Geochemical identification of a source rock affected by migrated hydrocarbons and its geological significance:Fengcheng Formation,southern Mahu Sag,Junggar Basin,NW China

The Fengcheng Formation is a crucial source rock and the primary reservoir for oil accumulation in the Mahu Sag.Crude oils are distributed throughout the Fengcheng Formation,ranging from the edge to the interior of the sag in the southern Mahu Sag.These crude oils originate from in-situ source rocks in shallowly buried areas and the inner deep sag.During migration,the crude oil from the inner deep sag affects the source rocks close to carrier beds,leading to changes in the organic geochemical characteristics of the source rocks.These changes might alter source rock evaluations and oil-source correlation.Based on data such as total organic carbon(TOC),Rock-Eval pyrolysis of source rocks,and gas chromatography-mass spectrometry(GC-MS)of the saturated fraction,and considering the geological characteristics of the study area,we define the identification characteristics of source rock affected by migrated hydrocarbons and establish the various patterns of influence that migrated hydrocarbons have on the source rock of the Fengcheng Formation in the southern Mahu Sag.The source rocks of the Fengcheng Formation are mostly fair to good,containing mainly Type II organic matter and being thermally mature enough to generate oil.Source rocks affected by migrated hydrocarbons exhibit relatively high hydrocarbon contents(S1/TOC>110 mg HC/g TOC,Extract/TOC>30%,HC:hydrocarbon),relatively low Rock-Eval Tmax values,and relatively high tricyclic terpane contents with a descending and mountain-shaped distribution.Furthermore,biomarker composition parameters indicate a higher thermal maturity than in-situ source rocks.Through a comparison of the extract biomarker fingerprints of adjacent reservoirs and mudstones in different boreholes,three types of influence patterns of migrated hydrocarbons are identified:the edge-influence of thin sandstone-thick mudstone,the mixed-influence of sandstone-mudstone interbedded,and the full-influence of thick sandstone-thin mudstone.This finding reminds us that the influence of migrated hydrocarbons must be considered when evaluating source rocks and conducting oil-source correlation.

Quantitative effect of kerogen type on the hydrocarbon generation potential of Paleogene lacustrine source rocks,Liaohe Western Depression,China

Kerogen types exert a decisive effect on the onset and capacity of hydrocarbon generation of source rocks.Lacustrine source rocks in the Liaohe Western Depression are characterized by thick deposition,high total organic carbon(TOC)content,various kerogen types,and a wide range of thermal maturity.Consequently,their hydrocarbon generation potential and resource estimation can be misinterpreted.In this study,geochemical tests,numerical analysis,hydrocarbon generation kinetics,and basin modeling were integrated to investigate the differential effects of kerogen types on the hydrocarbon generation potential of lacustrine source rocks.Optimized hydrocarbon generation and expulsion(HGE)models of different kerogen types were established quantitatively upon abundant Rock-Eval/TOC/vitrinite reflectance(R_(o))datasets.Three sets of good-excellent source rocks deposited in the fourth(Es4),third(Es3),and first(Es1)members of Paleogene Shahejie Formation,are predominantly types I-II_(1),II_(1)-II_(2),and II-III,respectively.The activation energy of types I-II_(2)kerogen is concentrated(180-230 kcal/mol),whereas that of type III kerogen is widely distributed(150-280 kcal/mol).The original hydrocarbon generation potentials of types I,II_(1),II_(2),and III kerogens are 790,510,270,and 85 mg/g TOC,respectively.The Ro values of the hydrocarbon generation threshold for type I-III source rocks gradually increase from 0.42%to 0.74%,and Ro values of the hydrocarbon expulsion threshold increase from 0.49%to 0.87%.Types I and II_(1)source rocks are characterized by earlier hydrocarbon generation,more rapid hydrocarbon expulsion,and narrower hydrocarbon generation windows than types II_(2)and III source rocks.The kerogen types also affect the HGE history and resource potential.Three types(conventional,tight,and shale oil/gas)and three levels(realistic,expected,and prospective)of hydrocarbon resources of different members in the Liaohe Western Depression are evaluated.Findings suggest that the Es3 member has considerable conventional and unconventional hydrocarbon resources.This study can quantitatively characterize the hydrocarbon generation potential of source rocks with different kerogen types,and facilitate a quick and accurate assessment of hydrocarbon resources,providing strategies for future oil and gas exploration.

The coupling control of biological precursors and environmental factors onβ-carotane enrichment in alkaline lacustrine source rocks:A case study from the Fengcheng formation in the western Junggar Basin,NW China

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation(Fm.)are typically alkaline lacustrine source rocks,which are typified by impressively abundantβ-carotane.Abundant β-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments.However,the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm.remains obscure.Based on a comprehensive investigation of the bulk,molecular geochemistry,and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes,we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm.The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm.The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane,which is suggested by a good positive correlation between the 2-methylhopane index,7-+8-methyl heptadecanes/C_(max),C_(29%),and β-carotane/C_(max)in sedimentary rocks and the predominance of cyanobacteria with abundantβ-carotene in modern alkaline lakes.The enrichment of β-carotane requires the reducing condition,and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold.The paleoclimate conditions do not considerably impact the enrichment of β-carotane,but they have some influence on the water's paleosalinity by affecting evaporation and precipitation.While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm.,paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.

Influence of Magmatic Intrusion on Abnormal Hydrocarbon Generation and Expulsion of Source Rock:A Case Study of the Dongying Sag,Bohai Bay Basin

How gabbro affects the generation and expulsion of hydrocarbons in muddy surrounding rocks is clarified by analyzing thin section,major and trace elements,total organic carbon(TOC),pyrolysis,extracts and vitrinite reflectance data from source rocks in the Chunxi area the Dongying Sag,Bohai Bay Basin,eastern China.The results show that a magma intrusion brings copious heat to the source rocks,which promotes abnormal maturation of organic matter(OM)and rapid hydrocarbon generation.The CH_(4)and H_(2)produced by gabbro alteration play a role in hydrocarbon generation of source rocks.The hydrothermal process during magma intrusion provides many different minerals to the source rock,resulting in carbonate-rich surrounding mudstone.The carbonate and clay minerals produced by volcanic mineral alteration jointly catalyze the hydrocarbon generation of the source rock.The high-temperature baking of the intrusion results in hydrothermal pressurization and hydrocarbon generation pressurization,causing many fractures in the surrounding rock.The generated oil and gas are discharged through the fractures under diffusion and pressure.Mantle-derived CO_(2)is also conducive to the expulsion of hydrocarbons because of its strong enrichment capacity for hydrocarbons.

Paleoenvironmental Characteristics of Paleogene Lacustrine Source Rocks in the Western Bozhong Sag,Bohai Bay Basin,China:Evidence from Biomarkers,Major and Trace Elements

The organic matter(OM)enrichment mechanisms and depositional environment characteristics of lacustrine source rocks in the western Bozhong Sag,Bohai Bay Basin in Northeast China remain controversial.To address these issues,based on Rock-Eval pyrolysis,kerogen macerals,H/C and O/C ratios,GC-MS,major and trace elements,the Dongying Formation Member(Mbr)3(E_(3)d_(3)),the Shahejie Formation mbrs 1 and 2(E_(2)s_(1+2)),and the Shahejie Mbr 3(E_(2)s_(3))source rocks in the western Bozhong Sag were studied.The above methods were used to reveal their geochemical properties,OM origins and depositional environments,all of which indicate that E_(2)s_(1+2)and E_(2)s_(3)are excellent source rocks,and that E_(3)d_(3)is of the second good quality.E_(3)d_(3)source rocks were formed under a warm and humid climate,mainly belong to fluvial/delta facies,the E_(3)d_(3)sediments formed under weakly oxidizing and freshwater conditions.Comparatively,the depositional environments of E_(2)s_(1+2)source rocks were arid and cold climate,representing saline or freshwater lacustrine facies,and the sediments of E_(2)s_(1+2)belong to anoxic or suboxic settings with large evaporation and salinity.During the period of E_(2)s_(3),the climate became warm and humid,indicating the freshwater lacustrine facies,and E_(2)s_(3)was characterized by freshwater and abundant algae.Moreover,compared with other intervals,the OM origin of E_(3)d_(3)source rocks has noticeable terrestrial input.The OM origin of the E_(2)s_(1+2)and E_(2)s_(3)are mainly plankton and bacteria.Tectonic subsidence and climate change have affected the changes of the depositional environment in the western Bozhong Sag,thus controlling the distribution of the source rocks,the geochemical characteristics in the three intervals of lacustrine source rocks have distinct differences.Overall,these factors are effective to evaluate the paleoenvironmental characteristics of source rocks by biomarkers,major and trace elements.The established models may have positive implications for research of lacustrine source rocks in offshore areas with few drillings.

Differences and identification on multi-time hydrocarbon generation of carboniferous-permian coaly source rocks in the Huanghua Depression,Bohai Bay Basin

Coal is a solid combustible mineral,and coal-bearing strata have important hydrocarbon generation potential and contribute to more than 12%of the global hydrocarbon resources.However,the deposition and hydrocarbon evolution process of ancient coal-bearing strata is characterized by multiple geological times,leading to obvious distinctions in their hydrocarbon generation potential,geological processes,and production,which affect the evaluation and exploration of hydrocarbon resources derived from coaly source rocks worldwide.This study aimed to identify the differences on oil-generated parent macerals and the production of oil generated from different coaly source rocks and through different oil generation processes.Integrating with the analysis of previous tectonic burial history and hydrocarbon generation history,high-temperature and high-pressure thermal simulation experiments,organic geochemistry,and organic petrology were performed on the Carboniferous-Permian(C-P)coaly source rocks in the Huanghua Depression,Bohai Bay Basin.The oil-generated parent macerals of coal's secondary oil generation process(SOGP)were mainly hydrogen-rich collotelinite,collodetrinite,sporinite,and cutinite,while the oil-generated parent macerals of tertiary oil generation process(TOGP)were the remaining small amount of hydrogen-rich collotelinite,sporinite,and cutinite,as well as dispersed soluble organic matter and unexhausted residual hydrocarbons.Compared with coal,the oil-generated parent macerals of coaly shale SOGP were mostly sporinite and cutinite.And part of hydrogen-poor vitrinite,lacking hydrocarbon-rich macerals,and macerals of the TOGP,in addition to some remaining cutinite and a small amount of crude oil and bitumen from SOGP contributed to the oil yield.The results indicated that the changes in oil yield had a good junction between SOGP and TOGP,both coal and coaly shale had higher SOGP aborted oil yield than TOGP starting yield,and coaly shale TOGP peak oil yield was lower than SOGP peak oil yield.There were significant differences in saturated hydrocarbon and aromatic parameters in coal and coaly shale.Coal SOGP was characterized by a lower Ts/Tm and C31-homohopane22S/(22S+22R)and a higher Pr/n C17compared to coal TOGP,while the aromatic parameter methyl dibenzothiophene ratio(MDR)exhibited coaly shale TOGP was higher than coaly shale SOGP than coaly TOGP than coaly SOGP,and coal trimethylnaphthalene ratio(TNR)was lower than coaly shale TNR.Thus,we established oil generation processes and discriminative plates.In this way,we distinguished the differences between oil generation parent maceral,oil generation time,and oil production of coaly source rocks,and therefore,we provided important support for the evaluation,prediction,and exploration of oil resources from global ancient coaly source rocks.

Sedimentary paleoenvironment and its control on organic matter enrichment in the Mesoproterozoic hydrocarbon source rocks in the Ordos Basin,southern margin of the North China Craton

The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city,North China Craton,is a potential source rock.Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms.In this study,we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations,mineral composition analysis,organic geochemistry,and elemental geochemistry.We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale.The Cuizhuang Formation was deposited in a late-stage,restricted basin environment during the rift phase,and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic-anoxic water environment,that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities,which promoted the accumulation of organic matter.However,the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions.The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities.Warm conditions promote continental weathering and increase marine productivity,thereby consuming oxygen in the bottom water.Moreover,acidic hydrothermal activity also helps to establish an anoxic environment.Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions,which may explain the development of source rocks in the Cuizhuang Formation.

Hydrocarbon Generation Characteristics of Coal-measure Source Rocks and their Contribution to Natural Gas:A Case Study of Middle and Lower Jurassic Targets from the Southern Junggar Basin Margin

