Next Article in Journal
Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China
Previous Article in Journal
Geochemical State of Wilga River Environment in Kraków (Poland)—Historical Aspects and Existing Issues
Previous Article in Special Issue
Source Rock Evaluation and Hydrocarbon Generation Model of a Permian Alkaline Lakes—A Case Study of the Fengcheng Formation in the Mahu Sag, Junggar Basin
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Minerals
Volume 11
Issue 8
10.3390/min11080909
minerals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential

by
Xiong Cheng
1,2,*,
Dujie Hou
1,2,*,
Xinhuai Zhou
3,
Jinshui Liu
4,
Hui Diao
4 and
Lin Wei
1,2
1
School of Energy Resources, China University of Geosciences, Beijing 100083, China
2
Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, Beijing 100083, China
3
Hainan Branch of China National Offshore Oil Corporation, Haikou 570100, China
4
Shanghai Branch of China National Offshore Oil Corporation, Shanghai 200050, China
*
Authors to whom correspondence should be addressed.
Minerals 2021, 11(8), 909; https://doi.org/10.3390/min11080909
Submission received: 29 July 2021 / Revised: 19 August 2021 / Accepted: 20 August 2021 / Published: 23 August 2021
(This article belongs to the Special Issue Sedimentology and Geochemistry of Organic Matter)
Browse Figures
Versions Notes

Abstract

:
Eocene coal-bearing source rocks of the Pinghu Formation from the W-3 well in the western margin of the Xihu Sag, East China Sea Shelf Basin were analyzed using Rock-Eval pyrolysis and gas chromatography–mass spectrometry to investigate the samples’ source of organic matter, depositional environment, thermal maturity, and hydrocarbon generative potential. The distribution patterns of n-alkanes, isoprenoids and steranes, high Pr/Ph ratios, abundant diterpanes, and the presence of non-hopanoid triterpanes indicate predominant source input from higher land plants. The contribution of aquatic organic matter was occasionally slightly elevated probably due to a raised water table. High hopane/sterane ratios and the occurrence of bicyclic sesquiterpanes and A-ring degraded triterpanes suggest microbial activity and the input of microbial organisms. Overwhelming predominance of gymnosperm-derived diterpanes over angiosperm-derived triterpanes suggest a domination of gymnosperms over angiosperms in local palaeovegetation during the period of deposition. The high Pr/Ph ratios, the plot of Pr/n-C17 versus Ph/n-C18, the almost complete absence of gammacerane, and the distribution pattern of hopanes suggest that the samples were deposited in a relatively oxic environment. Generally, fluctuation of redox potential is coupled with source input, i.e., less oxic conditions were associated with more aquatic organic matter, suggesting an occasionally raised water table. Comprehensive maturity evaluation based on Ro, Tmax, and biomarker parameters shows that the samples constitute a natural maturation profile ranging from marginally mature to a near peak oil window. Hydrogen index and atomic H/C and O/C ratios of kerogens suggest that the samples mainly contain type II/III organic matter and could generate mixed oil and gas.

1. Introduction

Source rocks are capable of generating petroleum [1]. Coal and coal measure mudstone are organically rich and generally considered as gas-prone source rock, with some exceptions [2,3,4,5]. The formation and distribution of source rocks is mainly controlled by sedimentary basin, palaeoclimate, palaeohydrodynamic conditions, productivity, and preservation of organic matter [6]. Therefore, geochemical characterization of the source rocks is a key to understanding their type and origin of organic matter, thermal maturity, capacity for hydrocarbon generation, palaeoceanographic, and paleoclimatic conditions [7,8].
The Xihu Sag is the most petroliferous region within the East China Sea Shelf Basin (ECSSB), China. The coal-bearing Pinghu Formation (E2p) of middle–late Eocene has been regarded as the primary source rock unit in the Xihu Sag. The E2p is organically rich, with large thickness, extensive distribution, and a proper thermal maturity [9,10]. Additionally, oil/gas-source rock correlations support the E2p as the primary source rock in the Xihu sag [11,12,13]. Thus far, a limited number of studies on biomarkers in the E2p source rocks have been documented. For instance, Fu [14] reported the occurrence of diterpenoids in coal measures. Zhu et al. [11] systematically studied molecular composition of Paleogene coal-bearing source rocks and discussed their significance for source input. Zhu et al. [9] documented the hydrocarbon potential of Paleogene source rocks, in which distributions of n-alkanes, steranes, and hopanes were simply discussed for investigating the origin of organic matter, depositional environment, and thermal maturity. They found that the source rocks are dominated by Type III-II2 kerogens (HI = 41–386 mg HC/g TOC) within sub-oxic to oxic environments (Pr/Ph = 1.08–9.78). However, C29 diasteranes in some samples were incorrectly attributed to C27 regular steranes by Zhu et al. [9]; therefore, the interpretation based on sterane distributions could be problematic. Cheng et al. [15] compared the geochemical characteristics between a coal and mudstone from the E2p and found that they contain predominantly terrigenous source materials with the effect of microbial reworking and were deposited under oxic conditions within an unstratified water column; the mudstone has slightly more algal material input and less oxic conditions during deposition than the coal. Kang et al. [16] studied the organic geochemical and petrological characteristics of the coals of the E2p. They found that the coals were deposited under relatively oxic peatland conditions (Pr/Ph = 3.33–9.23) with a predominance of terrestrial higher plants input and have entered the hydrocarbon generation threshold. However, a detailed biomarker study on the E2p source rocks for elucidating the organic matter source and depositional characteristics changes over time is lacking in the Xihu Sag.
In 2019, a petroleum exploration well (W-3) located in the Pinghu Slope, western margin of the Xihu Sag (Figure 1), penetrated the E2p, and many sidewall cores and a 15.1 m core, including an organic-rich mudstone/carbonaceous mudstone interval, were obtained. This provided a rare opportunity to investigate detailed biomarker distributions of the E2p to elucidate the successive changes in source input and depositional environment over time. In this study, the bulk geochemical characteristics and aliphatic hydrocarbon composition of 16 source rocks of the E2p were characterized to enhance understanding of the origin of organic matter, depositional environment, thermal maturity, and hydrocarbon generation potential. The results of this study could be beneficial for better understanding the formation mechanism and heterogeneity of the coal-bearing source rocks, which will provide guidance for predicting the formation and distribution of high-quality source rocks. In addition, they could be useful for any oil–oil and oil–source correlations stemming from future exploration in the Xihu Sag.

2. Geological Setting

The Cenozoic ECSSB is located off of the southeast coast of mainland China to the west of the Ryukyu Islands (Japan) [18] (Figure 1a). It is NNE-SSW trending and covers an area of 240,000 km2. The Xihu Sag is in the central east of the ECSSB with an area of 46,000 km2. The Xihu Sag involves five structural units of which the Western Slope, from north to south, can be further divided into the Hangzhou, Pinghu, and Tiantai slopes (Figure 1b,c). The Xihu Sag is the most petroleum-rich region in the ECSSB, where seven gas/condensate fields and 11 petroleum-bearing structures have been discovered with proved natural gas reserves of 103.3 × 109 m3, and the Pinghu Slope is the most petroliferous area in the Xihu Sag where a suite of oil and gas fields have been discovered (Figure 1b) [10].
The tectonic evolution of the Xihu Sag includes three main stages: (1) rifting stage during the Paleocene to late Eocene, (2) post-rift stage during late Eocene to late Miocene, and (3) regional subsidence stage since the end of Miocene [19]. The Xihu Sag was filled with Paleogene–Quaternary succession up to 10,000 m (Figure 2a). Of these strata, the E2p has a thickness of 600–5000 m, and consists of mudstones, carbonaceous mudstones, coal seams, siltstones, and sandstones (Figure 2b) deposited in tidal delta and tidal flat facies [20]. Seismic sequence and the archive of marine palaeontological fossil suggest that there were frequent transgressions and regressions during the deposition of the E2p [21], and the fluctuating water levels and intermittent flooding during the deposition of peat resulted in the interbeded coal, mudstone, and sandstone [22]. The mudstones, carbonaceous mudstones, and coals have been recognized as the major source rocks in the Xihu Sag [11,12,13], and the accumulative thickness of the coal measuring mudstones is up to 2500 m with an average value of >1000 m and the coals are up to 65 m in the E2p [23,24]. The siltstones and sandstones are the major reservoir intervals and the mudstones in the Pinghu and Huagang formations constitute the seal.

