Organic Matter Accumulation Mechanisms Research Articles

Main controlling factors evolution on high-quality source rocks development in the Shanan Sag, Bohaibay Basin, NE China: Implication from structure, depositional environment, and organic matter

Lacustrine source rocks have a significant oil potential and contribute to 95 % of oil reserves in China's offshore areas, Nature of these rocks is related to climate and structure and they are commonly heterogeneous. Mechanism of organic matter accumulation in these rocks has been a matter of debate. Based on Rock-Eval pyrolysis, organic petrographic examinations, gas chromatography-mass spectrometry (GC–MS) analysis, and trace elements analysis, this paper illustrates the paleoclimate, tectonic activity, paleo-productivity, and preservation conditions recorded in high-quality source rocks of the Shanan Sag. The aim is to understand the controlling factors on the deposition of these source rocks. The results indicated that the organic matter of source rocks in middle segment of the third member of the Shahejie Formation (Es3m) was mainly from phytoplankton (average content = 52.2 %), the development of source rocks was controlled by high paleo-productivity. The primary productivity was low during the period of the first and second members of the Shahejie Formation (Es1-2), and the high gammacerane index, and low Pr/Ph ratios indicated that the source rocks were mostly controlled by a good reduction environment with high salinity. The source rocks of the third member of the Dongying Formation (Ed3) were characterized by high terrestrial materials (average exinite content = 62 %) and low to moderate Pr/Ph ratios, which indicated the source rocks were affected by both paleo-productivity and preservation environment. A productivity-controlling model in Es3m was established along with a co-controlling model of productivity and preservation in Ed3, which was largely developed under the background of a warm-humid climate and strong depression-controlled fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity.

The mechanism of organic matter accumulation in the archipelago marine sediments: Insights from the Middle Devonian Givetian mudstone with low TOC in the Youjiang Basin, South China

Despite the fact that the mean values of total organic carbon (TOC) content in the Luofu Formation are relatively low, at or below 1.0 wt%, it still has the potential to be a veritable source rock. To elucidate the influences of various paleogeographic factors on sedimentary environments, 25 samples from Well A on the slope and 32 samples from Well B in the basin centre were selected for study. The samples were analyzed by thin section microscopy and field emission-scanning electron microscopy, along with X-ray diffraction and geochemical analyses (TOC content, sulfur content, organic petrography, inorganic carbon isotope and major and trace elements) in order to understand variations in and controls on the mineralogy, lithofacies, depositional environments, as well as the organic matter accumulation mechanism. The results of our study indicate that mudstone of the Luofu Formation was deposited into 4 lithofacies groups, namely as the siliceous lithofacies, mixed lithofacies, argillaceous lithofacies and calcareous lithofacies. The siliceous lithofacies mudstone was developed in the deep-water columns, while the mixed lithofacies mudstone was deposited on shallow-water columns. The organic matter accumulation model of the Luofu Formation can be further divided into the following four stages, each with its own unique characteristics: (1) The organic matter accumulation was caused by the relatively high productivity and dysoxic basin bottom water columns; (2) The organo-mineral complexes may promote the organic matter accumulation to some extent; (3) The organic matter enrichment was facilitated by the dysoxic/anoxic conditions and relatively high productivity from the basin to the slope; (4) The organic matter was preserved at shortened exposure duration of oxic conditions on the slope. The correlations between various geochemical indicators (redox, paleoproductivity and terrestrial debris flux) and TOC content suggest that oxic water columns and low productivity are the main reasons for the widespread mudstone with low TOC. The organic-rich mudstone, as the target stratigraphic section for exploration, may be deposited in the basin bottom water columns away from upwelling and close to hydrothermal activity in stage 3.

Palaeoenvironment characteristics of the Ediacaran Doushantuo Shale, Yichang area, South China: Implications for organic matter accumulation mechanism

The organic-rich Ediacaran Doushantuo Shale have been identified as a high-quality source rock and a main shale gas play in the Yichang area, South China, which was characterized by high TOC content, high thermal maturity, enormous cumulative thickness, and relatively high contents of brittle minerals. As the oldest shale gas stratum, the paleo-environmental characteristics and organic matter (OM) accumulation mechanism remain unclear in this special marine setting. In this study, core descriptions, TOC content, X-ray diffraction (XRD) mineralogical analysis, scanning electron microscopy (SEM) observation, and elemental analysis, including the major, trace, and rare earth elements, were combined to identify lithofacies, reconstruct paleo-environmental conditions, and document organic matter (OM) accumulation mechanisms for this potential Ediacaran Doushantuo Shale in the Yichang area, South China. Four major shale lithofacies were identified based on TOC content and mineral composition. The geochemical redox proxies, such as the enrichment factors of Uranium (U-EF) and ratio of organic carbon to total phosphorus (Corg/P), indicate that the Doushantuo Shale was probably deposited in a dysoxic-anoxic conditions, which is consist with the interpretation from size distribution of pyrite framboids. The terrigenous influxes suggest the Doushantuo Shale was deposited in arid climate in absence of chemical weathering. Meanwhile, the productivity proxies (biogenic SiO2, Ba, and P/Al) indicate that the organic-rich Doushantuo Shale was deposited under a relatively low primary productivity. In addition, hydrothermal input has limited influence on organic matter (OM) accumulation of the Doushantuo Shale, which was also reported in the Qinglinkou Section of Yichang area. It can be observed that the organic matter (OM) accumulation mechanism of the Doushantuo Shale, deposited after the Marinoan “Snow Earth” glaciation, in the Yichang area, South China, was mainly controlled by the redox conditions, which is much different from typical marine shales and transitional shales.

