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.


Introduction
With the advances in unconventional oil and gas development techniques, Precambrian shale oil and gas resources have been playing an increasingly important role in the global petroleum industry.Previous research have shown that the Velkerri Formation in northern Australia, which might be deposited in the same gulf and coeval to the Xiamaling Formation, is rich in shale oil and gas resources (Cox et al., 2022).The discoveries of substantial Precambrian hydrocarbon source rocks have challenged the established understanding that large-scale hydrocarbon source rocks were only distributed in the Phanerozoic strata.Consequently, these advances have significantly augmented the exploration and development potential of Proterozoic hydrocarbon source rocks.In recent years, the Proterozoic strata of various regions such as the McArthur and Adelaide Basins in northern Australia, the Taoudeni Basin in western Africa, the Great Lakes region in northern America, and the São Francisco Basin in Brazil have been the focus of exploration (Craig et al., 2013).
Significantly, over 200 Mesoproterozoic oil seepages have been identified in the Yanliao Basin (Wang and Han, 2011;Zhao et al., 2019).The bituminous sandstone reservoirs of the Xiamaling Formation are estimated to be as high as 0.7-1 billion tons (Liu et al., 2011).Therefore, the organic-rich shales of the Xiamaling Formation in the Yanliao Basin have been regarded as promising strata with the potential to form large-scale in situ oil and gas resources (Zhao et al., 2019).Especially, the depositions of organic-rich rocks across North Australia and North China have been attributed to a 30-myr-long marine euxinic event (Zhang et al., 2022).However, volcanic ashes within the Xiamaling Formation indicate that the organic enrichment is linked with large igneous provinces (LIPs) (Zhang et al., 2018).According to Wang et al. (2020), the deposition of organic-rich shale in the Xiamaling Formation can be attributed to nutrient loading from upwelling activities, which subsequently led to the development of oceanic anoxic conditions.Nonetheless, it is noteworthy that the predominant primary producers during the Mesoproterozoic were prokaryotes, specifically cyanobacteria, while the presence of eukaryotic primary producers was scarce (Knoll and Nowak, 2017).It was not until Cryogenian that eukaryotic marine primary productivity became dominant and consequently, the organic parent materials became diversified (Brocks et al., 2017).In addition, the Mesoproterozoic was characterized by a distinct difference in both the atmosphere, which was low in oxygen (Zhang et al., 2016) and the prevalence of anoxic water bodies, when compared to the Phanerozoic era (Planavsky et al., 2011).On the one hand, the anoxic ocean provides ideal circumstances for organic matter burial, which is comparable to the model of organic carbon accumulation during Cretaceous oceanic anoxic event 2 or organic matter preservation in the black sea (Murray, 1989;Raven et al., 2019;Wagner et al., 2013).On the other hand, the primary productivity is restricted throughout the Mesoproterozoic due to the nutrient throttling brought on by the ocean's widespread anoxia (Anbar and Knoll, 2002;Scott et al., 2008;Zerkle and Mikhail, 2017).Consequently, it holds significance to investigate the organic matter enrichment mechanism, hydrocarbon generation, and expulsion characteristics of the Xiamaling Formation.
The objective of this study is to provide novel insights into the provenance, paleoclimate, weathering intensity, and paleo-environment conditions, for the organic matter enrichment mechanism and source rocks potential of the Xiamaling Formation located within the Yanliao Basin, using scanning electron microscopy (SEM), pyrite morphology, pore structure, rock-eval pyrolysis, and elemental geochemistry.These insights would facilitate the exploration and subsequent development of hydrocarbon resources in the ancient marine strata.These findings will provide more data support that could facilitate the exploration and deployment of Mesoproterozoic oil and gas reserves located within the Yanliao Basin.