3. Materials and Methods

3.1. Samples

In this study, a total of 16 samples, including 11 sidewall cores and five cores, of the E2p (Figure 2b) were collected from the W-3 well situated in the Pinghu Slope of the Xihu Sag. The W-3 well was drilled with water and no oil-based mud was used. The sidewall cores were selected from a set of sidewall cores obtained during drilling based on their lithology. The rest five cores were collected from the dark color mudstone interval of the 4050.00–4067.95 m core (15.1 m recovered) continuous core. The samples consist of 12 mudstones, three carbonaceous mudstones, and one coal. The location of the well is shown in Figure 1b, and the origin of the samples is presented in Table 1. The samples were washed with deionized water, and then cleaned with a 9:1 dichloromethane methanol mixture before pretreatment for further analysis.

3.2. Total Organic Carbon, Rock-Eval Pyrolysis, Vitrinite Reflectance Measurement, and Elemental Composition of Kerogen

The samples were ground (<100 mesh) and homogenized prior to further treatment. The total organic carbon (TOC) contents were determined on samples pretreated with 10 vol% HCl using a LECO CS230 instrument. Source rock pyrolysis was performed using a Rock-Eval instrument following standard methods described by Peters [25]. Briefly, ground whole rock samples weighing up to about 100 mg are pyrolyzed at 300 °C for 3 min (yielding S1 peak), followed by programmed pyrolysis at 25 °C/min to 600 °C (yielding S2 peak), both in a helium atmosphere. Rock-Eval parameters, i.e., S1 (free hydrocarbons, mg HC/g rock), S2 (pyrolysis hydrocarbons, mg HC/g rock), and Tmax (temperature of maximum pyrolysis yield during the formation of S2 peak, °C) were determined, and the hydrogen index (HI = S2/TOC × 100, mg HC/g TOC) was calculated from the pyrolysis data and the TOC values (Table 1). Both TOC content and Rock-Eval pyrolysis were analyzed at the Experimental Center of China National Offshore Oil Corporation, Shanghai, China (ECCNOOC).
Mean random vitrinite reflectance (Ro) measurements were performed on the surface of vitrinite particles under oil immersion on a TIDAS S MSP400 microscope spectrometer and using standard procedures also at ECCNOOC.
Kerogens were isolated from the powdered samples using HCl/HF digestion. The elemental composition of kerogen (C, H, O and N) was measured using Elementar Vario Cube at China University of Petroleum, Beijing.

3.3. Solvent Extraction, Fractionation, and Gas Chromatography–Mass Spectrometry (GC–MS)

The rock powders were Soxhlet-extracted with chloroform for 72 h and then aliquots of the extractable organic matter (EOM) were separated into asphaltenes, saturates, aromatics, and polar compounds using column chromatography [26].
The saturated hydrocarbons were analyzed using a Thermo Trace DSQ II GC–MS at ECCNOOC. An HP-5ms GC column (30 m × 0.25 mm × 0.25 μm) was used to separate the hydrocarbons. The oven temperature was programmed as follows: 50 °C for 1 min, 50–120 °C at a rate of 20 °C/min, 120–250 °C at 3 °C/min, and 250–320 °C at 2 °C/min with a final hold of 20 min. The electron ionization source was operated at 70 eV. Helium was used as carrier gas at a flow rate of 1.0 mL/min. The MS was operated in selected ion monitoring mode. Identification of individual compounds was based on elution order and published mass spectra [17,27,28]. Biomarker parameters were calculated from peak areas integrated in the mass chromatograms using Chemstation.

4. Results

4.1. Bulk Geochemical Parameters

The bulk geochemical properties of the studied samples are presented in Table 1. The TOC values for the mudstones and carbonaceous mudstones range between 0.3 and 11.4 wt.% and the TOC content of the coal is 60.3 wt.%. The S1 + S2 and HI values of the samples vary between 0.6–208.3 mg HC/g rock and 91–479 mg HC/g TOC, respectively. The Tmax values range between 435 and 454 °C. The atomic H/C and O/C ratios for the kerogens are in the ranges of 0.80–1.12 and 0.10–0.22. The cross-plots of S2 versus TOC, HI versus Tmax and atomic H/C versus O/C are shown in Figure 3. Vitrinite reflectance (Ro) of the analyzed samples varies between 0.53% and 0.76% (Table 1). Ro and Tmax both increase gradually downwards (Figure 4a,b).

4.2. Molecular Composition of Aliphatic Hydrocarbons

The molecular composition, derived from GC-MS of the aliphatic fractions, provides valuable information for origin, depositional environment, and thermal maturity of organic matter preserved in the source rocks.

4.2.1. n-Alkanes and Acyclic Isoprenoids

The total ion chromatograms of representative aliphatic fractions and corresponding m/z 85 mass chromatograms are shown in Figure 5. n-Alkanes dominate the aliphatic fractions ranging from C14 to C35. One exception is the coal at depth of 3966 m in which diterpanes are the dominant components (Figure 5). The n-alkanes are dominated by components of intermediate to long chain lengths with >21 carbon atoms (56–84% of the total n-alkanes); the low-molecular weight n-alkanes (n-C14–n-C20) are present in lower proportions (16–44%, mainly <25% of the total n-alkanes; Table 2). The samples from the upper part of the E2p show evident odd over even n-alkane predominance, which gradually disappear with increasing depth (Figure 5). As a result, the carbon preference index (CPI) [29] decreases with depth (Figure 4c). The commonly used source indicators, e.g., terrigenous/aquatic ratio (TAR) [30], proxy aqueous (Paq) [31] and proxy wax (Pwax) [32], are highly variable, ranging from 0.73 to 3.89, 0.43 to 0.71 and 0.45 to 0.68, respectively (Figure 6b,c). As expected, TAR positively correlates with Pwax, and TAR and Pwax both negatively correlate with Paq (Supporting Information Figure S1). CPI, TAR, Paq, and Pwax are calculated as follows:
CPI = 1/2 × [(n-C25 + n-C27 + n-C29 + n-C31 + n-C33)/(nC26 + nC28 + nC30 + n-C32 + n-C34) + (n-C25 + n-C27 + n-C29 + n-C31 + n-C33)/(n-C24 + nC26 + nC28 + nC30 + nC32)]
TAR = (n-C27 + n-C29 + n-C31)/(n-C15 + n-C17 + n-C19)
Paq = (n-C23 + n-C25)/(n-C23 + n-C25 + n-C29 + n-C31)
Pwax = (n-C27 + n-C29 + n-C31)/(n-C23 + n-C25 + n-C27 + n-C29 + n-C31)
Acyclic isoprenoids, pristane (Pr), and phytane (Ph), are abundant, especially Pr, which is more prominent than n-C17 in most samples (Figure 5). The Pr/Ph ratios vary between 3.3 and 8.2, and a depth profile of Pr/Ph is presented in Figure 6d. The coaly samples are generally having higher Pr/Ph ratios. The ratios of Pr/n-C17 and Ph/n-C18 range from 0.65 to 7.62 and 0.24 to 0.98, respectively (Figure 7).