Geological Features, Paleosedimentary Environment, and Organic Matter Accumulation Mechanisms of the Lacustrine Shale Oil System: A Case Study of the Jurassic Dongyuemiao Member in the Sichuan Basin

Lacustrine shale has garnered significant attention due to its significant resource potential. The Jurassic shale in the Sichuan Basin is an important target for lacustrine shale exploration in China. However, previous studies have predominantly focused on the Da’anzhai member of the Ziliujing Formation, and little attention has been paid to the shale of other strata. This paper aims to address this gap by investigating the Jurassic Dongyuemiao member in the Sichuan Basin. The study systematically characterizes the geological properties of the Dongyuemiao shale system, reconstructs the paleosedimentary environment, identifies the key factors influencing organic matter (OM) enrichment and physical properties, and assesses its resource potential through comparative analysis. Our results show that the Dongyuemiao shale was deposited in an oxic and semi-humid sedimentary environment characterized by intense weathering conditions. The enrichment of OM in the shale is primarily controlled by redox conditions and salinity, with redox conditions playing the most crucial role in OM accumulation. In terms of pore system characterization, clay mineral-associated pores dominate the pore types in the Dongyuemiao shale, while two types of organic matter-associated pores are also widely observed. The development of framework grain-associated pores is limited and only present in certain siliceous and carbonate minerals. The shales of the Dongyuemiao member and the Da’anzhai member exhibit slight differences in TOC content. However, the kerogen in the Dongyuemiao member displays higher hydrocarbon generation potential, and the Dongyuemiao shale exhibits more favorable pore structure parameters. Overall, the Dongyuemiao shale does not exhibit any significant disadvantages compared to the Da’anzhai shale. Therefore, it deserves greater attention in future exploration endeavors. The research findings presented in this paper provide a solid theoretical foundation for expanding the exploration scope of lacustrine shale in the Sichuan Basin.

Organic matter accumulation mechanisms and source rock potential of the Mesoproterozoic Xiamaling Formation, Yanliao Basin, North China

The Mesoproterozoic Xiamaling Formation (∼1.4 Ga) is considered to be a set of promising strata for shale oil and gas exploration and production in the Yanliao Basin, North China Craton. This study aimed to investigate the organic matter accumulation mechanism, and hydrocarbon generation and expulsion characteristics of the Xiamaling Formation, using various methods, including scanning electron microscopy, rock-eval pyrolysis, and elemental geochemistry. Multiple geochemical proxies indicate that Unit 3—Unit 1 of the Xiamaling Formation was deposited in a climate that alternated between highly humid and semiarid conditions, with the strong-to-weak terrigenous influx. The redox conditions of bottom water were undulatory oxic in Unit 3 and the upper part of Unit 1, but anoxic in Unit 2 and the lower part of Unit 1. Sulfurization of organic matter was the main reason for the high TOC content in Unit 3. However, the high TOC content in Unit 2 and the lower part of Unit 1 was caused by an oxygen-deficient water column. Unit 2 and the lower part of Unit 1 exhibit a significant shale oil potential owing to low maturity, high hydrogen index (HI) values, and prevalence of type Ⅰ, II1, and II2 kerogens.

Organic matter accumulation mechanisms in the Wufeng-Longmaxi shales in western Hubei Province, China and paleogeographic implications for the uplift of the Hunan-Hubei Submarine high

