Geochemical characteristics of light hydrocarbons in cracking gases from chloroform bitumen A,crude oil and its fractions

The composition characteristics of light hydrocarbons from crude oil, chloroform bitumen A, saturated hydrocarbon fraction, aromatic hydrocarbon fraction, and asphaltene fraction during cracking have been studied systematically. The results revealed that the content of n-alkanes, branched alkanes and cycloalkanes in light hydrocarbons from the samples gradually decreased as the simulation temperature increased, and finally almost depleted completely, while the abundance of methane, benzene and its homologues increased obviously and became the main products. The ratios of benzene/ n-hexane and toluene/n-heptane can be used as measures for oil cracking levels. Variation characteristics of maturity parameters of light hydrocarbons, for example, iC₄/nC₄, iC₅/nC₅, isoheptane value, 2,2-DMC₄/nC₆, and 2-MC₆+3-MC₆/nC₇ for different samples with increasing pyrolysis temperature, are consistent with those in petroleum reservoirs, indicating that these parameters may be efficient maturity index.     

Cracking conditions of crude oil under different geological environments

There are mainly 3 kinds of existing states of oil generating from source rocks, that is, dispersive liquid hydrocarbon inside of source rock, dispersive liquid hydrocarbon outside of source rock and concentrated liquid hydrocarbon outside of source rock. Because of the differences in thermal history and medium conditions around, and the interaction of organic and inorganic matter, the liquid hydrocarbon with 3 kinds of existing state has different cracking conditions. The gas generation dynamics experiments of crude oil matching different mediums indicate that the distribution of activation energy of methane changes a lot according to medium difference. The carbonate has a main influence on oil cracking conditions and can largely reduce its activation energy, which reflects the lower cracking temperature of crude oil. The mudstone takes a second place and the sandstone is the smallest. The catalytic cracking function to the oil of the carbonate, of the mudstone and of the sandstone changes weaken in turn. The corresponding R _o values of main gas generation period in different mediums are as follows: 1.5%–3.8% with pure crude oil, 1.2%–3.2% with dispersive crude oil in carbonate, 1.3%～3.4% with dispersive crude oil in mudstone and 1.4%–3.6% with dispersive crude oil in sandstone. The influence of pressure to crude oil cracking is relatively complicated. In the low heating speed condition, pressure restrains the oil cracking and gas generation, but in the high heating speed condition, pressure has an indistinctive influence to the oil cracking and gas generation. Pressure also makes a different effort in different evolvement stage. Taking the middle and lower Cambrian source rocks in the Tarim Basin as an example, primary oil generating quantity is 2232.24×10⁸t, residual oil and oil cracking gas quantity is 806.21×10⁸t and 106.95×10¹²m³ respectively.     

Genetic features of gas-condensate reservoirs in the Kekeya Field,southwest depression in the Tarim Basin,China

Oils, condensates and natural gases in the Kekeya Field, southeast depression of the Tarim Basin were studied for their geochemical characteristics. According to the distribution analysis of the C₂/C₃ values with C₁/C₂ values, C₂/C₃ values with C₁/C₃ values, as well as C₂/C₃ values with dryness index, there are two different types of natural gases in the studied field, which are spatially regularly distributed. One is the oil cracking gas, located on shallow reservoirs over X ₅ ² reservoir, namely Upper oil legs; the other is kerogen cracking gas, located on X ₇ ² reservoirs, X₈ reservoirs and E₂ k reservoirs, namely Lower oil legs. In addition, the distribution patterns of molar concentration of oils and condensates with different carbon numbers of the n-alkanes in the Kekeya Field indicate that the crude oils have experienced several kinds of secondary alterations, which were closely related to the charging of gaseous hydrocarbons after petroleum accumulation. These results indicate that, based on the research of δ ¹³C values of individual hydrocarbons, heptane values and isoheptane values of light hydrocarbons and aromatic maturity parameters for oils, condensates and natural gases, oils and gases were charged at different geological time in the Kekeya Field.     

