Organic matter enrichment of the Late Triassic Yanchang Formation (Ordos Basin, China) under dysoxic to oxic conditions: Insights from pyrite framboid size distributions

Accepted Manuscript Full length article Organic matter enrichment of the Late Triassic Yanchang Formation (Ordos Basin, China) under dysoxic to oxic conditions: Insights from pyrite framboid size distributions Guo Chen, Wenzhe Gang, Yazhou Liu, Ning Wang, Chong Jiang, Jingbo Sun PII: DOI: Reference:

S1367-9120(18)30450-4 https://doi.org/10.1016/j.jseaes.2018.10.027 JAES 3688

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

8 April 2018 29 October 2018 31 October 2018

Please cite this article as: Chen, G., Gang, W., Liu, Y., Wang, N., Jiang, C., Sun, J., Organic matter enrichment of the Late Triassic Yanchang Formation (Ordos Basin, China) under dysoxic to oxic conditions: Insights from pyrite framboid size distributions, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes. 2018.10.027

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Organic matter enrichment of the Late Triassic Yanchang Formation (Ordos Basin, China) under dysoxic to oxic conditions: Insights from pyrite framboid size distributions Guo Chena, b

Wenzhe Ganga, b,*

Yazhou Liua, b

Ning Wanga, b Chong Jianga, b

Jingbo Suna, b

a. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China;

b. college of geosciences, China University of Petroleum, Beijing, 102249, China

*. Corresponding Author: Wenzhe Gang (w.z. Gang). E-mail: [email protected]

Graphic Abstract

Abstract Pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals. The various lithologies in the Chang 7 Member of the Upper Triassic Yanchang Formation contain abundant pyrite framboids, which provide an opportunity to understand the fluctuation in paleoredox conditions and relative sedimentation rates during deposition. The measured diameters of the pyrite framboids in the Chang 7 Member are characterized by large mean diameters (9.7 to 16.6 μm) with a wide distribution range that plots in the area indicative of dysoxic to oxic paleoredox conditions during deposition of the Chang 7 sediments. Based on correlation analysis, the strong correlation between paleoredox-sensitive trace elements ratios (V/Cr, U/Th, and Cu/Zn) and the mean size of framboids indicate that

the latter can serve as a reliable indicator of paleoredox conditions. The results of the framboid size distribution can also be interpreted through Crystal Size Distribution Theory (CSDT). According to CSDT, the relative sedimentation rate of the Chang 7 Member fluctuated during deposition in parallel with fluctuation in lacustrine levels. Furthermore, the strongly positive correlation between total organic carbon (TOC) and paleoredox conditions indicates that although relatively high sedimentation rate may have contributed to the dysoxic-oxic conditions that were harmful to the preservation of organic matter, but it may also have brought nutrients to increase productivity. Similarly, the correlation between TOC and relative sedimentation rates suggests that the relative sedimentation rates can affect organic matter enrichment in several ways. A high sedimentation rate is favourable for the preservation of organic matter, which can be prevented from oxidation, but large amounts of sediments can dilute the organic matter, which reduces the organic matter abundance in a potential source rock. Keywords: Chang 7 Member; pyrite framboids; paleoredox conditions; relative sedimentation rate; organic matter enrichment

1. Introduction Sediments of the Chang 7 Member deposited in the Late Triassic are recognized as the most important hydrocarbon generating source rocks in the Ordos Basin, China. In addition, the Chang 7 Member exhibits good hydrocarbon-generating potential for both oil and gas, while the maturity of the source rocks has remained within the oil generation window (Yang and Zhang, 2005; Wang, 2015; Yang et al., 2010b). Many

studies have been published on the sedimentology of the Chang 7 Member, as well as on source rock evaluation, petroleum migration, accumulation, and organic matter enrichment. And the study of organic matter enrichment mainly focused on paleoredox condition reconstruction by the characteristics of biomarkers, carbon and oxygen isotopes and inorganic elements, but paid less attention to authigenic minerals such as apatite and pyrite framboids (Song et al., 2002; Zhu et al., 2013; Ji et al., 2007; Li et al., 2012; Qiu et al., 2015a&b). In addition, paleoredox conditions and sedimentation rates have important roles in the preservation of organic matter and hence control the hydrocarbon generating process (Raiswell et al., 1988; Jones and Manning, 1994). But the variation in the paleoredox conditions and sedimentation rates of the Chang 7 Member are still uncertain, and previous results relating to paleoredox conditions were contradictory (Ji et al., 2007; Li et al., 2012; Qiu et al., 2015 a&b). Authigenic minerals, such as pyrite and apatite, form in different depositional environments, and can also be used to understand the organic matter enrichment (Jones and Manning, 1994; Wilkin and Barnes, 1996). Sedimentary pyrite generally displays two basic forms, namely euhedral and framboidal crystals. The euhedral pyrite is formed in a reducing environment with unsaturated amorphous iron sulfide (Fig. 1) (Berner, 1984; Wilkin and Barnes, 1996). Conversely, the genesis of pyrite framboids has been controversial. Biogenic theories held a dominant position in the first half century since the discovery of pyrite framboids, and included ‘Fe-sulfide gels’ (Schouten, 1946), the product of sulfate-reducing bacteria (Love, 1957), and the replacement of spherical organic