Organic matter accumulation exhibits different distribution patterns in time and space and tends to be controlled by many factors. This study investigated the mechanisms that control the accumulation of organic matter in the Xianfeng area of western Hubei Province, South China through sedimentological and geochemical analyses of wellbore samples. The results showed that the total organic carbon (TOC) content, mineral content, and elemental geochemical data of the Wufeng-Longmaxi Formations varied significantly with depth. Based on the sharp changes in the vertical profiles of the TOC content, Gamma-ray, and numerous geochemical parameters, as well as regional stratigraphy, this study divided the section from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation into six units (W1, W2, GYQ, L1, L2, and L3). The TOC content was positively correlated with redox proxies (DOPT, VEF, and MoEF) and primary productivity proxies (CuEF and NiEF), and negatively correlated with terrigenous input proxies (Al, K, and Ti). The correlation coefficients suggest that the primary factors affecting organic matter accumulation are redox conditions and paleoproductivity, while terrigenous input is a secondary controlling factor. This study proposed an organic matter accumulation model based on the organic matter characteristics, lithofacies, sedimentary environment, and geological setting of the study area. During the deposition of Unit W1, organic matter was well-preserved owing to the anoxic bottom water environment resulting from strong water mass restriction. During the deposition of Unit W2, the rising sea level led to the development of anoxic-euxinic bottom water conditions, and the joint influence of nutrients from upwelling in the open sea and volcanic eruptions enhanced productivity, resulting in the most intense accumulation of organic matter. Unit GYQ had low TOC content due to low productivity despite anoxic conditions. During the deposition of Unit L1, the uplift of the Hunan-Hubei Submarine High caused the depocenter to shift westward and the sea level in this area to drop, resulting in suboxic bottom water and relatively low productivity in the surface water layer. During the deposition of Unit L2, the regional sea level drop resulted in increased input and dilution of terrigenous clasts, thus leading to low TOC content. During the deposition of Unit L3, continuously enhanced terrigenous input due to climate warming and sea level drop became the major factor controlling the organic matter accumulation. In summary, the main factors controlling the organic matter accumulation differed significantly at different evolutionary stages. Compared with the Jiaoshiba area, the Xianfeng area in western Hubei experienced continuously increased terrigenous input, a less reductive water mass, a significant decrease TOC content, and a westward shift of the depocenter since the early stage of the Longmaxi Formation deposition. These changes may be related to the intense uplift of the Hunan-Hubei Submarine High triggered by the Kwangsian Orogeny. These results provide important implications for the understanding of the accumulation of organic matter in the Wufeng-Longmaxi Formations and paleogeographic changes.

Mechanism of organic matter accumulation in black shale of the Datangpo Formation: Insights from paleo-environmental variation during the Cryogenian non-glaciation

Mechanism of organic matter (OM) accumulation in the Cryogenian Datangpo Formation of the Nanhua Basin is poorly understood, restricting shale-gas exploration in these non-glacial deposits. To understand mechanism of OM accumulation, we carried out high-resolution analyses of geological and geochemical data from shales of the lower three members (I.II.III) of the Datangpo Formation across the basin. The results show that OM distribution in the basin is heterogeneous: (1) stratigraphically, total organic carbon (TOC) concentrations vary from moderate values in Member I, peaks in Member II, to lowest values in Member III; and (2) spatially, TOC concentrations in Member I increase with increasing depositional water depths, whereas they are high and low throughout Member II and Member III across the basin, respectively. The heterogeneity of OM accumulation in shales is interpreted to be controlled by complex paleo-environmental variations in water-mass exchange, paleoclimate, seawater redox state, and hydrothermal activities. Specifically, (1) ferruginous-euxinic conditions, intensive hydrothermal activities, and pronounced water-mass exchange between the basin and the open ocean favored OM accumulation in Member I; (2) expanded euxinic conditions, dramatic chemical weathering, and reduced water-mass exchange resulted in OM enrichment in Member II; and (3) less extensive anoxic conditions, reduced chemical weathering, and stagnant water-mass exchange led to OM depletion in Member III. This integrated conceptual model for OM accumulation in shales of the Datangpo Formation shows the influences of both primary productivity and redox conditions in the Nanhua Basin.

Paleodepositional conditions and organic matter accumulation mechanisms in the Upper Ordovician-lower Silurian Wufeng-Longmaxi shales, Middle Yangtze region, South China

The deposition of black shales has been widely attributed to primary productivity, preservation controlled by redox conditions, and detrital dilution controlled by sedimentation rates. It is difficult to distinguish the relative importance of productivity and redox conditions in organic matter accumulation. On the Upper Ordovician-Lower Silurian Wufeng-Longmaxi black shales, we combined petrographic features, major and trace elemental proxies, and organic carbon isotopic analysis to determine the relative importance of productivity versus anoxia on organic matter accumulation and the evolution of factors controlling organic matter accumulation across the Ordovician-Silurian boundary in the Middle Yangtze region. Our research also sheds light on the temporal variations in redox conditions that caused the Late Ordovician mass extinction.Productivity, redox conditions, detrital dilution, and basin restriction changes over time as the Wufeng-Longmaxi Formations were deposited (from upper Katian to Rhuddanian stage). Based on the stratigraphic profiles of productivity (biogenic Si and P/Al), redox conditions (Mo/Al, U/Th ratios, and DOPT values), detrital fraction flux (Al2O3), and the cross-plots of biogenic Si versus Co*Mn values, our findings indicate that organic matter accumulation was a combined effect of productivity and anoxia; however, the elevated productivity caused by upwelling that introduced plenty of nutrients to the oceans was the direct factor driving organic matter accumulation. Previous research found a simultaneous variation in stratigraphic profiles of productivity and redox proxies; however, in our datasets, we found an offset between productivity geochemical proxies (biogenic Si content) and redox conditions (Mo/Al). Biogenic Si profile peaks occur stratigraphically beneath Mo/Al profile peaks. Anoxia does not coexist with increased paleoproductivity across the Ordovician-Silurian boundary in the Middle Yangtze region, and redox conditions may be disrupted by local geological events.

