples in Nanbaxian structural area, but only one phase is full of oil and gas. The early inclusions appeared in the late stage of the extended edge (Ⅱ) (the early inclusions in the extended edge were small in shape, mainly liquid salt solution inclusions, which were not studied this time). The types of inclusions are mainly gas-liquid two-phase salt solution inclusions. The gas-liquid inclusions are relatively large, the organic inclusions are not developed, and only a few liquid hydrocarbon inclusions exist. The late inclusions occur in synchronous faults and secondary synchronous faults. Organic inclusions are well developed. The inclusions are dark brown with high gas-liquid ratio (mostly 10% ~ 35%). Some gaseous hydrocarbon inclusions and asphalt inclusions were also found. It reflects that the maturity of organic matter in inclusions in this period is high. According to the characteristics of organic inclusions, combined with the history of sedimentary burial and thermal evolution in this area, it is an important period for the formation of a large number of moisture and condensate oil. There are three phases of inclusions in wells Xian 5, Xian 6 and Xian 7 in Nanbaxian structure, but more organic inclusions are developed in the last two phases, which reflects two phases of oil and gas charging. The inclusions in Yingshidadan, siliceous cement (early stage) and timely veinlets are mostly salt solution inclusions (liquid phase and gas-liquid phase), colorless, light yellow and light brownish yellow, with undeveloped organic inclusions and only a few liquid hydrocarbon inclusions. The homogenization temperature of inclusions in salt solution is low, the average temperature is 74 ~ 8 1℃, and the salinity is 9.6% ~ 10.2%, which should be the product of the same evolution stage (the first inclusion). The color of inclusions in the secondary fracture of well Xian 7, the timely expansion margin of well Xian 5 and the siliceous cement of well Xian 6 are all dark, mainly gray, dark gray and dark brown, mainly gas-liquid two-phase hydrocarbon inclusions, with some asphalt and gaseous hydrocarbon inclusions (accounting for 10% ~ 15%). Compared with the gas-liquid brine solution inclusions in the same period, according to the organic and inorganic contents, The homogeneous temperature of inclusions in this period is low, which may be caused by the long-distance vertical migration of oil-rich fluids and the influence of shallow low-temperature strata. According to the characteristics of organic inclusions, the inclusions in this stage were formed at the same time as the inclusions in the first stage of Xiansan and Xiansi wells. The salinity of inclusions in siliceous cement, timely fracture (late stage) and intergranular cement fractures is also similar to that in salt solution (the average range is 104 ~ 1 12℃ and 13.0% ~ 13.5% respectively). Almost all the organic inclusions generated by brine solution inclusions are gaseous hydrocarbon inclusions and asphalt inclusions, which are black, dark brown and brownish black. The size and shape of the inclusions vary greatly. According to the characteristics of organic inclusions, they should be in the formation stage of a large amount of water and condensate oil. According to the characteristics of organic inclusions, the inclusions in this stage are the same as those in the second stage of Xiansan and Xiansi wells. According to the characteristics of organic inclusions, sedimentary burial history and thermal evolution history in this area, the oil and gas in the samples of Well Xian 5, Well Xian 6 and Well Xian 7 were filled in two stages, which were formed in E3-N 1 and N2 respectively.
The second is oil and gas migration
In the study, only one sample was measured in each well, so it is impossible to calculate the paleogeothermal gradient, and the layers and depths of samples in different regions are different, so it is difficult to compare the paleogeothermal levels in different regions. According to the homogeneous temperature of inclusions in different regions at the same time and combined with the relative temperature (homogeneous temperature/sample depth), the paleogeothermal changes in different regions of Lenghu and Nanbaxian structures are briefly analyzed. From the overall situation of average temperature and relative temperature, the paleogeothermal of Lenghu structure (from northwest to southeast) has a rising trend. From the submersible well 1 in the northwest to Lengsi well 1, Shen86 well and Lengqi well 1 in the southeast, the temperature generally increases gradually. The diagenetic temperature of the early expansion margin and the formation temperature of the late secondary fractures are the same. It shows that the thermal evolution degree of Lenghu structure gradually increases from northwest to southeast. Salinity has the same trend as paleotemperature, and the salinity in the early stage is lower than that in the late stage, indicating that the concentration of diagenetic fluid gradually increased in the late stage. However, from Paleogene to Neogene, the salinity of fluid gradually decreased (just opposite to the salinity change trend of formation water), so it can be seen that the fluid captured by Neogene inclusions did not come from this layer, but from the lower stratum.