substrates (Wilkin and Barnes, 1996). However, the majority of framboids attributed to microbial metabolisms contain fewer microcrystals (Popa et al., 2004). More recently, it is widely accepted that the pyrite framboids are formed by a greigite intermediate (Wang and Morse, 1996). Wilkin and Barnes (1996) suggest that four steps are involved in framboid formation: (i) initial nucleation and growth of iron monosulfide (FeS) microcrystals; (ii) transformation of the microcrystals to greigite (Fe3S4); (iii) aggregation of greigite into densely packed, spherical framboids; and (iv) conversion of greigite to pyrite (FeS2) (Fig. 1a). Generally, pyrite framboids are not precipitated as pyrite directly, but transformed through intermediate phases of iron sulfides (Wang and Morse, 1996). Because monosulfide nucleation and the formation of pyrite framboids must occur under reducing conditions, and the formation of greigite must exists in dysoxic conditions, thus pyrite framboids can be used as an indicator of paleoredox condition. The size distribution of pyrite framboids has proven to be an effective and viable way to identify the paleoredox conditions at the interface of the sediments with the water column (Wignall et al., 2005; Bond and Wignall, 2010; Tian et al., 2014; Wei et al., 2015). Under euxinic conditions, the framboids formed in the water column will drop down to the sediments’ surface and stop growing (Fig. 1b). Thus, the size of framboids formed in euxinic conditions is usually finer with a narrower range than those formed in oxic-dysoxic redox conditions. Moreover, a lot of research has suggested that framboid size will remain stable during deposition and diagenesis, except in salt marsh sediments (Wilkin et al., 1996). Finally, inorganic geochemical proxies of the paleoredox conditions were used

to compare the results of pyrite framboids in the Chang 7 sediments. A theoretical method proposed by Randolf and Larson (1988) is used to explain the crystal distribution in a chemical engineering system. A population equilibrium equation of numbers and sizes of crystals against growth time and growth rate is the basis of the crystal size distribution theory (CSDT), and the residence times and input or output of crystals in the system are the main factor of CSDT (Wilkin et al., 1996). In a relatively restricted system, the framboids only form immediately subjacent to oxic-anoxic interfaces. Thus, according to the standard deviation, skewness, and mean values of size distribution, we can deduce the relative growth time and then acquire the relative sedimentation rate (Nielsen, 1964; Wilkin, 1995; Schoonen and Barnes, 1991; Zhou and Jiang, 2009). Here, we present statistical data for framboids from the Chang 7 Member in order to study the fluctuation in paleoredox conditions and relative sedimentation rates, and then discuss the effects on organic matter enrichment by correlation analysis between TOC and paleoredox or sedimentation rates.

2. Geological Setting The Ordos Basin is located in the central northern China (Fig. 2) with an area of 320,000 km2 (Liu et al., 2004; Yang et al., 2010a). The Ordos basin is part of the North China Block, and is one of the most important terrestrial petroliferous basins in China (Qiu et al., 2014). In particular, the Upper Triassic Yanchang Formation contains a vast hydrocarbon resource potential (Yang and Deng, 2013; Zeng and Li, 2009; Zhong et al., 2013; Zhu et al., 2008).

During the Early to Middle Triassic period, the Ordos Basin was part of the North China Craton, which was dominated by a fluvial-lacustrine depositional system. In the period, the North China Block impacted with the Yangtze Block and integrated into a whole continent, which resulted in the formation of the Qinling Mountains (Yang, 2002). Hence, the Ordos Basin became a foreland basin and displayed the character of a low gradient in the north-eastern flanks and a high gradient in the south-western flanks. The progressive closure of the basin led to the development of a terrestrial lake basin during the Late Triassic (Zhang et al., 2006). Moreover, the intense regional tectonization led to the maximum lake extent during the early deposition of the Chang 7 Member, and hence deposition of sediment with abundant organic matter preserved in the deep to semi-deep subfacies (Yang and Zhang, 2005; Yang et al., 2010a; Fig. 2). Six tectonic units in the Ordos Basin have been identified, consisting of the Yimeng Uplift the Western Thrust Belt, the Tianhuan Depression, the Yishan Slope, the Weibei Uplift and the Jinxi Fault-Fold Belt. The study area, namely the Yanchi-Dingbian area, lies in the western region of the Tianhuan Depression and covers an area of 4,000 km2. The area is located at the margin of the lacustrine sediments, which are mainly of deep/semi-deep lacustrine and delta-front subfacies formed during the sedimentation period of the Yanchang Formation (Yang and Zhang, 2005). The stratigraphy of the Ordos Basin can be separated into two stages: the preceding stage is prior to the Permian (pre-299 Ma), at that which time the area of the Ordos Basin was a part of the North China Block, and the subsequent stage is

from the Permian (post-299 Ma), when the stratigraphy of the Ordos Basin changes from marine facies to terrestrial lake facies (Yang, 2001; Wan et al., 2006; Zou et al., 2010). Before the Permian, the Ordos Basin was filled by marine sediments consisting of Sinian/Ediacaran, Cambrian, Ordovician, and Carboniferous age units. Since the start of the Permian Period, the stratigraphy consists of Permian, Triassic, Jurassic, Cretaceous, Neogene and Quaternary age units (Yin and Nie, 1996; Zou et al., 2010). The key stratigraphic unit in the study area is the Yanchang Formation, which was deposited in the Ordos Basin during the late Triassic, and was dominated by terrestrial sedimentation with a thickness of approximately 1000 m (Fig. 3) (Zou et al., 2012; Qiu et al., 2015). According to key beds and rhythmic lithological alterations of the stratigraphy, the Yanchang Formation is divided into 10 members, which are named Chang 1 to Chang 10 (Guo et al., 2014; Ji et al., 2008; Qiu et al., 2015a). Based on U-Pb dating of zircons by the LA-ICP-MS method, where the zircons were collected from the tuff layers at the bottom of Chang 7 Member of the Yanchang Formation, the age of the tuffs is 228.2 ± 2.0 Ma, which corresponds to the period of Indosinian movement (Deng et al., 2013). The depositional period of the Chang 7 Member is about 2.1 Ma in duration, and the area of the lake basin reached its maximum depth during this period. As the area of the lake basin began to shrink, the lithology of the Chang 7 Member changes from oil shale to black shale, to carbonaceous shale, and to turbidite siltstone (Fan et al., 2018). The shale at the bottom of Chang 7 Member is known as the best hydrocarbon source rock in the Yanchang Formation and contains mainly humic-sapropel-type organic matter with maturity or Reflectance in oil (Ro)

values that range from 0.68 % to 1.12 % (Zhao et al., 1996; Duan et al., 2008; Duan, 2012).