4.2.2. Bicyclic to Pentacyclic Terpenoids

Drimane class bicyclic sesquiterpanes with carbon number of C14–C16 were present at extremely low abundance or undetected, and 8β(H)-homodirmane is almost the only visible compound (Supporting Information Figure S2).
Diterpanes, including bi-, tri-, and tetracyclic components, were detected in all samples analyzed (m/z 123). They include 8β(H)-labdane (βL), 4β(H)-19-norisopimarane (NIP), fichtelite (18-norabietane, NA), rimuane (R), 17-nortetracyclic diterpene (NT), pimarane (P), ent-beyerane (B), isopimarane (IP), 16β(H)-phyllocladane (βP), abietane (A), and 16α(H)-phyllocladane (αP) (Figure 8). ent-Kauranes (K) were not detected from the samples. The relative abundance of diterpanes varies significantly in the samples (Figure 5). The diterpanes/n-C19–20 ratios for the mudstones and carbonaceous mudstones are of 0.06–6.10, and the coal has a high ratio of 33.73 (Figure 6e).
The abundance of tricyclic terpanes (TT) and C24 tetracyclic terpane (TeT) are much lower relative to hopanes (Figure 9a). The distribution patterns of C19–C26TT vary between samples, with a dominance of C19–C21TT (coal and carbonaceous mudstones) or C23TT (mudstones). Although the C24TeT was co-eluted with the second isomer of C26TT, the former is much more abundant than the latter with the calibrated C24TeT/C26TT ratios of 1.09–17.26 (Table 2).
Angiosperm-derived triterpanes and their diagenetic derivatives, including 10β(H)-des-A-oleanane (dO), 10β(H)-des-A-lupane (dL), 10β(H)-des-A-ursane (dU) and oleanane (O), were identified in all samples at low and variable abundance (Figure 9a). The ratio of oleanane/C30 αβ-hopane (O/C30H) for the studied samples are of 0.05–0.43 (Figure 6f). Diterpanes predominate over these triterpenoids with the ratio of diterpanes to the sum of diterpanes plus non-hopanoid triterpanes [diterpanes/(diterpanes + non-hopanoid triterpanes)] [33] ranging from 0.86 to1.00 (Figure 6g).
Hopanes (H) with carbon numbers of C27–C34 (without C28) were detected in the studied samples (Figure 9a–e). Among them, C29–C31 hopanes are generally more abundant than the other homologues, homohopanes decrease exponentially with increasing carbon number, and rearranged hopanes, including 18α(H)-22,29,30-trinorneohopane (Ts), 18α(H)-30-norneohopane (C29Ts), and 17α(H)-diahopane, were present at trace amounts, except the three lowermost samples, as partly shown by the m/z 191 mass chromatograms (Figure 9e). The 18α(H)-22,29,30-trinorneohopane/(18α(H)-22,29,30-trinorneohopane + 17α(H)-22,29,30-trinorhopane) [Ts/(Ts + Tm)] ratios are of 0.05–0.43, and generally show an increasing trend with depth (Figure 4d). The C30 moretane/C30 αβ-hopane (C30M/C30H) ratios (0.14–0.49) decrease with increasing depth (Figure 4e). The C31 homohopane 22S/(22S + 22R) ratios range from 0.55 to 0.60 for the studied samples (Figure 4f), which are approximately equal to the endpoints (0.57–0.62) [34]. Gammacerane was almost absent in the sample set, and therefore the commonly used gammacerane index was not calculated.

4.2.3. Steroids

Low abundance of steroids was detected in the analyzed samples by m/z 217 mass chromatograms (Figure 9f–i). Among the steroids, C29 regular steranes predominate over C27 and C28 homologues, followed by C29 diasteranes. The relative abundances of the C27, C28 and C29 ααα20R steranes in the samples are 7–26%, 16–27%, and 47–76%, respectively (Table 2). The ratio of C30 αβ-hopane/C29 regular steranes (C30H/C29St) ranges from 1.46 to 12.36, with an average value of 4.46 and is displayed in stratigraphic order shown in Figure 6h. The C27/C29 ααα20R sterane ratios of the studied samples vary from 0.10 to 0.56 (Table 2; Figure 6i), and the samples with high ratios are characterized by low TOC values. The C29 steranes isomerization ratios, ααα20S/(20S + 20R) and ββ/(ββ + αα), for the samples range from 0.34 to 0.45 and 0.23 to 0.53, respectively (Table 2), having not reached equilibrium values. Interestingly, the ααα20S/(20S + 20R) ratios are relatively stable throughout the profile, except the initial increase with depth (<4000 m) (Figure 4g). In contrast, the C29 sterane ββ/(ββ+αα) ratios gradually increase downwards, except a negative shift of the three lowermost samples (Figure 4h). The ratio of C29 diasteranes/C29 steranes (diaC29/C29St; peak areas integrated on m/z 259 and 217 mass chromatograms, respectively) ranges from 0.14 to 0.31 and shows no regular correlation with depth (Figure 6j).

5. Discussion

5.1. Source of Organic Matter

5.1.1. Algae, Microorganisms, and Unspecified Higher Plants

Short-chain n-alkanes (< n-C20) predominantly originate from algae and microorganisms [35], mid-chain n-alkanes (n-C21–n-C25), especially n-C23 and n-C25, are reported to originate from submerged/floating aquatic macrophytes or Sphagnum [31], and long-chain n-alkanes (> n-C25) are typically considered to derive from epicuticular waxes of vascular plants [36], although some nonmarine algae (e.g., Botryococcus braunii) may also contain high molecular-weight n-alkanes [37]. The studied samples contain high abundance of long-chain (n-C26–35: 29–53%) and mid-chain (n-C21–25: 27–37%) n-alkanes and low abundance of short-chain homologues (n-C14–20: 16–44%) (Table 2). This indicates significant contributions from terrestrial higher plants and aquatic macrophytes/sphagnum and relatively low algal/bacterial sources input but with some variations.
TAR reflects relative input of terrigenous versus aquatic OM and high values are indicative of a higher plant input [30]. The relatively high TAR values (mainly >1.0) of the studied samples are consistent with significant source input from higher plants. However, the TAR values are highly variable (Figure 6b), indicating that input of aquatic OM was variable during deposition of these sediments. The Paq reflects the input of the non-emergent aquatic macrophytes relative to emergent and terrestrial plants [31]. For the studied samples, this proxy gives values ranging from 0.43 to 0.71 (Figure 6c), indicating significant submerged or floating macrophyte-derived source materials input. The Pwax reflects the relative proportion of waxy hydrocarbons derived from emergent macrophytes and terrestrial plants [32]. The high Pwax ratios (0.45–0.68) for the samples indicate prominent higher plant input in relation to bulk plant input at low water level. The good correlation among Paq, Pwax and TAR (Supporting Information Figure S1) suggest changes in source contribution of aquatic versus terrestrial plants-derived OM through time. CPI was affected by thermal maturation (see Section 5.3) and, therefore, is not reliable for interpreting source contribution. It is worth noting that thermal maturation favors the transformation of high molecular-weight n-alkanes to lower molecular-weight counterparts. Thus, the contributions from algae and microorganisms in the samples from the lower part of the profile would be slightly overestimated.
In the Pr/n-C17 versus Ph/n-C18 plot (Figure 7), all the studied samples are plotted in the terrigenous Type III zone, suggesting a dominance of terrigenous OM deposited in oxidizing environments with some fluctuation. This is consistent with the high Pr/Ph ratios. Furthermore, the occurrence of only trace amounts of tricyclic terpanes with high C24TeT/C26TT ratios also support a terrigenous source of OM in the samples [38].
C27 and C28 steroids are derived from phytoplankton, whereas C29 homologues are mainly derived from terrestrial plants [39]; in addition, C29 steranes may also originate from brown and certain green algae [40]. Thus, the relative composition of the regular steranes can be used to illustrate the type of OM contributing to the sediments. In this case, the C29 steranes are more likely to be derived from higher plants according to the tidal delta and tidal flat facies of the E2p, and the predominance of C29 over C27 and C28 steranes in the samples indicates major input of higher plant materials.
There is a marked correlation between the distribution of regular steranes and n-alkanes: the samples containing higher amounts of C27 steranes are characterized by more abundant low molecular-weight n-alkanes, as evidenced by the negative correlation between C27/C29 ααα20R sterane and TAR (Supporting Information Figure S1d). This suggests that steranes and n-alkanes are consistent in source interpretation. The lowermost three samples together with the samples at depth of 4024 m and 3864.5 m consistently contain relatively high concentrations of C27 regular steranes and low molecular-weight n-alkanes and hence the slightly higher C27/C29 ααα20R sterane and Paq ratios and lower TAR values (Figure 6b,c,i), suggesting that these samples have more contribution from aquatic OM. The increased aquatic OM could be related to the raised water table during deposition.
Additionally, the TOC values of the samples were observed to be positively related to TAR and Pr/Ph, and negatively related to C27/C29 ααα20R sterane ratios, respectively (Figure 10), supporting the interpretation of a dominant terrestrial OM input.
Hopanoids are derived from degradation of bacteriohopanepolyols in prokaryotes, and are indicative of bacterial OM input [41]. The hopane/sterane ratio indicates the contribution of prokaryotic versus eukaryotic organisms, and high values are associated with input of terrigenous and/or bacterially reworked OM [1]. The high C30H/C29St ratios for the samples analyzed suggest a significant contribution of bacterial biomass and/or bacterially reworked terrestrial OM. This is consistent with the oxic conditions of the nearshore coal-bearing transitional and marsh facies which are favorable for an enhanced degree of bacterial activity and which in turn produced relatively high amounts of hopanoid precursors. The presence of bicyclic sesquiterpanes also suggests the contribution of bacterial to the OM [42]. In addition, the occurrence of dO, dL, and dU further proved microbial activity, as degraded triterpenoids are thought to be the products of microbial photomimetic decomposition of pentacyclic triterpenoids [43].