Geochemical evolution during the cracking of crude oil into gas under different pressure systems

Two comparative simulation experiments(a normal atmospheric-pressure opening system and a 20 MPa closed system)were conducted to study the geochemical evolution of n-alkane,sterane,and terpane biomarkers in the process of oil cracking into gas under different pressures.With an initial experimental temperature set at 300°C,the temperature was increased to 650°C at a heating rate of 30°C/h.The products were tested every 50°C starting at 300°C,and a pressure of 20 MPa was achieved using a water column.The low-maturity crude oil sample was from the Paleogene system in the Dongying sag in eastern China.The threshold temperature obtained for the primary oil cracking process in both pressure systems was 450°C.Before the oil was cracked into gas,some components,including macromolecular n-alkanes,were cracked into medium-or small-sized n-alkanes.The secondary oil cracking of heavy hydrocarbon gases of C2–5to methane mainly occurred between 550°C to 650°C,and the parameters Ln(C1/C2)and Ln(C1/C3),as well as the dry coefficients,increased.Overpressure inhibited the oil cracking process.In the 20 MPa system,the oil conversion rate decreased,the temperature threshold for gas generation rose,and oil cracking was inhibited.Compared with the normal pressure system,high-carbon n-alkanes and other compounds in the 20 MPa pressure system were reserved.Furthermore,the parameters∑C21-/∑22+,Ln(C1/C2),and Ln(C1/C3),as well as the dry coefficients,decreased within the main temperature range.During secondary oil cracking(550°C to 600°C),the Ph/nC18and Pr/nC17decreased.High pressure influenced the evolution of the biomarkers Ts and Tm,C31homohopane,C29sterane,and their related maturity parameters to different extents during oil cracking under different temperature ranges.     

Identification of marine natural gases with different origin sources

Kinetic experiments of gas generation for typical samples of marine gas precursors including low-maturity kerogen, residual kerogen and oil as well as dispersed liquid hydrocarbon (DLH) in source rocks were performed by closed system, and the evolution trends of molecular and isotopic compositions of natural gases from different precursors against the maturity (R ₀%) at laboratory conditions were analyzed. Several diagrams of gas origin were calibrated by using the experimental data. A diagram based on the ratio of normal and isomerous butane and pentane (i/nC₄ ? i/nC₅) was proposed and used to identify the origins of the typical marine natural gases in the Sichuan Basin and the Tarim Basin, China. And the maturities of natural gases were estimated by using the statistical relationships between the gaseous molecular carbon isotopic data and maturities (δ¹³C-R ₀%) with different origins. The results indicate that the molecular and isotopic compositions of simulated gases from different precursors are different from each other. For example, the dryness index of the oil-cracking gas is the lowest; the dryness indices of gases from DLH and kerogen in closed system are almost the same; and the dryness index of gases from residual kerogen is extremely high, indicating that the kerogen gases are very dry; the contents of non-hydrocarbon gases in kerogen-cracking gases are far higher than those in oil-cracking and DLH-cracking gases. The molecular carbon isotopes of oil-cracking gases are the lightest, those of kerogen in closed system and GLH-cracking gases are the second lightest, and those of cracking gases from residual kerogen are the heaviest. The calibration results indicate that the diagrams of In(C₁/C₂)-In(C₂/C₃) and δ^{4 3}C₂-δ^{4 3}C₃-In(C₂/C₃) can discriminate primary and secondary cracking gases, but cannot be used to identify gas origin sources, while the diagram of i/nC₄ ? i/nC₅ can differentiate the gases from different precursors. The application results of these diagrams show that gas mixtures extensively exist in China, which involved the gases from multiple precursors and those from different maturity stages. For example, marine gases in the Sichuan Basin involve the mixture of oil-cracking gases and high-over-maturated kerogen gases, while those in the Tarim Basin involve not only the mixture of gases from multiple precursors, but also those from different maturity gases and post-reservoir alternations such as oxidized degradation and gas intrusion processes.     