3. Materials and Methods Thirteen samples from the Yan 56 well were selected from the Chang 7 Member drill core located in the western Ordos Basin (Fig. 2). To understand the variation in paleoredox and the sedimentation rate, continuous sampling was used in the Yan 56 well to obtain source rock samples including oil shale, dark mudstone, and silty mudstone. All samples were crushed and sieved with an 80-mesh sieve and then were analyzed for their TOC contents at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing). The finely powdered rock samples were blended with 10% (vol) HCl for one hour to eliminate the inorganic carbon. After that, all samples were washed with distilled water to remove all traces of HCl. Finally, the TOC analysis was performed using a Leco CS-230 carbon analyzer with 99.5 % oxygen as carrier gas under the temperature of 24 ℃ and relative humidity of 48 %. For quantitative mineralogical analyses, all of samples were ground in an agate mortar with more than 300-mesh sieve and then filled in the X-ray holder. A Bruker D2 equipped with Ni-filtered Cu-Kα tube and a curved graphite monochromator, running at 40 kV and 25 mA, was used to determine the mineral composition. The scan range was from 4.5 to 45° 2θ for the bulk samples, and the mineral identification method was on the basis of Moore and Reynolds (1997). In addition, samples were

analyzed for their major oxides and trace elements at the Analytical Laboratory Beijing Research Institute of Uranium Geology (ALBRIUG), China. Thirteen fused discs were prepared for analysis of their major oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, TiO2 and P2O5) with the Chinese National Standard (GB/T 14506.28-2010) by X-ray fluorescence (XRF) with an Axiosm AX XRF Trace elements were analyzed by ICP-MS (Element XR) after samples digestion and quantified based on the Chinese National Standard (GB/T 14506.30-2010). The sample powders were dissolved using HF (30 %) and HNO3 (68 %) AT 190 ℃ for 24 h.Then, the solution was heated to evaporate-off excess solvent with ultrapure water and then re-dissolved in 2 ml of 6 mol/L HNO3 in capped Teflon bombs at 150 ℃ for 48 h. Afterwards, the solutions were evaporated to near dryness again, and 1 ml of 6 mol/L HNO3 was added subsequently. Finally, the samples were for analysis (Jenner et al., 1990), and the detection limits for elements ranged from 0.1 × 10-12 to 9 × 10-12. The analyses were performed under the temperature of 23 ℃ and relative humidity of 38 %. All samples were selected from typical representative areas of core, then were polished by argon ion and observed under a field emission scanning electron microscope (FESEM) (Hitachi SU8010) with a back-scattered electron (BSE) detector at the Microstructure Laboratory for Energy Materials, China University of Petroleum (Beijing). The morphology of pyrite was observed, and the size distribution of pyrite framboids was measured. Because the polished surface randomly intersects the framboids, the measured size distribution of pyrite framboids are an approximation to

within a 10 % deviation (Cashman and Marsh, 1988). In all, 2403 pyrite framboids were photographed and measured, and the size distribution of each sample was extracted.

4. Results 4.1 Morphology of sedimentary pyrite Observation under a FE-SEM revealed that framboids are the dominant form of pyrite, while the euhedral form is subordinate in the Chang 7 Member source rocks. According to the classification of Wang et al. (2013), pyrites are divided into two categories including abiological and biomorphic type (usually seen in pyrite radiolarian or hexactinellid molds), and framboids and euhedral crystals are major types in abiological pyrite. In this study, framboidal and abiological aggregates are the main types of pyrite, with a few euhedral and biomorphic pyrite aggregates. Within these, variable pyrite morphologies were exhibited, including euhedral pyrites (Fig. 4A, B and E), fragmental pyrites (Fig. 4B-C), normal framboids (Fig. 4D, F-H, J), infilled framboids (Fig. 4C-E), polyframboids (Fig. 4K), and biomorphic pyrites (Fig. 4L). Single euhedral crystals mainly occur as octahedral (Po) and pyritohedral (Pp) shapes, and a majority of the pyrites are less than 10 μm in size. In addition, some fragmental pyrites are discovered near fractures, and this may be caused by tectonic compression (Fig. 4C). Pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals. According to the form of microcrystals in framboid, four types of normal framboids were usually observed, and these are: (1) spherical microcrystalline framboid (Fig. 4F); (2)