In order to study the hydrocarbon generation(HCGE)characteristics of coal-bearing basins,the coal-measure source rocks of the Middle Jurassic-Lower Jurassic(MLJ)of the piedmont thrust belt in the southern margin of the Junggar Basin in Northwest China are taken as research objects.More than 60 MLJ samples were collected from outcrops and wells.Total organic carbon(TOC),rock pyrolysis(Rock-Eval),organic petrological,vitrinite reflectance(%Ro),and hydrous pyrolysis were performed to analyze the relevant samples.The pyrolysis gases and liquid products were measured,and then the chemical composition,as well as carbon isotopes of the gases,were analyzed.The results indicate that the MLJ source rocks have the capacity for large-scale gas generation.In addition,for coal-measure source rocks,the heavier the carbon isotope of kerogen(δ^(13)C_(kerogen)),the lower the liquid hydrocarbon and hydrocarbon gas yield,and the easier it is to produce non-hydrocarbon gas.It is worth noting that when theδ^(13)C_(kerogen)in organic matter(OM)is relatively heavier,the fractionation of its products may become weaker in the evolutionary process.The vital contribution of the MLJ source rock to natural gas resources in the study area was further confirmed by comparing it with the Jurassic source gas.

The Lower Cambrian Xiaoerbulake Formation in the Tarim Basin as a potential carbonate source rock

The oil and gas exploration of the Middle and Lower Cambrian in the Tarim Basin reveals widely distributed source rocks with the Yuertusi Formation being recognized as high-quality source rocks that are distributed in a rather small range.The Xiaoerbulake Formation that is right under the Yuertusi Formation has also been eyed as potential high-quality source rocks and is studied through analyses focusing on the stratigraphic development,the abundance,type,and maturity of organic matter,and the paleoproductivity of a dark-colored algae dolomite within the formation.The results show that the dolomite is rich in organic matter of mainly types Ⅰ and Ⅱ kerogens.Although reached the high mature to over-mature stage,the dolomite was deposited in an anoxic sedimentary environment featuring a high paleoproductivity level and a high organic carbon burial efficiency,quite favorable for the development of high-quality source rocks.The study provides material evidence to the Middle-Lower Cambrian subsalt source rock-reservoir-caprock combination model for the Tarim Basin.

Prediction and quantification of effective gas source rocks in a lacustrine basin:Western Depression in the Liaohe Subbasin,China

Due to limited data on the geochemical properties of natural gas,estimations are needed for the effective gas source rock in evaluating gas potential.However,the pronounced heterogeneity of mudstones in lacustrine successions complicates the prediction of the presence and geochemical characteristics of gas source rocks.In this paper,the Liaohe Subbasin of Northeast China is used as an example to construct a practical methodology for locating effective gas source rocks in typical lacustrine basins.Three types of gas source rocks,microbial,oil-type,and coal-type,were distinguished according to the different genetic types of their natural gas.A practical three-dimensional geological model was developed,refined,and applied to determine the spatial distribution of the mudstones in the Western Depression of the Liaohe Subbasin and to describe the geochemical characteristics(the abundance,type,and maturation levels of the organic matter).Application of the model in the subbasin indicates that the sedimentary facies have led to heterogeneity in the mudstones,particularly with respect to organic matter types.The effective gas source rock model constructed for the Western Depression shows that the upper sequence(SQ2)of the Fourth member(Mbr 4)of the Eocene Shahejie Formation(Fm)and the lower and middle sequences(SQ3 and SQ4)of the Third member(Mbr 3)form the principal gas-generating interval.The total volume of effective gas source rocks is estimated to be 586 km^(3).The effective microbial,oil-type,and coal-type gas source rocks are primarily found in the shallow western slope,the central sags,and the eastern slope of the Western Depression,respectively.This study provides a practical approach for more accurately identifying the occurrence and geochemical characteristics of effective natural gas source rocks,enabling a precise quantitative estimation of natural gas reserves.

Application of C_(30)tetracyclic polyprenoids as effective biomarker in oil-to source rock correlation in the ZhuⅢdepression,Zhujiangkou Basin,northern South China

The northern South China Sea,including the Zhujiangkou Basin and the Beibuwan Basin,developed high-quality lacustrine source rocks during the Eocene rifting period.These source rocks are vital for hydrocarbon generation in the northern South China Sea.The ZhuⅠdepression in the Zhujiangkou Basin and the Beibuwan Basin typically exhibit high abundance of C_(30)4-methyl steranes.However,shales in the Eocene Wenchang Formation in the ZhuⅢdepression of the Zhujiangkou Basin contains lower quantities of high-quality lacustrine source rocks with 4-methyl steranes,which often co-elute with some pentacyclic triterpanes in gas chromatography-mass spectrometry(GC-MS).Therefore,the single 4-methylsterane parameter based on GC-MS cannot accurately distinguish organic source in the deep to semi-deep water lacustrine source rocks of the Wenchang Formation from other source rocks,thus impeding the recognition of their contributions to petroleum reservoirs.In this study,GC-MS of aliphatic hydrocarbons,palynofacies and algal identification,as well as stable carbon isotope data of organic matter were used to identify the algal species and construct the paleoclimate during deposition of the Wenchang Formation source rocks in the ZhuⅢdepression of the Zhujiangkou Basin.It is suggested that during the Wenchang Formation period,freshwater green algae prevailed in the lake,which is likely account for the relatively low content of 4-methyl steranes in the high-quality lacustrine source rocks.Controlled by the algal species,it is proposed that the content of C_(30)tetracyclic polyprenoids(TPP)can better indicate the quality of the Wenchang source rocks than C_(30)4-methyl steranes.Consequently,a relationship between the TPP index and the quality of the lacustrine source rocks in the Wenchang Formation of the ZhuⅢdepression was established.A higher TPP index indicates higher organic matter abundance and hydrogen index of the lacustrine source rocks.When applied to the origin analysis of oils in the ZhuⅢdepression,it is believed that the organic-rich deep lacustrine source rocks in the Wenchang Formation made great contribution to the transitional zone crude oils in the Wenchang A and Wenchang B depressions.

Preliminary Study of Chemical Elements Distribution in Petroleum Source Rocks Donga and Yogou Formations of the Termit Sedimentary Basin (Est-Niger)

XRF and EDX analyses were carried out on 18 batches of representative raw samples to determine the distribution of major chemical elements in the petroleum source rocks of Donga and Yogou formations of Termit sedimentary basin. The chemical composition of these formations is dominated by silicon (Si), aluminum (Al) and iron (Fe). This is consistent with the oxide composition, which is also dominated by silicon oxide (SiO2), aluminum oxide (Al2O3) and iron monoxide (FeO). No less important chemical elements are calcium (Ca), potassium (K), sulfur (S), titanium (Ti), magnesium (Mg), manganese (Mn) and barium (Ba), as well as some of their oxides. All these major chemical elements are carried by silicate detrital minerals associated with pyrite and goethite and/or clay minerals such as kaolinite and interstratified illite, smectite and chlorite. This trend is illustrated by the values of the Si/Al and SiO2/Al2O3 ratios.

The influence of organic sources and environments on source rock deposition during the periods of Cretaceous–Eocene and Oligocene–Miocene,northern Kalimantan

The sedimentary system of Kalimantan has undergone significant development since the Oligocene.Previous research have largely ignored the capacity of the Cretaceous–Eocene sediments to produce hydrocarbons,focusing instead primarily on the Oligocene–Miocene coal as the principal source rocks.Shales and coals from the outcrops in the northern margin of Kalimantan were analyzed with palynological and geochemical methods to characterize the palaeoenvironmental and palaeoecological differences between the Cretaceous–Eocene and the Oligocene–Miocene samples.The high proportion of Cheirolepidoaceae,Schizaeoisporites and Ephedripites in the pollen assemblage from the Cretaceous–Eocene outcrops reflects an arid tropical/subtropical climate.The relatively low abundances of gymnosperm-derived biomarkers including isopimarane,β-phyllocladane,β-kaurane,suggest the gymnosperm features in flora.High C_(27)/C_(29)ααα20R sterane ratios,(C_(19)–C_(29))tricyclic terpanes/C_(30)αβhopane and extremely low oleanane/C_(30)αβhopane,bicadinane T/C_(30)αβhopane,and diterpenoid abundance indicate that there was a dominance of algae relative to higher plants in the organic matter.The gymnosperm-derived biomarkers,including isopimarane,β-phyllocladane,β-kaurane,suggest that palaeovegetation during this period was dominated by gymnosperms.The saline and reducing conditions in the bathyal and abysmal sea,evidenced by rather low Pr/Ph and high Gammarerane index,are beneficial for the preservation of hydrogen-rich organic matter.It is presumed that the Cretaceous–Eocene shales had great hydrocarbon generation potential in the southern South China Sea.During the period of Oligocene to Miocene in the Zengmu Basin and the Baram-Sabah Basin,the climate changed to a dominant humid and warm condition,which is corroborated by abundant pollen of Florschuetzia and Magnastriatites hawardi.Low C_(27)/C_(29)ααα20R sterane ratios,(C_(19)–C_(29))tricyclic terpanes/C_(30)αβhopane,and high oleanane/C_(30)αβhopane,bicadinane T/C_(30)αβhopane suggest that the palaeovegetation was dominated by angiosperms including the mangrove plants.The extremely abundant higher plants provide ample terrigenous organic matter for the formation of coal-measures in delta facies.The low gammacerane index and high Pr/Ph indicate the fresh and sub-oxic water in delta-neriticabysmal faces,which is not beneficial for the accumulation of hydrogen-rich organic matter.Thus,the Oligocene–Miocene marine argillaceous rocks can be potential sources of natural gas.

Evaluation of the oil and gas preservation conditions, source rocks, and hydrocarbongenerating potential of the Qiangtang Basin: New evidence from the scientific drilling project

The Qiangtang Basin of the Tibetan Plateau,located in the eastern Tethys tectonic domain,is the largest new marine petroliferous region for exploration in China.The scientific drilling project consisting primarily of well QK-1 and its supporting shallow boreholes for geological surveys(also referred to as the Project)completed in recent years contributes to a series of new discoveries and insights into the oil and gas preservation conditions and source rock evaluation of the Qiangtang Basin.These findings differ from previous views that the Qiangtang Basin has poor oil and gas preservation conditions and lacks high-quality source rocks.As revealed by well QK-1 and its supporting shallow boreholes in the Project,the Qiangtang Basin hosts two sets of high-quality regional seals,namely an anhydrite layer in the Quemo Co Formation and the gypsum-bearing mudstones in the Xiali Formation.Moreover,the Qiangtang Basin has favorable oil and gas preservation conditions,as verified by the comprehensive study of the sealing capacity of seals,basin structure,tectonic uplift,magmatic activity,and groundwater motion.Furthermore,the shallow boreholes have also revealed that the Qiangtang Basin has high-quality hydrocarbon source rocks in the Upper Triassic Bagong Formation,which are thick and widely distributed according to the geological and geophysical data.In addition,the petroleum geological conditions,such as the type,abundance,and thermal evolution of organic matter,indicate that the Qiangtang Basin has great hydrocarbon-generating potential.

Gas source of the Middle Jurassic Shaximiao Formation in the Zhongjiang large gas field of Western Sichuan Depression:Constraints from geochemical characteristics of light hydrocarbons

The Zhongjiang gas field is a typical large gas field in terrigenous strata of the Western Sichuan Depression.It remains debatable which member of the Upper Triassic Xujiahe Formation served as the source rocks and how significant the member contributed to the gas accumulations in the Zhongjiang gas field.In this study,we analyzed the essential characteristics of the Lower Jurassic source rocks and the geochemical features of light hydrocarbons in natural gas from the 2nd(T_(3)χ^(2))and 4th members(T_(3)χ^(4))of the Upper Triassic Xujiahe Formation(T_(3)χ),as well as the Middle Jurassic Shaximiao(J_(2)s)and Qianfoya(J_(2)q)formations.Based on this,we explored the sources of the natural gas in the Zhongjiang gas field and determined the natural gas migration patterns and their effects on the properties of light hydrocarbons in the natural gas.The results indicate that the Lower Jurassic lacustrine source rocks of the Zhongjiang gas field contain humic organic matter,with vitrinite reflectance(R_(0))values ranging from 0.86%to 0.98%.Samples meeting the criterion for effective source rocks[total organic carbon(TOC)content≥0.75%]exhibited an average TOC content of merely 1.02%,suggesting significantly lower hydrocarbon generation potential than source rocks in the underlying T3x,which show higher thermal maturity and TOC contents.For natural gas samples from T_(3)χ^(2),T_(3)χ^(4),J_(2)s,and J_(2)q reservoirs,their C_(5-7)iso-alkane content was significantly higher than their n-alkane content,and their methylcyclohexane(MCH)index ranged from 59.0%to 77.3%,indicating the predominance of methylcyclohexane in C_(7)light hydrocarbons.As indicated by the origin identification and gas-source correlation based on the geochemical features of light hydrocarbons,the natural gas in the Zhongjiang gas field is typical coal-derived gas.The gas from the primary pay zone of the Shaximiao Formation,with significantly high K_(1),(P_(2)+N_(2))/C_(7),and P_(3)/C_(7)values,predominantly originated from the 5th member of the T3x and migrated in the free phase,with a small amount possibly sourced from the Lower Jurassic source rocks.The dissolution and adsorption during gas migration led to a decrease in the aromatic content in C_(6-7)light hydrocarbons and an increase in the isoheptane values.Therefore,their effects must be considered when determining the gas origin and thermal maturity based on the aromatic content in C_(6-7) light hydrocarbons and iso-heptane values.

Evolutions of sedimentary facies and palaeoenvironment and their controls on the development of source rocks in continental margin basins:A case study from the Qiongdongnan Basin,South China Sea

Hydrocarbon resources in the Qiongdongnan Basin have become an important exploration target in China.However,the development of high-quality source rocks in this basin,especially in its deep-water areas,are still not fully understood.In this study,evolutions of sedimentary facies and palaeoenvironment and their influences on the development of source rocks in diverse tectonic regions of the Qiongdongnan Basin were investigated.The results show that during the Oligocene and to Miocene periods,the sedimentary environment of this basin progressively varied from a semi-closed gulf to an open marine environment,which resulted in significant differences in palaeoenvironmental conditions of the water column for various tectonic regions of the basin.In shallow-water areas,the palaeoproductivity and reducibility successively decrease,and the hydrodynamic intensity gradually increases for the water columns of the Yacheng,Lingshui,and Sanya-Meishan strata.In deep-water areas,the water column of the Yacheng and Lingshui strata has a higher palaeoproductivity and a weaker hydrodynamic intensity than that of the Sanya-Meishan strata,while the reducibility gradually increases for the water columns of the Yacheng,Lingshui,and Sanya-Meishan strata.In general,the palaeoenvironmental conditions of the water column are the most favorable to the development of the Yacheng organic-rich source rocks.Meanwhile,the Miocene marine source rocks in the deep-water areas of the Qiongdongnan Basin may also have a certain hydrocarbon potential.The differences in the development models of source rocks in various tectonic regions of continental margin basins should be fully evaluated in the exploration and development of hydrocarbons.

Factors controlling the formation and evolution of source rocks in the Shahezi Formation,Xujiaweizi fault depression,Songliao Basin

The types and quality of source rocks in the Shahezi Formation are the key factors affecting the distributions of various deep gas reservoirs in the Xujiaweizi fault depression in Songliao Basin.To clarify the quality differences and origins of different types of source rocks in the Shahezi Formation,this study reconstructed the sedimentary and water environment,determined the controlling effects of fault activity,sedimentary facies,and paleo-sedimentary environment on the quality of various source rocks,by making full use of seismic,logging,core,organic geochemical and element geochemical analysis.The results show that two types of source rocks developed in the Shahezi Formation,namely,mudstones and coals.The mudstones have a relatively high abundance of organic matter,which consists of type-Ⅱ kerogen and partial type-Ⅲ kerogen,and are concentrated in Sha-I Member.The coals have a high abundance of organic matter,which consist of type-Ⅲ kerogen,and are mainly distributed in Sha-Ⅱ Member.During the deposition of Sha-I Member,intense fault activity formed arrow,deep-water lacustrine basins with high salinity and strong reducibility on the downthrow sides of faults.During the deposition of Sha-II Member,fault activity progressively weakened,and the areas of lacustrine basins enlarged to their maximum values and became wide,shallow-water basins with low salinity and low reducibility.The development of source rocks was controlled by fault activity,sedimentary facies,and paleo-sedimentary environment.Fault activity formed accommodation space on the downthrown sides of faults for mudstone accumulation,thus determining mudstone thickness.The sedimentary environment controlled the organic matter input and determined the distribution of mudstones and coals.The paleo-sedimentary environment,which consisted of paleo-salinity,as well as paleo-water depth and redox conditions,affected the accumulation and preservation of organic matter and is the main controlling factor for the quality difference of various source rocks in the Shahezi Formation.