5.1.2. Gymnosperm and Angiosperm

Tri- and tetracyclic diterpenoids are derived from conifer (gymnosperm) resins. Diterpenoids with labdane and isopimarane skeleton have been identified in all conifer families except the former family Cephalotaxaceae [44]. Additionally, labdanes could also be originated from microorganisms when accompanied by the C15–C24 homologues [45]. Regular abietane-type diterpenoids are very widespread in conifer families, and they are major components of Pinaceae resins [46,47]. Diterpenoids with phyllocladane structures widely occur in Podocarpaceae, Araucariaceae, Cupressaceae, and Taxodiaceae but are absent in the Pinaceae [44,47]. C19 diterpanes, e.g., NIPs, NA, and NT, could be derivatives of corresponding C20 diterpenoid precursors (e.g., conifer resin acids) by decarboxylation and reduction reactions [27,48]. However, NIPs could have additional C19 precursors present in gymnosperms [49]. Non-hopanoid triterpenoids containing O, L, and U skeletons have long been considered as typical biomarkers for angiosperms [50,51]. They are common constituents of leaf waxes, wood, roots, and bark of angiosperms. Des-A-triterpenoids are probably formed during diagenesis from C30 precursors [52], although unidentified C24 precursors could be present [51].
In the studied samples, βL (C20) was present but without C15–C24 homologues, indicating a conifer origin of labdane. Therefore, the occurrence of diterpanes with different structural types, distribution patterns, and relative abundance in the analyzed samples reflects varying contributions of various conifer families (Figure 6e and Figure 8). The presence of dO, dL, dU, and O in the studied samples (Figure 9a) ascertains the contribution of angiosperms. However, the taxanomical differentiation between gymnosperm and angiosperm families is not possible based on the molecular composition as they are not species-specific [53,54].
Due to the diagnostic features of diterpanes (conifers) and non-hopanoid triterpanes (angiosperms), the ratio of diterpanes/(diterpanes + non-hopanoid triterpanes) can be used to semi-quantitatively estimate the relative contribution of gymnosperms (specifically conifers) and angiosperms [33]. The abundance of diterpanes relative to n-alkanes show large variations (0.06–33.73), suggesting varying contributions of conifers to the sedimentary OM. However, the diterpanes/(diterpanes + non-hopanoid triterpanes) ratio shows extremely high and uniform values throughout the depth profile (above 0.97, with a few exceptions: 3723 m, 3924 m, 3966.5 m and 4238 m, Figure 6g), indicating an overwhelming predominance of conifers over angiosperms in palaeovegetation. The occasionally lower ratios for the four samples appear to imply variations in compositional changes in the local plant community, where the relative contribution of angiosperms was slightly higher.
In conclusion, the composition of biomarkers in the studied samples suggest predominantly terrigenous organic matter input with subordinate algae and microorganisms. Among the terrigenous organic matter, gymnosperm-derived dominates over angiosperm-derived. Some fluctuations exist as certain intervals contain a greater amount of algae.

5.2. Depositional Environment

The Pr/Ph ratio is the most used proxy for redox conditions of water column during deposition [55]. However, it has some limitations due to the influence of additional source materials and thermal maturation [56,57]. Pr/Ph ratio < 0.8 indicates saline to hypersaline conditions and/or reducing conditions, while ratio > 3 reflects terrigenous plant input deposited under oxic to suboxic environments [58]. The high Pr/Ph ratios (3.3–8.2) for the studied samples are indicative of terrigenous OM input under oxic conditions. The vertical variation of Pr/Ph ratios (Figure 6d) indicate periodic changes in redox potential, which might be associated with the fluctuation of the water table during deposition, as consistent with the changes in relative abundance of aquatic OM input. This interpretation further supports the frequent transgression and regression during the deposition of the E2p as suggested by the seismic sequence and the occurrence of marine palaeontological fossils [21]. The cross-plot of Pr/n-C17 versus Ph/n-C18 (Figure 7) suggests terrigenous source materials deposited in an oxidizing environmental condition as well. The simple distribution pattern of hopanes characterized by high Tm and C29–C31H and the exponential decrease of homohopanes without C35H (Figure 9a–e) are characteristics of hopanoids associated with oxic-type environments [58].
Diasteranes are formed by the rearrangement of steroids during catagenesis by the catalyzation of clay minerals [59]. Highly anoxic environments are favorable for the formation of diasteranes because, within an oxic-type environment, sterenes will be left unreduced and hence available for rearrangement reaction [38,60]. Additionally, high thermal maturity can also result in the enrichment of diasteranes [61]. In this study, the effect of thermal maturity on the diaC29/C29St ratios could be negligible, as the ratio has no positive relationship with thermal maturity/depth (Figure 6j). The mudstones rich in clay have diaC29/C29St ratios of 0.16–0.31 which are not consistently higher than that of the coal and carbonaceous mudstones with low clay and high TOC contents (0.14–0.24, Figure 11a), suggesting that the content of clay minerals does not necessarily exclusively control diasterane formation. Interestingly, there is a positive and negative correlation between the diaC29/C29St and Pr/Ph ratios for the mudstones and coaly samples, respectively (Figure 11b). Therefore, it is possible that oxicity of the depositional environment is a major factor for the enrichment of diasteranes in the clay samples, while the content of clay minerals controls the formation of diasteranes in coaly samples.

5.3. Thermal Maturity

Vitrinite reflectance, Rock–Eval pyrolysis Tmax, and varying biomarker proxies have been widely used to evaluate the thermal maturity of source rocks [58]. A comprehensive thermal maturity evaluation suggests that the studied E2p samples constitute a natural maturation profile from the marginally mature to mid-mature (onset of oil window to near peak oil window).
The studied samples have Ro values ranging from 0.53 to 0.76% and therefore are of sub-bituminous A to high volatile bituminous B rank. Tmax values range from 435–454 °C and are thus in agreement with the vitrinite reflectance data [62]. All samples analyzed are therefore in the early mature to mid-mature range.
The CPI, homohopane 22S/(22S+22R), and moretane/hopane ratios are reliable maturity indicators during the immature to low mature stage, decreasing with increasing thermal maturity [29,58]. The moretane/hopane ratios decrease from ~0.8 in immature sediments to <0.15 in mature sediments and petroleum [63]. The ααα20S/(20S + 20R) and ββ/(ββ + αα) ratios based on C29 steranes gradually increase during maturation and meet the endpoints (0.52–0.55 and 0.67–0.71, respectively) at the peak oil window [64]. The C31H 22S/(22S + 22R) ratios for the samples are in equilibrium, suggesting that they have entered the oil window [34]. The elevated CPI and C30M/C30H values for the upper samples gradually decrease in a downward trend, together with the increasing trend of the C29 ββ/(ββ + αα) steranes ratio without reaching equilibrium points, this all suggesting a natural thermal maturity gradient ranging from marginally mature to the middle oil generation window. This interpretation agrees well with the Ro values.
The Ts/(Ts + Tm) ratios broadly increase with depth (Figure 4d), reflecting increasing maturity with depth; however, the significant fluctuation of Ts/(Ts + Tm) ratios indicate that this parameter could also be affected by other factors, such as lithology (clay minerals) [58]. The relatively constant C29 ααα20S/(20S + 20R) steranes’ ratios throughout the profile but without reaching the equilibrium values were unexpected. No plausible explanation is available here, but it might be related to the effects of lithofacies and the depositional environment.

