A Novel Redox Indicator based on Relative Abundances of C31 And C32 Homohopanes in the Eocene Lacustrine Dongying Depression, East China

A suite of oils and bitumens from the Eocene Shahejie Formation (Es) in the Dongying Depression, East China was geochemically characterized to illustrate the impact of source input and redox conditions on the distributions of pentacyclic terpanes. The fourth member (Es4) developed under highly reducing, suldic hypersaline conditions, while the third member (Es3) formed under dysoxic, brackish to freshwater conditions. Oils derived from Es4 are enriched in C 32 homohopanes (C 32 H), while those from Es3 are prominently enriched in C 31 homohopanes (C 31 H). The C 32 H/C 31 H ratio shows positive correlation with homohopane index (HHI), gammacerane index (G/C 30 H), and negative correlation with pristane/phytane (Pr/Ph) ratio, and can be used to evaluate oxic/anoxic conditions during deposition and diagenesis. High C 32 H/C 31 H ratio (> 0.8) is an important characteristic of oils derived from suldic, hypersaline anoxic environments, while low values (< 0.8) indicate non-suldic, dysoxic conditions. Extremely low C 32 H/C 31 H ratios (< 0.4) indicate strong oxic conditions of coal depsoition. Advantages to use C 32 H/C 31 H ratio as redox condition proxy compared to the HHI and gammacerane indexes are wider valid maturity range, less sensitive to biodegradation inuence and better differentiation of reducing from oxic environments. Preferential cracking of C 35 -homohopanes leads HHI to be valid in a narrow maturity range before peak oil generation. No C 35 homohopane can be reliably detected in the Es4 bitumens when vitrinite reectance is > 0.75%, which explains the rare occurrence of high HHI values in Es4 source rocks. Gammacerane is thermally more stable and biologically more refractory than C 30 hopane, leading G/C 30 H ratio more sensitive to maturation and biodegradation than C 32 H/C 31 H ratio. Meanwhile, both HHI and gammacerane index cannot differentiate level of oxidation. The C 32 H/C 31 H ratio can be applied globally as a novel redox proxy in addition to the Dongying Depression. The purpose of the present study is to explore the geochemical signicance of the C 32 /C 31 homohopane ratio in the determination of redox conditions during deposition of source rocks, and the factors governing the C 32 /C 31 homohopane ratio during thermal maturation or alteration processes. the C 35 H/C 34 H and C 32 H/C 31 H ratios drop to 0.93 and 0.99, respectively, with 28% and 12% of reduction. The C 35 H/C 34 H and C 32 H/C 31 H ratios in pyrolysates at 320°C are 0.89 and 0.82, a 31% and 26% of reduction. Hydrous pyrolysis experiments performed by Peters and Moldowan [8] for Monterey shale delivered the similar results. The unheated siliceous member (GC-MS No. 767) has C 31 to C 35 homohopanes in relative percentage of 30.8, 25.2, 17.3, 8 and 18.7, respectively. Once heated at 290°C, relative percentage of C 31 to C 35 homohopanes becomes 38.9, 27.1, 17.6, 8.2 and 8.2, respectively. The C 32 H/C 31 H ratio drops from 0.82 to 0.7 with 14.5% of reduction, while the HHI and C 35 H/C 34 H ratios drop from 18.7 to 8.2 and from 2.3 to 1.0, with 56.1% and 57.2% of reduction. The pyrolysis results are consistent with our observations for the Es4 source rocks where no reliable HHI can be obtained when TT/PT ratios are > 0.4 and C 32 H/C 31 H ratios decrease accordingly with increasing TT/PT ratios. However, much slow reduction of C 32 H/C 31 H ratio as compared to HHI and C 35 H/C 34 H ratios attests a wider valid range of C 32 H/C 31 H ratio during maturation.