Study of genetic evolution of oil inclusion and density of surface oil by measurement of fluorescence lifetime of crude oil and oil inclusion

By using fluorescence lifetime image microscope (FLIM) and time-correlated single photon counting (TCSPC) technique, we measured fluorescence lifetime of crude oils with density of 0.9521–0.7606 g/cm³ and multiple petroleum inclusions from Tazhong uplift of Tarim Basin. As indicated by the test results, crude oil density is closely correlated with average fluorescence lifetime following the regression equation Y=–0.0319X+0.9411, which can thus be used to calculate density of oil inclusions in relation to fluorescence lifetime and density of corresponding surface crude. For type A oil inclusions showing brown-yellow fluorescence from Tazhong 1 well in Tarim Basin, their average fluorescence lifetime was found to be 2.144–2.765 ns, so the density of surface crude corresponding to crude trapping these oil inclusions is 0.852–0.873 g/cm³, indicating that they are matured oil inclusions trapped at earlier stage of oil formation. For type B oil inclusions with light yellow-white fluorescence, their average fluorescence lifetime was found to be 4.029–4.919 ns, so the density of surface crude corresponding to crude trapping these oil inclusions is 0.784–0.812 g/cm³, indicating that they are higher matured oil inclusions trapped at the second stage of oil formation. For type C oil inclusions showing light blue-green fluorescence, their average fluorescence lifetime was found to be 5.063–6.168 ns, so the density of surface crude corresponding to crude trapping these oil inclusions is 0.743–0.779 g/cm³, indicating that they are highly-matured light oil inclusions trapped at the third stage of oil formation.     

Geochemical characteristics of pyrolysis gas from epimetamorphic rocks in the northern basement of Songliao Basin,Northeast China

The Xushen gas field, located in the north of Songliao Basin, is a potential giant gas area for China in the future. Its proved reserves have exceeded 1000×10⁸ m³ by the end of 2005. But, the origin of natural gases from the deep strata is still in debating. Epimetamorphic rocks as a potential gas source are widely spreading in the northern basement of Songliao Basin. According to pyrolysis experiments for these rocks in the semi-confined system, gas production and geochemistry of alkane gases are discussed in this paper. The Carboniferous-Permian epimetamorphic rocks were heated from 300°C to 550°C, with temperature interval of 50°C. The gas production was quantified and measured for chemical and carbon isotopic compositions. Results show that δ ¹³C₁ is less than ?20‰, carbon isotope trend of alkane gas is δ ¹³C₁<δ ¹³C₂<δ ¹³C₃ or δ ¹³C₁<δ ¹³C₂>δ ¹³C₃, these features suggest that the gas would be coal-type gas at high-over maturity, not be inorganic gas with reversal trend of gaseous alkanes (δ ¹³C₁>δ ¹³C₂>δ ¹³C₃). These characteristics of carbon isotopes are similar with the natural gas from the basin basement, but disagree with gas from the Xingcheng reservoir. Thus, the mixing gases from the pyrolysis gas with coal-typed gases at high-over maturity or oil-typed gases do not cause the reversal trend of carbon isotopes. The gas generation intensity for epimetamorphic rocks is 3.0×10⁸–23.8×10⁸ m³/km², corresponding to R _o from 2.0% to 3.5% for organic matter.     