5.4. Hydrocarbon Generative Potential and Exploration Implications

The remaining petroleum potential of the Pinghu Formation samples can be characterized using TOC contents and S2. TOC and S2 values for the studied samples are generally greater than 0.5 wt% and 2.0 mg HC/g rock (Table 1), suggesting a range in source rock potential from fair to very good (Figure 3a).
There is now consensus that coal seams and coal-bearing strata can generate liquid hydrocarbons [65]. Oil generation capacity of coaly source rocks depends on hydrogen content in the kerogen, and those hydrogen-rich coaly source rocks are often rich in liptinite (e.g., resinite, sporinite, cutinite) and/or perhydrous vitrinite [65,66]. The source potential of immature kerogens with an HI between 150 and 300 mg/g is generally considered to be gas and oil [67].
The elemental composition of kerogen, the atomic H/C versus O/C diagram, is the most familiar method of classifying kerogen type (Figure 3b) [68]. Rock-Eval derived HI provides a rapid measurement of hydrogen content, and the HI versus Tmax cross-plot (Figure 3c) reflects the organic matter type as well. Most of the samples are plotted in the Type II organic matter domain but near the boundary of Type III (Figure 3b,c), suggesting predominantly Type II/III kerogen. The coal and carbonaceous mudstones are characterized by higher HI values (Figure 3a,c), which is commonly observed for coaly samples [69]. The samples with relatively high hydrogen content might be due to high amounts of resinite, as suggested by the abundant diterpanes in the aliphatic fractions (Figure 5) and the maceral composition of other samples taken from this formation [16,23]. The relatively hydrogen-rich nature (Type II/III kerogen) together with maturity at the early to near peak oil window of the samples analyzed suggest that they could be capable of generating some liquid hydrocarbons and significant natural gas at a higher maturity stage [25].
In summary, the composition of biomarkers suggests that the studied samples contain predominantly terrestrial higher plants’ (especially gymnosperms rather than angiosperms) derived organic matter with subordinate algae/microorganisms deposited in a relatively oxic, fluvial-deltaic environment. An appropriate water table favorable for peat formation resulted in source rocks with higher TOC and HI values, whereas, during occasional increased water levels, mudstone containing more algae but lower TOC and HI values was developed. These, therefore, suggest that a stable peat environment is the key for high-quality source rock formation in the Xihu Sag. A combination with sedimentologic studies will give further constraints on the distribution of excellent source rocks for petroleum exploration. In addition, not only natural gas but also light oils are worth exploring due to the relatively hydrogen-rich nature of the E2p source rocks. However, appropriate thermal maturity is a prerequisite.

6. Conclusions

The investigation of the coal-bearing source rocks of the Eocene Pinghu Formation from the W-3 well, western margin of the Xihu Sag, East China Sea Shelf Basin has yielded important new results regarding their organic matter source, depositional environment, thermal maturity, and hydrocarbon generative potential.
Source-related biomarker parameters suggest a dominant contribution from terrigenous higher plant organic matter with subordinate algae/microorganisms in the studied source rock samples. Contributions from aquatic organic matter were increased in certain intervals as suggested by reduced Pr/Ph and TAR and elevated Paq and C27 steranes, which might indicate the occasional rise of the water table. The occurrence of diterpanes and non-hopanoid triterpanes (oleanane, des-A-oleanane, -ursane, and -lupane) within the source rocks indicates the contribution of conifers (gymnosperms) and angiosperms. The predominance of gymnosperms in the peat-forming vegetation is evidenced by the dominance of diterpenoids over non-hopanoid triterpanes.
The high Pr/Ph ratios, Pr/n-C17 versus Ph/n-C18 cross-plot, dominance of C29 steranes, distribution of hopanes, and almost complete absence of gammacerane suggest that the source rocks were deposited in relatively oxic environments. Variations in redox potential during deposition were proven by significant fluctuation of Pr/Ph in the profile. Diagenetic rearrangement of steroids to diasteranes could be controlled by both clay minerals and oxicity of the depositional environment.
Vitrinite reflectance, Rock-Eval Tmax, and molecular thermal maturity proxies indicate that the source rock samples constitute a natural maturation profile ranging from a marginally mature to a mid-mature stage.
Kerogen elemental composition and HI values indicate that the source rock samples have Type II/III kerogen, suggesting that they are gas- and oil-prone and are capable of generating oil and gas during thermal maturation. Overall, our results suggest that source rocks with high source potential were formed in a stable peat formation environment. Natural gas and crude oil are worth exploring, which depend on the thermal maturity of the source rock.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/min11080909/s1, Figure S1: Cross-plots of TAR versus Paq (a) and Pwax (b), Paq versus Pwax (c), and TAR versus C27/C29 ααα20R sterane (d), Figure S2: Partial m/z 123 mass chromatograms showing distribution of bicyclic sesquiterpanes and diterpanes in representative samples.