Geological setting
The North China Craton underwent cratonzation at the end of the Archean, followed by a series of tectonic processes such as riftings, subductions, accretions, and collisional processes during the Paleoproterozoic and Mesoproterozoic eras (Zhai et al., 2020).It is suggested that the formation of the North China Craton margin rift and the orogenic belt is associated with the reorganization and splitting of the Columbia (Nuna) supercontinent (Zhai et al., 2020).The Yanliao Basin, located at the northern edge of the North China Craton (Figure 1), contains the oldest Changcheng System, the Jixian System, and Xiamaling Formation, which was deposited directly on the crystalline basement (Zhai, 2019).
The Xiamaling Formation is widely distributed in the Yanliao Basin, with its most comprehensive and thickest stratigraphic sequence observed in the Xiahuayuan and Zhaojiashan regions of northwestern Hebei (Figure 1(a)), where it can reach a thickness of nearly 600 m (Fan, 2015).The dark shale found in most regions of the Yanliao Basin has a thickness ranging from 100 to 300 m and serves as one of the most significant source rocks in the Mesoproterozoic (Fan, 2015).It is in parallel unconformable contact with the underlying limestone of the Tieling Formation and the overlying carbonate rocks of the Changlongshan Formation (Zhai et al., 2020).In this study, we have adopted the division scheme proposed by Zhang et al. (2019), which divides the Xiamaling Formation into six units based on lithological and geochemical characteristics.This scheme can be compared with the lithological stratigraphic framework of four members (Fan, 2015).Unit 1 corresponding roughly to Member IV is both being dominated by gray-black, gray-green silt-shale with thin-laminated marlstone lens and stromatolitic features in the upper part.The black shale and mudstone in the upper part of Member III correspond to Unit 2. However, Unit 3 corresponding to the lower part of the Member III, is dominated by interbedded black shale and siliceous rock.Red mudstones and green shales with marl lens comprise Unit 4, which corresponds roughly to Member II.The iron formations of Unit 5 correspond to the upper part of Member I, while the silt mudstones of Unit 6 correspond roughly to the lower part of Member I.The intrusive diabase dyke in the upper part of Member III yielded U-Pb ages of 1320 ± 6 Ma, as well as K-bentonite (Unit 3) and a tuff bed (Unit 2) in Member III with U-Pb ages of 1392.2 ± 1 and 1384.4 ± 1.4 Ma, respectively (Zhang et al., 2019; Figure 2).

Samples
We have collected 92 field samples from Units 3-1 of the Xiahuayuan section.The exterior of each sample was carefully removed to prevent potential weathered components.

Major and trace element analyses
For major elements (Na, Mg, Al, P, K, Ca, Ti, Mn, Fe), the lost-on-ignition (LOI) was determined by combusting for 1 h at 980 °C to correct the elemental abundances.Then, 50 mg of each sample were dissolved in 2 ml 8 N nitric acid.After heating at 150 °C for 6 h, the digestive liquid was manipulated to facilitate complete dissolution in 1 ml perchloric acid and 1 ml nitric acid until the liquid became a colorless and transparent solution for measuring using inductively coupled plasma optical emission spectrometer (ICP-OES).Analytical errors were monitored using AGV-2, GSR-1, and GSR-5 standards, and were generally less than 5%, except for P and K, which were ≤15%.

Extraction of organic sulfur
For extraction of organic sulfur, ∼200 g bulk sample powders were washed with 10% NaCl solution to remove adsorbed sulfate.The remaining sample materials underwent a two-step acidification procedure using excess 6 N HCl, followed by a solution of 3:1 6 N HCl and 40% HF to dissolve carbonate and clay minerals for 2 h at 60 °C.This operation was repeated until all effervescence ceased and then the remnant was heated for 2 h with 6 N HCl to minimize fluoride precipitation.The residual sample materials were then washed with deionized water before being separated to extract pure kerogens using heavy liquids (KBr + ZnBr) with densities of 2.0-2.1 g/cm 3 .The inorganic sulfides in the kerogens, primarily pyrites, were removed using the chromium reduction method; see Canfield et al. (1986) for more details.In order to quantify the content of organic sulfur, the pyrite-free kerogen extracts were combusted in a Parr bomb apparatus at ∼25 atm oxygen to oxidize organically bound sulfur to sulfate, then the dissolved sulfate was trapped as BaSO 4 (Cai et al., 2022).
Observation of organic matter, pyrite, and pore structure Eight samples from the Xiamaling Formation were mechanically polished and coated with approximately 5-nm-thick conductive carbon to enhance the electrical conductivity, and then were observed using a Nova NanoSEM 450 scanning electron microscopy equipped with an energy-dispersive spectrometer (SEM-EDS) to characterize the features of organic matter, pyrite, and pore structure.Typical operating conditions were: a 15 kV accelerating voltage, a working distance of 5.5-6.5 mm, and a tilt angle of 0°.

Pyrolysis experiments
Rock pyrolysis measurement was performed using OG-2000 V at Yangtze University.The pyrolysis temperature was programmed to increase from 300 °C (maintained for 3 min) to a final temperature of 600 °C at a rate of 50 °C per minute.The hydrocarbon that is released during thermal heating at 300 °C is referred to as the free hydrocarbon (S1), while the hydrocarbons produced during pyrolysis between 300 and 600 °C are known as residual hydrocarbon (S2).

Data presentation
The enrichment factor (EF) is a common measure for eliminating the influence of detrital content and indicating the extent of elemental enrichment or depletion.It can be calculated using the following equation (Tribovillard et al., 2006): element X EF = (X/Al) sample /(X/Al) PAAS .The enrichment factor (EF) of element X, represented by X EF , is normalized using Post-Achaean Australian Shale (PAAS) data.Element X is enriched or depleted if values of X EF are greater or less than 1.0, respectively.