octahedral microcrystalline framboid (Fig. 4G); (3) cubic microcrystalline framboid (Fig. 4H); and (4) disordered microcrystalline framboid (Fig. 4J). Usually, the framboids composed of cubic microcrystals are smaller than the other three framboids, and their diameter sizes are commonly less than 6.5 μm. Furthermore, according to the arrangement of the microcrystals, the framboids can be divided into two types: (1) formed by orderly microcrystals (Fig. 4F); and (2) formed by disorderly microcrystals (Fig 4.G-J). Infilled framboids are usually round with small holes which suggest that infilled framboids may have formed by microcrystal enlargement of normal framboids. Moreover, some large polyframboid aggregates occur in a few samples (Fig. 4K), which may inherit the morphology of trace fossils and scattered coal wood fragments (Reolid, 2014). The polyframboid aggregates are not discussed further because most of them formed during the diagenetic process and have no connection with environmental conditions during deposition (Reolid, 2014). A few biomorphic aggregates are observed in the samples, and these pyrite aggregates contain obvious shell structure (Fig. 4L). 4.2 Pyrite framboid size distribution and physical properties Under an SEM equipped with a BSE, the diameters of the framboids and microcrystals from the Chang 7 Member sediments were measured, and subsequently, were statistically analyzed. In all samples, the diameters of the framboids range from 3.3 to 39.7 μm, but mainly cluster in the range of 5.0-17.0 μm. More than 85% of the pyrite framboids are larger than 5.0 μm in size. The statistics of the size distribution, such as the mean diameter, standard deviation, and skewness, have been calculated for

each sample in the Chang 7 Member. The variation in mean size, standard deviation, and skewness range from 9.7 to 16.6 μm, 3.43 to 6.12 and 0.14 to 1.89, respectively, with an average of 13.11 μm, 4.53, and 0.75, respectively. The diameter of the framboids (D) and the constituent microcrystals (d) of all the framboids were also measured. The major ratios of D/d range from 7.8 to 15.6, with an average of 12.4, and the narrow range of D/d indicates that the size of the framboids increases by the regrowth of microcrystals rather than the number of microcrystals. When the microcrystals are predominantly octahedral and pyritohedral, the major ratios of D/d are more than 10, while when the microcrystals are predominantly cubic, the ratios of D/d are usually less than 10. These differences may be caused by redox conditions, nucleation time or the growth rate of the microcrystals. The diameter of the framboid (D), diameter of microcrystal (d), and the packing coefficient of the microcrystals (φ) can be used to calculate the number of microcrystals (NM) with the equation: N M=φ(D/d)3 (Wilkin et al., 1996). According to the ratios of D/d from each sample, as well as a φ of 0.74 (Wilkin et al., 1996), the number of microcrystals is between 351 and 2809. Combined with the pyrite mass fraction (CP) measured by XRD, the number of framboids (N) corresponds to 106.5±0.5 per gram of source rock. 4.3 TOC and inorganic geochemical proxies TOC content is the most direct proxy for assessing the hydrocarbon source potential (Dembicki, 2009). The TOC values of all samples vary from 1.93% to 8.52%, with an average of 5.47%, which indicates that most samples have good

hydrocarbon-generating potential (Jarvie, 1993). The concentrations of iron (Fe) and aluminum (Al) measured by XRF analysis can be used to calculate the available reactive Fe (Fear) using the equation Fear=Fetotal-Altotal*[Fe/Al]upper

crust.

Fetotal represents the concentration of Fe in the

sample, Altotal represents the concentration of Al in the sample, [Fe/Al]upper

crust

represents the ratio of Fe/Al in the upper crust, where the value is taken as 0.44 (Tribovillard et al., 2015). The concentration of available reactive Fe in the samples range from 0.73% to 4.40%, with an average of 2.00%. Meanwhile, the vanadium/chromium (V/Cr), uranium/thorium (U/Th) and copper/zinc (Cu/Zn) ratios, which are considered to be proxies for the paleoredox conditions, were calculated from the trace element analyses. The ratios range from 1.10 to 2.10, 0.39 to 0.85 and 0.52 to 1.33, respectively (Table 2). Barium (Ba) and phosphorus (P) are the most important trace elements in organisms and can be used as indicators to evaluate the biological paleoproductivity of the sedimentary water column. The biogenic sourced Ba (Baxs) is obtained by normalizing to Al, which is defined as Baxs=Batotal-Altotal*[Ba/Al]upper

crust,

where

[Ba/Al]upper crust represents the ratio of Ba/Al in the upper crust (Murray and Leinen, 1996; McLennan, 2001). The concentration of Baxs range from 210.42 to 626.92 ppm. Moreover, organic P (Porg) is also obtained by normalizing to Al, and the Porg were found to be in the range of 578.8 - 2433.2 ppm with an average of 1668.8 ppm (McLennan, 2001). Because Al and Zirconium (Zr) are mainly derived from aluminosilicate clay

minerals and silt-sized minerals, respectively, the Zr/Al ratios can be used as an indicator of coarser-grained, terrestrial debris input (Rachold and Brumsack, 2001; Calvert and Pedersen, 2007). The Zr/Al ratios of the samples from the Chang 7 Member range from 9.05 to 29.97 with an average of 16.84.

5. Discussion 5.1 Mean size of framboids as an indicator of paleoredox conditions Based on the formation mechanism of pyrite framboids, the framboids can be used as an indicator of paleoredox conditions. Due to the instability of the fluids, the framboids will settle to the surface of sediments after having crystallized to a certain size and will become buried (Fig. 1b). This process results in framboids formed in reducing conditions being much smaller than framboids formed in oxic-dysoxic conditions (Wilkin et al., 1996). Hence, the crossplot of mean size of framboids versus standard deviation or skewness can be used to distinguish the paleoredox conditions (Wilkin et al., 1996), and the size distribution of framboids from the Chang 7 Member are given in Table 1. In Fig. 6, the pyrite framboids from modern euxinic and oxic-dysoxic environments have been plotted, as have the samples from the sediments of the Chang 7 Member. It is clear that pyrite framboids from euxinic environments exhibit only slight changes in standard deviation compared to anoxic and oxic-dysoxic environments (Wilkin et al., 1996). Based on Fig. 5, the size distribution of framboidal pyrite Chang 7 Member is characterized by large mean diameters (9.7 to 16.6 μm) with a wide distribution range that plots in an area distant from the euxinic environment. This indicate that the paleoredox conditions were

