Preservation Of Organic Matter Research Articles

Lipid biomarkers as organic matter source indicators of estuarine mangrove ecosystems

ABSTRACT Surface sediments from three mangrove forests and three adjoining stations of the Cochin estuary were analysed for lipid biomarkers along with bulk parameters to delineate organic matter (OM) sources. Fine-grained sediments dominated in mangrove forests due to low hydrodynamic energy conditions compared to estuaries. The protein/carbohydrate ratio >1 at most of the sites implied that a major fraction of OM was freshly originated. Lipid/carbohydrate ratios >1 indicated the better food quality of OM. Depleted δ13C, higher organic carbon-to-nitrogen ratios (>20), tannin and lignin content and lipid biomarkers implied OM inputs from mangrove vegetation. Occurrence of unique mangrove biomarkers like taraxerol in estuarine sediments reflected mangrove input via tidal export of OM. Prominent relative abundance of steroidal ketones, triterpenoid ketones and other degraded triterpenoids pointed towards active diagenetic transformations of sedimentary OM. Significantly higher content of biochemical components and biomarkers in mangrove systems was due to the dominance of fine-grained sediments and better preservation of OM under anoxic conditions. The study could provide baseline information on the significance of OM sequestration and preservation potential of mangrove ecosystems, and hence it is recommended to formulate sustainable management strategies to conserve the existing mangrove forest cover along the Kerala coast.

Plio‐Pleistocene Ocean Circulation Changes in the Gulf of Alaska and Its Impacts on the Carbon and Nitrogen Cycles and the Cordilleran Ice Sheet Development

AbstractThe modern Gulf of Alaska (GOA) is a Cordilleran Ice Sheet (CIS) region, estimated to be important for nutrient cycling and CO2 exchange. Little is known of the GOA evolution over the Pliocene and Pleistocene as well as its impact on the CIS development, when other evidence for changing North Pacific circulation has emerged. We analyzed Integrated Ocean Drilling Program Expedition 341 Site U1417 sediments, which extend through the Plio‐Pleistocene transition (4–1.7 Ma), focusing on productivity‐related biomarkers (alkenones, brassicasterol), siliceous microfossils and bulk carbon and nitrogen stable isotopes. Our results show two dominant water column regimes: one characterized by high silica and low organic matter (OM) preservation, containing microorganism remains from a mix of habitats (4–3.7 Ma) and a second characterized by low biogenic silica and increased OM preservation of microorganisms from dominantly open ocean habitats (3.33–3.32 Ma and 2.8–1.66 Ma). An increase of phytoplankton diversity (3.7–3.35 Ma, 3.19–2.82 Ma) characterizes the two transitions of water column conditions, from oxygenated to reductive, that we attribute to a change from ocean mixing to strong stratified conditions with some occasional mixing. The biogeochemical changes in the GOA follow 400 and 100 kyr eccentricity cycles which are also reflected in changes in the CIS. We conclude that the CIS expansion created high nutrient low chlorophyll conditions in the GOA during the Mid Piacenzian Warm Period and the early Pleistocene. In turn, positive feedbacks increased marine productivity export, atmospheric CO2 drawdown and further CIS expansion.

Genetic Mechanism of Pyrite in the Shale of the Longmaxi Formation and Its Influence on the Pore Structure: A Case Study of the Changning Area, South Sichuan Basin of SW China

Pyrite is a mineral that is commonly found in shale gas reservoirs. Its genetic mechanism and impact on pore and organic matter in shale gas reservoirs are critical for shale gas exploration. This study selects the Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin by comprehensively using a scanning electron microscope (SEM), X-ray diffraction (XRD), total organic carbon (TOC), and image processing technology. The type and characteristics of pyrite in shale reservoirs are studied, the sedimentary environment and genetic mechanism of pyrite are analyzed, and the influence of pyrite formation on organic matter enrichment and reservoir formation is evaluated. The results showed that pyrite in shale primarily forms framboidal pyrite, euhedral pyrite, and subhedral pyrite, with particle sizes ranging from 1 to 15 μm. The maximum framboid diameter (MFD) is less than 20 μm, with the average particle size distribution of 3–5 μm. These parameters indicate the vulcanization and blocking environment. The reducing environment promotes organic matter enrichment and preservation. Framboidal pyrite has two genetic sequences: rich organic matter and poor organic matter. The development of organic matter will limit the continuous radial growth of pyrite and is conducive to the protection of pores, and the formation of pyrite can reduce the activation energy of kerogen reaction and catalyze the hydrocarbon generation of organic matter, resulting in higher gas content. The framboidal pyrite content can be used to predict high-quality shale gas reservoirs.