Major and trace elements
The Xiamaling Formation displays significant vertical variations in the concentrations of both major and trace elements, as demonstrated in Figures 2 to 4 and the Supplemental Data Set.The aluminum (Al) contents varied between 3.09 and 10.01 wt.% with an average value of 5.51 wt.%.Meanwhile, titanium (Ti) contents range from 0.12 to 0.52 wt.%, with an average value of 0.29 wt.%.It is worth mentioning that Unit 3 exhibits notably low Ti contents.Additionally, the Ti/Al ratio varies from 0.04 to 0.44, with an average value of 0.06.
The C-value exhibits a range of 0.26-1.29,with an average value of 0.58.The values decrease in an upward direction from Unit 3, with an average value of 0.58, to Unit 2, with an average value of 0.51.Conversely, the values increase in an upward direction in Unit 1, with an average value of 0.65.The Sr/Rb ratios display a narrower range, varying from 0.34 to 2.02, with an average value of 0.61.The pattern of variation in Sr/Rb ratios is less pronounced than that observed in the C-value range (Figure 2).
Regarding redox-sensitive trace elements (Figure 3), the U/Th ratios exhibit a range of 0.15-1.26,with an average value of 0.47.Similarly, the Ni/Co ratios range from 0.48 to 38.76, with an average value of 8.20, and display a comparable pattern of variation to that observed in the U/Th ratios.In relation to nutrient elements, the Zn EF , Cu EF , Ni EF , and P EF exhibit a range of 0. 37-13.35, 0.26-19.65, 0.18-13.59,and 0.12-15.68,respectively.These elemental ratios and enrichments follow a similar trend, with the maximum value observed in Unit 2 of the Xiamaling Formation.

The content of organic sulfur
In order to assess the sulfurization degree of organic matter, organic sulfur content is expressed in the form of an organic S: C atomic ratio.The organic S: C atomic ratios exhibit a range of 0.4-2.24,with an average value of 0.99.

Characteristics of organic matter, pyrite, and pore structure
Following a detailed polarizing microscope study, organic matter layers in Unit 3 alternate with the chert and clay layers (Figure 5(a) and (b)), respectively, while organic matter layers in Unit 2 occupy a considerable proportion (Figure 5(c)).The results of SEM observation indicate that the presence of pyritic framboids in the Xiamaling Formation is limited to Unit 2, while Units 1 and 3 exhibit a scarcity or absence of these structures (Figure 5(d)-(i)).The pyrite displays various morphologies, including normal framboids (Figure 5(f)), overgrown framboids (Figure 5(g)), euhedral pyrites (Figure 5(e) and (h)), and scatter pyrites (Figure 5(d) and (i)).Various reservoir spaces were identified in the shales, including interpartical pores, intrapartical pores, intercrystal pores and microfissures (Figure 6).Interpartical pores are always developed between clay and mineral grains with irregular shapes; intrapartical pores are mostly in the clay mineral flakes, organic matter and coarse detrital particles; intercrystal pores are mainly observed in the pyrite, carbonate and Quartz; microfissures are also found in organic matter, and clays which can connect the pores to improve the connectivity.

Rock-eval pyrolysis
Total organic carbon (TOC) contents vary from <0.1 to 10.8%, with a mean of 2.63% (Figures 2  and 7).The T max values, which range from 430 to 457 °C with an average value of 440 °C and exhibit a decreasing upward trend from Unit 3 to Unit 2 (Figure 8), are noteworthy.Additionally, the hydrogen index (HI), calculated as HI = 100 × S2/TOC, and the production index (PI), calculated as PI = S1/(S1 + S2), exhibit ranges of 20-799 with an average value of 326 and 0.003-0.5 with an average of 0.08, respectively.

Provenance
The chemical composition of clastic sedimentary rocks is predominantly regulated by the lithologies of their source, which comprise a distinctive assemblage of sedimentary, igneous, and metamorphic rocks.Trace and rare elements, such as Zirconium (Zr), Thorium (Th), Hafnium (Hf), and Lanthanum (La), remain stable during geological processes like weathering, deposition, and diagenesis (Wang et al., 2020;Zhang et al., 2021).Furthermore, these elements exhibit immobility and dissimilar concentrations and/or ratios in various parent rocks (Cullers and Berendsen, 1998).As a result, the bivariate diagram of Zr/Sc versus Th/Sc can be utilized to elucidate sedimentary recycling and mineral composition variances (Mclennan et al., 1993).According to Zr/Sc and Th/Sc ratios (Figure 4(a)), it can be inferred that the degree of sediment recycling during the deposition of the Xiamaling shales was insignificant, thus signifying their potential for use in provenance identification.The La/Th-Hf bivariate plots are commonly employed for determining the category of source rocks (Floyd and Leveridge, 1987).Based on the La/Th-Hf plot (Figure 4(b)), it can be observed that the majority of the samples from Unit 2 and Unit 3 of the Xiamaling Formation are located in close proximity to the mixed felsic/basic source area, while those from Unit 1 are positioned within the acidic arc source.In summary, the sources of weathering products of the Xiamaling Formations changed from the mixed felsic/basic source in Units 2 and 3 to the single acidic arc source in Unit 1.

Paleoclimate and weathering
The paleoclimate has a significant impact on the sedimentary geochemical background, chemical weathering intensity, and influx of terrigenous materials (Yan et al., 2010).The C-value is frequently utilized for this purpose, where sediment C-values greater than 0.8 signify a humid climate (Li et al., 2020;Tao et al., 2017).Values ranging from 0.6 to 0.8, 0.4 to 0.6, and 0.2 to 0.4 are linked to semihumid, semiaridsemihumid, and semiarid climate, respectively, while values less than 0.2 indicate arid paleoclimate (Qiu et al., 2015).The C-values of Unit 3 in the Xiamaling Formation exhibit a decreasing trend from 1.23 to 0.33 with an average of 0.61, indicating a shift from a humid to semiarid paleoclimate (Figure 2).However, the C-values in Unit 2 of the Xiamaling Formation are relatively low, ranging from 0.26 to 0.81 with an average of 0.51, suggesting a semiarid-semihumid climate.In Unit 1 of the Xiamaling Formation, the C-values show an increase and fluctuation from 0.29 to 1.29 with an average of 0.65, indicating a transition to humid paleoclimate.
Furthermore, the Sr/Rb ratio is a useful indicator of paleoclimate because Rb is generally stable during weathering and Sr is more prone to being lost in warm and humid conditions (Chen et al., 2011;Liu et al., 2022).In contrast, in arid climates where weathering is less pronounced, Sr is more likely to be retained, resulting in an increasing Sr/Rb ratio (Zhang et al., 2020).The range of Sr/Rb ratios in Unit 3 is 0.44-2.93,with an average value of 0.88, whereas the Sr/Rb ratios in Unit 2 range from 0.36 to 1.54 with an average value of 0.59.The Sr/Rb ratios of Unit 1 range from 0.34 to 0.79 with an average value of 0.47.These proxies suggest that the Xiamaling Formation was primarily deposited in a humid climate, with some periods of Unit 3 experiencing even more humid conditions (Figure 2).

Paleo-environment conditions
Terrigenous influx.The impact of clastic materials input on the organic and mineral compositions of shales and its role in controlling organic matter accumulation are significant, because clastic material can act as a dilutant and/or control the burial rate.For the stability of the chemical properties, elements such as Al, Ti, and Zr have made them extensive choices as proxies for indicating detrital influx (Lash and Blood, 2014;Rimmer, 2004).Al is primarily derived from aluminosilicate clay minerals found in fine-grained sediment (Cox et al., 2016).Ti, on the other hand, is present in both clay minerals and sand-and silt-sized grains, such as ilmenite and rutile (Wang et al., 2017).The stratigraphic variations in Al and Ti content exhibit similar patterns, implying that Ti is sourced from detrital aluminosilicates.The terrigenous input proxies both display higher values in the lower part of Unit 3 and the upper part of Unit 1, corresponding to a humid climate indicated by the high C-value (Figure 2).In humid climates, the enhanced fluvial processes resulted in high terrigenous clastic inputs at high sedimentation rates.The precipitation of authigenic clays (i.e., aluminosilicate minerals) in situ is typically associated with high Al content and low concentrations of Sc, Ti, and Th, which are inherited from seawater.Consequently, Ti serves as a reliable proxy for determining siliciclastic grain size and sedimentation rate when normalized to Al (Murphy et al., 2000).A higher Ti/Al value indicates larger grains and a faster sedimentation rate (Liu, Yao et al., 2019).The Ti/Al ratios display a decreasing trend upward in Unit 3, followed by an increasing-upward trend in Units 1 and 2 (Figure 2).The Ti and Al contents, as well as the Ti/Al ratios, suggest that increased fluviation led to a higher terrigenous influx in the lower part of Unit 3 and upper part of Unit 1. Conversely, the upper part of Unit 3, the entirety of Unit 2, and the lower part of Unit 1 display lower Ti and Al contents and Ti/Al ratios, suggesting reduced detrital flows.These findings suggest that the deposition is in deep water, away from the shore, likely due to the rising sea level during this period, which limited the influx of detrital materials into the depositional site.
Paleoredox conditions.Various methods, such as pyrite morphology and trace elemental elements, have been used to evaluate paleo-oceanic redox conditions under which the fine-grained siliciclastic sediments were deposited.Generally, diagenetic framboids forming under oxic-suboxic conditions, with a larger mean size of 7.7 ± 4.1 μm and a broader size range, contrast syngenetic framboids forming under euxinic which have a smaller mean size of 5.0 ± 1.7 μm and a narrower size range (Guan et al., 2014;Wignall and Newton, 1998).Thus, the diameter of diagenetic framboids can serve as a dependable indicator of paleoenvironmental conditions (Bond and Wignall, 2010).The normal pyrite framboids in Unit 2 comprised tiny spherical microcrystals with less variation, reflecting a syngenetic origin within the anoxic water column and sediment-water interface (Figure 5(f)).However, the overgrown framboids (Figure 5(g)) in Unit 2, and euhedral pyrites with a homogeneous inner structure (Figure 5(e) and (h)) in Units 3 and 1 were formed during early diagenesis.In remarkable contrast, the abundantly scattered pyrites (Figure 5(d) and (i)) that are only distributed within organic matters of Units 3 and 1 indicate genesis through microbial sulfate reduction (MSR) in sediments, reflecting relative oxic bottom water.Under oxic conditions, pyrites formed slowly in sediment pore water where the supply of sulfate and reactive iron was limited.In addition, due to the mutual support of sediment particles, the pyrites were scattered with a wide range of diameters and the aggregation of pyrite microcrystals was not obvious.
Several redox-sensitive elements (RSE) such as Mo, U, V, Ni, and related ratios are frequently utilized to reconstruct paleoredox history in ancient sedimentary archives, because these elements are less soluble in a reducing depositional environment (Bennett and Canfield, 2020;Cai et al., 2015).In this study, we employed U/Th, Ni/Co, and the molybdenum enrichment factor (Mo EF ) to develop a comprehensive overview (Figure 3) and provide a detailed interpretation of the redox bottom-water conditions during the deposition of Units 3 to 1 of the Xiamaling Formations.U and Mo exhibit dissimilar enrichment behaviors during depositional processes, despite both being enriched in anoxic conditions (Tribovillard et al., 2006).Authigenic uptake of U commences at the Fe 2+ -Fe 3+ redox boundary, while authigenic Mo can be converted to particle reactive thiomolybdate in the presence of free H2S in the water column (Tribovillard et al., 2006).The Mo content of marine sediments during Phanerozoic was found to be less than 25 ppm in non-euxinic conditions and more than 100 ppm in the stable euxinic environment.However, the threshold for euxinic conditions might be dropped to an average value of 35 ppm or even lower in the Mesoproterozoic, due to the depletion of RSE in pervasively anoxic conditions (Planavsky et al., 2014;Scott et al., 2008).The lower part of Unit 1 and Unit 2, which exhibit the highest enrichment of Mo, so reflect anoxic to euxinic conditions.The similar transition of redox conditions is also proved by iron speciation (Wang et al., 2017).Additionally, the pattern of Mo and Mo EF suggests that Unit 3 and the upper part of Unit 1 of the Xiamaling Formation were formed under oxic conditions.However, as shown by V depletion, the bottom water of Unit 3 was oxygenated in an oxygen-minimum zone (OMZ) environment (Wang et al., 2017).
Benthic redox conditions have a considerable impact on the Ni/Co and U/Th ratios.Ni/Co ratios exhibit thresholds of <5, 5-7, and >7, representing oxic conditions, dysoxic conditions, and anoxic conditions, respectively (Jones and Manning, 1994).In Unit 2, Ni/Co ratios range from 4.52 to 31.24 with an average value of 13.14, indicating anoxic-dominated conditions that are consistent with the Mo EF trend.Similarly, Ni/Co ratios in Unit 1 display a similar trend to Mo EF , ranging from 0.48 to 7.04 with an average value of 2.33, indicating euxinic conditions in the lower part of Unit 1 and oxic conditions in the upper part of Unit 1.However, the Ni/Co ratios in Unit 3 exhibit variability, ranging from 2.37 to 38.76 with an average value of 7.91, indicating redox condition fluctuations that differ from those of Mo EF .The enrichment is predominantly associated with the detrital influx and generally occurs as insoluble Th 4+ during diagenesis (Tribovillard et al., 2006).The U/Th ratio is a commonly used proxy to infer redox conditions, with a high ratio indicating strong reducing conditions (Algeo and Liu, 2020).The U/Th ratios of the Xiamaling Formation exhibit high values of ≤1.26 in Unit 2, and low values in Unit 3 (≤0.87)and Unit 1 (≤0.55).The redox proxies, including Mo EF , Ni/Co, and U/Th, demonstrate similar trends with low values in Unit 3, a significant increase in Unit 2, and a subsequent decrease in Unit 1.These proxies suggest a transition from fluctuant oxic conditions under OMZ in Unit 3 to anoxic conditions in Unit 2, and finally, a change to mixed anoxic-oxic conditions in Unit 1.
Primary productivity.The formation of organic matter-rich sediment on the seafloor is a function of the primary productivity in the surface water to some extent (Lü and Liu, 2022;Schoepfer et al., 2015).To characterize this, various geochemical proxies such as phosphorus (P), and concentrations of trace elements like Zn, Cu, and Ni are utilized (Ge et al., 2022).P, Zn, Cu, and Ni all primarily enter sediments with dead organisms, but their biogeochemical cycles are different under different redox conditions (Algeo and Maynard, 2004;Piper and Perkins, 2004;Schoepfer et al., 2015).As a result, they are frequently used in combination to track changes in primary productivity under different redox conditions (Ge et al., 2022).
Zn, Ni, and Cu are micronutrient-limiting elements and have been widely used as tracers to assess primary productivity, as they are biologically essential elements for marine life with the potential to influence diverse biogeochemical processes, including nitrogen fixation, nitrogen uptake, carbon fixation, and methanogenesis, in both modern and past oceans (John et al., 2022;Piper and Perkins, 2004;Tyson, 2005).Moreover, Zn, Ni, and Cu could be delivered into the sediment mainly in the form of organometallic complexes.These elements may be released into the water column during the mineralization of organic matter, and if the water column contains hydrogen sulfide, they may be converted to sulfides and subsequently preserved in the sediment (Algeo and Maynard, 2004;Piper and Perkins, 2004).Therefore, Zn EF , Ni EF , and Cu EF are reliable proxies to reconstruct primary productivity, especially in anoxic even euxinic conditions.The Zn EF , Ni EF , and Cu EF exhibit similar patterns of variation from Unit 3 to 1, increasing initially and then decreasing with height (Figure 3).Unit 2 displays the highest Zn EF , Ni EF , and Cu EF , indicating high primary productivity.Zn EF , Ni EF , and Cu EF fluctuations in Unit 3 are considerably high when compared to variations in Unit 1, which is consistent with TOC variation.
Phosphorus (P) can be deposited in association with decayed microorganisms as an essential nutrient element in metabolic activities, and its concentration is thus a widely used indicator of primary productivity (Tyrrell, 1999).Generally, P tends to be released into the water column along with the mineralization of organic matter in reducing water, leading to the underestimation of primary productivity (Algeo and Ingall, 2007).However, P will be adsorbed on the Fe-Mn (oxyhydro-) oxides under oxic conditions, so it will be enriched in the sediment and effectively reflect the primary productivity (Schoepfer et al., 2015).The P EF exhibits an increasing trend upward from Unit 3 to 2, ranging from 0.15 to 8.05 with an average value of 3.51 (Figure 3).Then Unit 1 shows an extremely low value of P EF , ranging from 0.12 to 2.06 with an average value of 1.10, suggesting poor productivity.Overall, the varying pattern of P EF is similar to that of Zn EF , Ni EF , and Cu EF patterns, exhibiting high paleoproductivity during Unit 2, even though Unit 2 is anoxic conditions, which may lead to an underestimation of P enrichment.
The deposition of Unit 2 was characterized by high productivity, which has been attributed to upwelling (Wang et al., 2020).Previous studies have demonstrated that upwelling can enhance the rapid reproduction of calcareous, siliceous, and phosphatic organisms (Schoepfer et al., 2012(Schoepfer et al., , 2013;;Sweere et al., 2016).The abundant nutrient-limiting elements transported by upwelling from deep water masses to the surface, including Ni, Cu, P, and Zn, have played a significant role in promoting productivity (Schoepfer et al., 2013).Böning et al. (2012) studied the upwelling productivity offshore Peru and suggested that the average Ni EF and Cu EF were 1.3 and 1.6, respectively.Co and Mn contents are generally used to recognize upwelling events in modern and ancient coastal systems, where sediments exhibit a low Co × Mn value of ≤0.4 ppm•wt.%,as a result of the upwelling conveyor belt's increased Co × Mn removal (Sweere et al., 2016).Furthermore, the OMZ may function as a conveyor belt for the transportation of dissolved Mn and potentially Co toward the open marine, thereby expediting the depletion of these elements (Brumsack, 2006).Therefore, the average Co × Mn values of 0.15 ppm•wt.%during Unit 2 suggest that upwelling events may have contributed to high primary productivity.Additionally, Unit 3 was considered to have been deposited in an open basin, with the Co × Mn values ranging from 0.01 to 0.25 ppm•wt.%suggest that upwelling events and/or OMZ may have also influenced the deposition during this period (Figure 7).

Organic matter accumulation model of the Xiamaling formation
Organic matter enrichment is regarded as multifaceted physical and chemical processes, which is primarily influenced by surficial primary productivity, benthic redox conditions, and sedimentation rate (Ge et al., 2022).Moreover, the sulfurization mechanism of organic matter has recently attracted attention for its role in the facilitation of carbon burial (Raven et al., 2018).
In Unit 3, there exist strongly positive correlations between organic S: C and TOC content in the Xiahuayuan sections, with R 2 values of 0.57, suggesting that organic sulfurization enhances organic matter burial (Figure 7).There are two types of sulfurization mechanisms: rapid sulfurization occurring over a timescale of days near dynamic redox interfaces, and gradual sulfurization occurring over thousands of years in sediments (Raven et al., 2018).Generally, fresh organic matter containing more reactive functional groups, such as aldehydes, alcohols, and conjugated double bonds, is expected to be better preserved through rapid sulfurization, which reduces microbial degradation (Raven et al., 2019).During the deposition of Unit 3, intense sulfurization of organic matter, as reflected by high organic S: C ratios, result in more reducing sulfur being incorporated into organic matter, which ultimately enhanced organic carbon burial near the dynamic redox interface.Organic sulfurization provides a plausible explanation for the high TOC of Unit 3.
The organic matter accumulation in Unit 2 was attributed to a combination of anoxic bottomwater conditions, high primary productivity, and low terrigenous influx resulting from the rising sea level.During the deposition of Unit 2, the depth of water deepened rapidly controlled by transgression in the Yanliao Basin, leading to anoxic bottom water (Gao et al., 2021).The U/Th redox proxies exhibit positive correlations with TOC content, suggesting that redox conditions favored the preservation of organic matter and were crucial for organic matter accumulation (Figure 7).The high primary productivity proxies of Unit 2 indicate an increased organic matter deposition flux, which was attributed to nutrient-laden upwelling that led to a surface water bloom of marine organisms.The upwelling setting brought abundant nutrients from deep waters, promoting the blooming of marine organisms, which significantly enhanced primary productivity and the transformation of excess organic matter to the bottom water (Sweere et al., 2016).Nonetheless, the TOC contents of the Xiamaling Formation exhibit only a weak positive correlation with the P EF , even during Unit 2, where an upwelling event occurred, indicating that redox conditions, rather than primary productivity, were the primary controlling factor for organic matter accumulation.
During the deposition of Unit 1, the study area underwent a large-scale regression and transformed into a shallow shelf, resulting in gradual from anoxic to oxic conditions in the water columns (Gao et al., 2021).The relatively warm and humid paleoclimate and exposure to landmass resulted in stronger chemical weathering and increased terrestrial detrital input, consistent with the variations of the C-value.Terrigenous influx proxies exhibit upward-increasing trends, which correspond to decreasing TOC, and negative correlations exist between the TOC content and the Al content (Figure 7), indicating the adverse dilution effect of clastic matter input on organic matter burial.Abundant detrital input rapidly increased the sedimentation rate of the mudstones/shales and resulted in a strong dilution effect on organic carbon content, which was unsuitable for organic matter accumulation.Additionally, the upwelling was disrupted indicated by increased Co × Mn values, which led to less nutrient-rich water and decreased marine primary productivity, as evidenced by a significant decline in primary productivity proxies.Furthermore, the elevated atmospheric oxygen level and weathering increased the input flux of oxidants, resulting in the descent of the redox interface and degradation of organic matter during Unit 1 (Liu, Tang et al., 2019, Liu et al., 2020).

Evaluation of total organic matter and source rock potential
In order to gain a clearer understanding of the source rock potential of the Xiamaling Formation, we compared its organic geochemical characteristics to those of the Velkerri Formation and Qingbaikou Formation (Cox et al., 2016;Huo et al., 2020).The Velkerri Formation in the Precambrian hydrocarbon-bearing McArthur Basin was recently assessed as of ∼68 million  (Cox et al., 2016); the Qingshankou Formation in the Songliao Basin, Northeast China (Huo et al., 2020;Zeng, 2020).cubic meters of oil and ∼222.56 billion cubic meters of gas (Cox et al., 2022).The paleogeographic reconstructions indicate that the Xiamaling and Velkerri Formations were deposited in the same low-latitude gulf (Mitchell et al., 2020).Additionally, promising results and industrial oil flow have been achieved from the deep lacustrine facies Qingbaikou shale oil in the Gulong Sag, Songliao Basin (Huo et al., 2020).
Our analysis of T max values indicates that the Xiamaling Formation is primarily composed of low thermal maturity, whereas the Velkerri Formation ranges from low to highly mature and the Qingshankou Formation is mature (Figure 9).The PI parameter is a reliable indicator of shale oil content, suggesting that the generating oil potential of Qingbaikou Formation is relatively high (Figure 9(d)).Additionally, we observed a decreasing trend in both T max and PI with increasing height from Unit 3 to Unit 2 of the Xiamaling Formation, indicating an increase in thermal maturity with greater burial depth, as well as variable retention and expulsion of hydrocarbons (Figure 8).However, this trend breaks down at Unit 1, where T max and PI increase with height but still show low maturity.We hypothesize that the upper and lower samples may have undergone loss of organic matter due to transient magmatic heating from hydrocarbon generation and expulsion (Fan, 2015).The occurrence of dolerite sills in various landmasses, including the Yanliao Basin and McArthur Basin, may indicate a large igneous province (Cox et al., 2016;Zhang et al., 2018).
The pyrolysis yield (S2) is also used to evaluate the OM abundance of source rocks, indicating that Unit 2 of the Xiamaling Formation has good to excellent potential for generating hydrocarbons, which is consistent with the high TOC content (Figure 9(c)).Preservation of organic matter can be evaluated using the Hydrogen Index (HI).High values of HI were observed in the upper part of Unit 2 and the lower part of Unit 1 of the Xiamaling Formation, which was consistent with the euxinic condition (Wang et al., 2017).According to the HI-T max diagram, the Xiamaling shales mainly show type I, II 1 , and II 2 kerogens, which are distinct from type I and II 1 kerogens of the Qingbaikou Formation and type II 1 and II 2 kerogens of the Velkerri Formation (Figure 9(a)).Low HI values are typically associated with a high degree of organic matter decomposition, but low TOC concentrations can also cause low HI values (Wang et al., 2017).Overall, the TOC, S2, HI, and PI contents of Unit 2 and the lower part of Unit 1 are similar to those of the Qingbaikou Formation, but higher than those of the Velkerri Formation (Figure 9(b) and (c)), suggesting that the Xiamaling shales are good to excellent hydrocarbon source rocks with intermediate oil-prone properties compared to the other two formations.

Conclusions
1. Multiple geochemical proxies and pyrite framboid morphology indicate that Unit 3-Unit 1 of the Xiamaling Formation was deposited in a climate fluctuating between highly humid and semiarid, corresponding strong-to-weak terrigenous influx.Unit 3 was deposited in oxic conditions in an open basin.In contrast, as the result of large-scale marine transgression, Unit 2 and the lower part of Unit 1 were deposited under anoxic conditions.The upper part of Unit 1 was deposited under oxic conditions associated with regressions and a drop in sea level.2. Organic matter accumulation in Unit 3 is controlled by the intense sulfurization of organic matter, resulting in the co-variations in TOC content with organic S: C ratios.In contrast, the organic matter enrichment of Unit 2 and the lower part of Unit 1 was mainly due to anoxic conditions.3. Based on a comparison with other petroliferous basins, Unit 2 and the lower part of Unit 1 of the Xiamaling Formation exhibit low maturity, high hydrogen index (HI) values, and are dominated by type I, II 1 , and II 2 kerogens, showing a significant shale oil potential.

Figure 1 .
Figure 1.(a) The Geological map and study sections of the Xiamaling Formation in North China Craton (Modified after Gao et al. (2021)).(b) The location of the Yanliao Basin (red rectangle).(c) Paleogeography of the Yanliao Basin in North China Craton during the Xiamaling Formation with the location of study sections, modified after (Lyu et al., 2021).(d) Green shales with carbonate concretions in Unit 1, Xiahuayuan section.(e) Black shales with four tuff layers (U-Pb ages of 1384.4 ± 1.4 Ma) in Unit 2 (Zhang et al., 2015).(f) Alternating black shales and cherts in Unit 3, Xiahuayuan section.

Figure 2 .
Figure 2. Stratigraphic variations of lithology, TOC content, and geochemical proxies of paleoclimate (C-value, Sr/Rb), terrigenous influx (Al, Ti content, and Ti/Al ratios), and upwelling event (Co × Mn) from Unit 3 to 1 in the Xiahuayuan section.

Figure 3 .
Figure 3. Stratigraphic variations of geochemical proxies of redox conditions (Mo, U/Th, and Ni/Co), primary productivity (Zn, Cu, Ni, P, and enrichment factors) from Unit 3 to 1 in the Xiahuayuan section.

Figure 5 .
Figure 5. Photomicrographs of lithological features and pyrite of the Xiamaling formation.(a-c) were taken under plane-polarized light.(a) Organic matter layer between clastic minerals in Unit 3. (b) Organic matter layer alternating with silica-cemented sediments in Unit 3. (c) Organic flakes of the mudstone in Unit 2. SEM photos of pyrite morphology in the Xiamaling Formation (d-i).(d, e) Scatter pyrites in organic matter and euhedral pyrites near Mn-bearing carbonate in Unit 1, respectively.(f, g) Normal framboid pyrites and overgrown framboid pyrites in Unit 2, respectively.(h, i) Euhedral pyrites and scatter pyrites in the organic matter layer in Unit 3, respectively.

Figure 7 .
Figure 7. Crossplots of (a) TOC content versus organic S: C. (b) TOC content versus U/Th; (c) TOC content versus P EF ; (d) TOC content versus Al content in the Xiahuayuan section.

Figure 8 .
Figure 8. Stratigraphic variations of T max , HI, and PI from Unit 3 to Unit 1 of the Xiamaling formation, including data from the previous study(Wang et al., 2017).

Figure 9 .
Figure 9. Crossplots of (a) T max versus hydrogen index.(b) TOC content versus hydrogen index.(c) TOC content versus S2.(d) T max versus production index.Compilation of data from the Xiamaling Formation in the Yanliao Basin (Wang et al., 2017), North China; the Velkerri Formation in the McArthur Basin, North Australia(Cox et al., 2016); the Qingshankou Formation in the Songliao Basin, Northeast China(Huo et al., 2020;Zeng, 2020).