Organic geochemistry and controlling factors of alkaline source rocks within sequence stratigraphic framework in the Fengcheng Formation, Mahu sag, Junggar Basin, NW China

The Fengcheng Formation (P1 f) source rock, which was deposited in an alkaline water body, is one of the most significant source rocks in the Mahu sag, Junggar Basin. However, its evolution and mechanism of organic matter accumulation are still not understood. Using seismic, core, and log data, P1 f is divided into 3 third-order sequences, i.e., SQ1, SQ2, and SQ3, from bottom to top. Pyrolysis data of core samples shows that SQ1 and SQ3 mainly are characterized by poor to fair source rocks, with Type III to Type II2 kerogen, while SQ2 mainly are characterized by fair to good to excellent source rocks, with Type II2 to Type I kerogen. The biomarker parameters of 40 source rock samples from different sequences suggest that the reducibility of sedimentary water is characterized by SQ2 > SQ1 > SQ3. The organic matter contribution in different sequences is obviously different. SQ1 organic matter contain more bacteria and algae than that of SQ2 and SQ3, and terrigenous higher plants may have contributed to SQ3 organic matter. Lake level changes and water salinity dominate organic matter origin, accumulation, and preservation. During SQ1 deposition, the lake level was the highest, the salinity of sedimentary water was moderate, and the organic matter contained more conventional algae sources. During SQ2 deposition, the lake level was the lowest and the salinity was the highest, and the organic matter contained special algae characterized by C29 regular steroids. The lake level of SQ3 was higher than that of SQ2, the salinity was the lowest, and terrigenous higher plants may have contributed to organic matter during this period.

Mechanism of Organic Matter Accumulation in Black Shales of the Yuertusi Formation in the Tarim Basin: Insights From Paleoenvironmental Variation During the Early Cambrian

The paleoenvironment during the Early Cambrian is closely related to the accumulation mechanism of organic matter (OM) from the Lower Cambrian black shales. However, paleoenvironment remains a controversial issue. Here, we reported a lot of detailed data of sedimentary stratigraphy and geochemistry of the Lower Cambrian Yuertusi Formation in the Aksu area, Tarim Basin. The Yuertusi Formation from the Yutixi outcrop consists mainly of silicalite at the base, two sets of black shales, and crystalline dolostone. Based on the redox conditions traced by U/Th, V/Cr, Ni/Co, and V/Sc, the hydrothermal activity traced by Ce/Ce*, Cr/Zr, U/Th, Fe/Ti, and (Fe + Mn)/Ti ratios, as well as paleo-productivity traced by Ba, Cu, Rb/Sr, and other parameters, variations were observed in the depositional environments of the Yuertusi Formation: 1) the silicalite at the base was deposited under an euxinic condition and intense hydrothermal activity. Mo-U co-variation analysis revealed that the north margin of Tarim Basin belonged to the unrestricted marine during the Early Cambrian, 2) the lower black shales were deposited under an oxygen-poor condition and weak hydrothermal activity, and 3) the upper black shales were deposited under oxygen-poor, sub-oxic conditions and almost no hydrothermal activity. Although the hydrothermal activity improved paleo-productivity, the TOC values were low on the whole, which may be due to the intense hydrothermal activity that damaged the formation of source rocks. Comprehensive studies showed a gradually oxidizing environment and weakening paleo-productivity during the Yuertusi Formation deposited. The anoxic conditions were conducive to the preservation of OM, and the high-quality source rocks represented by the black shales of the Yuertusi Formation were formed, especially the first set of black shales. However, the enrichment of OM may be affected by the intense hydrothermal activity.

Petrographical and geochemical characterization of the Upper Permian Longtan formation and Dalong Formation in the Lower Yangtze region, South China: Implications for provenance, paleoclimate, paleoenvironment and organic matter accumulation mechanisms

The Upper Permian Longtan and Dalong Formations are considered prospective targets for shale gas exploration and production in the Lower Yangtze region, South China. However, the paleoclimate change and the mechanism of organic matter accumulation in the shales are rarely reported. In this study, the varying provenance, paleoclimate, paleoenvironment, primary productivity, and the mechanism of organic matter accumulation of the Longtan and Dalong Formations were investigated using X-ray diffraction (XRD), polished section and scanning electron microscope (SEM), pyrite morphology, and elemental geochemical data.Multiple geochemical proxies indicate that the Longtan Formation was deposited in marine–continental transitional facies under a highly humid climate and intermediate-to-strong weathering conditions. The water column was oxic with low paleoproductivity and high terrigenous influx. The rapid rise in sea level during the deposition of the lower Dalong Formation resulted in an anoxic water column, high productivity, and low terrigenous input under a semiarid–semihumid climate and weak-to-moderate weathering conditions. The oceanic upwelling elevated primary productivity and was crucial in the organic matter accumulation. Therefore, the total organic carbon (TOC) is high in the lower Dalong Formation. However, the later sea level fall at the upper Dalong Formation led to a dramatic change in the sedimentary environment, which is characterized by a moderately humid climate, intermediate weathering conditions, decreased productivity, a dysoxic–oxic water column, and enhanced detrital flux.The correlations between TOC content and geochemical proxies suggest that the enrichment of organic matter in the Longtan Formation was mainly affected by terrigenous influx and sedimentation rate. The high TOC enrichment in the lower Dalong Formation was because of oxygen-deficit water column, high productivity, and low terrigenous influx. In the upper Dalong Formation, a combined effect of the decrease in primary productivity and relatively oxygen-enriched conditions was not conducive to the organic matter accumulation. They gave rise to low concentration of organic matter. However, the positive correlations of TOC vs. Cu/Al and TOC vs. Ni/Al indicate that paleoproductivity was nevertheless the most critical factor controlling the enrichment of organic matter.

Effect of paleoclimate and paleoenvironment on organic matter accumulation in lacustrine shale: Constraints from lithofacies and element geochemistry in the northern Qaidam Basin, NW China

Organic matter enrichment in lacustrine shale is significantly influenced by paleoclimate and paleoenvironment. In this study, a total of 24 shale samples were vertically collected from the YQ-1 drilling well in the northern Qaidam Basin, north-western China. The high-resolution geochemistry analyses were performed to provide a detailed geological description of these samples through lithofacies interpretation, total organic carbon (TOC) content testing, element geochemistry analysis, and mineralogical composition characterization. The results show that four lithofacies types can be identified in the Middle Jurassic Shimengou Formation, which consist of the brown oil shale with horizontal bedding, the black shale with well-developed lamination, the grey-black mudstone with horizontal and wavy bedding, and the carbonaceous mudstone with plant debris. The former two types were preserved in the Upper Member and were mainly deposited in a deep and semi-deep lake environment, whereas the latter two types were preserved in the Lower Member and were deposited in a littoral-shallow lake. The TOC content, paleo-salinity, redox action, and paleo-productivity of the shales in the Upper Member are higher than those in the Lower Member, and other parameters, such as the chemical weathering degree and terrestrial clastic input, are higher in the shales of the Lower Member. Based on lithofacies and geochemical data, the shales in the Lower Member were deposited in a small, shallow freshwater lake within an oxygen-rich and relatively wet and hot paleoclimate, whereas the shales in the Upper Member were deposited in a large, deep, and saline water lake with dysoxic-anoxic conditions and a relatively arid and cold paleoclimate. The organic matter content in the Lower Member was primarily determined by the amount of detritus input with a low paleo-productivity owing to a strong weathering, whereas that in the Upper Member was dominated by anoxic processes under a good preservation and a low deposition rate. The model reflecting the relationship between accommodation and sediment-fill is employed to analyse the mechanism of organic matter accumulation. Three depositional stages are distinguished within the Shimengou Formation corresponding to overfilled, underfilled, and balanced-filled sediments with average TOC contents of 0.84 %, 4.95 %, and 6.09 %, respectively. This study reveals the dominated factors controlling organic matter enrichment in a continental basin and proposes a TOC distribution mechanism, which could provide a guide for the shale gas and shale oil exploration. • High-resolution geochemistry and detailed geological description were performed. • Paleoclimate evolution of Middle Jurassic Shimengou Formation was analysed. • Detritus inputs and anoxic degree were predominant factors for influencing TOC. • Three depositional stages of the Shimengou Formation were subdivided. • The model of lacustrine organic matter accumulation was proposed.

Elemental geochemical evidence for controlling factors and mechanisms of transitional organic matter accumulation: The upper Permian Longtan Formation black shale in the Lower Yangtze region, South China

The marine-continental transitional shale in the upper Permian Longtan Formation is not only regarded as an excellent source rock, but also one of the crucial layers of shale gas exploration in the Lower Yangtze region, South China. However, controlling factors and mechanisms of transitional organic matter (OM) accumulation in such a setting are not well understood. In this study, the characteristics of total organic carbon (TOC) and elemental geochemistry of 22 rock samples collected from one core and two different types of outcrops are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that shales developed in a tidal flat-lagoon environment display high TOC contents ranging from 6.16% to 10.10% (average 8.50%), whereas these formed in a deltaic environment exhibit medium-high TOC contents varying from 0.93% to 6.70% (average 3.26%). These data also elucidate that the target shales in the Longtan Formation were deposited in a complex paleoenvironment with strong water-mass restriction that was mainly characterized by warm and humid paleoclimate, high primary productivity, oxic-to-dysoxic conditions, and a high sedimentary rate. Besides, the transitional OM accumulation is not determined by a single factor, but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoproductivity, paleoredox, and sedimentary rate. All these factors will directly or indirectly affect OM supply or preservation. Finally, the “integrated model” for transitional OM accumulation is proposed. The model stresses two aspects: on one hand, the warm and humid paleoclimate not only facilitates the growth of higher plants, but also accelerates the chemical weathering rate of the parent rocks and increases the input of nutrients to the water column, which is conducive to the blooms of lower aquatic organisms. Both higher plant debris and lower aquatic organisms together provide abundant OM sources. On the other hand, although the oxidized water environment is usually unfavorable for OM preservation, a higher sedimentary rate can greatly shorten OM exposure time in the decomposition region of aerobic bacteria, and a number of organic matters cannot be oxidized or degraded through the rapid burial. These findings also add to our knowledge that despite the oxygenated water environment during transitional shale deposition, TOC contents are not necessarily lower.

Factors controlling the organic matter accumulation across the Pliensbachian-Toarcian transition in the Qiangtang Basin, Tibetan Plateau

The Lower Jurassic oil shales in the Bilong Co area are the most important hydrocarbon source rocks in the southern Qiangtang Basin, Tibetan Plateau. The onset of the deposition of these shales was closely related to the global warming during the Pliensbachian-Toarcian transition; however, the mechanism of organic matter accumulation in these shales remains poorly constrained. In this study, a multiproxy geochemical study was performed on the organic-rich shales spanning the Pliensbachian/Toarcian boundary in the Bilong Co section in order to assess the organic matter sources, detrital influx, paleosalinity, paleoredox conditions, and paleoproductivity during the deposition of these shales. The distribution patterns of the n-alkanes, as well as the H/C and O/C ratios of the kerogen, suggest that the organic matter primarily originated from marine plankton/algae. The detrital proxies (K/Al, Si/Al, Ti/Al, Zr/Al, and Zr/Rb) indicate relatively stationary sea level and stable continental weathering intensity along the section. The paleosalinity proxies indicate that the organic-rich shales were deposited under marine salinity conditions with little freshwater input. The covariation patterns of the redox proxies (MoEF, UEF, ReEF, DOPT, and Corg/Ptot) suggest that the organic-rich shales were deposited under suboxic conditions, while the MoEF-UEF and Mo-TOC plots suggest a weakly hydrographically restricted depositional setting. The productivity proxies (Ptot, CuEF, ZnEF, and CdEF), as well as CdEF/MoE-CoEF*MnEF plot indicate low-to-moderate primary productivity, indicating that the organic matter accumulation was driven by preservation rather than by productivity. The development of the ocean deoxygenation in the Bilong Co area was probably the result of sluggish oceanic circulation and an enhanced thermocline on the shelf between the Qiangtang central uplift and the Bangong-Nujiang Ocean. The climatic changes during the Pliensbachian-Toarcian transition led to the onset of organic matter accumulation in the Bilong Co area, while the hydrographically restricted depositional setting associated with good preservation conditions may have been the primary reason for the massive deposition of the Lower Jurassic oil shales in the southern Qiangtang Basin.

Geochemical Characteristics of the Middle Devonian Dacaozi-Tanshanping Shale Strata in the Yanyuan Basin, Southwest China: Implications for Organic Matter Accumulation and Preservation

How the geochemical characteristics of organic matter shale for the carbonate platform facies remain uncertain, which restricts an integrated reconstruction of the model of organic matter accumulation and preservation. Here, we present new results from element geochemical fingerprinting and integrated analyses of paleoclimate, paleoproductivity, paleoredox environment, and terrigenous input of the targeted Middle Devonian Tanshanping and Dacaozi Formations in the Ninglang-Yanyuan Basin, Southwest China. It is worth noting that although the carbonate platform connects with the open sea partially, the redox environment will not be completely controlled by relative sea level variations. Specially, the paleoclimate, paleoproductivity, and paleoredox conditions are the main controlling factors of the accumulation and preservation of organic matter. In view of the paleoclimate indexes, we suggest that both a relatively warm-humid climate characterized by intensified chemical weathering conditions and a higher terrigenous input are identified as two major drivers forcing the reductive environment in the sedimentary waterbody. Finally, a comprehensive model is established for providing new insights into the mechanism of organic matter accumulation and preservation for the carbonate platform facies. The paleoredox environment, paleoproductivity, paleoclimate, and terrigenous input are believed to have exerted a very considerable force on reconstructing the model of organic matter accumulation and preservation for the carbonate platform facies. Specially, the coupling interactions between the paleoproductivity and redox condition are thus also stressed. We found that the preservation condition is much more important than the paleoproductivity, resulting in the degree of organic matter enrichment. Even if the paleoproductivity of a sedimentary waterbody of a depositional period of the Dacaozi Formation was higher, the TOC concentrations were relatively low due to the poor preservation condition by fall of the sea level and increase of the terrigenous input. In another aspect, the better preservation condition of the Tanshanping Formation makes the TOC concentrations higher in the case of lower paleoproductivity in the sedimentary waterbody.

Geochemical characteristics and mechanism of organic matter accumulation of marine-continental transitional shale of the lower permian Shanxi Formation, southeastern Ordos Basin, north China

The marine-continental transitional shale is widely distributed in the Lower Permian Shanxi Formation of the southeast Ordos Basin. The primary factors controlling organic matter accumulation of the Shanxi shale are still obscure. In this study, we used mineralogy and organic geochemistry, coupled with major and trace element geochemistry to analyze the paleoenvironment and their effects on organic matter enrichment, and established the accumulation model of organic matter in the Shanxi transitional shale. Results show that the mineralogical composition of the Shanxi shale is dominated by clay and quartz (averaging of 62.8 and 33.3%, respectively). The Shanxi shale has relatively high TOC contents and thermal maturity (Ro) (averaging of 1.25% and 2.66%, respectively), and predominantly contains type III kerogen. The significantly low paleoproductivity indicates that this factor plays not the primary role of organic matter accumulation. Ti and Al concentrations indicate a strong terrestrial input, simultaneously bringing abundant terrestrial organic matter, which is favourable for its enrichment. The warm and humid climate condition enhanced higher plant growth and provenance weathering. The δU, U/Th, V/Cr and Ni/Co values show dysoxic-oxic environments, suggesting relatively poor preservation conditions for organic matter. Comprehensive analysis shows that the terrestrial organic matter input, paleoclimate and redox conditions are the primary factors controlling the organic matter accumulation of the Shanxi shale. • The paleoproductivity is significantly low during the Shanxi shale deposition. • The terrestrial detrital input is strong, carrying abundant terrestrial organic matter. • The paleoclimate and redox conditions triggered organic matter accumulation to some extent. • An accumulation model of organic matter in the Shanxi Formation was established.

Controlling Factors and Formation Models of Organic Matter Accumulation for the Upper Permian Dalong Formation Black Shale in the Lower Yangtze Region, South China: Constraints from Geochemical Evidence.

The black shale in the upper Permian Dalong Formation is considered as an excellent source rock in the Lower Yangtze region. However, mechanisms of organic matter (OM) accumulation in such a setting are scarcely understood. Here, the characteristics of total organic carbon (TOC) and elemental geochemistry of 33 rock samples from GD1 well are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that the lower and middle parts of Dalong Formation (section A) display high TOC contents ranging from 1.19 to 6.45% (average 3.19%), whereas the upper part (section B) exhibits medium TOC contents varying from 1.18 to 4.90% (average 2.09%). These data also elucidate that the target shales were deposited in a complex paleoenvironment with moderate to strong water-mass restriction that was characterized by warm and semiarid–semihumid paleoclimate, high biotic productivity, fluctuating plaeoredox conditions, and a relatively high sedimentary rate. Compared to the organic-rich shales from section A mainly developed under an anoxic condition, shales from section B formed in an oxic-to-dysoxic water environment exhibited a comparatively higher sedimentary rate. Moreover, among all these factors that might affect OM accumulation, the paleoredox conditions appear to be the dominant controlling factors for section A, whereas the biotic productivity, paleoredox conditions, and sedimentary rate are the main controlling factors for section B. Finally, two formation models for OM accumulation in Dalong Formation shale in the Lower Yangtze region are proposed. The “preservation model” for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The “integrated model” for OM accumulation in section B stresses that greater biotic productivity intensifies respiratory oxygen consumption in a water column and a higher sedimentary rate can greatly shorten OM exposure time for respiration by oxygen, both of which cause OM accumulation under an oxidizing water environment. These findings also add to our knowledge that despite the oxygenated water environment during shale deposition, TOC contents are not necessarily lower.

Accumulation of organic compounds in paddy soils after biochar application is controlled by iron hydroxides

Soil acidity is one of the vital factors that influence organic matter transformation and accumulation. Long-term studies on the mechanisms of biochar's effects on soil organic matter (SOM) accumulation dependent on pH values are lacking. A four-year column experiment was conducted without and with biochar application (11.3 Mg ha−1 crop−1) in acid (pH = 5.24) and alkaline (pH = 8.22) soils under paddy rice/wheat annual rotation. To explore organic matter accumulation mechanisms, SOM pools were extracted (physical-chemical fractionation) and their chemical structures were analyzed using advanced solid-state 13C nuclear magnetic resonance (13C NMR) techniques. Biochar increased the proportion of aromatic carbon (C) in all SOM pools, which led to an increased C content in two soils. The elevated pH after biochar application (∆pH = 1.03) increased Fe (III) oxidation and precipitation, and therefore, stimulated amorphous Fe content in 53-μm pool in the acid soil. This change increased the interaction between organic compounds and Fe (hydr)oxide, which impeded bacteria access to substrates, and in turn, promoted SOM accumulation in the acid soil. Conversely, low Fe (hydr)oxide availability resulted in the decomposition of the labile substrates (di-O-alkyl C, NCH, and OCH) in mobile humic acids via microbial respiration, thereby lowering the effect of SOM sequestration in the alkaline soil. Our study revealed that organic matter accumulation after biochar amendment is not solely dependent on the chemical recalcitrance of biochar, but also is controlled by the transformation of Fe (hydr)oxide in SOM pools.

Palaeoenvironmental analysis of the Lower Silurian Longmaxi Formation in the Zhaotong area of the Sichuan Basin, South China: Implications for organic matter accumulation mechanisms

Geochemical data have been used as the most effective proxies for determining palaeoclimate and palaeoenvironmental conditions. In this article, we conduct a systematic review on a variety of geochemical data of sediments that are considered proxies for tracing palaeoclimate and palaeoenvironmental information. Comprehensive studies have been carried out on the palaeoproductivity, redox conditions, palaeoclimate, and the acquired data on the enrichment mechanism of organic matter have been summarized. The general mechanisms of the geochemical data regarded as the palaeoclimate and palaeoenvironmental archives include the characteristics of mineral composition, major and trace elements, and rare earth elements. The palaeoproductivity index, including the Ba and P, indicate that the organic‐rich shale was deposited in a high‐productivity water column. The trace elements revealed that the Longmaxi units recorded variable redox conditions from anoxic to aerobic. The elements V, U, Th, Ni, and Co together with the U/Th, V/Cr, Ni/Co, and V/(V + Ni) are essential indicators for defining the redox conditions. The NASC‐normalized REE + Y distribution indicates that the Longmaxi Formation was slightly altered by detrital materials in reduced and acidic environments. The Sr/Cu and MgO/CaO all reveal that the palaeoclimate was gradually transformed from humid to dry heat. The Sr/Ba ratio implies that the water depth fluctuates frequently at the early stage of sedimentation. All geochemical data integrated with the sedimentary characteristics show that anoxic conditions, high palaeoproductivity, and relatively humid palaeoclimates contributed to the accumulation of organic‐rich shale. We propose that jointly utilizing various geochemical and sedimentary indices can expand the outlook of the palaeoenvironment and provide a better understanding of the organic matter accumulation mechanisms.

Influence of palaeoclimate and hydrothermal activity on organic matter accumulation in lacustrine black shales from the Lower Cretaceous Bayingebi Formation of the Yin’e Basin, China

The lacustrine black shales of the Lower Cretaceous Bayingebi Formation are an important hydrocarbon source rock in the Yin’e Basin. It is important to understand the mechanism of organic matter accumulation in these shales to reconstruct the palaeoenvironment and palaeoclimate in the Early Cretaceous in northwestern China and search for potential oil and gas resources. Based on petrological, elemental geochemical and organic geochemistry analyses of typical field profile samples, the influence of palaeoclimate and hydrothermal activity on organic matter accumulation in different sedimentary units (Unit I, II and III) is revealed. Unit II has the highest total organic matter abundance (TOCavg = 5.95%), mainly type I kerogen (lamalginite), which is in the immature to low mature stage. Organic carbon isotope data (δ13Corg), element ratios (Sr/Cu and C-value) and weathering indexes (including CIA, CIW and PIA) together revealed that the region experienced a semihumid to semiarid palaeoclimate in the middle of the Early Cretaceous. The increase in rainfall in Unit II enhanced the abundance of lake algae and surrounding terrestrial vascular vegetation and elevated the lake level, which contributed to adequate sources and favourable preservation conditions for organic matter accumulation. Several lines of evidence, including high sulfur contents, the combination of vein pyrite and micritic dolomite, positive Eu anomalies, and element geochemical correlations, confirm the existence of intermittent low–temperature white smoker–type hydrothermal activity. The decrease of oxygen content in lake bottom water may be related to the input of hydrothermal fluid, which provided necessary nutrient elements for biological productivity. According to palaeontological and radioisotope dating data, it can be inferred that the black shales of the Bayingebi Formation are the product of the combined effect of global palaeotemperature rise in the early Aptian stage of the Early Cretaceous and intermittent low–temperature hydrothermal activity.