Pyrite Characteristics and Its Environmental Significance in Marine Shale: A Case Study from the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formation in the Southeast Sichuan Basin, SW China

Pyrite, as a characteristic mineral in organic-rich marine shale, is a significant index for the interpretation of paleoredox conditions. In this study, based on drilling cores and focused ion beam-scanning electron microscopy (FIB-SEM), the occurrence, diameter and particle size distribution of pyrites from 32 samples obtained from the Wufeng–Longmaxi Formation in the southeast Sichuan Basin were analyzed. The results show that pyrite displays various occurrences at the macro-scale and micro-scale. At the macro-scale (mm–cm), pyrite laminations, nodular pyrites and lenticular pyrites can be found from drilling cores. At the micro-scale (nm–µm), the common occurrences of pyrite are pyrite framboids, euhedral pyrites and infilled pyrite framboids. According to the formation mechanism of pyrites, pyrites can be divided into syngenetic pyrites and diagenetic pyrites. The infilled pyrite framboids are categorized as diagenetic pyrites. The mean pyrite framboid diameters (Mean, D) range from 2.94 µm to 5.33 µm (average of 4.26 µm), with most samples showing pyrite framboid diameters from 3.5 μm to 4.8 μm. Most of the diameters of the framboid microcrystals (Mean, d) are less than 0.4 µm. Therefore, according to the (Mean, D) and the (Mean, d), the pyrite framboids can be divided into three sizes: syngenetic framboids (SF, D < 4.8 µm, d ≤ 0.4 µm), early diagenetic framboids (EDF, D > 4.8 µm, d > 0.4 µm) and late diagenetic framboids (LDF, D < 4.8 µm, d > 0.4 µm). Additionally, box-and-whisker charts of the diameter, standard deviation/skewness value of the mean diameter of pyrite framboids (Mean, D) and the ratio of trace elements indicate that the sedimentary water body was a euxinic–dysoxic environment. Euxinic conditions dominated the Wufeng Formation to the lower part of the Long11-3 section, which is beneficial for the preservation of organic matter. However, the middle-upper part of the Long13-Long12 sub-member is a dysoxic sedimentary environment.

Geochemical characteristics of organic-rich intervals within the Cryogenian non-glacial Datangpo Formation in southeastern Yangtze Block-implications for paleoenvironment and its control on organic matter accumulation

To understand the mechanism of organic matter enrichment in the organic-rich intervals of the Cryogenian non-glacial strata, we report high-resolution geochemical data of the well-developed non-glacial Datangpo Formation at a section in the southeastern margin of the Yangtze Block, including iron (Fe) speciation, sulfur and organic carbon isotopic compositions and elemental contents. The organic-rich intervals are mainly distributed in the basal manganese (Mn) carbonate and Mn-bearing shales (intervalⅠ), and overlying black shales (intervalⅡ). The paleo-redox reconstruction shows a transition from ferruginous-dominated watermass during deposition of the IntervalⅠto euxinic-dominated watermass during deposition of the IntervalⅡ, which provides good conditions for the preservation of organic matter. A variety of geochemical indicators demonstrate that during deposition of the IntervalⅠ, elevated primary productivity in the surface water driven by hydrothermal activities and improved post-glacial basinal watermass exchange, coupled with some organic carbon influx from the dissolved organic carbon reservoir, collectively promoted the enrichment of organic matter. For the interval Ⅱ, enhanced chemical weathering accompanied by persistently warm climatic conditions exerted a major control on the organic matter enrichment by importing abundant nutrients, resulting in enhanced surface-water primary productivity. In addition, favorable sedimentation rates also made positive contributions to the organic-rich black shales in the intervalⅡ. Based on a combined analysis of paleoclimates, hydrological conditions and hydrothermal activities, this study puts forward organic matter enrichment models for the organic-rich intervals deposited during the Cryogenian non-glacial period.