dysoxic to oxic character during the deposition of the Chang 7 Member which may be related to the high sedimentation rate. 5.2 Paleoredox-sensitive trace elements Inorganic chemical analyses also can be used to evaluate the paleoredox conditions during sedimentation of siliciclastic sediments (Dypvik, 1984; Dill, 1986; Jones and Manning, 1994). The use of the trace elements V, Ni, and Cr as indicators of paleoredox conditions has been researched for years, and many proxies have been established to evaluate the paleoredox conditions during sedimentation (Tribovillard et al., 2006; Algeo and Maynard, 2004; Zhou and Jiang, 2009). V usually occurs as V5+ in oxic environments, while it is mostly in the form of V4+ or V3+ in reducing environments. Cr occurs as the oxidized oxyanion chromate (CrO42-) in oxic environments, and thus under oxic conditions, Cr migrates from the sediments to the water column, which reduces the relative concentration of Cr in the sediments. Hence, compared to Cr, the relative concentration of V in sediments deposited in anoxic environments is higher than that in oxic environments. Likewise, U and Th have a similar relationship as V and Cr. Therefore, the V/Cr and U/Th ratios also can serve as indicators of paleoredox conditions. The higher V/Cr and U/Th ratios are, the higher the reducing conditions. Usually, the ratios of V/Cr and U/Th vary from 1.20 - 2.00 and 0.3 - 0.7, indicating dysoxic conditions (Hatch and Leventhal, 1992; Rimmer, 2004). Based on the V/Cr and U/Th ratios from the analyzed samples, the paleoredox conditions for the Chang 7 Member source rocks are dysoxic to oxic, which is the same as the paleoredox conditions obtained from the mean sizes of framboids (Fig. 7).

In addition, the Cu/Zn can also be used as a paleoredox indicator, increasing with more reducing conditions, but no stable boundary value has been confirmed because of the complicated geochemical basin conditions. Based on correlation analysis between paleoredox-sensitive trace elements and the mean size of framboids, the mean size of framboids shows a strong negative correlation with the V/Cr and U/Th ratios and a weak negative correlation with the Cu/Zn ratio. This indicates that the mean size of framboids is an effective method to reconstruct the paleoredox conditions during sedimentation (Table 3 and Fig. 7). 5.3 Framboid size distribution as an indicator of the sedimentation rate A theoretical method proposed by Randolf and Larson (1988) is used to explain the crystal distribution in a chemical engineering system. A population equilibrium equation of numbers and sizes of crystals is the basis of the crystal size distribution theory (CSDT), and the residence times and input or output of crystals in the system are the main factors of CSCT (Wilkin et al., 1996). In the discussion of Wilkin et al. (1996), diameter of crystals (D), mean growth rate (G), initial nucleation density (n0) and mean growth time (τ) are the variates in the equilibrium equation: ln n=ln n0-D/Gτ Conditions necessary for the application of the above equation include: (1) the formation of framboids is a natural, unseeded, successive crystallization process; (2) both detrital input and output enhance or reduce framboids in a system; (3) neither Fe nor S is the limiting factor during the formation of framboids. Based on the above theory, if the formation of framboids obeys the preconditions

above, the plot of ln n versus D displays a negative linear relationship, the slope represents the -1/Gτ and the intercept represents the ln n0. Hence, if either G or τ is given, the other can be calculated. Thus, the cumulative number curve determined from the statistics of the framboids can be used to compute the framboid population density (n). Coupled with the mean size of framboids, the negatively correlated liner part of the crossplot of ln n versus D can be used to confirm the slope and intercept (Wilkin and Barnes, 1994; Wilkin, 1995). The majority of pyrite in the Chang 7 Member exists in framboidal form, which is well preserved in barely observed fractures. These observations confirm that most framboids crystallized during the early stage of diagenesis rather than remobilizing from terrigenous detritus or forming in the water column. Moreover, in oxic water, pyrite from terrestrial flux are labile, thus pyrite cannot make up a flux to the redox interface of euxinic basin (Kaplan et al., 1963). In addition, Sulfur (S) and Fe availability is the most important factor in the formation of framboids. Because of the ubiquity of framboids in the samples, the availability of S and Fe seems to be not limiting. All samples show a high TOC with an average of 5.24 %, suggesting that there is abundant organic matter for bacterial sulfate reduction to provide S (Canfield et al., 1992). In addition, frequent hydrothermal activity associated with volcanic activity provided abundant S (Zhang et al., 2009), and hence, the concentration of S exceeds the amount needed for the formation of pyrite. Moreover, the available reactive Fe proposed by Tribovillard et al. (2015) is also displayed in Table 2, and Fe is deemed to be reactive with its concentrations exceeding 0.5%. The available