Brittleness characteristics of tight oil siltstones

Rock brittleness directly affects reservoir fracturing and its evaluation is essential for establishing fracturing conditions prior to reservoir reforming. Dynamic and static brittleness data were collected from siltstones of the Qingshankou Formation in Songliao Basin. The brittle–plastic transition was investigated based on the stress–strain relation. The results suggest that the brittleness indices calculated by static elastic parameters are negatively correlated with the stress drop coefficient and the brittleness index B₂, defined as the average of the normalized Young’s modulus and Poisson’s ratio, is strongly correlated with the stress drop. The brittleness index B₂, Young’s modulus, and Poisson’s ratio correlate with the brittle minerals content; that is, quartz, carbonates, and pyrite. We also investigated the correlation between pore fluid and porosity and dynamic brittle characteristic based on index B₂. Pore fluid increases the plasticity of rock and reduces brittleness; moreover, with increasing porosity, rock brittleness decreases. The gas-saturated siltstone brittleness index is higher than that in oil- or water-saturated siltstone; the difference in the brittleness indices of oil- and water-saturated siltstone is very small. By comparing the rock mechanics and ultrasonic experiments, we find that the brittleness index obtained from the rock mechanics experiments is smaller than that obtained from the ultrasonic experiments; nevertheless, both decrease with increasing porosity as well as their differences. Ultrasonic waves propagate through the rock specimens without affecting them, whereas rock mechanics experiments are destructive and induce microcracking and porosity increases; consequently, the brittleness of low-porosity rocks is affected by the formation of internal microcrack systems.     

The distribution characteristic and its significance of compound specific isotopic composition of aromatic hydrocarbon from marine source rock and oil in the Tarim Basin,western China

Aromatic hydrocarbons are generally main distillation of crude oil and organic extract of source rocks. Bicyclic and tricyclic aromatic hydrocarbons can be purified by two-step method of chromatography on alumina. Carbon isotopic composition of individual aromatic hydrocarbons is affected not only by thermal maturity, but also by organic matter input, depositional environment, and hydrocarbon generation process based on the GC-IRMS analysis of Upper Ordovician, Lower Ordovician, and Cambrian source rocks in different areas in the Tarim Basin, western China. The subgroups of aromatic hydrocarbons as well as individual aromatic compound, such as 1-MP, 9-MP, and 2,6-DMP from Cambrian-Lower Ordovician section show more depleted 13 C distribution. The 13 C value difference between Cambrian-Lower Ordovician section and Upper Ordovician source rocks is up to 16.1‰ for subgroups and 14‰ for individual compounds. It can provide strong evidence for oil source correlation by combing the 13 C value and biomarker distribution of different oil and source rocks from different strata in the Tarim Basin. Most oils from Tazhong area have geochemical characteristics such as more negative 13C9-MP value, poor gammacerane, and abundant homohopanes, which indicate that Upper Ordovician source rock is the main source rock. In contrast, oils from Tadong area and some oils from Tazhong area have geochemical characteristics such as high 13C9-MP value, abundant gammacerane, and poor homohopanes, which suggest that the major contributor is Cambrian-Lower Ordovician source rock.     

Cyclic loading test of self-centering precast segmental unbonded posttensioned UHPFRC bridge columns

Three 1/3-scale precast segmental bridge columns, manufactured with ultrahigh-performance fiber-reinforced concrete (UHPFRC) incorporating river sand and coarse aggregate, were tested under cyclic loading. Energy dissipation (ED) bars, embedded in ultrahigh-performance concrete (UHPC) grout, maintained continuous across segment joints and unbonded at the bottom joint. Self-centering prestressing force was provided by unbonded posttensioning (PT) tendons. The research parameters included PT force level and the amount of ED bars. Test results showed that all the specimens exhibited no less than 8% drift capacities, which were remarked with the first fracture of ED bars. No obvious cracking and limited UHPFRC spalling were observed. Both PT force level and the amount of ED bars have notable effects on stiffness, lateral strength, and ductility. Increased PT force may improve ductility with the total axial loading ratio less than 0.08. All PT tendons were elastic and no yield or rupturing was found, but the stress loss was significant. The equivalent unbonded length can be evaluated with 0.007d_bf_y for ED bars embedded in UHPC grout. The rotation of the bottom joint dominated lateral deformation and the contribution of joint sliding can be neglected. The contribution λ_ED of ED bars to lateral strength should be no more than 25% to maintain self-centering capacity.     