Geochemistry and organic petrology of organic-rich shales of the Upper Jurassic – Lower Cretaceous Bazhenov Horizon in the Frolov Region, West Siberian Basin: Implications for the reconstruction of the organic matter origin and paleoredox conditions

The objective of this study is the reconstruction of paleodepositional conditions that have prevailed in the Bazhenov Sea during the sedimentation of the organic matter-rich black shales of the Upper Jurassic-Lower Cretaceous Bazhenov Horizon from the western West Siberian Basin, with the aim to gain insights into the primary forces and mechanisms favouring enhanced organic matter preservation and accumulation. Elemental analysis, rock pyrolysis and petrographic observations were carried out on a total of 298 core samples from a 60.6 m thick well section, covering black shales of the Tutleim Formation and parts of the under- and overlying Abalak and Frolov Formations from the south-western West Siberian Basin (Frolov region). The results reveal systematic depth variation of sedimentary and diagenetic conditions, e.g. detrital influx and paleoredox conditions.The sediments were deposited in a marine environment with a significant contribution of authigenic minerals and subordinate detrital influence. The organic matter composition suggests an aquatic and bacterial origin with minor input of land plant-derived material. The enrichment of redox-sensitive inorganic and organic compounds results from reducing and periodically sulfidic conditions in the bottom and uppermost pore water columns during deposition. Sections that show evidence of anoxia or euxinic conditions simultaneously represent the lowest terrigenous influence, suggesting that anoxia is closely linked to sea-level fluctuations in the course of eustatic transgression-regression cycles. A distinctive correlation pattern of organic matter quantities and redox proxies suggests that organic matter accumulation and preservation are mainly controlled by paleoredox conditions and, therefore, regression-transgression cycles. The most prolific source rock sections are defined as the central Tutleim units, T4 and T5, marking the transition from marine transgression to regression at the Upper Jurassic-Lower Cretaceous boundary.

Depositional Environment and Organic Matter Enrichment of Early Cambrian Niutitang Black Shales in the Upper Yangtze Region, China

Natural gas generation is the result of organic matter degradation under the effects of biodegradation and thermal degradation. Early Cambrian black shales in the Upper Yangtze Region are rich in organic matter and have shown great shale gas potentiality in recent years. Nevertheless, the enrichment mechanism and distribution of organic matter in these black shales between different sedimentary settings, such as intra-platform basin, slope, and deep basin, are still poorly understood. In this paper, based mainly on elemental geochemistry, a comprehensive study of the marine redox conditions, primary productivity, sedimentation rate, terrigenous input, hydrothermal activity, and water mass restrictions was conducted on the Early Cambrian Niutitang black shale in the Upper Yangtze Region. Our data showed that an intra-platform basin received a higher terrigenous input and that it deposited under more restricted conditions than the slope and deep basin settings. The primary productivity in the slope and deep basin settings was higher than that in the intra-platform basin setting. In the intra-platform basin, the productivity increased from its inner part to its margin. For the slope and deep basin settings, the high paleoproductivity generated large amounts of organic matter and its preservation was synergistically affected by the redox conditions. In contrast to the slope and deep basin, the preservation of organic matter in the inner part of the intra-platform basin was mainly controlled by redox conditions because the paleoproductivity in it was much lower than in the slope and deep basin settings. The intra-platform basin margin was the most favorable area for accumulating organic matter.

Differential enrichment mechanisms of organic matter in the Chang 7 Member mudstone and shale in Ordos Basin, China: Constraints from organic geochemistry and element geochemistry

Source rocks in the seventh member of the Upper Triassic Yanchang Formation (Chang 7 Member) are characterised by lacustrine organic-rich mudstone and organic-rich shale in the Ordos Basin. There are obvious differences in organic and inorganic geochemical characteristics between mudstone and shale in the Chang 7 Member, indicating that they are significantly different in genetic mechanism. Presently, the difference of organic-matter enrichment mechanism between shale and mudstone in the Chang 7 Member is still obscure. In this study, we explored the organic-matter accumulation mechanisms in the Chang 7 Member mudstone and shale by investigating their parent-rock properties, palaeoproductivity, palaeoenvironment and dilution conditions using biomarkers and elemental geochemical data. According to our analysis results, the mudstone and shale parent rocks are composed of sedimentary rock and granite. The material source of the mudstone was mainly from the south of the Ordos Basin, whilst the source of the shale was mainly from the south and northeast of the basin. The organic-matter enrichment of the shale was affected by high palaeoproductivity and anoxic environment conducive to the preservation of organic matter. The palaeoproductivity during the deposition of the mudstone was moderate, and the dilution caused by the high sedimentation rate was the main reason for its low abundance of organic matter. Affected by frequent volcanic activities, a grater algae bloom occurred in the process of the shale deposition, which made the palaeoproductivity during shale deposition higher than that of the mudstone deposition. In addition, the degradation of a large quantity of algae resulted in a reducing environment during shale deposition. Therefore, the high palaeoproductivity and anoxic preservation conditions resulted in large amounts of aquatic organisms in the shale, resulting in high organic-matter abundance in the shale. The palaeoclimate during mudstone deposition was warmer and wetter than during shale deposition, which was more conducive to parent-rock weathering and the transportation and deposition of terrigenous detritus. Therefore, the organic matter of mudstone is a mixture of terrestrial higher plants and aquatic organisms. However, the high input rate of terrigenous detritus caused the dilution of organic matter in the mudstone. The mudstone was mainly deposited under weakly oxidising to weakly reducing conditions. The moderate palaeoproductivity, oxygen-containing preservation conditions and dilution caused by the high sedimentation rate made the aquatic and terrestrial organic matter in the mudstone easily oxidised, degraded and diluted. Therefore, the differences of primary productivity and preservation conditions make the organic-matter abundance of the mudstone lower than that of the shale.