Cosmic Contributions to the Deposition of Petroleum Source Rocks: Review and Analysis

The development of globally distributed Phanerozoic petroleum source rocks is concentrated on time intervals, which correlate convincingly with climatic driven glaciation epochs of Earth’s history, repeated every 150 million years, and during sea level high stands and maxima of global magmatism with a period of 300 million years. The 150 million year periodicity appears to be related to the path of the solar system through the spiral arms of the Milky Way and the 300 million year periodicity to changes of the spiral system. The spiral arms are preferred birth places of new stars, of which the larger ones have only smaller lifespans. Their preliminary deaths ended with explosions and selectively with the development of so-called white dwarfs, neutron stars or black holes. The times of the explosions of intermediate (sun-like) stars can be determined by measuring the present brightness of the dwarfs. Not surprisingly the last two maxima of recordable near solar system star explosions took place during the presumably spiral arms driven glacial epochs in Eocene to present and Upper Jurassic times. Such near solar system star explosions may have been the source of intense neutrino showers, cosmic rays and star dust. This dust contained all kinds of chemical elements, including phosphorus and uranium. Such cosmic phosphorus may have supported, through fertilizing, the distribution of life on Earth additionally to local phosphorus resources via bloom of biota in lakes and oceans and the enhanced growth of plants on land across all climatic zones. Subsequently it maintained the development of petroleum source rocks of all organic matter types within black shales and coals. Via the distribution of remnants of exploding stars—mainly white dwarfs, but neutron stars and black holes have to be counted as well—a cosmic contribution can therefore casually linked to the deposition of petroleum source rocks on Earth, not only purely correlatively by their contemporaneous appearances.

Impact of microorganism degradation on hydrocarbon generation of source rocks:A case study of the Bozhong Sag,Bohai Bay Basin

The discovery of the Bozhong 19-6 gas field,the largest integrated condensate gas field in the eastern China in 2018,opened up a new field for the natural gas exploration deep strata in the Bohai Bay Basin,demonstrating there is a great potential for natural gas exploration in oil-type basins.The ethane isotope of the Bozhong 19-6 condensate gas is heavy,showing the characteristics of partial humic gas.In this paper,aimed at the source rocks of the Bozhong 19-6 gas field in the Bohai Bay Basin,the characteristics of the source rocks in the Bozhong 19-6 structural belt were clarified and the reason are explained from impact of microorganism degradation on hydrocarbon generation of source rocks why the condensate oil and gas had heavy carbon isotope and why it showed partial humic characteristics was explored based on the research of parent materials.The following conclusions were obtained:The paleontology of the Bozhong 19-6 structural belt and its surrounding sub-sags is dominated by higher plants,such as angiosperm and gymnosperm.During the formation of source rocks,under the intensive transformation of microorganism,the original sedimentary organic matter such as higher plants was degraded and transformed by defunctionalization.Especially,the transformation of anaerobic microorganisms on source rocks causes the degradation and defunctionalization of a large number of humic products such as higher plants and the increase of hydrogen content.The degradation and transformation of microorganism don't transform the terrestrial humic organic matter into newly formed“sapropel”hydrocarbons,the source rocks are mixed partial humic source rocks.As a result,hydrogen content incrased and the quality of source rocks was improved,forming the partial humic source rocks dominated by humic amorphous bodies.The partial humic source rocks are the main source rocks in the Bozhong 19-6 gas field,and it is also the internal reason why the isotope of natural gas is heavy.

Supply of terrigenous organic matter from tidal flat to the marine environment:An example of neritic source rocks in the Eocene Pinghu Formation,Xihu Depression,East China Sea Shelf Basin

The terrigenously-dominated marine shales which were deposited in the lower Eocene Pinghu Formation were thought to be a potential source rock in the Xihu Depression of the East China Sea Shelf Basin.However,the exceptionally high total organic carbon content(TOC,>6%on average)of the tidal sand ridge samples was not compatible with their sedimentary environment,indicating coal-bearing sedimentary debris may have been transferred from the coast to the ocean.In this study,new sights into the origins and supply of organic materials in the coastal environment were proposed in the neritic organic matter of the Eocene Pinghu Formation.A discriminant model was developed using plynofacies analysis data to pinpoint the source of organic materials in marine source rocks.The discrimination results suggested that marine mudstones were associated with tidal flat mudstones rather than deltaic ones.The biomarker characteristics of mudstones deposited in various environments support this assertion,indicating that the supply of plant materials in tidal flats is the primary organic matter source for the marine environment.The organic matter abundance was elevated in tidal flats due to their superior preservation conditions.Additionally,the lithological assemblage of tidal flats suggests that tidal currents can scour marshes and then transport dispersed terrigenous organic materials to neritic areas.These findings indicate that coal-bearing sedimentary debris was likely transferred from the coast to the ocean,and tidal currents are thought to be the dominant mechanism driving organic matter from the tidal to the marine environment.