Chromatographic separation of diasteriomeric isoprenoids for the identification of fossil oil contamination

In aliphatic hydrocarbon fractions of crude oils eight acyclic isoprenoid alkanes were separated from accompanying n-alkanes and iso-alkanes by high-performance glasscapillary gas chromatography. Four of these: 2,6,10-trimethyltetradecane (V), norpristane (IV), pristane (III), and phytane (II) could be resolved further to reveal a doublet produced by the different diastereoisomers.In these doublets the front peak represents stereoisomers formed during maturation of crude oil while the rear peak indicates the respective hydrocarbons carrying the original biogenic precursor configuration. This appearance of doublets demonstrates the loss of stereospecificity in hydrocarbons derived from phytol (I). By using the front peaks as diagenetic ‘maturity markers’ attributable to fossil fuels, hydrocarbon mixtures extracted from sea water samples contaminated with fossil oil could be investigated in detail. The quantitative relationship between recent biogenic and fossil fuel hydrocarbons could be determined in extracts in the lower boiling point range.     

Kinetics of hydrocarbon generation for Well Yingnan 2 gas reservoir,Tarim Basin,China

Well Yingnan 2, an important exploratory well in the east of Tarim Basin, yields high commercial oil and gas flow in Jurassic. Natural gas components and carbon isotopic composition indicate that it belongs to sapropel type gas. Because this region presents many suits of hydrocarbon source rocks, there are some controversies that natural gases were generated from kerogen gas or crude oil cracking gas at present. By using the kinetics of hydrocarbon generation and carbon isotope, natural gas of Well Yingnan 2 is composed mainly of crude oil cracking gas, about 72%, it is generated from secondary kerogen gas of Cambrian-Lower Ordovician source rock and crude oil cracking gas of Mid-Upper Ordovician oil reservoir. The main oil and gas filling time is 65 Ma later in the Jurassic gas reservoir of Well Yingnan 2, so the gas reservoir belongs to late accumulation and continuous filling type.

     

Genetic features of gas-condensate reservoirs in the Kekeya Field,southwest depression in the Tarim Basin,China

Oils, condensates and natural gases in the Kekeya Field, southeast depression of the Tarim Basin were studied for their geochemical characteristics. According to the distribution analysis of the C₂/C₃ values with C₁/C₂ values, C₂/C₃ values with C₁/C₃ values, as well as C₂/C₃ values with dryness index, there are two different types of natural gases in the studied field, which are spatially regularly distributed. One is the oil cracking gas, located on shallow reservoirs over X ²₅ reservoir, namely Upper oil legs; the other is kerogen cracking gas, located on X ²₇ reservoirs, X₈ reservoirs and E₂ k reservoirs, namely Lower oil legs. In addition, the distribution patterns of molar concentration of oils and condensates with different carbon numbers of the n-alkanes in the Kekeya Field indicate that the crude oils have experienced several kinds of secondary alterations, which were closely related to the charging of gaseous hydrocarbons after petroleum accumulation. These results indicate that, based on the research of δ ¹³C values of individual hydrocarbons, heptane values and isoheptane values of light hydrocarbons and aromatic maturity parameters for oils, condensates and natural gases, oils and gases were charged at different geological time in the Kekeya Field.

     

Effects of surface and sunken crude oil on the behaviour of a sea urchin

Studies have been made of the effects of exposure to various forms of crude oil on the righting behaviour of Paracentrotus lividus and its reactions towards the presence of oil. Prolongation of the righting response was recorded in animals exposed to contact with surface or sunken fresh crude oil or to their water soluble fractions. No such effect was recorded on exposure to weathered oils and results indicate that the more volatile components of crude oil were responsible for this effect. Paracentrotus showed no avoidance reaction to the presence of sunken oil in its vicinity. The likely ecological significance of these results is discussed.     