Paleoproductivity proxies and alkenone precursors in the Western Mediterranean during the Early-Middle Pleistocene transition

Multidisciplinary analyses (taxonomic analysis of coccolithophore assemblages, stable oxygen isotopes, marine and terrestrial biomarkers) have been carried out on sediments from Ocean Drilling Program Site 975 in the Algero-Balearic basin, through late marine isotope stage (MIS) 20-19 (800–756 ka). The aim is to compare coccolithophore paleoproductivity proxies, such as C37 alkenone concentration and nannofossil accumulation rate (NAR), and understand their relationship with paleoceanographic condition and paleoenvironmental changes, alkenone-producing precursors and unsaturated C37 alkenone compounds. The patterns of C37 alkenones and NAR provide reliable information on past paleoproductivity changes since coccolith dissolution and organic matter preservation were excluded as relevant processes at the site. This is testified by the high values of Nannofossil Dissolution Index and relation between C37 alkenone concentration and Alcohol Preservation Index, the latter used as a proxy of sea bottom ventilation in the basin. A weak mismatching between NAR and C37 alkenone concentration records has been observed and related to paleoenvironmental factors and ecological preferences of alkenone-producing species. Temperature variations mostly controlled the alternating interspecific abundance variations of these taxa through glacial-interglacial and stadial-interstadial climate phases. The percentage abundances of alkenone-producing species, Gephyrocapsa caribbeanica and Gephyrocapsa with open central area (mainly G. margerelii-G. muellerae) strongly co-varied with the percentages of C37:2 and C37:3, respectively during warm and cool-cold periods, suggesting their prominent role in producing these unsaturated C37 alkenone compounds. Moreover, Gephyrocapsa spp. with open central area > 3 μm were likely the main C37:4 producers during the colder late MIS 20 stadial and stadial phases. Other factors in addition to temperature influenced the paleoproductivity proxy patterns. The oceanographic condition established during MIS 20-MIS 19 deglaciation and the more nutrient-rich surface waters during the orbitally-controlled organic-rich layer deposition in the early MIS 19 enhanced primary productivity leading to higher production and preservation of total C37 alkenones.

Quantitative analysis of paleoenvironment of Qingshankou Formation in northern Songliao Basin, Northeastern China

The Qingshankou Formation in the Songliao Basin of northeastern China is an important layer for unconventional oil exploration. To solve the problem of the main controlling factors of shale oil enrichment, to quantitatively analyze the paleosalinity, paleoclimate, paleoredox environment, provenance, and lake productivity, and to clarify the relationship between sedimentary environment and total organic carbon, we have analyzed quantitative reconstruction of the sedimentary environment by using comprehensive analysis of multiple indicators with major and trace elements, fossils, and biomarkers. The results are as follows: the paleosalinity index w(Sr)/ w(Ba) ratio is 0.4–4, the w(B)/ w(Ga) value is 0.7–8.5, and the w(Pr)/ w(Ph) value is 0.8–2.8, which mainly stand for fresh brackish to saline water. The paleoclimate CIW′ (Chemical index of weathering, CIW = [A12O3/(AI203 + CaO+Na20)] × 100 [molecular proportions]) value is 90–100, indicating a warm and humid climate. The redox index w(V)/ w((V + Ni)) value is 0.54–0.89, which mainly belongs to the dysoxic environment. The provenance input of terrigenous debris index w(Ti)/ w(Al) value is 0.026–0.034, mainly standing for low terrigenous debris input. The production index CAL (enrichment degree of Cu) value is 1–3, belonging to high production. We conclude that the humid climate, the fresh brackish to saline and dysoxic water, and the high lake production provide favorable geologic conditions for biological reproduction and then provide a foundation for the formation, enrichment, and preservation of organic matter. The quantitative reconstruction of the paleoenvironment of the Qingshankou Formation has theoretical significance for Gulong shale oil exploration and development.