Introduction
Numerous biomarker parameters are available in the literature to diagnose organic facies and depositional environments. The n-alkane distribution [1], ratio of pristane (Pr) to phytane (Ph) [2], relative abundances of C 27 , C 28 and C 29 steranes [3], occurrence of 28,30-bisnorhopane [4], C 30 tetracyclic polyprenoids [5] and C 30 steranes [6], elevated gammacerane and C 35 -homohopane abundance [7,8], are commonly used indicators among many others. Hopane distributions, in particular, have been the focus of biomarker investigations for decades because of their ubiquitous occurrence in geological samples and their unique precursors in organisms and sensitivity to redox conditions at the time of sediment deposition [9]. Hopanes are primarily derived from biochemical processes in bacteria and cyanobacteria [10], and their diagenetic and catagenetic evolution after burial in sediments is relatively well understood [11,12]. Origin, redox conditions, and thermal alteration are the three main factors controlling the nal distributions of hopanes in the source rock extracts (bitumens) and related oils [9][10][11]. C 29 17α,21β(H) 30-norhopane (C 29 H) and C 30 17α,21β(H) hopane (C 30 H) are typically the most abundant hopane components in oils and bitumens. The relative abundance of C 29 H and C 30 H appears to be sensitive to lithological changes with C 29 H/C 30 H ratios of > 1.0 characterizing evaporitic-anoxic carbonate source rocks, while lower values (< 1.0) indicates a clay-rich siliciclastic source [13]. Homohopanes (C 31 H-C 35 H) are generally believed to originate from bacteriohopanepolyols, which are synthesized as membrane lipids by bacteria [10][11][12]. The C 35 homohopanes are selectively preserved by sulfur incorporation under anoxic conditions, while lower carbon homologues are preferentially preserved under suboxic to oxic conditions due to side chain cleavage [12,[14][15]. Dominant C 31 homohopanes without C 34 and C 35 homohopanes usually indicates terrigenous plant-derived organic matter, especially in coaly strata [16][17], while the dominance of C 33 [19] further proposed that a C 31 H:C 33 H:C 35 H ternary diagram, coupled with a hopane odd/even predominance parameter, is a good depiction of overall homohopane carbon number distributions. They applied the distributions of homohopanes for oil-oil and oil-source rock correlation in the North Sea. However, the causes of homohopane compositional differences and their signi cance in geochemical application remain poorly understood.
Generally, oils and bitumens derived from highly reducing environments of deposition contain high relative abundance of C 32 -C 35 homohopanes, while those from freshwater lacustrine environments have low abundance of homohopanes, especially C 33 -C 35 homohopanes [20][21]. The preservation of distinctive carbon numbers of extended hopanes appears to be controlled by the availability of free oxygen during deposition [7]. For example, the marine Triassic Filletino oil from the Adriatic Sea shows elevated C 32 homohopane and gammacerane [22]. However, variations of the C 32 /C 31 homohopane ratio in geological samples and its application to assess depositional environment has not been fully explored.
The Dongying Depression in the Bohai Bay Basin, east China, is a proli c oil production province where the source rocks were deposited under variable conditions from strongly reducing, anoxic hypersaline water to relatively oxic freshwater during basin evolution. The geochemical features of source rocks and oils from this basin have been thoroughly investigated [23][24][25][26], and C 32 /C 31 homohopane ratios show a wide range of variation in oils and source rock extracted bitumens. The purpose of the present study is to explore the geochemical signi cance of the C 32 /C 31 homohopane ratio in the determination of redox conditions during deposition of source rocks, and the factors governing the C 32 /C 31 homohopane ratio during thermal maturation or alteration processes.