Comparing the salinity of salt solution inclusions in different horizons, it is found that the newer horizons, the lower the salinity, reflecting the vertical migration characteristics of fluids (migration channels are fractures and fissures). Generally speaking, the salinity of Nanbaxian area gradually decreases from southwest to northeast, reflecting that the migration direction of underground fluid is Xian 3→ Xian 7→ Xian 5→ Xian 6→ Xian 4 well. Along this direction, the maturity of oil and gas also gradually increases, which can also be reflected from the characteristics of organic inclusions, that is, the color of organic inclusions gradually darkens, the contents of gaseous hydrocarbons and asphalt inclusions increase, and the gas-liquid ratio of gas-liquid two-phase organic inclusions tends to increase gradually (Table 6-2).
Table 6-3 Classification Table of Fluid Inclusions in Different Periods
Three. Characteristics of biomarkers in organic inclusions
Hydrocarbon-bearing pores in reservoirs can be divided into open pores and closed pores (Pan Changchun et al., 1997). The oil and gas composition in open pores changes with the continuous injection of oil and gas composition; Because the pores are closed, the oil and gas components in the closed pores remain unchanged. In the same pore, the distribution of oil and gas components is also very complicated. Oil and gas components can be divided into free state and bound state. Oil phase and mineral phase will inevitably interact to form a contact interface. Polar components (mainly asphaltenes and other macromolecules) containing N, S and O elements in the oil phase can be preferentially adsorbed on the surface of mineral particles. This adsorbed component is relatively stable and is not easy to be replaced and changed. The adsorbed polar macromolecules can adsorb and carry nonpolar or weakly polar oil and gas components such as saturated hydrocarbons and aromatic hydrocarbons, and these components remain relatively unchanged, which can be considered as bound oil and gas components. Free oil and gas components are not affected by contact interface, and can be freely mixed with foreign components (Pan Changchun et al., 1997).
The composition of organic inclusions represents the composition of oil and gas at the time of capture. In this paper, organic inclusions are analyzed by chromatography and color-mass spectrometry, the geochemical characteristics of original (paleo) oil and gas components in reservoir rocks are restored, the geochemical evolution process of oil and gas components in reservoir rocks is revealed, and the oil source of oil and gas reservoirs is traced. However, the experimental methods and techniques need to be further improved. The ideal situation is to determine the composition and biomarkers of single or contemporaneous organic inclusions, but at present, only the composition of whole rock-like organic inclusions can be determined in China. In this way, it is bound to be difficult to explain the test results of inclusions formed in different evolution stages.
1. Analysis process and experimental conditions
Firstly, it was separated into individual sand grains, and the particle size of 100 ~ 150 g was screened out. Then it was extracted with dichloromethane+methanol for 48 hours, and then treated with hydrochloric acid (12 hours). Then treat with potassium dichromate+concentrated sulfuric acid (washing liquid, strong oxidant) (12 hours), clean, dry (below 40℃), extract with dichloromethane+methanol for 48 hours, grind the sample, extract with dichloromethane+methanol for 48 hours, and the extract is oil-gas inclusion.
Saturated hydrocarbon components were separated by alumina/silica gel column chromatography (ether was washed with petroleum). Then the saturated hydrocarbons were analyzed by gas chromatography and gas chromatography-mass spectrometry.
Table 6-4 Parameter Table for Analysis of Oil and Gas Inclusions in Northern Margin of Qaidam Basin by GC and GC-MS
Gas chromatographic conditions: HP6890GC chromatograph, 30 m×0.32 mm inner diameter, 0.25μm, HP-5 column. Heating procedure: the initial temperature is 70℃, and the temperature is kept for 2 minutes, then the temperature is raised to 290℃ at 4 ℃/ min, and the temperature is kept for 45 minutes, and the carrier gas is nitrogen.
Analysis conditions of GC-MS: Micromass Peat Form II (product of VG Company) is connected with HP5890 chromatography. Chromatographic column: 30 m×0.25 mm inner diameter, 0.25μm coating thickness, HP-5 ms, helium as carrier gas. Heating program: keep 70℃ for 5 minutes, heat it to 120℃ at 8 ℃/ min, then heat it to 290℃ at 2 ℃/ min, and keep it for half an hour.
2. Test results
Chromatographic characteristics of (1) saturated hydrocarbon. Figures 6- 1 to 6-7 are saturated hydrocarbon chromatograms of inclusion samples. Chromatographic characteristics can be roughly divided into two categories: one is Leng 7 1, Leng 4 1, Xian 6 and Xian 3 wells. Saturated hydrocarbons have bimodal characteristics, mainly post-peak type, reflecting the low maturity of organic matter. The other is samples from Well Xian 4 and Well Xian 5, which are unimodal, indicating high maturity. It can also be seen from Figure 6-8 that the ratios of Pr/n C 17 and Ph/n C 18 of samples from Well Xian 4 and Well Xian 5 are low, indicating that the maturity is high. These two types of inclusions may be the products of two periods of oil and gas charging.
(2) Color-mass spectrum characteristics. C27-C28-C29 sterane content is an important parameter to distinguish different crude oil and source rocks. It can be seen from Table 6-4 that C29 sterane is the main organic inclusion in Lenghu and Nanbaxian structural areas, and C27 sterane is close to C28 sterane, indicating that terrigenous organic matter input accounts for a large proportion. From the triangle diagram of sterane C27-C28-C29 (Figure 6-9), the regular sterane composition of organic inclusions is basically consistent with that of crude oil, indicating that the geochemical characteristics of original crude oil (organic inclusions) are consistent with that of crude oil. In Nanbaxian structural area, the Ts/Tm value gradually increases from Lengsi 1 well to Lengqi 1 well, and from Xian3 → Xian5 → Xian6 → Xian4 (CZ-6 sample) well, which is consistent with the horizontal change trend of oil and gas maturity. However, the Ts/Tm value of the shallow-buried Xian 4CZ-5 sample suddenly decreased, which may be related to the different types and quantities of organic inclusions and their later changes.
Fig. 6 chromatogram of saturated hydrocarbon in inclusion of Lengsi 1 well 2457.96m (E 1+2 L).
Fig. 6-2 Saturated hydrocarbon chromatogram of inclusion in Lengqi 1 Well 2906.2m (E3 X 1).
Fig. 6-3 Chromatogram of Saturated Hydrocarbon of 2909.7 m(N 1 s) Inclusions in Well Xian 3.
Fig. 6-4 Chromatogram of Saturated Hydrocarbon in 2838 438+0 m (E3 X2) Inclusions in Well Xian 6.
Fig. 6-5 chromatogram of inclusion111134m (n1s) saturated hydrocarbon in Well Xian 4.
Fig. 6-6 Chromatogram of Saturated Hydrocarbon Inclusion in Well Xian 4+0806 ~1818m (n1s).
Fig. 6-7 chromatogram of saturated hydrocarbon in 2880.45 ~ 2887.5438+0 m (n1s) inclusions in Well Xian 5.
Fig. 6-8 relationship between inclusions Pr/n C 17 and Ph/n C 18.
Figure 6-9 Regular Sterane Composition Diagram of Organic Inclusion Samples and Nanbaxian Crude Oil Samples