Hydroclimate variability in the United States continental interior during the early Eocene Climatic Optimum

The early Eocene (56.0 to 47.8 million years ago) was punctuated by a series of transient episodes of rapid global warming superimposed on the long-term early Cenozoic warming trend, culminating in the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 million years ago). Details of the hydroclimate regime operating during the EECO are poorly constrained, especially for continental interior sites. The Green River Formation (GRF) of Utah and Colorado was deposited in a suite of large, unusually productive lakes that offer an ideal opportunity to study the hydrological response to warming. Here we report the hydrogen isotopic composition (δ 2 H) of leaf wax (long-chain n -alkanes) and algal (phytane) lipids preserved in the organic-rich Mahogany Zone (49.3 to 48.7 Ma) and use these data to reconstruct precipitation and lake water δ 2 H records, respectively. We observe large inter-site variations in algal and leaf wax δ 2 H values (~50 to 75‰), suggesting that additional local controls influence precipitation and/or lake water δ 2 H (e.g., salinity). Intriguingly, leaf wax and algal lipid δ 2 H values show little variation through the Mahogany Zone, implying a relatively stable hydrological regime during the latter phase of the EECO. This contrasts with the more variable hydrological regime that prevailed during early Eocene hyperthermals. Unlike the EECO, the early Eocene hyperthermals in the Uinta region do not coincide with the deposition of organic-rich sediments. This suggests that a stable hydrological regime during the EECO may enable the preservation of organic matter within continental-interior lake systems, potentially leading to an important negative climate feedback during the early Eocene and other greenhouse climates. • Gradual variations in leaf wax δ 2 H values indicate a stable hydrological cycle during the latter phase of theEECO. • However, leaf wax and algal δ 2 H values exhibit large lake inter-site variability (~50 to 75‰). • Stable hydroclimate may have promoted organic matter burial within the lake system. • High organic carbon burial may have acted as an important negative climate feedback.

Characteristics of Lithofacies in Deep Shale Gas Reservoirs in the Southeast Sichuan Basin and Their Influence on Pore Structure

The characteristics of lithofacies in the shale reservoir of the Wufeng and Longmaxi formations located in the Luzhou gas field were studied using the three-end-member method (mineralogical components). In addition, the microscopic characteristics and pore genesis of different lithofacies were studied and compared using TOC, FESEM, and digital core images. The results of the present study showed the development of five lithofacies: 1) quartz-rich argillaceous shale; 2) quartz/clay-mixed shale; 3) calcareous/clay-mixed shale; 4) calcium-rich argillaceous shale; and 5) clay-rich siliceous shale. The degree of pore development between lithofacies was highly heterogeneous. In addition, data indicated that total shale porosity increased with the increase in TOC. Thus, the pores were mainly related to organic matter. The enrichment of framboidal pyrite in quartz-rich shale (S1, S-3) and mixed shale facies (M-2, M-3) is essential during the formation of high-quality reservoirs. On the other hand, the content of organic matter in clayey shale was low, which does not favor the development of high-quality reservoirs. The differences in lithofacies pore structures are controlled by the sedimentary environment. The strong retention and reduction environment are the most favorable features for the formation of organic-rich siliceous shale lithofacies, which promote the preservation of organic matter and the development of reservoirs.

The Volcanic Impacts on the Formation of Organic-Rich Shales From the Freshwater to Saline Lakes: Cases Study in the Ordos and the Junggar Basins