reactive Fe in Chang 7 Member sediments ranges from 0.73 to 4.40 %, with an average of 2.00 %, which indicates high contents of Fear, at the same time suggesting that Fe is also not a limiting factor. Since both S and Fe are not limiting factors in the formation of pyrite framboids, the differences in the framboid size distribution may be caused by the crystal growth rate (G) and crystal growth time (τ). From the observations of pyrite framboids, we plotted the framboid frequency distribution (Fig. 5) and the cumulative frequency curve (Fig. 8), and then calculated the slope -1/Gτ of the negative linear regression of ln n versus D (Table 4). Because pyrite framboids crystallize at the anoxic-oxic interface, if the interface is below the surface of the sediments, the crystallization of framboids occurs in the few centimeters near the interface, and thus, the growth time controls the relative sedimentation rate. Because Fe and S are not limiting conditions in this environment, and if the crystal growth rate is constant, the size distribution of framboids is only controlled by growth time, which indicates the residence time of framboids at the oxic-euxinic interface (Wilkin and Barnes, 1997). Based on the discussion of Wilkin et al. (1996), the residence time at the oxic-anoxic interface is determined by the sedimentation rate. If the sedimentation rate is slow, the residence time is long, whereas a rapid sedimentation rate indicates a short residence time. In addition, it is not possible to calculate the absolute value of the residence time, and thus a relative sedimentation rate will be calculated (Table 4). The results of the linear regression of ln n versus D are shown in Table 4. The slope of the linear regression varies from -0.46 to -0.17, with an average of -0.31, and the relative growth time of pyrite

framboids and the sedimentation rate are in the range of 0.60 - 1.62 and 0.62 - 1.66, respectively. According to the results, the relative sedimentation rate in Chang 7 Member is fluctuant, and the fastest relative sedimentation rate in sample YD 13 being more than double that in the slowest sample YD 7 (Table 4). This indicates that YD 7 has a longer growth time for pyrite framboids than YD 13, and hence the diameter of framboids in YD 7 is larger than that in YD 13 (Table 2). This result is consistent with the size distribution of framboids measured by SEM analyses. Moreover, because the Zr and Al elements are derived from clay minerals and silt-sized minerals respectively, the ratios of Zr/Al can be used as an indicator of coarser-grained, terrestrial detrital input (Rachold and Brumsack, 2001; Calvert and Pedersen, 2007). Thus, the strong positive correlation between Zr/Al and the relative sedimentation rate records the faster sedimentation rate, and the higher terrestrial input. This suggests the size distribution of framboids is an effective complementary method to research the relative sedimentation rate. 5.4 Controls on organic matter enrichment Based on the above research on pyrite framboid, the variation in paleoredox conditions and relative sedimentation rate have been achieved, thus the controlling factors of organic matter enrichment can be discussed. The organic matter abundance shows positive correlation with the framboid mean size, suggesting that dysoxic-oxic conditions correspond to higher abundances of organic matter. The results tend to contradict the generally accepted view that reducing paleoredox conditions result in a more advantageous setting for the

preservation of organic matter. But, the dysoix-oxic paleoredox condition of the Chang 7 Member is favourable for the organism explosion. The strong positive correlation between TOC and paleoproductivity proxies, such as Porg, Baxs and Mo (Table 3 and Fig. 7), verifies that the dysoxic-oxid paleoredox condition improves the paleoproductivity, and thus increases the organic matter abundance. Furthermore, by means of deciphering the size distribution of pyrite framboids, the above research has shown that the relative sedimentation rate of Chang 7 Member source rocks was fluctuant. The complicated correlation between the relative sedimentation rate and TOC shows that when the relative sedimentation rate is less than 1, the TOC will increase with the increasing relative sedimentation rate, whereas when the relative sedimentation rate is greater than 1, the TOC will decrease with the increasing relative sedimentation rate (Fig. 10). The result indicates that the a favourable sedimentation rate can protect the organic matter by avoidance of being oxidized, but that the rapid sedimentation rate can dilute the organic matter, which reduces the organic matter abundances in source rocks.

6. Conclusions In the Chang 7 Member source rocks, framboids are the dominant form of pyrite, with subordinate euhedral pyrite. The ubiquity of pyrite framboids provides an opportunity to study the fluctuations in depositional paleoredox conditions and sedimentation rates. The size distribution of framboidal pyrite from the Chang 7 Member is characterized by large mean diameters (9.7 to 16.6 μm), with a wide distribution range. These size data plot in an area that is distant from the euxinic

environments, indicating that the paleoredox conditions had a dysoxic to oxic character during the Chang 7 sedimentation. In addition, the strong correlation between paleoredox-sensitive trace element ratios (V/Cr, U/Th, and Cu/Zn) and the mean size of framboids shows that the mean size of framboids can serve as a reliable indicator for paleoredox conditions. The residence time in the oxic-anoxic interface is another key factor in controlling the size distribution of framboid. Crystal Size Distribution Theory (CSDT) proposed by Randolf and Larson (1988) provides an effective method to calculate the growth time and the relatively sedimentation rate of framboids. Because the S and Fe elements in water column are not limiting factors, the relative sedimentation rate deciphered from the size distribution of framboids is fluctuant during the Chang 7 sedimentation. Furthermore, the positive correlation between Zr/Al and the relative sedimentation rate records the faster sedimentation rate and the higher terrestrial input, which shows that the size distribution of framboids is an effective complementary method to research the relative sedimentation rate. According to the correlation analysis between TOC and proxies for the paleoredox conditions and the relative sedimentation rate, the high organic matter abundance is controlled by high paleoproductivity and favourable sedimentation rate. Because the study area was located at the lake margin, more terrestrial debris could rapidly flow into lake, which would have been beneficial for the preservation of organic matter. In addition, the nutrient supply delivered by terrestrial input during deposition could have improved the biological paleoproductivity, which would have

brought about high organic matter abundances.

Acknowledgement This work was financially supported by the PetroChina Changqing Oilfield Company, and thank them for providing samples and data assess for permission to publish this work. We thank Sean Johnson and another anonymous reviewer for their insightful comments and suggestions to improve this work, as well as Professor Khin Zaw for handling the manuscript and valuable suggestions. The authors are also grateful for the technical assistance of Liu Mu from Analytical Laboratory Beijing Research Institute of Uranium Geology.