Author Contributions

Conceptualization, X.C. and D.H.; methodology, X.C. and H.D.; software, X.C.; investigation, X.C., D.H., H.D. and L.W.; resources, X.Z., J.L. and H.D.; data curation, X.C. and H.D; writing—original draft preparation, X.C.; writing—review and editing, X.C. and D.H.; visualization, X.C. and H.D.; supervision, D.H.; project administration, D.H., J.L. and H.D.; funding acquisition, X.C., D.H., J.L., H.D. and L.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Major Project, Grant No. 2016ZX05027-001-003; China Postdoctoral Science Foundation, Grant No. 2019M660735; and Fundamental Research Funds for the Central Universities, Grant No. 26522019013 and 26522018214.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the three anonymous reviewers for their constructive comments as well as Rita Xu, Assistant Editor, for handling the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Tissot, P.B.; Welte, D.H. Petroleum Formation and Occurrence; Springer: Berlin/Heidelberg, Germany, 1984. [Google Scholar]
  2. Difan, H.; Dajiang, Z.; Jinchao, L.; Xiaoming, H. Hydrocarbon genesis of Jurassic coal measures in the Turpan Basin, China. Org. Geochem. 1991, 17, 827–837. [Google Scholar] [CrossRef]
  3. Clayton, J.L.; Rice, D.D.; Michael, G.E. Oil-generating coals of the San Juan Basin, New Mexico and Colorado, U.S.A. Org. Geochem. 1991, 17, 735–742. [Google Scholar] [CrossRef]
  4. Killops, S.; Woolhouse, A.D.; Weston, R.J.; Cook, R.A. A Geochemical Appraisal of Oil Generation in the Taranaki Basin, New Zealand. AAPG Bull. 1994, 78, 1560–1585. [Google Scholar]
  5. Petersen, H.I.; Hertle, M. A Review of the Coaly Source Rocks and Generated Petroleums in the Danish North Sea: An Underexplored Middle Jurassic Petroleum System? J. Pet. Geol. 2018, 41, 135–154. [Google Scholar] [CrossRef]
  6. Katz, B.J. Controlling Factors on Source Rock Development—A Review of Productivity, Preservation, and Sedimentation Rate. In The Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and Consequences; SEPM Society for Sedimentary Geology: Tulsa, OK, USA, 2005; Volume 82, pp. 7–16. [Google Scholar]
  7. Fathy, D.; Wagreich, M.; Ntaflos, T.; Sami, M. Provenance Characterization of Campanian Lacustrine Organic-Rich Mudstones on the Southern Tethyan Margin, Egypt. ACS Earth Space Chem. 2021, 5, 197–209. [Google Scholar] [CrossRef]
  8. Fathy, D.; Wagreich, M.; Ntaflos, T.; Sami, M. Paleoclimatic variability in the southern Tethys, Egypt: Insights from the mineralogy and geochemistry of Upper Cretaceous lacustrine organic-rich deposits. Cretac. Res. 2021, 126, 104880. [Google Scholar] [CrossRef]
  9. Zhu, X.; Chen, J.; Li, W.; Pei, L.; Liu, K.; Chen, X.; Zhang, T. Hydrocarbon generation potential of Paleogene coals and organically rich mudstones in Xihu sag, East China Sea Shelf basin, offshore eastern China. J. Pet. Sci. Eng. 2020, 184, 106450. [Google Scholar] [CrossRef]
  10. Tao, S.-Z.; Zou, C.-N. Accumulation and distribution of natural gases in Xihu Sag, East China Sea Basin. Pet. Explor. Dev. 2005, 32, 103–110. [Google Scholar]
  11. Zhu, Y.; Li, Y.; Zhou, J.; Gu, S. Geochemical characteristics of Tertiary coal-bearing source rocks in Xihu depression, East China Sea basin. Mar. Pet. Geol. 2012, 35, 154–165. [Google Scholar] [CrossRef]
  12. Cheng, X.; Hou, D.; Zhao, Z.; Chen, X.; Diao, H. Sources of Natural Gases in the Xihu Sag, East China Sea Basin: Insights from Stable Carbon Isotopes and Confined System Pyrolysis. Energy Fuels 2019, 33, 2166–2175. [Google Scholar] [CrossRef]
  13. Xu, H.; George, S.C.; Hou, D.; Cao, B.; Chen, X. Petroleum sources in the Xihu Depression, East China Sea: Evidence from stable carbon isotopic compositions of individual n-alkanes and isoprenoids. J. Pet. Sci. Eng. 2020, 190, 107073. [Google Scholar] [CrossRef]
  14. Fu, N. Diterpenoid compounds in coal measures and condensates in Xihu Sag of East China Sea Basin. China Offshore Oil Gas 1994, 8, 21–28. [Google Scholar]
  15. Cheng, X.; Hou, D.; Zhou, X.; Liu, J.; Diao, H.; Jiang, Y.; Yu, Z. Organic geochemistry and kinetics for natural gas generation from mudstone and coal in the Xihu Sag, East China Sea Shelf Basin, China. Mar. Pet. Geol. 2020, 118, 104405. [Google Scholar] [CrossRef]
  16. Kang, S.; Shao, L.; Qin, L.; Li, S.; Liu, J.; Shen, W.; Chen, X.; Eriksson, K.A.; Zhou, Q. Hydrocarbon Generation Potential and Depositional Setting of Eocene Oil-Prone Coaly Source Rocks in the Xihu Sag, East China Sea Shelf Basin. ACS Omega 2020, 5, 32267–32285. [Google Scholar] [CrossRef]
  17. Cheng, X.; Hou, D.J.; Zhao, Z.; Jiang, Y.H.; Zhou, X.H.; Diao, H. Higher Landplant -Derived Biomarkers in Light Oils and Condensates from the Coal-Bearing Eocene Pinghu Formation, Xihu Sag, East China Sea Shelf Basin. J. Pet. Geol. 2020, 43, 437–451. [Google Scholar] [CrossRef]
  18. Weilin, Z.; Kai, Z.; Xiaowei, F.; Chunfeng, C.; Minqiang, Z.; Shunli, G. The formation and evolution of the East China Sea Shelf Basin: A new view. Earth-Sci. Rev. 2019, 190, 89–111. [Google Scholar] [CrossRef]
  19. Zhou, Z.W.; Z.hao, J.H.; Yin, P.L. Characteristics and Tectonic Evolution of the East China Sea; Elsevier: Oxford, UK, 1989. [Google Scholar]
  20. Abbas, A.; Zhu, H.; Zeng, Z.; Zhou, X. Sedimentary facies analysis using sequence stratigraphy and seismic sedimentology in the Paleogene Pinghu Formation, Xihu Depression, East China Sea Shelf Basin. Mar. Pet. Geol. 2018, 93, 287–297. [Google Scholar] [CrossRef]
  21. Yu, S. Depositional genesis analysis of source rock in Pinghu Formation of Western Slope, Xihu Depression. Earth Sci. 2020, 45, 1722–1736. [Google Scholar]
  22. Wang, Y.; Qin, Y.; Yang, L.; Liu, S.; Elsworth, D.; Zhang, R. Organic Geochemical and Petrographic Characteristics of the Coal Measure Source Rocks of Pinghu Formation in the Xihu Sag of the East China Sea Shelf Basin: Implications for Coal Measure Gas Potential. Acta Geol. Sin. -Engl. Ed. 2020, 94, 364–375. [Google Scholar] [CrossRef]
  23. Liu, J.; Kang, S.; Shen, W.; Qin, L.; Zhou, Q.; Li, S.; Ding, F.; Shao, L. Petrology and hydrocarbon significance of the coaly source rocks from the Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin. Energy Explor. Exploit. 2020, 38, 1295–1319. [Google Scholar] [CrossRef]
  24. Jiang, Y.; Diao, H.; Zeng, W. Coal source rock conditions and hydrocarbon generation model of Pinghu Formation in Xihu Depression, East China Sea Basin. Bull. Geol. Sci. Technol. 2020, 39, 30–39. [Google Scholar]
  25. Peters, K.E.; Cassa, M.R.; Magoon, L.B.; Dow, W.G. Applied Source Rock Geochemistry. In The Petroleum System—From Source to Trap; American Association of Petroleum Geologists: Tulsa, OK, USA, 1994; Volume 60, pp. 93–120. [Google Scholar]
  26. Cheng, X.; Hou, D.; Xu, C.; Wang, F. Biodegradation of tricyclic terpanes in crude oils from the Bohai Bay Basin. Org. Geochem. 2016, 101, 11–21. [Google Scholar] [CrossRef]
  27. Barrick, R.C.; Hedges, J.I. Hydrocarbon geochemistry of the Puget Sound region—II. Sedimentary diterpenoid, steroid and triterpenoid hydrocarbons. Geochim. Et Cosmochim. Acta 1981, 45, 381–392. [Google Scholar] [CrossRef]
  28. Noble, R.A.; Alexander, R.; Kagi, R.I.; Knox, J. Tetracyclic diterpenoid hydrocarbons in some Australian coals, sediments and crude oils. Geochim. Et Cosmochim. Acta 1985, 49, 2141–2147. [Google Scholar] [CrossRef]
  29. Bray, E.E.; Evans, E.D. Distribution of n-paraffins as a clue to recognition of source beds. Geochim. Et Cosmochim. Acta 1961, 22, 2–15. [Google Scholar] [CrossRef]
  30. Bourbonniere, R.A.; Meyers, P.A. Sedimentary geolipid records of historical changes in the watersheds and productivities of Lakes Ontario and Erie. Limnol. Oceanogr. 1996, 41, 352–359. [Google Scholar] [CrossRef] [Green Version]
  31. Ficken, K.J.; Li, B.; Swain, D.L.; Eglinton, G. An n-alkane proxy for the sedimentary input of submerged/floating freshwater aquatic macrophytes. Org. Geochem. 2000, 31, 745–749. [Google Scholar] [CrossRef]