High organic matter abundance is necessary for large-scale hydrocarbon accumulation and enrichment in shale oil. Comparing conventional marine basins with lacustrine ones, the formation of organic matter in the latter usually changes rapidly with strong heterogeneity, which brings great challenges to identify sweet spot prediction and resource evaluation. In addition, water columns with different redox conditions also affect the burial and preservation of organic matter during the deposition of lacustrine shales, especially under the impact of ephemeral critical geological events such as volcanic activities. Therefore, determining the properties and the influencing factors on the water columns is one of the key scientific issues in revealing the differential enrichment of organic matter in such basins. By comparing the petrological and geochemical characteristics of organic-rich shales between a typical freshwater and a saline lacustrine basin, this study analyzes the depositional environment and water column properties during the burial and preservation of organic matter in the Ordos and Junggar basins. The results demonstrate that volcanic activity intensifies the degree of hypoxia during the formation of organic-rich matter, which in turn affects organic matter preservation. The sulfate reduction index (SRI) indicates that the organic-rich shale of the Chang seven Member (Ch7) of Yanchang Formation in the Ordos Basin and the Lucaogou Formation (P2l) in the Jimsar Sag of Junggar Basin in freshwater-saline environments have a certain intensity of sulfate reduction (BSR) (SRI <1.375). The organic matter consumed by BSR is lower than the preserved organic matter, resulting a higher TOC content. However, excessive volcanic activity or the input of hydrothermal fluids caused strong BSR (SRI >1.375) of the Fengcheng Formation (P2f) in saline lake of the Mahu Sag in Junggar Basin. This indicates that a large amount of organic matter was degraded and consumed, causing more iron oxides (and other iron-containing minerals) dissolve gradually to form pyrite, reducing the TOC content. This study enabled us to understand the shale oil enrichment, sweet spot prediction and evaluation, and propose a better theoretical and practical model for hydrocarbon accumulation in shale formations, to guide future exploration efforts.

Organic Matter Enrichment of Black Shale at the Turn of Ordovician-Silurian in the Paleosedimentary Center in Southern Sichuan Basin, Upper Yangtze Area

Abstract Herein, integrated vertical variation characteristic analysis was conducted by measuring total organic carbon (TOC), major and trace elements in Upper Ordovician-Lower Silurian (Wufeng-Longmaxi formations) black shale in Weiyuan, Luzhou, and Changning areas, southern Sichuan Basin to clarify the control of organic matter (OM) enrichment in different sublayers. According to the cycle boundary, it was divided into member 1 (submember 1 and submember 2) and member 2 from early to late Longmaxi formation, while it was divided into 4 sublayers from early to late during submember 1 depositional period. Under the warm and humid paleoclimate, high paleoproductivity, and gradually oxic paleoredox conditions, the TOC content (0.22-3.27 wt% in Luzhou area) in Wufeng formation increased gradually from early to late under the cocontrolling of tectonic framework and stagnant sea. After the glacial period, the warm and humid climate of Wufeng formation continued in Longmaxi formation, and TOC content reached the highest in sublayer 1 (2.02-4.02 wt%, 4.78 wt%, and 6.45 wt% in Weiyuan, Luzhou, and Changning areas, respectively), due to the high paleoproductivity and high sea level caused by melting glaciers, causing the extremely anoxic environment. The OM was preserved best. It remains higher in sublayer 2 (2.39 wt%, 3.69-4.18 wt%, and 3.5-3.86 wt% in three areas, respectively) because of the anoxic environment caused by deep water and the high paleoproductivity, but the temporary hot and dry environment has a certain negative impact on the OM preservation. Under the stable paleoproductivity, sublayer 3 has a character of water decreasing slowly, and the suboxic environment led to the decrease of TOC content (3.82-5.28 wt% and 3.77-4.13 wt% in Luzhou and Changning areas, respectively). Meanwhile, that in the yuan area became deeper and the TOC content became relatively higher (1.92-3.33 wt%). Under the open environment, sublayer 4 was controlled by lower sea level caused by regression, resulting in lower TOC content (1.31-4.57 wt%, 1.64-3.52 wt%, and 3.88-4.49 wt% in three areas, respectively). However, the global regression of the submember 2 period, high terrigenous debris dilution, and oxic environment lead to the no preservation of OM. Generally speaking, the enrichment of OM is not only affected by the changes of global sea level and paleoproductivity to some extent (Longmaxi shale) but also controlled by the tectonic framework in Sichuan Basin (Wufeng shale). The findings of this study can help for a better understanding of the OM enrichment mechanism and provide a theoretical basis for the evaluation of high-quality source rocks.