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Figure Captions Fig.1. (a) Formation paths of euhedral and framboidal pyrite; (b) the formation model of pyrite framboids under different bottom water environments (modified by Wilkin and Barnes, 1996 and Wang et al., 2013) Fig. 2. Map showing the location of the Ordos Basin, tectonic units, sedimentary facies during the period of the Chang 7 deposition, and the location of the sample well Yan 56 (modified from Yang et al., 2010). Fig. 3. Triassic stratigraphy of the Yanchang Formation, showing the Upper Triassic strata, lithology and thickness of each member, sedimentary facies, and lake-level fluctuations (modified from Yang et al., 2017). Fig. 4. Morphology of sedimentary pyrite from the Chang 7 Member source rocks (A) Single euhedral pyrite crystal with pyritohedral shape, 2961.2 m; (B) single euhedral pyrite crystal with octahedral shape, 2961.2 m; (C) infilled framboids and fragmental pyrite near fractures, 2973.7 m; (D) infilled framboid and normal framboid, 3040.4 m; (E) single pyrite framboid with pyritohedral microcrystal and infilled framboid, 3009.8 m; (F) single pyrite framboid with orderly spherical microcrystals, 2961.2 m; (G) single pyrite framboid with disordered pyritohedral microcrystals, 3040.4 m; (H) single pyrite framboid with disordered cubic microcrystals, 2973.7 m; (I) detail of cubic and octahedral microcrystal in pyrite framboid, 3009.8 m; (J) single pyrite framboid with randomly oriented microcrystals, 3040.4 m; (K) polyframboid aggregates composed of morphological round-shaped framboids less than 10 μm joined by randomly oriented microcrystals less than 1 μm, 3063.6 m; (L) pyritized microfossil, 3040.4 m. Samples were selected from different depths in Yan 56 well.

Fig. 5. Pyrite framboid size distribution and mean size, standard deviation, and skewness in the Chang 7 Member source rocks. Fig. 6. (a) Plot of the mean versus the standard deviation of the framboid size distribution; (b) plot of the mean versus the skewness of the framboid size distribution. The samples from modern euxinic environments (green markers) and anoxic and oxic-dysoxic environment (blues markers) are clearly distinguished by mean size and standard deviation or skewness. (modified from Wilkin et al., 1996 and Wignall et al., 2015) Fig. 7. The variation of paleoredox-sensitive trace elements, paleoproductivity proxies, and the mean size of framboids from the Chang 7 Member source rocks in the Yan 56 well. Fig. 8. Cumulative frequency curve and framboid size distribution plot from the Chang 7 Member source rocks (YD 2 shown here as an example). Fig. 9. Relative growth time and sedimentation rate reconstructed based on pyrite framboid diameters and the variation in Zr/Al and TOC from the Chang 7 Member sediments (GT = growth time, SR = sedimentation rate). Fig. 10. Bivariate TOC versus relative sedimentation rate of Chang 7 source rock samples

Fig.1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Table 1 Pyrite content and descriptive statistics of framboid size distributions of samples from the Chang 7 Member source rocks Depth

Pyrite

Samples

Mean

Standard

Framboids Skewness

(m)

Concentration (%)

(μm)

Deviation

YD 1

2961.2

1.44

9.7

3.59

0.48

125

YD 2

2973.7

2.96

13.0

4.43

0.64

155

YD 3

2987.4

3.23

16.6

4.20

0.14

147

YD 4

2992.5

2.58

12.5

5.57

1.31

125

YD 5

3001.6

3.39

14.9

4.36

0.21

272

YD 6

3009.8

3.62

15.4

6.12

0.73

378

YD 7

3018.3

2.90

13.1

5.59

0.86

203

YD 8

3032.1

2.68

10.4

3.34

0.79

111

YD 9

3040.4

2.66

11.1

4.35

1.00

156

YD 10

3048.9

2.38

14.0

4.10

1.38

93

YD 11

3054.3

4.57

12.2

4.96

1.89

344

YD 12

3063.6

1.62

14.9

4.86

0.17

189

YD 13

3068.2

2.77

12.6

3.43

0.40

105

2.98

13.11

4.53

0.75

184

Average

Measured

Table 2 Inorganic geochemical proxies and TOC of samples from the Chang 7 Member source rocks Samples

YD 1

YD 2

YD 3

YD 4

YD 5

YD 6

YD 7

YD 8

YD 9

YD 10

YD 11

YD 12

YD 13

Depth (m)

2961.2

2973.7

2987.4

2992.5

3001.6

3009.8

3018.3

3032.1

3040.4

3048.9

3054.3

3063.6

3068.2

V (ppm)

162

126

189

162

140

155

165

137

150

161

182

167

94.3

153.1

Cr (ppm)

77.0

87.0

171.8

114.0

96.6

126.0

108.0

77.0

102.7

104.0

158.3

111.0

45.0

100.7

U (ppm)

5.0

5.0

8.7

5.6

7.3

6.0

8.7

7.6

5.5

6.4

14.2

10.1

6.6

7.4

Th (ppm)

5.9

8.6

22.3

10.2

10.8

11.0

13.3

9.6

9.2

8.5

33.8

16.0

9.0

11.8

Cu (ppm)

104.0

74.5

94.9

132.0

85.8

53.1

102.0

84.6

103.0

93.9

126.0

108.0

71.6

94.9

Zn (ppm)

90.4

130.7

182.5

157.1

99.8

77.0

114.6

79.8

104.0

93.0

203.2

116.1

53.8

130.0

Ba (ppm)

579.5

950.9

791.4

1033.1

756.5

798.2

826.4

732.5

632.2

710.6

936.6

728.0

814.1

791.5

P (ppm)

659.1

2509.3

2443.9

2170.1

1734.8

1973.2

1493.2

1270.6

1772.7

1458.3

2400.7

1799.0

1091.1

1752.0

Zr (ppm)

134.0

117.0

156.0

91.9

211.0

186.1

98.6

203.0

97.3

186.0

158.0

179.0

291.0

161.0

Al (%)

9.23

8.75

9.17

10.15

9.28

10.81

9.47

9.83

8.36

9.88

9.28

9.70

10.40

9.56

Fe (%)

7.31

7.11

7.01

8.87

4.97

5.49

5.87

4.69

6.44

5.80

5.65

5.28

6.16

6.21

V/Cr

2.10

1.45

1.10

1.42

1.45

1.23

1.53

1.78

1.46

1.55

1.15

1.50

2.10

1.52

U/Th

0.85

0.58

0.39

0.55

0.68

0.55

0.65

0.79

0.60

0.75

0.42

0.63

0.73

0.63

Cu/Zn

1.15

0.57

0.52

0.84

0.86

0.69

0.89

1.06

0.99

1.01

0.62

0.93

1.33

0.73

578.8

2433.2

2364.2

2081.8

1654.1

1879.2

1410.9

1185.0

1699.9

1372.4

2320.0

1714.6

1000.6

1668.8

210.4

601.1

424.6

626.9

385.5

365.8

447.6

339.5

297.8

315.4

565.4

340.0

397.9

409.1

2.0

13.2

15.1

18.3

4.3

7.0

7.0

6.1

10.0

4.1

12.7

4.8

7.6

8.3

14.52

13.38

17.01

9.05

22.75

17.21

10.41

20.66

11.64

18.83

17.02

18.46

27.97

16.84

Fear (%)

3.25

3.26

2.98

4.4

0.89

0.73

1.70

0.37

2.76

1.45

1.57

1.01

1.58

2.00

TOC (%)

3.34

6.14

7.17

6.93

4.71

7.48

6.86

3.67

6.06

3.80

8.52

4.55

1.93

5.47

Reactive iron

Terrestrial input proxy

Paleoproductivity proxies

Paleoredox proxies

Fundamental datas

Average

Porg (ppm) Baxs (ppm) Mo (ppm) Zr/Al (ppm/%)

Table 3 Correlation analysis among V/Cr, U/Th, Cu/Zn, P, Ba xs, Mo, mean size of framboids (D), and TOC from the Chang 7 Member source rocks V/Cr

U/Th

Cu/Zn

Porg

Baxs

Mo

Mean

TOC

V/Cr

1

0.777

0.805

-0.862

-0.476

-0.480

-0.624

-0.765

U/Th

0.877

1

0.834

-0.893

-0.621

-0.685

-0.535

-0.804

Cu/Zn

0.805

0.834

1

-0.877

-0.583

-0.510

-0.538

-0.752

Porg

-0.862

-0.893

-0.877

1

0.707

0.684

0.519

0.715

Baxs

-0.476

-0.621

-0.583

0.707

1

0.864

0.168

0.580

Mo

-0.480

-0.685

-0.510

0.684

0.864

1

0.218

0.614

Mean

-0.624

-0.535

-0.538

0.519

0.168

0.218

1

0.318

TOC

-0.765

-0.804

-0.752

0.715

0.580

0.614

0.318

1

Table 4 Results of the linear regression of framboid population density versus framboid diameter from the Chang 7 Member source rocks 0

Slope

Relative growth time

Relative sedimentation rate

(no./cm )

-1/Gτ

(relative to average)

(relative to average)

Ln n Samples

Depth (m)

4

YD 1

2961.2

21.35

-0.42

0.66

1.51

YD 2

2973.7

20.84

-0.19

1.44

0.69

YD 3

2987.4

23.92

-0.33

0.84

1.19

YD 4

2992.5

20.67

-0.26

1.08

0.92

YD 5

3001.6

24.18

-0.36

0.79

1.27

YD 6

3009.8

22.70

-0.28

1.00

1.00

YD 7

3018.3

20.13

-0.17

1.62

0.62

YD 8

3032.1

20.92

-0.39

0.73

1.38

YD 9

3040.4

20.71

-0.22

1.27

0.79

YD 10

3048.9

22.88

-0.35

0.81

1.23

YD 11

3054.3

22.16

-0.29

0.96

1.04

YD 12

3063.6

23.46

-0.31

0.90

1.12

YD 13

3068.2

25.12

-0.46

0.60

1.66

Average

-0.31

Table 5 Correlation analysis among Zr/Al, the relative growth time (GT) and sedimentation rate (SR) and TOC from the Chang 7 Member source rocks Zr/Al

Relative GT

Relative SR

TOC

Zr/Al

1

-0.736

0.780

-0.612

Relative GT

-0.736

1

-0.958

0.593

Relative SR

0.780

-0.958

1

-0.737

TOC

-0.612

0.593

-0.737

1

Highlights (1) The mean size of pyrite framboids is related to the paleoredox conditions. (2) Inorganic elemental paleoredox proxies show strongly correlation with mean size of framboids. (3) According to the Crystal Size Distribution Theory (CSDT), the size distribution of pyrite framboids is related to relative sedimentation rate. (4) A favourable sedimentation rate is beneficial to organic matter enrichment, even though the dysoxic to oxic paleoredox condition adverse to the preservation of organic matter.