Geological Background
The Dongying Depression, with an area of approximately 5700 km 2 , is a typical half-graben lacustrine depression situated in the southeast Jiyang Sub-basin, Bohai Bay Basin, East China (Fig. 1). It is an asymmetric depression with a steeply faulted zone in the northern side and a very gentle slope in the southern side. It is bounded by several structural highs and consists of the Lijin, Minfeng, Niuzhuang and Boxing sags and a central anticline. Detailed structural evolution, sediment deposition, and petroleum generation and accumulation in the depression have been well documented [27][28][29][30][31]. Brie y, sediments in the 4. Results

Terpane distributions in oils and source rock extracts
The m/z 191 mass fragmentograms of representative oil samples in the saturated hydrocarbon fraction exhibit high proportions of pentacyclic terpanes (PT) relative to tricyclic terpanes (TT) (Fig. 2). The distributions of pentacyclic terpanes in all samples are dominated by C 30 H with a general depletion from C 31 to C 35 homohopanes. Minor amounts of 18α(H)-trisnorneohopane (Ts), 17α(H)-trisnorhopane (Tm), C 29 and C 30 17β, 21α(H) hopanes (moretanes), C 30 17α (H)diahopane (C 30 D) and C 29 18α(H)-30-norneohopane (C 29 Ts) are also present. The main difference between Es3 and Es4 oils is the relative abundance of homohopanes and gammacerane. The Es3 oils are characterized by relatively low concentrations of gammacerane and little or no C 35 homohopane, showing a typical feature of freshwater lacustrine clastic source rocks. In contrast, the Es4 oils are enriched in gammacerane and relatively high levels of C 35 homohopane, suggesting hypersaline source rock deposition.
The m/z 191 chromatograms of saturated hydrocarbon fractions from representative source rock extracts exhibit very different features from the oils (Fig. 3). Only samples from wells FY1 and NY1 at relatively shallow depth bear the same features as oils from the Es3 and Es4 reservoirs, respectively, while deeply buried samples from well LY1 show drastically altered pentacyclic terpanes distribution patterns. The Es3 source rocks in the LY1 well show signi cantly depleted Tm and C 29 H and relatively concentrated Ts, C 29 Ts and C 30 D, while homohopanes remain relatively constant; however, the Es4 source rocks show substantial depletion of C 29 -C 35 regular hopanes with rearranged hopanes, especially Ts as a dominant peak. The C 34 and C 35 homohopanes are absent in a few of the deepest samples. Meanwhile, the abundance of tricyclic terpanes is relatively increased as compared to the pentacyclic terpanes (Fig. 3).
A few commonly used molecular parameters from the saturated hydrocarbon fraction in the studied samples are listed in Table 1. The C 32 H/C 31 H ratios vary from 0.59 to 0.96 with an average value of 0.72 in oils from the Es3 reservoir; while they range from 0.78 to 1.22 with an average value of 0.96 in oils from the Es4 reservoir (Table 1). While some overlap exists between the two reservoirs likely due to mixing of charge (some known mixed oils were excluded in this study), the systematic difference in C 32 H/C 31 H may re ect intrinsic differences in source rock depositional environment. However, the differences in bitumens are less obvious. High C 32 H/C 31 H ratios occur only in limited samples of the Es4 source rocks possibly due to thermal maturity in uence. Figure 4 shows diagrams that compare the C 32 H/C 31 H ratio with other parameters that are sensitive to redox conditions. The homohopane index (HHI) values, expressed as the percent abundance of C 35 hopanes relative to the summed C 31 to C 35 hopane abundances [8], for the Es3 oils are in the range of 2.9-11.5% (average 6.3%) and those for the Es4 oils range from 7.6-22.4% (average 14.2%) ( Table 1). The difference of HHI among the Es3 and Es4 source rock extracts is much less dramatic with an average value of 7.0% and 8.0%, respectively. Interestingly, a nearly linear correlation between C 32 H/C 31 H ratio and HHI was observed for the studied samples (Fig. 4A). Gammacerane is present in very different concentrations in the studied samples. The gammacerane index, calculated as the ratio of gammacerane to C 30 hopane (G/C 30 H), varies between 0.04 and 0.48 in the Es3 oil samples and from 0.63 to 1.1 in the Es4 oils (Table 1). Lower obvious with correlation coe cients of 0.66 and 0.69 for the Es3 and Es4 oils, respectively (Fig. 4B).