Depositional Paleo-Environments of Lower Cambrian Qiongzhusi Formation in the Western Middle Yangtze Block and Its Controlling Effect on the Organic Matter Enrichment

No systematic comparative study has been conducted on the factors controlling organic matter enrichment in the different depositional environments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block, leading to a large discrepancy in our understanding. Based on organic geochemical and elemental analyses of core, outcrop, rock, and mineral samples from the slope, deep-water shelf, and shallow-water shelf, in this study, comparative analysis of the organic matter content, sedimentological characteristics, and depositional paleoenvironments of the Lower Cambrian Qiongzhusi Formation in the western Middle Yangtze Block was conducted, and the main controlling factors and models of the organic matter enrichment were investigated. The results revealed that the organic matter enrichment in the Qiongzhusi Formation was jointly controlled by redox conditions, water restriction, upwelling currents, terrigenous inputs, and paleo-productivity, but the main factors controlling the enrichment during the different periods were significantly different. (1) During the deposition of the Qiong 1 Member, the extensional rifting was strong, and the sea level was always high. The low degree of terrigenous dilution and anoxic conditions favored organic matter preservation. In this period, the upwelling currents were the main factor controlling organic matter enrichment. The paleo-productivity decreased as the intensity of the upwelling currents gradually weakened from the slope to the shelf, leading to a decrease in the total organic carbon (TOC) content and thereby a gradual decrease in the biogenic silica content of the shale. (2) During the deposition of the Qiong 2 Member, the extensional rifting weakened, and the sea level continued to drop. The upwelling currents, terrigenous input, and redox conditions were all important factors controlling the organic matter enrichment in the region. From the slope to the shelves, the conditions favorable for organic matter enrichment gradually worsened, and the TOC content gradually decreased, with the lithofacies gradually transitioning from biogenic siliceous shale to clayey shale or clayey-calcareous shale. (3) During the deposition of the Qiong 3 Member, the Yangzi Platform underwent a filling and leveling-up process, and the redox conditions played a major role in controlling the organic matter enrichment. The entire region was dominated by an oxygen-rich environment, and the conditions were no longer favorable for organic matter preservation, leading to a low average TOC content. Overall, the spatial variability of the TOC content was closely associated with changes in the depositional paleoenvironment caused by sea-level changes.

Stylolitization structures as possible migration routes of organic matter in microbial genesis sedimentary deposites

It was studied that the possibilities of preservation, migration and transformation of organic matter (OM) in rocks of Precambrian age, the textural and structural characteristics of which are inextricably linked with the physiological characteristics of microorganisms, and the role of stylolithization structures in these processes. The similarity of modern stromatolites with ancient ones in the mechanism of deposition of carbonate micrite and the formation of porous frameworks allowed us to establish that a significant amount OM of cyanobacterial lamins could be preserved after the buildings were immersed in the sedimentation basin and transformed into sedimentary strata. OM was concentrated on numerous relict surfaces between micrite carbonate secretions deposited by microorganisms during life. Subsequent transformations were expressed in the recrystallization of the carbonate substance, accompanied by a significant loss of primary voidness, oustion and redistribution of OM, as well as clay material deposited from the suspension. During detailed petrographic studies, it was observed how clay-organic material (COM), displaced by recrystallization, concentrated on the crystal faces and in intercrystalline pores in certain areas and composed numerous lenticular layers in contact with each other, forming surfaces of complex configurations - rudimentary microstylolites. According to the authors, recrystallization is one of the main catalysts of stillolithization processes, because it creates mineral aggregates in which the pressure on the COM enclosed between them varies greatly at different points. This contributes to the occurrence of the phenomenon of differentiated dissolution under pressure, more intensive transformation of OM in areas with maximum load and, as a consequence, excessive release of CO, which in turn leads to saturation of pore solutions with carbon dioxide and initiation of chemical dissolution. Probably, this is how stylolites and associated fractured voidness are formed in stromatolites, which is available for the migration of OM.

Organic Matter Burial in Deep-Sea Fans: A Depositional Process-Based Perspective

Organic matter burial in the deep-sea fan sediments is an important component of the long-term carbon cycle. Although there is increasing recognition of the importance of organic matter in deep-sea sediments, a major focus has been on mudstones, commonly interpreted as the background sediments, deposited by pelagic or hemipelagic vertical suspension fallout in low-energy fan environments. Emerging evidence suggests that relatively coarse-grained sediment gravity flow deposits (e.g., turbidites and hybrid event beds) can also store a significant quantity of organic carbon, implying that a wide range of depositional processes can result in the concentration and enrichment of organic matter in submarine fans. However, the role of these processes on carbon burial is still not fully understood. This review aims to discuss the impact of three widely documented deep-sea depositional mechanisms/processes, namely vertical suspension settling, grain-by-grain (incremental aggradation), and the en-masse deposition on distribution, burial, and preservation of organic matter in deep-marine deposits. Organic matter accumulated from slowly settling suspension in mud caps (Te or H5 divisions of turbidites and hybrid beds, respectively) is prone to higher oxidation compared to the carbon buried in sandy components of turbidity currents (Ta-Tc units) and hybrid beds (H2/H3 divisions). The burial of organic matter in sandy parts of the deposits has important implications for understanding the fundamental physical processes that control carbon accumulation and preservation in deep-marine rock record.

The C–S–Fe system evolution reveals organic matter preservation in lacustrine shales of Yanchang Formation, Ordos Basin, China

Dissimilatory iron reduction (DIR) and bacterial sulfate reduction (BSR) are vital links in the cycle of the C–S–Fe system. They have been extensively studied in relation to the preservation of organic matter (OM) in marine anoxic environments. However, there has been little attention on DIR and BSR in ancient continental lakes with complex geologic factors. This paper tentatively reveals the differential OM enrichment of lacustrine shale in the Triassic Chang 7 Member of Ordos Basin. The dynamic evolution of the C–S–Fe system is investigated by mean of element geochemistry, a sequential extraction procedure for iron components, and cluster analysis. The results show that (1) the BSR triggered by the exogenous-inputting sulfate formed H2S in continental lake basins, and the enhanced H2S concentration which prompted the strongly anoxic body is particularly conducive for the preservation of OM in the Chang 7 Member. (2) Deposition of the Triassic Chang 7 Member organic-rich shale took place in a predominantly anoxic ferruginous environment. The reactive irons of pyrites (Fepy) and siderites (Fecarb) were produced during the mineralization of organic matter. The components of the Fepy and Fecarb reflect orderly dynamic evolution of the interaction between BSR and DIR. With the increase of sulfate input, the BSR reaction intensity increased, while DIR reaction intensity decreased. That can be quantitatively characterized by the value of Fepy/FeHR.

Effect of in-situ hydrocarbon generation over geochemical properties of the reservoired natural gas: A case study from the Ordos Basin, China

In a few wells, natural gas in the Permian Taiyuan Formation limestone (Pt-L) has the most depleted ethane carbon isotope. Saturated hydrocarbon gas chromatography (SH-GC), element and carbon isotope analysis of kerogen were conducted to investigate the source and sedimentation environment of the Pt-L, and its influence on the geochemical characteristics of gases. Compositions and carbon isotope values of gases produced in the Permian Taiyuan Formation limestone development area (Pt-LDA) were also studied for comparison with the Pt-L. The Pt-L is mixed source of marine and terrestrial provenance. The sedimentary environment is mainly reducing, which promotes the preservation of organic matter. Natural gas produced in the Pt-LDA is dominated by coal-derived gas generated from the Carboniferous-Permian coal strata. Gases from a few wells with more depleted carbon isotopic values may have generated from marine algae and bacteria in the Pt-L.

Upper Cretaceous-Paleocene Unconventional Oil Shales in Egypt and Jordan: Insights into Organic Geochemistry

The present paper investigates the unconventional hydrocarbon potential and characteristics of Campanian–Paleocene oil shales at five outcrop sections in central Egypt and Jordan. One-hundred and seven outcrop samples were analyzed for their content of total organic carbon and total sulfur. Twenty-four samples were selected for further geochemical treatment, including Rock-Eval pyrolysis, and six samples were subjected to molecular analyses using gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS). The distribution of aliphatic hydrocarbon biomarkers indicated that most of the organic matter in the studied samples had a marine origin under anoxic to suboxic conditions. The results indicated that the upper Campanian Duwi Formation in the Quseir area of central Egypt and the Maastrichtian-Paleocene Muwaqqar Chalk Marl (MCM) Formation in the Lajjun area of central Jordan had a relatively high hydrocarbon generating potential with a TOC content of up to 20%wt of immature Type I/II kerogen. Biomarker studies of the investigated formations revealed that the origin of the organic matter is marine algae with a minor contribution of bacterial biomass. The terrigenous/aquatic ratio (TAR) indicated the marine origin of the Jordanian organic matter (MCM and Al-Hisa Phosphorite Formation formations). By contrast, the Duwi Formation in Egypt and the Amman silicified limestone Formation in the Umm Qais section of Jordan showed that these localities were affected by a more terrigenous influx to the principal marine organic matter compared with other rock units. In addition, in the Abu Tartur section the Duwi Formation showed a significant microbial reworking of organic matter reducing the actual amount of organic matter and its hydrocarbon generating potentiality. Clear correlations between nature and the preservation of organic matter is noticed, where the high organic matter mainly occurs in regional paleo-upwelling areas, strongly related to an increase in marine phytoplanktonic/algal and bacterial productivity.