Crude-oil hydrocarbon composition characteristics and oil viscosity prediction in the northern Songliao Basin

Crude oil hydrocarbon composition characteristics and oil viscosity prediction are important bases in petroleum exploration.A total of 54 oil/heavy-oil samples and 17 oil sands were analyzed and quantified using both comprehensive 2D gas chromatography(GC×GC)and comprehensive 2D gas chromatography/time-of-flight mass spectrometry(GC×GC/TOFMS).The results show that crude oil in the West slope is mainly heavy oil and its hydrocarbon composition is characterized overall by paraffinsmono-aromaticsnaphthenesnon-hydrocarbonsdi-aromaticstri-aromaticstetra-aromatics.Aromatics are most abundant and non-hydrocarbons are least abundant,whilst content differences among paraffins,naphthenes,aromatics,and non-hydrocarbons are less than 15%.There are two types of heavy oil,secondary type and mixing type.Biodegradation is the main formation mechanism of heavy oil.Biodegradation levels cover light biodegradation,moderate biodegradation,and severe biodegradation.With increasing biodegradation,paraffin content decreases while contents of aromatics and nonhydrocarbons increase.In contrast,naphthene content increases first and then decreases with increasing biodegradation.In severe biodegradation stage,naphthenes decrease more quickly than aromatics and non-hydrocarbons.This provides a new method for studying oil/heavy-oil biodegradation mechanism and biodegradation resistance of different hydrocarbons at different biodegradation stages.In the Longhupao-Daan terrace and Qijia-Gulong depression,most crude oil is conventional oil.Its composition is dominated by paraffins with the lowest content of aromatics.In some casual oil wells from the Longhupao-Daan terrace,crude oil from Saertu oil reservoirs is moderately biodegraded whereas crude oil from Putaohua oil reservoir is lightly biodegraded.Chemical parameters using saturate hydrocarbons and aromatics are usually not suitable for determining organic type and thermal maturity of biodegraded oil,especially of moderately or severely biodegraded oil,whilst Ts/(Ts+Tm)ratio can be used to determine thermal maturity of both conventional crude oil and heavy oil.     

Modeling of the thermal structure of continental lithosphere

The thermal structure of continental lithosphere (the temperature, heat flows, and heat generation in the crust and lithosphere) is reconstructed from geothermal, seismic, and petrologic data. The first step is the determination of the temperature profile from absolute P and S wave velocities (T _{P, S} ). The T _{P, S} profile is then adjusted to a thermophysical model of conductive transfer. In addition, the surface heat flow and the T _{P, S} profile are used to determine heat generation, thicknesses of crustal layers, and heat flow components in the crust and lithosphere. A feature inherent in the solution of the thermophysical inverse problem obtained in this paper is the use of constraints derived from the temperature reconstruction by seismic data inversion. As a result, the analytical dependence of the temperature on depth, the intensity of radiogenic heat sources in the crust, and heat flow components in the crust and lithosphere are determined.     

Geochemical characteristics and accumulation of marine oil and gas around Halahatang depression,Tarim Basin,China

There exists a petroleum system rich of oil and gas around Halahatang depression, where the oil and gas possess obvious local distinctions of properties in different parts. The research proved that the discovered crude oil and natural gas in the region derived mainly from O₂₊₃ source rock, and the differences of its properties were controlled by the oil and gas filling intensity. The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases: late Caledonian-Early Hercynian epoch, late Hercynian epoch, and Yanshan-Himalayan epoch. In the first phase, the oil and gas derived mostly from Cambrian source rock, which formed the primary ancient oil reservoirs, then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present. In the second phase, the O₂₊₃ source rock of Manjia’er depression started its generation of hydrocarbon, which accumulated in the high position of Tabei up-lift afterwards, and then biodegradated to heavy oil in the late Hercynian epoch. In the last phase, the O₂₊₃ source rock of southern Halahatang depression and margin of Manjia’er depression started its peak of generating liquid hydrocarbon, which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch, and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading, to lead to obvious crude oil divergence.

     

Formation mechanism and geochemical characteristics of shallow natural gas in heavy oil province,China

Shallow gas reservoirs are distributed widely in Chinese heavy oil-bearing basins. At present, shallow gas resources have opened up giant potentials. The previous researches indicate the intimate genetic relationship between shallow gas and heavy oil. Shallow gas resources are generated from crude oil degraded by anaerobic microscopic organism, it belongs to biogenic gas family of secondary genesis, namely heavy oil degraded gas. Shallow gas resources are usually distributed in the upward position or the vicinity of heavy oil reservoirs. They are mainly of dry gas, which are composed of methane and only tiny C ₂ ⁺ heavy hydrocarbon and relatively higher contents of nitrogen gas. Generally, methane isotopes are light, whose values are between biogenic gas and thermal cracking gas. Ethane isotopes are heavy, which mixed possibly with thermogenic gas. Carbon dioxide bear the characteristics of very heavy carbon isotope, so carbon isotopic fractionation effects are very obvious on the process of microscopic organism degradation crude oil. The heavy oil degraded gas formation, a very complex geological, geochemical and microbiological geochemical process, is the result of a series of reactions of organic matter-microbes and water-hydrocarbon, which is controlled by many factors.     

Geochemical characteristics and accumulation of marine oil and gas around Halahatang depression,Tarim Basin,China

There exists a petroleum system rich of oil and gas around Halahatang depression, where the oil and gas possess obvious local distinctions of properties in different parts. The research proved that the discovered crude oil and natural gas in the region derived mainly from O₂₊₃ source rock, and the differences of its properties were controlled by the oil and gas filling intensity. The comprehensive study result shows the oil and gas reservoirs of the region mainly underwent three important accumulation phases: late Caledonian-Early Hercynian epoch, late Hercynian epoch, and Yanshan-Himalayan epoch. In the first phase, the oil and gas derived mostly from Cambrian source rock, which formed the primary ancient oil reservoirs, then suffered strong degradation and remained a great quantity of pyrobitumen in the high position of Tabei uplift in the present. In the second phase, the O₂₊₃ source rock of Manjia’er depression started its generation of hydrocarbon, which accumulated in the high position of Tabei up-lift afterwards, and then biodegradated to heavy oil in the late Hercynian epoch. In the last phase, the O₂₊₃ source rock of southern Halahatang depression and margin of Manjia’er depression started its peak of generating liquid hydrocarbon, which mostly accumulated in the trap formed before the Indo-China and Yanshan epoch, and in somewhere the heavy oil suffered dilutions in various degrees or serious gas invading, to lead to obvious crude oil divergence.     

Geochemical study on oil-cracked gases and kerogen-cracked gases (II)—Discrimination methods between oil-cracked gases and kerogen-cracked gases

In the processes of discrimination between oil-cracked gases and kerogen-cracked gases, Behar and Pinzgofer et al.’s results were adopted in the former researches, in which the ratio of C₂/C₃ is basically a constant while the ratio of C₁/C₂ gradually increases in the course of primary cracking of kerogen. Otherwise in the course of secondary cracking of oil, the ratio of C₂/C₃ increases rapidly while C₁/C₂ keeps relatively stable. Our study on analogue experiment shows that, whether it is oil or kerogen, in its process of gas generating by cracking, the ratios of C₂/C₃, C₁/C₂ or C₁/C₃ will all be increased with the growth of thermal conditions. In comparison, the ratio of C₂/C₃, which is affected by genetic type to some comparatively less extent, mainly responds to the maturity of gases, while the value of C₂/C₃ is about 2, and that of C₂/iC₄ is about 10, and the corresponding value of R _o is about 1.5%–1.6%. The influence of gas source on C₂/C₃ is less than that of gas maturity, otherwise C₁/C₂ (or C₁/C₃) is obviously affected by cracking matrices. The ratios of C₁/C₂, C₁/C₃ of oil-cracked gases are less than that of kerogen-cracked gases, under the condition that the ratios of C₂/C₃ are similar in value, so are the value of dryness indexes. There exists wide diffidence between this view and the former discrimination method in theory. The analysis of the spot sample indicates that we can apply the above basic view to dealing efficiently with the problem of the discrimination between oil-cracked gas and kerogen-cracked gas.