  32. Zheng, Y.; Zhou, W.; Meyers, P.A.; Xie, S. Lipid biomarkers in the Zoigê-Hongyuan peat deposit: Indicators of Holocene climate changes in West China. Org. Geochem. 2007, 38, 1927–1940. [Google Scholar] [CrossRef]
  33. Bechtel, A.; Sachsenhofer, R.F.; Kolcon, I.; Gratzer, R.; Otto, A.; Püttmann, W. Organic geochemistry of the Lower Miocene Oberdorf lignite (Styrian Basin, Austria): Its relation to petrography, palynology and the palaeoenvironment. Int. J. Coal Geol. 2002, 51, 31–57. [Google Scholar] [CrossRef]
  34. Seifert, W.K.; Moldowan, J.M. The effect of thermal stress on source-rock quality as measured by hopane stereochemistry. Phys. Chem. Earth 1980, 12, 229–237. [Google Scholar] [CrossRef]
  35. Cranwell, P.A. Organic geochemistry of Cam Loch (Sutherland) sediments. Chem. Geol. 1977, 20, 205–221. [Google Scholar] [CrossRef]
  36. Eglinton, G.; Hamilton, R.J. Leaf epicuticular waxes. Science 1967, 156, 1322–1335. [Google Scholar] [CrossRef] [PubMed]
  37. Gelpi, E.; Schneider, H.; Mann, J.; Oró, J. Hydrocarbons of geochemical significance in microscopic algae. Phytochemistry 1970, 9, 603–612. [Google Scholar] [CrossRef]
  38. Philp, R. Geochemical characteristics of oils derived predominantly from terrigenous source materials. Geol. Soc. Lond. Spec. Publ. 1994, 77, 71–91. [Google Scholar] [CrossRef]
  39. Huang, W.-Y.; Meinschein, W.G. Sterols as ecological indicators. Geochim. Et Cosmochim. Acta 1979, 43, 739–745. [Google Scholar] [CrossRef]
  40. Volkman, J.K. A review of sterol markers for marine and terrigenous organic matter. Org. Geochem. 1986, 9, 83–99. [Google Scholar] [CrossRef]
  41. Ourisson, G.; Albrecht, P.; Rohmer, M. The Hopanoids: Palaeochemistry and biochemistry of a group of natural products. Pure Appl. Chem. 1979, 51, 709–729. [Google Scholar] [CrossRef]
  42. Alexander, R.; Kagi, R.I.; Noble, R.; Volkman, J.K. Identification of some bicyclic alkanes in petroleum. Org. Geochem. 1984, 6, 63–72. [Google Scholar] [CrossRef]
  43. Philp, R.P. Fossil Fuel Biomarkers. Methods in Geochemistry and Geophysics; Elsevier: Amsterdam, The Netherlands, 1981. [Google Scholar]
  44. Otto, A.; Wilde, V. Sesqui-, Di-, and triterpenoids as chemosystematic markers in extant conifers—A review. Bot. Rev. 2001, 67, 141–238. [Google Scholar] [CrossRef]
  45. Dimmler, A.; Cyr, T.D.; Strausz, O.P. Identification of bicyclic terpenoid hydrocarbons in the saturate fraction of Athabasca oil sand bitumen. Org. Geochem. 1984, 7, 231–238. [Google Scholar] [CrossRef]
  46. Otto, A.; Simoneit, B.R.T. Biomarkers of Holocene buried conifer logs from Bella Coola and north Vancouver, British Columbia, Canada. Org. Geochem. 2002, 33, 1241–1251. [Google Scholar] [CrossRef]
  47. Diefendorf, A.F.; Leslie, A.B.; Wing, S.L. A phylogenetic analysis of conifer diterpenoids and their carbon isotopes for chemotaxonomic applications. Org. Geochem. 2019, 127, 50–58. [Google Scholar] [CrossRef]
  48. Noble, R.A.; Alexander, R.; Kagi, R.I.; Nox, J.K. Identification of some diterpenoid hydrocarbons in petroleum. Org. Geochem. 1986, 10, 825–829. [Google Scholar] [CrossRef]
  49. Weston, R.J.; Philp, R.P.; Sheppard, C.M.; Woolhouse, A.D. Sesquiterpanes, diterpanes and other higher terpanes in oils from the Taranaki basin of New Zealand. Org. Geochem. 1989, 14, 405–421. [Google Scholar] [CrossRef]
  50. Simoneit, B. Cyclic terpenoids of the geosphere. Methods Geochem. Geophys. 1986, 24, 43–99. [Google Scholar]
  51. Killops, S.D.; Raine, J.I.; Woolhouse, A.D.; Weston, R.J. Chemostratigraphic evidence of higher-plant evolution in the Taranaki Basin, New Zealand. Org. Geochem. 1995, 23, 429–445. [Google Scholar] [CrossRef]
  52. Trendel, J.M.; Lohmann, F.; Kintzinger, J.P.; Albrecht, P.; Chiarone, A.; Riche, C.; Cesario, M.; Guilhem, J.; Pascard, C. Identification of des-A-triterpenoid hydrocarbons occurring in surface sediments. Tetrahedron 1989, 45, 4457–4470. [Google Scholar] [CrossRef]
  53. Otto, A.; Walther, H.; Puttmann, W. Sesqui- and diterpenoid biomarkers preserved in Taxodium-rich Oligocene oxbow lake clays, Weisselster basin, Germany. Org. Geochem. 1997, 26, 105–115. [Google Scholar] [CrossRef]
  54. Regnery, J.; Püttmann, W.; Koutsodendris, A.; Mulch, A.; Pross, J. Comparison of the paleoclimatic significance of higher land plant biomarker concentrations and pollen data: A case study of lake sediments from the Holsteinian interglacial. Org. Geochem. 2013, 61, 73–84. [Google Scholar] [CrossRef]
  55. Didyk, B.; Simoneit, B.; Brassell, S.; Eglinton, G. Organic geochemical indicators of paleoenvironmental conditions of sedimentation. Nature 1978, 272, 216–222. [Google Scholar] [CrossRef]
  56. Goossens, H.; de Leeuw, J.; Schenck, P.A.; Brassell, S. Tocopherols as likely precursors of pristane in ancient sediments and crude oils. Nature 1984, 312, 440–442. [Google Scholar] [CrossRef]
  57. Koopmans, M.P.; Rijpstra, W.I.C.; Klapwijk, M.M.; de Leeuw, J.W.; Lewan, M.D.; Sinninghe Damsté, J.S. A thermal and chemical degradation approach to decipher pristane and phytane precursors in sedimentary organic matter. Org. Geochem. 1999, 30, 1089–1104. [Google Scholar] [CrossRef]
  58. Peters, K.E.; Walters, C.C.; Moldowan, J.M. The Biomarker Guide: Volume 2: Biomarkers and Isotopes in Petroleum Systems and Earth History, 2nd ed.; Cambridge University Press: Cambridge, UK, 2004; Volume 2. [Google Scholar]
  59. Rubinstein, I.; Sieskind, O.; Albrecht, P. Rearranged sterenes in a shale: Occurrence and simulated formation. J. Chem. Soc. Perkin Trans. 1975, 1, 1833–1836. [Google Scholar] [CrossRef]
  60. Moldowan, J.M.; Sundararaman, P.; Schoell, M. Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany. Org. Geochem. 1986, 10, 915–926. [Google Scholar] [CrossRef]
  61. Seifert, W.K.; Michael Moldowan, J. Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Et Cosmochim. Acta 1978, 42, 77–95. [Google Scholar] [CrossRef]
  62. Wan Hasiah, A. Oil-generating potential of Tertiary coals and other organic-rich sediments of the Nyalau Formation, onshore Sarawak. J. Asian Earth Sci. 1999, 17, 255–267. [Google Scholar] [CrossRef]
  63. Mackenzie, A.S.; Patience, R.L.; Maxwell, J.R.; Vandenbroucke, M.; Durand, B. Molecular parameters of maturation in the Toarcian shales, Paris Basin, France—I. Changes in the configurations of acyclic isoprenoid alkanes, steranes and triterpanes. Geochim. Et Cosmochim. Acta 1980, 44, 1709–1721. [Google Scholar] [CrossRef]
  64. Seifert, W.; Moldowan, J. Use of biological markers in petroleum exploration. Methods Geochem. Geophys. 1986, 24, 261–290. [Google Scholar]
  65. Wilkins, R.W.T.; George, S.C. Coal as a source rock for oil: A review. Int. J. Coal Geol. 2002, 50, 317–361. [Google Scholar] [CrossRef]
  66. Snowdon, L.R. Oil from Type III organic matter: Resinite revisited. Org. Geochem. 1991, 17, 743–747. [Google Scholar] [CrossRef]
  67. Hunt, J.M. Generation of gas and oil from coal and other terrestrial organic matter. Org. Geochem. 1991, 17, 673–680. [Google Scholar] [CrossRef]
  68. Tissot, B.; Durand, B.; Espitalie, J.; Combaz, A. Influence of Nature and Diagenesis of Organic Matter in Formation of Petroleum. AAPG Bull. 1974, 58, 499–506. [Google Scholar]
  69. Sykes, R.; Snowdon, L.R. Guidelines for assessing the petroleum potential of coaly source rocks using Rock-Eval pyrolysis. Org. Geochem. 2002, 33, 1441–1455. [Google Scholar] [CrossRef]
Minerals 11 00909 g001 550
Figure 1. Map showing (a) tectonic sketch of the East China Sea Shelf Basin (ECSSB), (b) Pinghu Slope of the Xihu Sag, ECSSB, and (c) structural cross-section of the Xihu Sag (A–B, shown in Figure 1a,b). (adapted from [17]).
Figure 1. Map showing (a) tectonic sketch of the East China Sea Shelf Basin (ECSSB), (b) Pinghu Slope of the Xihu Sag, ECSSB, and (c) structural cross-section of the Xihu Sag (A–B, shown in Figure 1a,b). (adapted from [17]).
Minerals 11 00909 g001
Minerals 11 00909 g002 550