Most samples in the present study have phytane concentrations higher than pristane and the Pr/Ph ratio varies from 0.14 to 1.14 in all studied samples. The average Pr/Ph ratios of the Es3 and Es4 oils are 0.55 and 0.25, respectively, while slightly higher values of Pr/Ph were observed for bitumens with an average value of 0.59 and 0.56 for the Es3 and Es4 source rocks, respectively (Table 1). Overall low Pr/Ph ratios suggest that Shahejie Formation source rocks formed under reducing depositional environments. A general negative correlation between C 32 H/C 31 H and Pr/Ph ratios occurs in the studied sample suite (Fig. 4C).
Good correlation between C 32 H/C 31 H and those well-established redox sensitive parameters implies that the C 32 H/C 31 H ratio can serve as another geochemical proxy for the depositional redox condition assessment.

Maturity impact on terpane related parameters
Measured vitrinite re ectance (%Ro) values in core samples from well NY1 are in the range of 0.44-0.67%, and those from well NY1 and LY1 are 0.48-0.76% and 0.52-0.86% (Fig. 5A) [30][31]. While the exact value of %Ro might be suppressed due to the nature of lacustrine Type I kerogen [29], molecular ratios such as Ts/(Ts + Tm) and the ratio of tricyclic terpanes to pentacyclic terpanes (TT/PT) show good correlations with vitrinite re ectance in the studied wells. The TT/PT ratio values in well NY1 vary from 0.01 to 0.35, while those from well LY1 increase from 0.1 at 3580 m to 1.14 at 3830 m (Fig. 5B). Depth pro les of Ts/(Ts + Tm) from wells NY1 and LY1 have been plotted in Fig. 5C, and this parameter increases from about 0.4 at 3300 m in well NY1 to greater than 0.95 at 3800 m in well LY1. While the organic input and redox conditions have inevitably exerted some in uence on TT/PT and Ts/(Ts + Tm) ratios in lacustrine source rocks, good correlation with %Ro and depth suggests that thermal maturity plays the dominant role on the variety of these ratios. The Ts/(Ts + Tm) ratios from the Es3 and Es4 oils are very similar within a range of 0.33-0.54 and 0.30-0.51, respectively. Similarly, all oils have TT/PT ratios below 0.15. Relatively low Ts/(Ts + Tm) and TT/PT ratios suggest generally low maturity of oil in the Dongying Depression.
Here the TT/PT ratio has been applied as a maturity determinant for oils, because vitrinite re ectance cannot be directly measured. A plot of TT/PT and C 32 H/C 31 H shows no correlation for the studied oils. However, elevated TT/PT ratios correspond to low C 32 H/C 31 H ratios in the bitumen samples, especially the Es4 source rocks, indicating preferential cracking of high molecular-weight homohopanes (Fig. 6A). The correlation between TT/PT ratio and HHI is also very weak. An issue for HHI is that no reliable C 35 homohopane can be detected from some Es4 bitumens due to high maturity (Fig. 6B). Extensive cracking of homohopanes at high maturity makes HHI unusable as a redox indicator in mature source rocks. There is no obvious correlations between TT/PT and G/C 30 H ratios in all oils and Es3 source rock extracts. However, the Es4 source rock extracts fall in two categories. Some samples have high G/C 30 H ratios but low TT/PT ratios, re ecting hypersaline conditions, while others show linear correlation between G/C 30 H and TT/PT ratios, suggesting maturity related variation due to preferential cracking of C 30 H (Fig. 6C).

Why the C 32 H/C 31 H ratio is needed
Redox conditions are known to affect the distribution of terpanes [8]. Our data from the Dongying Depression illustrate that high values of C 32 H/C 31 H correspond with high values of HHI and G/C 30 H, suggesting that C 32 H/C 31 H is sensitive to depositional environment as well. However, HHI and gammacerane index have been applied as depositional environment indicators for decades, why the C 32 H/C 31 H ratio is needed. Our rst concern is maturity sensitivity. The thermal maturity in uence on HHI is a well-documented phenomenon. In our studied samples, no C 34 and C 35 homohopanes can be reliably detected in the Es4 bitumens when Ro > 0.75% in well LY1, which limits the utility of HHI as redox indicator. The decrease of HHI with increasing thermal maturity has also been reported in related mature oils [8,34]. While maturity in uence on C 32 H/C 31 H has not been systematically documented, the correlation between C 32 H/C 31 H and TT/PT ratios in our studied samples suggests that C 32 H/C 31 H ratio suffers similar in uence as the C 35  High gammacerane abundance is a strong indicator of hypersalinity and/or water column strati cation during deposition of sediments [7,36]. Gammacerane mainly originates from tetrahymanol in bacterivorous ciliates living in hypersaline water [37]. High G/C 30 H ratios in the Es4 source rocks and related oils (Table 1) re ect salinity strati cation and are a marker for photic zone anoxia during source rock deposition, which is supported by the high sulfur content of the Es4 oils [25]. Low gammacerane index values (G/C 30 H generally < 0.4) in the Es3 source rocks and related oils indicate no strati ed water or very low salinity in the palaeolake during the deposition of sediments. However, gammacerane is thermally more stable than C 30 hopane. Once hopane thermal cracking was initiated, gammacerane index increases linearly with maturity (Fig. 6). Zhang et al. [38] noted that some abnormally high gammacerane indexes in source rock bitumen might be caused by preferential cracking of C 30 H and cannot be regarded as a proxy for depositional environment in the Dongying Depression. While maturity inevitably affects relative abundance of C 31 and C 32 homohopanes, thermal stability difference between them is less signi cant than the difference between C 30 H and gammacerane [11], which makes validity range of the C 32 H/C 31 H ratio less sensitive to maturation than the G/C 30 H ratio.
The second consideration is biodegradation in uence. Homohopane distributions can be altered by biodegradation. Peters et al. [39] demonstrated that biodegradation can result in selective loss of low molecular weight homologs, while C 35 homohopanes are more resistant. The HHI increase dramatically with the extent of biodegradation because C 35 homohopanes are demethylated less readily than their lower homologs. Similarity, gammacerane has much higher biodegradation resistivity than other regular hopanes and becomes the dominant component in the m/z 191 mass chromatograms of heavily biodegraded oils [40]. While heavily biodegraded oils have not been selected in the present study, selective preservation of C 35 homohopanes and gammacerane have been noted from biodegraded oils in the Dongying Depression [32]. However, biodegradation preference between C 31 and C 32 homohopanes is much less distinctive compared to compounds in the HHI and G/C 30 H. Therefore, the C 32 H/C 31 H ratio is more robust than HHI and G/C 30 H in biodegraded oils.
The third advantage to use the C 32 H/C 31 H ratio is its sensitivity in redox conditions. High C 35 -homohopane indices are typical of marine, low Eh environments of deposition. The elevated C 35 -homohopanes for the lacustrine oil indicate a highly reducing source rock depositional environment, most likely related to hypersalinity during the deposition. However, in anoxic, freshwater lacustrine environments, this enhanced preservation of higher hopane homologs does not occur, probably because the appropriate mechanism for sulfur incorporation is not operative [8]. Therefore, low C 35 -homohopane index does not imply the oxic depositional system. Similarly, high gammacerane index may re ect hypersaline and strong reducing conditions in lacustrine depositional system, but low gammacerane index does not necessary re ect oxic conditions. On other hand, the C 32 H/C 31 H ratio can differentiate reducing from oxic depositional environments in a similar manner as Pr/Ph ratio. The Pr/Ph ratio is one of the most commonly used geochemical parameters and has been widely invoked as an indicator of redox conditions in the depositional environment and source of organic matter [2]. Organic matter derived from terrigenous plants would be expected to have high Pr/Ph ratios of > 3.0 (oxic conditions), while organic matter formed under anoxic conditions normally has low Pr/Ph ratios of < 1.0 [2]. High C 32 H/C 31 H ratio (> 0.8) indicates reducing conditions, while low C 32 H/C 31 H ratio (< 0.8) re ects oxic conditions and extremely low C 32 H/C 31 H ratio (< 0.4) is indicative of coal (see further discussion in next section).

Does C 32 H/C 31 H ratio work for other petroleum systems
The geochemical signi cance of the C 32 H/C 31 H ratio as a redox proxy needs more supportive data from different environments. Here are a few case histories The relationship among HHI, G/C 30 H and C 32 H/C 31 H explored here may further clarify the reducing intensity. Both C 33 and C 34 homohopanes are absent in the Himmetoğlu and Gölpazarı oil shales but C 35 homohopanes were well preserved, whereas no C 35 homohopanes can be detected from the Beypazarı oil shale and the C 35 homohopanes are lower than C 34 homohopanes in the Bahçecik oil shale (Fig. 7B). The G/C 30  Only one low mature sample (#549 in original paper) out of nine studied oils has C 32 H/C 31 H ratio > 1.0 (Fig. 7C). Data from the Monterey oils may suggest that hypersalinity likely facilitates the preservation of C 32 homohopanes, although maturity interference with the distribution of homohopanes cannot be ruled out.
ten Haven et al. [42] studied the Messinian marl (late Miocene) formed in an evaporitic basin from Utah, USA. All samples have very low Pr/Ph ratios (< 0.1), high abundance of gammacerane, and extended hopanes maximizing at C 35 , indicating hypersaline environments. While no relative abundance of homohopanes has been mentioned in the original paper, elevated C 32 homohopanes with C 32 H/C 31 H ratio > 1.0 was illustrated in the m/z 191 mass chromatograms for the Rozel Point oil (Fig. 2 in ten Haven et al. [42]). Mello et al. [9] studied a wide range oils from the major Brazilian offshore basins. Group III oils are characterized by a slight even/odd preference of n-alkanes, the predominance of phytane over pristane, high concentration of gammacerane, and C 35 H/C 34 H ≥ 1.0. Inferred depositional environment for the source rock of this oil group is marine evaporitic under hypersaline conditions. Again, no C 32 H/C 31 H ratio has been measured in the original paper, but the m/z 191 mass chromatograms for Group III oils show similar abundance of C 31 and C 32 homohopanes (Fig. 5 in Mello et al. [9]).
On the other hand, samples from typical oxic depositional environments have very low C 32 H/C 31 H ratio. Peters and Molddowan [8] used the Brac and Famoso bitumens derived from oxic carbonate sediments as comparison for HHI variation. Their C 35 H/C 34 H ratios are 0.50 and 0.65, respectively, and C 32 H/C 31 H ratios are 0.46 and 0.71, respectively (Fig. 7D). Both C 35 H/C 34 H and C 32 H/C 31 H ratios are systematically lower than these formed under highly reducing conditions. Wang [17] recorded anomalous hopane distributions in marine sediments of the Meishan section at the Permian-Triassic boundary. While no full homohopane distribution has been documented, the C 32 H/C 31 H ratios in 67 samples ranging from 0.38 to 0.72 (average 0.57), showing typical feature of coals and soils. These hopanes were originated from acidi ed soil and peat and signi ed the end-Permian mass extinctions and marine ecosystem collapse [17]. A coal from early Eocene in the Fushun Basin, North China has been illustrated in Fig. 7D for the comparison purpose. It is a low maturity coal with vitrinite re ectance about 0.5%. The C 31 homohopanes account for 82% of extended hopanes and the C 32 H/C 31 H ratio is only 0.15. The unusual enrichment of C 31 -homohopanes results in extremely low C 32 H/C 31 H ratio (< 0.4) is a de ned terrigenous organic matter, especially in coals and soils under strong oxic conditions.
A plot of C 32 H/C 31 H ratio vs. HHI using above mentioned case studies shows positive correlation, indicating that both parameters can be applied for redox condition assessment (Fig. 7E). Limited data in the present study (Fig. 4A), coupled with these published in literature, show a clear boundary between strong reducing and suboxic conditions at C 32 H/C 31 H ratio of 0.8, whereas a boundary of redox conditions is di cult to de ne on the base of HHI.

Mechanisms for the C 32 H/C 31 H ratio as a redox proxy
Preferential preservation of C 35 homohopanes was attributed to the incorporation of sulfur into the bacteriohopanoid side chain during diagenesis, which mainly occurs in marine sediments formed under anoxic depositional conditions [14]. Abnormally high HHI encountered in lacustrine saline/hypersaline sediments [21], such as the Es4 source rocks and related oils, may share the same mechanism as the marine one, where sulfurization prevails in the organicrich source rocks. However, this enhanced preservation of C 35 homohopanes does not occur in freshwater sediments deposited under oxic or suboxic conditions. A narrower carbon number range of homohopane distributions commonly ranging from C 31 to C 33 and relatively lower HHI as illustrated by the Es3 petroleum system indicate unique freshwater bacterial and/or terrigenous organic matter input.
The mechanisms for the C 32 H/C 31 H ratio as a redox proxy are generally similar to HHI but operate in slightly different ways. The availability of free oxygen during deposition likely plays the dominant role on the C 32 H/C 31 H ratio variation. As homohopanes are mainly derived from bacteriohopanetetrols, diagenetic and catagenetic alteration during burial result in complex reactions including sulfurization, cyclization, side chain cleavage and condensation [15]. Farrimond et al. [12] revealed that both Priest Pot (freshwater lake) and Framvaren Fjord (sul dic) samples released high amounts of C 32 and C 35 bound homohopanes by hydropyrolysis from kerogen, but the relative abundance of C 35 homohopanes is much higher for samples from Framvaren Fjord than from Priest Pot. The C 30 and C 35 are biohopanoids incorporated into the kerogen, while C 32 hopanoic acids and hopanols are the main diagenetic products. The C 31 , C 33 , and C 34 are likely derived by side-chain cleavage of higher molecular weight bound hopanoids (C 35 and/or C 32 ) during the hydropyrolysis procedure [12]. Richnow et al. [43] noted that oxygen-linked hopanoids showed a greater proportion of side-chain shortened homologues, which is consistent with early diagenetic products that had become incorporated into the macromolecular structure. Thiel et al. [44] found high concentrations of free C 32 bis-homohopanoic acids occur in microbial mats at methane seeps in anoxic Black Sea waters. The carboxyl group might be reduced to C 32 -homohopanes under anoxic conditions, otherwise, the C 32 acid would be oxidized to C 31 -homohopane if free oxygen was available [8]. Therefore, the C 32 H/C 31 H ratio ultimately preserved in organic matter and related oil may be sensitive to oxic vs. anoxic conditions.

Conclusions
Source rocks of the Eocene Shahejie Formation in the Dongying Depression originated from a range of depositional environments. The fourth member (Es4) formed under highly reducing, sul dic and strati ed hypersaline water. The bitumen and oil derived from Es4 are characterized by high abundances of gammacerane, C 35 homohopanes, and low Pr/Ph (< 0.5). The third member (Es3) formed under dysoxic, brackish to freshwater with relatively more higherplant input and/or bacterially reworked organic matter. Bitumen and oil from the Es3 member show distinct opposite features compared to the Es4 counterpart.
The C 32 H/C 31 H ratios are linearly correlated to homohopane index (HHI), gammacerane index (G/C 30 H) and Pr/Ph ratios, suggesting that C 32 H/C 31 H can serve as a novel depositional environment proxy. High C 32 H/C 31 H ratios (> 0.8) may indicate sul dic, anoxic hypersaline conditions, while low ratios (< 0.8) re ect sub-oxic to oxic conditions in brackish to freshwater.
The mechanisms governing C 32 H/C 31 H variation are like those for HHI, G/C 30 H and Pr/Ph but operate in slightly different ways. The availability of free oxygen during deposition likely plays a dominant role on variation of the C 32 H/C 31 H ratio. When free oxygen is available under oxic or suboxic conditions, the precursor bacteriohopanetetrol is oxidized to a C 32 acid, followed by loss of the carboxyl group to form the C 31 homohopane or, if oxygen is depleted, the C 32 homolog is preserved While the C 32 H/C 31 H ratio can be in uenced by secondary alteration, it is not only much more robust than HHI and gammacerane index during thermal maturation and biodegradation, but also more sensitive to differentiate reducing from oxic conditions.

Declarations
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