Differences in and factors controlling organic matter enrichment in the Ziliujing Formation shale in the Sichuan Basin

Lacustrine shale oil and gas are important fields for unconventional exploration and development in China,and organic-rich shale deposition lays down the critical foundation for hydrocarbon generation.There are two sets of shale,the Dongyuemiao and Da’anzhai Members,in the Ziliujing Formation in the Sichuan Basin.To identify the differential enrichment characteristics of organic matter and clarify its controlling factors,geochemical analyses of organic and inorganic geochemical analyses were performed.The results showed that the total organic carbon content of the Dongyuemiao shale(1.36%)is slightly higher than that of the Da’anzhai shale(0.95%).The enrichment of organic matter in the two shales resulted from the comprehensive controls of paleoproductivity,paleoenvironment,and terrigenous input,but different factors have different effects.In addition,driven by climate,the change in the sulfate concentration in the bottom water further led to the different intensities of bacterial sulfate reduction in early diagenesis.This made a great difference regarding organic matter accumulation in the two members.In general,climate may have played a dominant role in organic matter enrichment in the two sets of shale.

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.

Source of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian black shale in western Hubei Province,China:Insights from geochemical and petrological analysis

To improve the understanding of the relationship between silica source and organic matter accumulation,the origin of silica and its implications for organic matter enrichment in the Upper OrdovicianLower Silurian(O_(3)w-S_(1)l)black shale in western Hubei Province in the middle Yangtze area,were investigated through geochemical and petrological analyses.The results show that the O_(3)w-S_(1)l black shale is mainly composed of five lithofacies with varying graptolite abundance,total organic carbon(TOC),and silica contents.Biogenic silica and terrigenous siliciclastic input constitute the main silica sources in O_(3)w-S_(1)l black shale and they exhibit an upward inverse variation trend interpreted to be related to sea-level changes.Moreover,with the increase in biogenic silica content or decrease in terrigenous siliciclastic input,TOC values in black shale initially rise and then fall,which is different from originally expected simple linear relationship.We infer that organic matter enrichment and the distribution of silica from different origins are controlled by sea-level changes and variations in terrigenous input in a continental shelf with little hydrothermal influence.An environment with appropriate sea level and terrigenous input should be most favorable for organic matter accumulation,rather than one with excessive high sea level and less terrigenous input.

Paleoenvironment and organic matter enrichment of the Carboniferous volcanic-related source rocks in the Malang Sag,Santanghu Basin,NW China

Volcanic activity was quite frequent during the deposition of the Late Carboniferous Ha’erjiawu Formation in the Santanghu Basin.The petrology and organic and inorganic geochemical indicators were used to investigate hydrocarbon potential,paleoenvironmental conditions and organic matter enrichment of the mudstones.The results show that the oil generation capacity of the Ha’erjiawu Formation mudstones,which has abundant oil-prone organic matter(TypeⅡkerogen with hydrogen index values mainly ranging from 250 to 550 mg HC/g TOC)in mature stage(Tmax values mainly ranging from 435 to 450℃),is considerable.The Ha’erjiawu Formation was deposited in a dysoxic,freshwater-mildly brackish,and warm-humid environment.During its deposition,the Ha’erjiawu Formation received hydrothermal inputs.The volcanic hydrothermal activities played an important role in the organic matter enrichment.In addition,the total organic carbon(TOC)is significantly positively correlated with the felsic mineral content,but it is negatively correlated with the carbonate mineral content and C27/C29 ratios,indicating that terrigenous organic matter input also contributed to the primary productivity in the surface water.Therefore,the formation of the high-quality source rocks in the Ha’erjiawu Formation was jointly affected by the hydrothermal activity and the terrigenous organic matter input.

Organic matter enrichment in the first member of the Xiagou formation of the lower Cretaceous in the Jiuquan Basin, China

The sources and enrichment of organic matter in a sediment core in the first member of the Xiagou Formation(K_1g^1) from the Chang 2-2 borehole of the Jiuquan Basin,NW China,have been examined using Rock-Eval,maceral,carbon isotopes and biomarker data.This data indicates that highly variable organic matter sources and preservation conditions in response to climate change.TOC content,HI,and δ^(13)C value were strongly correlated with the abundance of gammacerane,woody organic matter content,steranes/hopanes ratio,and C_(29) sterane content.This correlation demonstrates the importance that the control of the salinity of the depositional environment and organic matter sources can have upon the enrichment,type,and carbon isotopic composition of organic matter.In the Jiuquan Basin's relatively high temperature and arid climate,high salinity lakes with high primary productivity of algae,planktons,and bacteria,and good organic matter preservation conditions(anoxic bottom water) resulted in the enrichment of isotopically-light algae-bacterial organic matter.In the Jiuquan Basin's regions with a relatively low temperature and wet climate,fresh lakes with low primary productivity of algae,planktons,and bacteria received significant terrigenous high plants input,resulting in the deposition of a low abundance of isotopically heavier terrestrial organic matter.

Environmental Effects on Differential Organic Matter Enrichment of the Lower Cambrian Shale,Tarim Basin,NW China

Early Cambrian shale is an important petroleum source rock around the world.Because of little drilling data and poor seismic data,until recently,organic matter enrichment of the Lower Cambrian Yuertusi and Xishanbulake formations shale is still an enigma in the Tarim Basin,northwestern China.Total organic carbon(TOC),major and trace element data of Cambrian shale samples from five boreholes have been analyzed to decipher the mechanism of the organic matter enrichment.The results show that the shales deposited in the western restricted intraplatform have much higher TOC contents(3.2%-19.8%,on average 11.0%)than those from the eastern basin(2.2%-10.2%,on average 4.5%).The paleoproductivity proxies(Ba,Ba/Al,P/Al)in the western restricted platform are much higher than those in the eastern basin.The trace element indicators such as V/Cr,Ni/Co,Mo-TOC and Mo_(EF)-U_(EF)suggest an anoxic environment across the basin,but a more restricted environment in the western intraplatform.The paleoproductivity rather than anoxic condition and hydrothermal activity are concluded to have resulted in the differentiation of the organic matter enrichment from the western intraplatform to eastern basin in the early Cambrian shales;the restricted environment was favorable for paleoproductivity and preservation of organic matter.

Factors Controlling Organic Matter Enrichment in Alkaline Lacustrine Source Rocks:A Case Study of the Late Paleozoic Fengcheng Formation in the Junggar Basin,NW China

The late Paleozoic Fengcheng Formation shale(LPF shale)in the Junggar Basin,NW China,is the oldest alkaline source rock discovered in the world,providing a unique perspective with which to explore organic matter(OM)enrichment in alkaline lake environments.Combined with the organic carbon isotope profile and paleoenvironmental proxies,this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment,accompanied by frequent volcanic activity.High TOC values are concentrated in two intervals with frequent fluctuations in OM types.A negative excursion due to changes in sedimentary OM source is found in the δ^(13)C_(org) profile.The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu.This implies that the extreme arid climate has led to high salinity,resulting in strong reducibility and changes in paleontological assemblages,which in turn controlled the differential enrichment of OM.The Fengcheng Fm.high-quality source rocks are the result of the combined action of climatic events,volcanism,high-salinity water environment and superior hydrocarbon-generating organisms.The results provide new insights into the formation conditions of terrestrial alkaline high-quality source rocks and the factors controlling alkaline OM enrichment.

Constraints of palaeoenvironment on organic matter of Benxi Formation shale and discussion on enrichment mechanism under different facies

As a hydrocarbon-rich sedimentary basin in China,the Ordos Basin has enormous potential for shale gas resources.The shale of the Upper Carboniferous Benxi Formation is rich in organic matter,however,its palaeoenvironment and organic matter enrichment mode are yet to be revealed.In this study,the geochemical characteristics of the shale of the Benxi Formation in the east-central part of the Ordos Basin were analyzed to investigate its palaeoenvironment.At the same time,the organic matter enrichment modes in different sedimentary facies were compared and analyzed.The results indicate that:1)the shale of the Benxi Formation was mainly deposited on the continental margin and strong terrestrial clastic input;2)the deposition period of the Benxi Formation shale had a hot and humid climate with high palaeoproductivity and local volcanic hydrothermal fluid,and a high sedimentation rate with the strong stagnant environment.The bottom water was in dysoxic conditions and a semi-saline deposition environment;3)multiple factors,such as palaeoproductivity,volcanic hydrothermal,redox conditions,and palaeosalinity interact to influence the enrichment of shale organic matter in Benxi Formation;4)the organic matter enrichment modes of continental,marine-continental transitional,and marine shales can be classified into three types:“production mode”,“hybrid mode of preservation and production”,and“preservation mode”,respectively.This study provides a reference for the organic matter enrichment mode,shale gas formation conditions,and core area evaluation in these marine-continental transitional shales,and also offers new guidance for exploration ideas for shale gas in different sedimentary facies.

Reconstruction of sedimentary paleoenvironment of Permian Lucaogou Formation and its implications for the organic matter enrichment in south-eastern Junggar Basin,China

The Permian Lucaogou Formation represents one of the most important hydrocarbon source rock intervals in the Junggar Basin,although the sedimentary paleoenvironment and organic matter enrichment mechanism of the Lucaogou Formation remain controversial.We studied the temporal evolution of the sedimentary paleoenvironment in the Lucaogou Formation by analyzing the elemental composition and total organic carbon content of 27 hydrocarbon source rock samples from the J305 well in the Jimsar Sag.Using these data,we found that the Lucaogou Formation overall was deposited in a semisaline to saline,reducing lake basin under an arid climate.We identified five organic matter-enriched intervals,which can be correlated with the parameters that indicate a wetter climate and a more anoxic lake environment.To compare sedimentary environments spatially,we compiled environmental indicators from 10 cores and outcrops in three sags around the Bogda Mountains.The compilation shows that the organic matter-enriched Jimsar Sag experienced a more arid climate and a more saline and anoxic lake environment during the deposition of the Lucaogou Formation,which was possibly controlled by the paleogeographic position.We conclude that the spatially arid climate and anoxic environment induced organic matter burial in the Jimsar Sag,while temporal events of a more humid climate and more anoxic environment triggered the enrichment of organic matter in some intervals of the Lucaogou Formation.

Sedimentary environment and model for lacustrine organic matter enrichment:lacustrine shale of the Early Jurassic Da’anzhai Formation,central Sichuan Basin,China

Based on the analysis of element geochemistry and total organic carbon(TOC),this study investigates the main factors controlling organic matter(OM)enrichment,reconstructs the evolution process of the sedimentary environment,and proposes a dynamic OM enrichment model of the Jurassic Da’anzhai(D)Formation,Sichuan Basin.The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment,but with some peculiarities,such as a hotter climate in the D1 member and more anoxic lake water in the D2a member.The sedimentary evolution of the Da’anzhai Formation can be divided into a fluctuating sedimentary stage,a stable sedimentary stage and a reef-building stage.The D2a member showed the strongest hypoxia,the weakest weathering,the largest amount of terrestrial inputs,and the highest TOC content.The TOC is positively correlated with reducing conditions and terrestrial inputs,negatively correlated with weathering.Based on these findings,it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin.Thus,the anoxic deep water,high terrestrial inputs,and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.

Resolving the influence of nitrogen abundances on sediment organic matter in macrophyte-dominated lakes, using fluorescence spectroscopy

A controlled experiment was designed to resolve the influence of nitrogen abundance on sediment organic matters in macrophyte-dominated lakes using fluorescence analysis.Macrophyte biomass showed coincident growth trends with time, but different variation rates with nitrogen treatment. All plant growth indexes with nitrogen addition（N, NH4Cl100, 200, 400 mg/kg, respectively） were lower than those of the control group. Four humiclike components, two autochthonous tryptophan-like components, and one autochthonous tyrosine-like component were identified using the parallel factor analysis model. The results suggested that the relative component changes of fluorescence in the colonized sediments were in direct relation to the change of root biomass with time. In the experiment, the root formation parameters of the plants studied were significantly affected by adding N in sediments, which may be related to the reason that the root growth was affected by N addition.Adding a low concentration of N to sediments can play a part in supplying nutrients to the plants. However, the intensive uptake of NH4^＋may result in an increase in the intracellular concentration of ammonia, which is highly toxic to the plant cells. Hence, our experiment results manifested that organic matter cycling in the macrophyte-dominated sediment was influenced by nitrogen enrichment through influencing vegetation and relevant microbial activity.

Genesis and mechanisms of organic matter enrichment of the Hongshuizhuang Formation in the Zhoukoudian area of Jingxi sag,North China

The Hongshuizhuang Formation is a Meso-Neoproterozoic high-quality source rock in the North China Craton(NCC),comprising abundant organic matter.The present study focuses on the analysis of the Hongshuizhuang Formation in the Zhoukoudian area of the Jingxi sag,discussing its genesis and mechanisms of organic matter enrichment through geological and geochemical methods.The Hongshuizhuang Formation is divided into three members from bottom to top respectively as the lower,middle and upper member.Trace elements analysis indicate that the Hongshuizhuang Formation developed restricted neritic facies in an extensional environment within a continental island arc under a post-orogenic background.The lower and upper members were deposited in a relatively shallow,strongly retained water mass within a suboxic and saline environment influenced by subhumid climate,while the middle member was deposited in an anoxic deep-water environment with relatively low salinity and weak restrictions.The PAAS-normalized rare earth element distributions of the middle and upper members show an enrichment of LREEs and a depletion of HREEs,and a low mean Y/Ho ratio with a positive Eu anomaly,indicating that the regional deposition has been affected by hydrothermal fluids.The negativeδ13 C andδ18 O values and the positive87 Sr/86 Sr values confirm that the deposition of the middle and upper members was accompanied by crustal hydrothermal activities.Accumulation of organic matter and enrichment of Baxsand P indicate that productivity is the basis of organic matter enrichment in the Hongshuizhuang Formation,where high-quality source rocks are concentrated in the middle member.In addition,reduced water mass controls the preservation of organic matter.Hydrothermal activity,humid climate,and salinity support a higher primary productivity and the formation of reduced water masses.However,due to limitations in depth,the high-quality source rocks in the Jingxi sag are thinner than the Jibei sag.

Sedimentary environment and major controlling factors of organic matter-rich shale from the Wufeng-Longmaxi formation in eastern Sichuan Basin,China

The major controlling factors of organic matter and its enrichment model of the black shale from the Wufeng-Longmaxi Formation were explored by investigating the vertical variation characteristics,as well as major element and trace element abundances in the Wuxi Bailu section.The results show that the sedimentary tectonic setting of the Wufeng-Longmaxi Formation in the north-east margin of the upper Yangtze platform is located on the active continental margin,which is a passive continental margin and continental island arc.The parent rock in the source area is mainly felsic volcanic rocks mixed with small amounts of sedimentary recycling materials.Due to increased plate activity and a drop in sea levels,terrigenous pyroclastic input increased.The palaeoclimate was semi-humid,and a robust dysoxic-reduction environment and a high level of palaeoproductivity,causing the formation of the organic-rich shale in the Wufeng Formation.At the base of the Longmaxi Formation,the sedimentary water body was affected by global transgression,showing a strong anoxic-reductive environment,and the paleoclimate was a warm and humid condition.The palaeoproductivity level was high,resulting in the formation of organic shale.Due to the sea level drop at the top of the Longmaxi Formation,the sedimentary water was in an oxic-reduced environment,but the input of terrigenous pyroclastic matter increased.Because the paleoclimate was warm and humid and the palaeoproductivity level was high,organic-rich shale was formed.The findings demonstrate that terrigenous clastic input circumstances,palaeoproductivity conditions,and paleo-redox conditions had the greatest influence on the enrichment of organic matter in the Wufeng-Longmaxi Formation.Thus,organic matter enrichment was controlled by multiple paleoenvironmental factors.

A case study on how astronomical cycles affect the organic carbon accumulation

The paleoclimate change impacts the sedimentary environment and process,which in turn control the accumulation of organic carbon.Numerous studies have shown that the paleoclimate is controlled by astronomical cycles.Hence,understanding how these cycles impact the accumulation of organic carbon is a critical question that requires in-depth discussion.Previous studies have shown that Milankovitch cycle can be revealed from the sediments of the 7^(th)Oil Member(Chang 7 Member for short)of the Triassic Yanchang Formation in the southern Ordos Basin,suggesting that the deposition was controlled by astronomically-forced climate changes.Building on previous research,this study collected natural gamma(GR)data of Chang 7 Member from the N36 Well to further analyze astronomical cycles,combined with X-ray fluorescence(XRF)analysis and total organic carbon(TOC)tests,to reconstruct the paleoenvironment and analyze the organic matter enrichment characteristics.The results of this contribution show that,paleoclimate,paleo-redox conditions,paleo-water level,paleo-productivity and sediment accumulation rate(SAR)collectively controlled the enrichment of organic matter.Notably,this study identified the presence of eccentricity,obliquity,precession,and the 1.2 Myr long obliquity cycle in the Chang 7 Member.These cycles controlled the paleoenvironmental changes at different timescales and influenced the enrichment of organic matter,which has implications for subsequent energy exploration.