Figure 2. A generalized stratigraphy of the Xihu Sag (a) and stratigraphic column showing lithology of the middle–upper Eocene Pinghu Formation from well W-3 and sample locations (b).
Figure 2. A generalized stratigraphy of the Xihu Sag (a) and stratigraphic column showing lithology of the middle–upper Eocene Pinghu Formation from well W-3 and sample locations (b).
Minerals 11 00909 g002
Minerals 11 00909 g003 550
Figure 3. Cross-plots of S2 versus TOC (a), atomic H/C versus O/C (b), and HI versus Tmax (c) for source rocks of the Pinghu Formation from well W-3, Xihu Sag. See Figure 3c for the legend.
Figure 3. Cross-plots of S2 versus TOC (a), atomic H/C versus O/C (b), and HI versus Tmax (c) for source rocks of the Pinghu Formation from well W-3, Xihu Sag. See Figure 3c for the legend.
Minerals 11 00909 g003
Minerals 11 00909 g004 550
Figure 4. Variation of vitrinite reflectance Ro (a), Tmax (b), CPI (c), Ts/(Ts + Tm) (d), C30 moretane/C30 αβ-hopane C30M/C30H (e), C31 αβ-homohopane 22S/(22S + 22R) (f), and C29 steranes ααα20S/(20S + 20R) (g) and ββ/(ββ + αα) (h) with depth for the studied source rocks.
Figure 4. Variation of vitrinite reflectance Ro (a), Tmax (b), CPI (c), Ts/(Ts + Tm) (d), C30 moretane/C30 αβ-hopane C30M/C30H (e), C31 αβ-homohopane 22S/(22S + 22R) (f), and C29 steranes ααα20S/(20S + 20R) (g) and ββ/(ββ + αα) (h) with depth for the studied source rocks.
Minerals 11 00909 g004
Minerals 11 00909 g005 550
Figure 5. Total ion chromatograms and m/z 85 mass chromatograms of aliphatic fractions of the samples. The prominent peaks eluting between n-C19 and n-C21 have been identified as tri- and tetracyclic diterpanes derived from conifer resins.
Figure 5. Total ion chromatograms and m/z 85 mass chromatograms of aliphatic fractions of the samples. The prominent peaks eluting between n-C19 and n-C21 have been identified as tri- and tetracyclic diterpanes derived from conifer resins.
Minerals 11 00909 g005
Minerals 11 00909 g006 550
Figure 6. Variation of TOC (a), TAR (b), Paq (c), Pwax (c), Pr/Ph (d), diterpanes/n-C19–20 Di/n-C19–20 (e), oleanane/C30 hopane O/C30H (f), diterpanes/(diterpanes + non-hopanoid triterpanes) [Di/(Di + Tri)] (g), C29 steranes/C30 hopane C29St/C30H (h), C27/C29 ααα20R sterane (i), and C29 diasteranes/C29 steranes diaC29/C29St (j) with depth for the studied source rocks.
Figure 6. Variation of TOC (a), TAR (b), Paq (c), Pwax (c), Pr/Ph (d), diterpanes/n-C19–20 Di/n-C19–20 (e), oleanane/C30 hopane O/C30H (f), diterpanes/(diterpanes + non-hopanoid triterpanes) [Di/(Di + Tri)] (g), C29 steranes/C30 hopane C29St/C30H (h), C27/C29 ααα20R sterane (i), and C29 diasteranes/C29 steranes diaC29/C29St (j) with depth for the studied source rocks.
Minerals 11 00909 g006
Minerals 11 00909 g007 550
Figure 7. Cross-plot of Pr/n-C17 versus Ph/n-C18 for the source rocks of the Pinghu Formation.
Figure 7. Cross-plot of Pr/n-C17 versus Ph/n-C18 for the source rocks of the Pinghu Formation.
Minerals 11 00909 g007
Minerals 11 00909 g008 550
Figure 8. Partial m/z 123 mass chromatograms showing distribution of diterpanes.
Figure 8. Partial m/z 123 mass chromatograms showing distribution of diterpanes.
Minerals 11 00909 g008
Minerals 11 00909 g009 550
Figure 9. Partial m/z 191 (a–e) and 217 (f–i) mass chromatograms showing the distribution of tricylic terpanes, pentacyclic triterpanes and steranes. TT: tricyclic terpane; TeT: tetracyclic terpane; dO: 10β(H)-des-A-oleanane, dL: 10β(H)-des-A-lupane, dU: 10β(H)-des-A-ursane; Ts: 18α(H)-22,29,30-trinorneohopane; Tm: 17α(H)-22,29,30-trinorhopane; O: oleanane, H: αβ-hopane; M: mortane (βα-hopane).
Figure 9. Partial m/z 191 (a–e) and 217 (f–i) mass chromatograms showing the distribution of tricylic terpanes, pentacyclic triterpanes and steranes. TT: tricyclic terpane; TeT: tetracyclic terpane; dO: 10β(H)-des-A-oleanane, dL: 10β(H)-des-A-lupane, dU: 10β(H)-des-A-ursane; Ts: 18α(H)-22,29,30-trinorneohopane; Tm: 17α(H)-22,29,30-trinorhopane; O: oleanane, H: αβ-hopane; M: mortane (βα-hopane).
Minerals 11 00909 g009
Minerals 11 00909 g010 550
Figure 10. Cross-plots of TOC versus TAR (a) Pr/Ph, (b) and C27/C29 ααα20R sterane, (c) for the studied source rocks.
Figure 10. Cross-plots of TOC versus TAR (a) Pr/Ph, (b) and C27/C29 ααα20R sterane, (c) for the studied source rocks.
Minerals 11 00909 g010
Minerals 11 00909 g011 550
Figure 11. Cross-plots of C29 diasteranes/C29 regular steranes (diaC29/C29St) versus TOC (a) and Pr/Ph (b) for the studied source rocks.
Figure 11. Cross-plots of C29 diasteranes/C29 regular steranes (diaC29/C29St) versus TOC (a) and Pr/Ph (b) for the studied source rocks.
Minerals 11 00909 g011
Table
Table 1. Origin and bulk properties of the Eocene Pinghu Formation source rock samples from well W-3, Xihu Sag.
Table 1. Origin and bulk properties of the Eocene Pinghu Formation source rock samples from well W-3, Xihu Sag.
Depth
(m)		Sample TypeLithologyTOC (%)Tmax (°C)S1 (mg/g)S2 (mg/g)S1 + S2 (mg/g)HI (mg/g)Atomic
H/C		Atomic
O/C		Ro
(%)
3723Sidewall coreMudstone0.814360.150.750.9093ndnd0.57
3864.5Sidewall coreMudstone0.294350.190.410.60141ndnd0.58
3924Sidewall coreMudstone1.904400.662.453.111290.840.100.58
3966Sidewall coreCoal60.284418.36199.92208.283320.920.110.60
3966.5Sidewall coreCarbonaceous
mudstone		9.094404.0143.5147.524791.030.170.58
4024Sidewall coreMudstone0.494410.290.470.76960.820.170.64
4063.8CoreMudstone2.824430.614.875.481730.900.130.63
4064.3CoreCarbonaceous
mudstone		11.394442.9130.8533.762710.900.140.66
4064.5CoreMudstone1.494450.311.792.101201.120.220.67
4064.7CoreMudstone0.844450.220.941.16112ndnd0.66
4065CoreCarbonaceous
mudstone		7.414451.8419.6821.522660.900.110.66
4095Sidewall coreMudstone3.294451.366.898.252090.880.100.60
4175Sidewall coreMudstone3.404491.455.356.801570.870.100.68
4180Sidewall coreMudstone0.964530.570.871.44910.800.140.70
4238Sidewall coreMudstone0.904540.530.931.461030.800.130.74
4260.9Sidewall coreMudstone1.074540.861.342.201250.800.130.75
Table
Table 2. Partial source related parameters of the studied source rock samples from the Xihu Sag.
Table 2. Partial source related parameters of the studied source rock samples from the Xihu Sag.
Depth (m)n-C14–20/
n-alkanes		n-C21–25/
n-alkanes		n-C26–35/
n-alkanes		C24TeT/
C26TT		C27
(%)		C28
(%)		C29
(%)		C27
(%)		C27/C29
ααα20R
37230.210.330.461.56141967140.21
3864.50.440.270.291.09162361160.27
39240.160.280.562.24121771120.17
39660.160.320.514.277177670.10
3966.50.210.390.406.199167590.12
40240.360.300.341.38262747260.56
4063.80.180.330.491.787187470.10
4064.30.190.300.513.107167670.10
4064.50.170.330.501.23131968130.19
4064.70.190.310.501.20182161180.29
40650.180.280.535.44111971110.15
40950.240.300.4617.26132266130.19
41750.240.340.417.73142165140.22
41800.310.330.361.85242749240.50
42380.290.360.352.04222651220.43
4260.90.360.360.292.98192655190.36
C24TeT/C26TT: 1/2 × [(C24 TeT + C26TT) − C26TT]/C26TT (see Figure 9a); O/C30H: oleanane/C30 αβ-hopane; Di/(Di + Tri): diterpanes/(diterpanes + non-hopanoid triterpanes); C27 (%): the relative percentage of C27 ααα20R sterane over the total of C27 to C29 ααα20R steranes; C27/C29 ααα20R: C27 ααα20R/C29 ααα20R sterane.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Cheng, X.; Hou, D.; Zhou, X.; Liu, J.; Diao, H.; Wei, L. Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential. Minerals 2021, 11, 909. https://doi.org/10.3390/min11080909

AMA Style

Cheng X, Hou D, Zhou X, Liu J, Diao H, Wei L. Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential. Minerals. 2021; 11(8):909. https://doi.org/10.3390/min11080909

Chicago/Turabian Style

Cheng, Xiong, Dujie Hou, Xinhuai Zhou, Jinshui Liu, Hui Diao, and Lin Wei. 2021. "Geochemical Characterization of the Eocene Coal-Bearing Source Rocks, Xihu Sag, East China Sea Shelf Basin, China: Implications for Origin and Depositional Environment of Organic Matter and Hydrocarbon Potential" Minerals 11, no. 8: 909. https://doi.org/10.3390/min11080909

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop