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.     

Evaluation of hydrocarbon generation of the Xiamaling Formation shale in Zhangjiakou and its significance to the petroleum geology in North China

The semi-closed pyrolysis simulation system under constant pressure was conducted to explore the characteristics and mechanisms of hydrocarbon generation from Xiamaling Formation shale in Xiahuayuan,North China.The experiment results indicate the oil generated by the Xiamaling Formation shale in oil window should be classified as "aromatic-intermediate" type,whereas the decreasing of dry coefficient can be ascribed to the cracking of residual bitumen in source rock in the stage of high to post maturity.The amount of hydrocarbon gas generated from residual bitumen can be up to 1-2 m3 per ton rock in high to post mature stage by calculating hydrogen contents in the kerogen,the expelled hydrocarbon,and the residual hydrocarbon.This reveals the importance of residual bitumen as a gas source during high to post mature stage of the kerogen evolution,and also as the broad exploration prospect of these gases.This research highlights the attention should be paid to oil/gas reservoirs sourced from residual bitumen of organic-rich source rock in high mature stage,even the primary oil/gas reservoirs considered as the main exploration targets in middle-upper Proterozoic sediments of North China.     

The deep structures and oil-gas prospect evaluation in the Qiangtang Basin, Tibet

The Qiantang Basin is now one of the topics of general interest in petroleum exploration in China. This paper reports a comprehensive study of geophysical and geological survey data recently obtained in this area and, combined with INDEPTH-3 deep survey results, comes to the following conclusions： 1） The hydrocarbon source formations, reservoirs, and overlying strata and their association within the basin are quite good, local structures are developed, and, therefore, the region is favorable for forming and preserving oil and gas accumulations. Faults are not a fatal problem. The future main target strata are the middle-deep structural strata composed of Upper-Triassic and middle Jurassic rocks; 2） A new classification has been made for second-order tectonic sequences inside the basin to disavow the central Qingtang uplift. It is noted that the main structures at the surface are orientated NW-SE and the crustal structure can be described as three depressions, three risees, and one deep depression, of which the prospective zone with the most potential is the inner main subsided belt and its two sides; 3） Comparatively intensive interaction between the crust and mantle and volcanic and thermal activities in the northern basin play a very important role in petroleum evaluation. The southern deeper sedimentation and less thermal activity make this area a more perfect zone for oil exploration; 4） Currently, the most important objective is determining the physical properties of the deep strata, the status of oil and gas accumulations, the source of the hydrocarbons, and the relationship between the upper and lower structures; and 5） The Lunpola Tertiary basin may be favorable for oil accumulations because petroleum may migrate from marine strata on two sides.     

Quantitative studies of hydrocarbon loss of the Silurian bitumen sandstone in the Tarim Basin

The evolution of Tarim Basin experienced several tectonic movements,of which Caledonian movement caused a serious damage to reservoirs of Silurian. The earlier reservoirs size and the later hydrocarbon loss are two key factors to exploration prospect of Silurian. On the basis of net thickness of bituminous sandstone and recovery coefficient of crude oil converting to bitumen,the scale of the hydrocarbon loss of palaeo-accumulation in bitumen sandstone of Silurian was calculated (86.30×108t). The deter-mination of net thickness of bitumen sandstone was completed mainly by several methods,such as physical property demarcation,observing lithology,grains with oil inclusion (GOI) technology,and quantitative grains fluorescence (QGF) technology. The samples of black sandstone and white sand-stone were collected and compared by means of the methods mentioned above. The results showed that there was little hydrocarbon,or none,in the white sandstone. The recovery coefficient was ob-tained by the analyses of plentiful physical parameters of crude oil and bitumen sandstone,based on the hypothesis that nonhydrocarbon compounds and bitumen were invariable in the process of bitu-men forming. The calculated result,which is more scientific and actual,indicates the exploration pros-pect is more promising in Silurian of the Tarim Basin.     

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 Ro 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 2232.24×10 8 t,residual oil and oil cracking gas quantity is 806.21×10 8 t and 106.95×10 12 m 3 respectively.     

Preliminary study on the origin identification of natural gas by the parameters of light hydrocarbon

The light hydrocarbon composition of 209 natural gas samples and individual light hydrocarbon carbon isotopes of 53 natural gas samples from typical humic-sourced gas and sapropelic-sourced gas in the four basins of China have been determined and analyzed.Some identification parameters for humic-sourced gas and sapropelic-sourced gas are proposed or corrected.The differences of compound-specificδ13C value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded.The humic-sourced gas has the distribution ofδ13C benzene > -24‰,δ13C toluene >-23‰,δ13C cyclohexane >-24‰andδ13C methyl cyclohexane >-24‰,while the sapropelicsourced gas has the distribution ofδ13C benzene <-24‰,δ13C toluene <-24‰,δ13C cyclohexane <-24‰and δ13C methyl cyclohexane <-24‰.Among the components of C7 light hydrocarbon compound,such as normal heptane(nC7),methyl cyclohexane(MCH)and dimethyl cyclopentane(ΣDMCP),etc,relative contents of nC7 and MCH are influenced mainly by the source organic matter type of natural gas.Therefore,it is suggested that the gas with relative content of nC7 of more than 30%and relative content of MCH of less than 70%is sapropelic-sourced gas,while gas with relative content of nC7 of less than 35%and relative content of MCH of more than 50%is humic-sourced gas.Among components of C5-7 aliphatics, the gas with relative content of C5-7 normal alkane of more than 30%is sapropelic-sourced gas,while the gas with relative content of C5-7 normal alkane of less than 30%is humic-sourced gas.These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.     

Discovery of “Enveloping Surface of Oil and Gas Overpressure Migration” in the Songliao Basin and its bearings on hydrocarbon migration and accumulation mechanisms

The Fuyang oil layer of the Songliao Basin is a tight and low-permeability sandstone pay zone formed in the fluvial-shallow water delta environment.In the formation are mainly lithologic reservoir and tight reservoir.The lacustrine-mudstone of K2qn1 is a good source rock and also acts as a good regional cap rock.The Fuyang oil layer is a typical upper-source and lower-reservoir pattern distributed in a large area.Based on a large number of exploration and development data, a macroscopic enveloping surface is found developed in the Fuyang oil layer, which is below K2qn1.The effective reservoirs within the enveloping surface are commonly saturated with oil, and below the enveloping surface are mainly water layers.The distance from the enveloping surface to the bottom of the source rock is usually 100-350 m and at most 550 m.Through the research of the distribution patterns and the physical properties of the sandbodies above or beneath the source rock, it is concluded that: 1) the enveloping surface is the boundary of the overpressure hydrocarbon migration; 2) the spacial distribution of the pressure release beds controls the direction and the distance of the overpressure hydrocarbon migration; 3) tight oil reservoirs and lenticular oil reservoirs are mainly formed inside the envelope surface, whereas, conventional reservoirs are formed outside the envelope surface as a result of the buoyancy hydrocarbon migration.The discovery of the "overpressure hydrocarbon migration enveloping surface" and the concepts of overpressure hydrocarbon migration and buoyancy hydrocarbon migration not only challenge the old notion that "hydrocarbon migrates along the faults and is distributed along fault belts" in the Fuyang oil layer of the Songliao Basin, give a new explanation to the long-distance-oil-downwards migration (hundreds of meters) and expand the exploration potential of the Fuyang oil layer, and provide a rational guidance to the exploration of syncline plays, but also better categorize tight oil/gas and conventional reservoirs in all of the key elements related to hydrocarbon migration, accumulation, reservoir characteristics and oil and gas spatial distribution.     

Geochemical characteristics and families of the Paleozoic oil seepage and solid bitumen in the Southern Guizhou Depression,SW China

Fifteen oil seepage and solid bitumen samples in the Southern Guizhou Depression were analyzed with GC–MS. Characteristics of molecular markers and carbon isotopes are discussed systemically. The results showed that the oil seepage and solid bitumen samples in the Southern Guizhou Depression could be divided into two families: Ordovician and Siluric samples, and Permian samples. The two families are different in alkanes distribution, biomarkers, aromatic hydrocarbon composition,and stable carbon isotopes; differences mainly caused by source rock variation.     

Carbon isotopic compositions of 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin: Evidence for the source formed in a strongly reducing environment

Although 1-alkyl-2,3,6-trimethylbenzenes and a high relative amount of 1,2,3,4-tetramethylbenzene have been detected in marine oils and oil asphaltenes from Tabei uplift in the Tarim Basin, their bio-logical sources are not determined. This paper deals with the molecular characteristics of typical ma-rine oil asphaltenes from Tabei and Tazhong uplift in the Tarim Basin and the stable carbon isotopic signatures of individual compounds in the pyrolysates of these asphaltenes using flash pyrolysis-gas chromatograph-mass spectrometer (PY-GC-MS) and gas chromatograph-stable isotope ratio mass spectrometer (GC-C-IRMS), respectively. Relatively abundant 1,2,3,4-tetramethylbenzene is detected in the pyrolysates of these marine oil asphaltenes from the Tarim Basin. δ 13C values of 1,2,3,4-tetrame-thylbenzene in the pyrolysates of oil asphaltenes vary from-19.6‰ to-24.0‰, while those of n-alkanes in the pyrolysates show a range from-33.2‰ to-35.1‰. The 1,2,3,4-tetramethylbenzene in the pyro-lysates of oil asphaltenes proves to be significantly enriched in 13C relative to n-alkanes in the pyro-lysates and oil asphaltenes by 10.8‰―15.2‰ and 8.4‰―13.4‰, respectively. This result indicates a contribution from photosynthetic green sulfur bacteria Chlorobiaceae to relatively abundant 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin. Hence, it can be speculated that the source of most marine oil asphaltenes from the Tarim Basin was formed in a strongly reducing water body enriched in H2S under euxinic conditions.     

The Xiamaling oil shale generated through Rhodophyta over 800 Ma ago

A suit of oil shales, predominated by black argillaceous silicalite and finely laminated black-brown shale, has been discovered in a set of carbonaceous-siliceous mudstone formations (350 m in thickness) in the third member of Xiamaling Formation of the Upper Proterozoic Qingbaikou Series (900―873 MaBP), Xiahuayuan, Hebei Province, China. The oil shale, combustible with strong bitumen odour, has su- per-high TOC contents ranging from 21.4% to 22.9%, bitumen “A” contents from 0.58% to 0.88% and oil length from 5.29% to 10.57%. The ultrathin section observation of the shale and the identification of its kerogen demonstrate that its hydrocarbon-generative parent material is mainly benthonic Rhodophyta whose specific tetrasporangia are legible and abundant. It is rarely reported in the literature that such a hydrocarbon-generative parent material, composed mainly of Rhodophyta and with extraordinarily high contents of TOC and bitumen “A”, developed into a set of high-quality source rocks. The extracts of the oil shale are characteristic of richness in 17α(H)-diahopanes and n-alkyl tricyclic terpenoids but low in steranes. Such a biomarker feature is obviously different from that of the extracts from other Proterozoic marine carbonate source rocks of the studied area. Since the biological constitution of this oil shale is rather simple, it is clear that these biomarkers most likely represent to certain extent the specific mo- lecular constitutions of the benthonic Rhodophyta identified in the ultrathin sections of the samples. Studies on its lithologic association and depositional sequences suggest that this suit of the carbona- ceous-siliceous mudstone formation, which contains oil shales, was probably developed in an under- compensation deep-bay environment when a maximum transgression occurred during the formation of the third member of Xiamaling Formation. The high concentration of SiO2 in this organic-rich rock and the positive correlation between TOC and some trace elements such as P, Cu, Ni, W and Mo indicate that this suit of rocks was affected by activities of bottom thermal currents as deposited.     

Mn content of reservoir calcite cement: A novel inorganic geotracer of secondary petroleum migration in the tectonically complex Junggar Basin (NW China)

Electronic probe microanalysis(EPMA) results of reservoir calcite cement from fourteen core samples in the Junggar Basin show that Mn-content varies largely between different samples from below the detect limitation to 4.14%,while it displays a generally good correlation with oil-gas shows.This,therefore,likely indicates that concentration of the Mn-content of the calcite cement has a close rela-tion to the intensity of petroleum fluid charging during hydrocarbon secondary migration.In order to assess this hypothesis,oxygen and strontium isotopic measurements on sixteen calcite veins host in source sequences were carried out to investigate the feature of the oil-source petroleum fluid.Analytical results imply that during hydrocarbon generation and migration,deep hot fluid has dissolved volcanic minerals interlined between mudstone source rocks.As Mn is a kind of typical trace element enriched in volcanic rocks,it is reasonable to conclude that the petroleum fluid formed in the source sequences would be Mn-rich.Consequently,calcite cements precipitated from such Mn-rich petroleum fluid would be Mn-rich accordingly.Due to the geologic chromatographic effect during migration along reservoir rocks,the decreasing of the Mn-content of the reservoir calcite cements indicates the migration direction.Then,this novel geotracer was further successfully applied in the study of hydrocarbon migration in the Junggar Basin in combination with organic geochemical analyses during the hydrocarbon migration.The Mn content of the reservoir calcite cement appears promising as a novel inorganic geotracer for the petroleum migration.This paper represents a search for novel indicators of secondary petroleum migration in tectonically complex basins based on fundamentals of the reservoir fluid-rock interactions.     

Effects of uranium on hydrocarbon generation of hydrocarbon source rocks with type-III kerogen

Organic-inorganic interaction exists universally and is important in the process of mineral resources formation.It is the essential reason why organic oil,gas,coal and inorganic uranium coexist,accumulate,and mineralize in the same sedimentary basins.Hydrocarbon-generating simulation experiment was conducted using low-mature hydrocarbon source rock containing kerogen type III with uranium(UO2CO3 solution)added to study the effects of uranium on the hydrocarbon generation of hydrocarbon source rocks.Experiment results show that uranium can enhance the yield of gas hydrocarbon,promote the total gas output,and increase the total hydrocarbon production(mass or volume).Uranium may lower the hydrocarbon generation threshold temperature and lead to the generation of liquid hydrocarbon in the relative low temperature of hydrocarbon source rock.Uranium can enhance the yield of saturated hydrocarbon,promote the low molecular weight hydrocarbons generating,and in turn increase the content of CH4 and the content of dry gas of the generated hydrocarbons.Uranium is one of the potential inorganic accelerating factors of the immature hydrocarbons.     

Raman characteristics of hydrocarbon and hydrocarbon inclusions

The Raman spectrograms of hydrocarbon standard samples show that: (1) the Raman spectrogram of normal paraffin has very strong peaks of methyl and methylene (from 2700 cm-1 to 2970 cm-1); (2) branch methyl has the particular peak of 748 cm-1±; (3) six cyclic has the particular peak of 804 cm-1±; (4) phenyl has two particular peaks of 988 cm-1± and 3058 cm-1± and the 988 cm-1± peak is stronger than the 3058 cm-1± peak; and (5) hexene has three alkenyl spectrum peaks of 1294 cm-1±, 1635 cm-1± and 2996 cm-1±, with the 1635 cm-1± peak being the strongest, showing that the number of carbon in hy-drocarbon does not affect its Raman spectrogram, and the hydrocarbon molecular structure and base groups affect its Raman spectrogram, the same hydrocarbons (such as normal paraffin) have the same Raman spectrogram; the types (such as CH4, C2H6, C3H8) and the content of hydrocarbon in oil inclu-sions are not estimated by their characteristic Raman peaks. According to the Raman spectrograms of hydrocarbon compositions, the Raman spectrogram of hydrocarbon inclusion can be divided into five types: saturated hydrocarbon Raman spectrogram, fluoresce Raman spectrogram, saturated hydro-carbon bitumen Raman spectrogram, bitumen Raman spectrogram, and ethane Raman spectrogram. And according to the characteristics of Raman spectrogram, hydrocarbon inclusions can be divided into five types: saturated hydrocarbon inclusion, less saturated hydrocarbon (oil or gas) inclusion, saturated hydrocarbon bitumen inclusion, bitumen inclusion, and methane water inclusion.     

Classification and origin of natural gases from Lishui Sag,the East China Sea Basin

Natural gases discovered up to now in Lishui Sag,the East China Sea Basin,differ greatly in gaseous compositions,of which hydrocarbon gases amount to 2%―94%while non-hydrocarbon gases are dominated by CO2.Their hydrocarbon gases,without exception,contain less than 90%of methane and over 10%of C2 heavier hydrocarbons,indicating a wet gas.Carbon isotopic analyses on these hydrocarbon gases showed thatδ13C 1 ,δ13C 2 andδ13C 3 are basically lighter than-44‰,-29‰and-26‰, respectively.The difference in carbon isotopic values between methane and ethane is great,suggesting a biogenic oil-type gas produced by the mixed organic matter at peak generation.δ13C CO2 values of nonhydrocarbon gases are all heavier than-10‰,indicating a typical abiogenic gas.The simulation experiment on hydrocarbon generation of organic matter in a closed gold-tube system showed that the proportion of methane in natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit is obviously higher than that in natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit,consequently the proportion of heavier hydrocarbons of the former is remarkably lower than that of the latter.Moreover, δ13C 1 values of natural gases produced by terrigenous organic matter in the Lingfeng Formation marine deposit are about 5‰heavier than those of natural gases derived from the aquatic and terrigenous mixed organic matter in the Yueguifeng Formation lacustrine deposit whileδ13C 2 andδ13C 3 values of the former are over 9‰heavier than those of the latter.Currently the LS36-1 oil-gas pool is the only commercial oil-gas reservoir in Lishui Sag,where carbon isotopic compositions of various hydrocarbon components differ greatly from those of natural gases produced by the Lingfeng Formation organic matter but are very similar to those of natural gases derived from the Yueguifeng Formation organic matter,therefore,natural gases in the LS36-1 oil-gas pool are mainly derived from the Yueguifeng Formation lacustrine source rock rather than the Lingfeng Formation marine or Mingyuefeng Formation coal-measures source rocks.     

Geochemical features and genesis of the natural gas and bitumen in paleo-oil reservoirs of Nanpanjiang Basin, China

Bitumen from the Nanpanjiang Basin occurs mainly in the Middle Devonian and Upper Permian reef limestone paleo-oil reservoirs and reserves primarily in holes and fractures and secondarily in minor matrix pores and bio-cavities. N2 is the main component of the natural gas and is often associated with pyrobitumen in paleo-oil reservoirs. The present study shows that the bitumen in paleo-oil reservoirs was sourced from the Middle Devonian argillaceous source rock and belongs to pyrobitumen by crude oil cracking under high temperature and pressure. But the natural gas with high content of N2 is neither an oil-cracked gas nor a coal-formed gas generated from the Upper Permian Longtan Formation source rock, instead it is a kerogen-cracked gas generated at the late stage from the Middle Devonian argilla- ceous source rock. The crude oil in paleo-oil reservoirs completely cracked into pyrobitumen and methane gas by the agency of hugely thick Triassic deposits. After that, the abnormal high pressure of methane gas reservoirs was completely destroyed due to the erosion of 2000--4500-m-thick Triassic strata. But the kerogen-cracked gas with normal pressure was preserved under the relatively sealed condition and became the main body of the gas shows.     

Interactions between hydrocarbon-bearing fluids and calcite in fused silica capillary capsules and geological implications for deeply-buried hydrocarbon reservoirs

During the burial processes of deep/ultra-deep hydrocarbon reservoirs,the interactions between hydrocarbon-bearing fluids and reservoirs significantly affect the quality evolution of hydrocarbons and reservoirs;thus,this topic requires further investigation.In this study,the continuous evolution and the coupling mechanisms in various anhydrous and hydrous nC₁₆H₃₄-(water)-(calcite)systems in fused silica capillary capsules(FSCCs)were investigated using laser Raman spectroscopy,fluorescence color analysis,and fluorescence spectroscopy,and the mineral alterations were analyzed using scanning electron microscopy(SEM).The experimental results show that extensive organic-inorganic interactions occur in the systems if water is present,and different inorganic components have different effects on hydrocarbon degradation.Distilled water promotes freeradical thermal cracking and steps oxidation,forming more low-molecular-weight hydrocarbons,CO₂,and organic acids(e.g.,acetic acids)but suppresses the free-radical cross-linking,generating less high-molecular-weight hydrocarbons.However,in the presence of CaCl₂ water,the yields of hydrocarbon gases are lower than in the distilled water system because high concentrations of Ca ions inhibit the generation of free radicals.Calcites,which exhibit different surface reactivities in different fluid conditions,affect hydrocarbon degradation in different ways.In the anhydrous nC16H34-calcite system,calcites promote the generation of both hydrocarbon gases and high-molecular-weight hydrocarbons.In contrast,in the hydrous nC16H34-distilled(CaCl₂)watercalcite system,calcites promote the generation of hydrocarbon gases and suppress the generation of high-molecular-weight hydrocarbons.Calcite also reacts with organic acids via surface reactions to form secondary pores.Therefore,except for the formation temperature and pressure,organic-inorganic interactions are controlled by multiple factors,such as the water saturation,water type,water salinity,and the mineral content,resulting in different evolutions of the hydrocarbon degradation and reservoir properties.     

Distribution and significance of C~_(40) alkanes in the extracts of Cretaceous source rocks from the Songliao Basin

Source rock extracts and crude oils from the Songliao Basin were analyzed by high-temperature gas chromatography (HTGC), gas chromatography-mass spectrometry (HTGC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IRMS), for high molecular-weight alkanes. The distributions of n-alkanes in the Nenjiang Formation extracts are in the C14―C63 range; a bimodal distribution occurs in the C-21 and C21―40 regions. The C30―C37 n-alkanes are accompanied by C29―C35 hopanes, whereas the high molecular-weight C45―C47 n-alkanes co-occur with abundant isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The high δ 13C values of the n-alkanes and the microscopic maceral compositions indicate a highly diversified organic source input for the Nenjiang Formation source rocks, ranging from aquatic plants, blue alge-bacteria, to land plant material. In contrast, n-alkanes in the rock extracts of the Qingshankou Formation are characterized by a single modal distribution, with relatively low abundances of C29―C35 hopanes, but high molecular-weight isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The relatively low δ 13C values of C22―C44 n-alkanes and organic material compositions indicate that the source rocks in the Qingshankou Formation contain dominantly type I algal organic matter. The relative abundance of C 40 compounds in source rocks changes little at low maturity stage, but decreases drastically at higher maturity levels, with a concurrent reduction in the odd/even carbon predominance. In crude oils, in contrast, the relative abundance of C 40 compounds appears to relate closely with the oil source and oil viscosity.     

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(R0%)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 ln(C1/C2)-ln(C2/C3)andδ13C2-δ13C3-ln(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.     

Controls of Late Jurassic-Early Cretaceous tectonic event on source rocks and seals in marine sequences,South China

Thermal evolution of source rocks and dynamic sealing evolution of cap rocks are both subjected to tectonic evolution.The marine sequences in South China have experienced superposed structural deformation from multiple tectonic events.To investigate the effectiveness of preservation conditions,it is of great importance to understand the controls of key tectonic events on the dynamic evolution of cap rocks.This paper discusses the controls of Late Jurassic-Early Cretaceous(J3-K1) tectonic event on source and cap rocks in marine sequences in South China based on the relationships between J3-K1 tectonic event and the burial history types of the marine sequences,the hydrocarbon generation processes of marine source rocks,the sealing evolution of cap rocks,the preservation of hydrocarbon accumulations,and the destruction of paleo-oil pools.The study has the following findings.In the continuously subsiding and deeply buried areas during the J3-K1 period,marine source rocks had been generating hydrocarbons for over a long period of time and hydrocarbon generation ended relatively late.At the same time,the sealing capacity of the overburden cap rocks had been constantly strengthened so that hydrocarbons could be preserved.In the areas which suffered compressional deformation,folding and thrusting,uplifting and denudation in J3-K1,the burial history was characterized by an early uplifting and the hydrocarbon generation by marine source rocks ended(or suspended) during the J3-K1 period.The sealing capacity of the cap rocks was weakened or even vanished.Thus the conditions for preserving the hydrocarbon accumulations were destroyed.The continuously subsiding and deeply buried areas during the J3-K1 period are the strategic precincts of the petroleum exploration in marine sequences in South China.     

The scientific connotation of oil and gas formations under deep fluids and organic-inorganic interaction

As a relatively stable craton block in the earth system, the petroliferous basin is influenced by the evolution of the earth system from the early development environment of source rocks, hydrocarbon formation, and reservoir dissolution to hydrocarbon accumulation or destruction. As a link between the internal and external factors of the basin, deep fluids run through the whole process of hydrocarbon formation and accumulation through organic-inorganic interaction. The nutrients carried by deep fluids promote the bloom of hydrocarbon-generating organisms and extra addition of carbon and hydrogen source, which are beneficial to the development of high-quality source rock and enhancement of the hydrocarbon generation potential. The energy carried by the deep fluid promotes the early maturation of the source rock and facilitates the hydrocarbon generation by activation and hydrogenation in high-mature hydrocarbon sources. The dissolution alteration of carbonate rocks and clastic reservoirs by CO_2-rich deep fluids improves the deep reservoir space, thus extending the oil and gas reservoir space into greater depth. The extraction of deeply retained crude oil by deep supercritical CO_2 and the displacement of CH_4 in shale have both improved the hydrocarbon fluidity in deep and tight reservoirs. Simultaneously, the energy and material carried by deep fluids(C, H, and catalytic substances) not only induce inorganic CH_4 formation by Fischer-Tropsch(F-T) synthesis and "hydrothermal petroleum" generation from organic matter by thermal activity but also cause the hydrothermal alteration of crude oil from organic sources. Therefore, from the perspective of the interaction of the earth's sphere, deep fluids not only input a significant amount of exogenous C and H into sedimentary basins but also improve the reservoir space for oil and gas, as well as their enrichment and accumulation efficiencies.