Constrained by Water Column Stratification

Jul 2, 2015 - Wuxi, Jiangsu 214000, People's Republic of China. ∥. Exploration and Development Institute of Jianghan Oilfield, China Sinopec Company...
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2,3,6-/2,3,4-Aryl Isoprenoids in Paleocene Crude Oils from Chinese Jianghan Basin: Constrained by Water Column Stratification Hong Lu,*,† Chenchen Shen,†,‡ Zhirong Zhang,§ Ming Liu,∥ Guoying Sheng,† Ping’an Peng,† and Chang Samuel Hsu*,⊥,# †

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, China Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Wuxi Institute of Petroleum Geology, Petroleum Exploration and Production Research Institute of Sinopec (PEPRIS), Sinopec, Wuxi, Jiangsu 214000, People’s Republic of China ∥ Exploration and Development Institute of Jianghan Oilfield, China Sinopec Company, Wuhan, Hubei 430079, People’s Republic of China ⊥ Department of Chemical and Biomedical Engineering, Florida A&M University/Florida State University, Tallahassee, Florida 32310, United States # State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (Beijing), Changping, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: 2,3,4-Trimethyl aryl isoprenoid (AIP) hydrocarbons as well as their 2,3,6 homologues were found in most Paleocene crude oils from the Jianghan Basin, China. We interpret their concurrence as evidence for the presence of both purple sulfur bacteria (PSB, family of Chromatiaceae) and green sulfur bacteria (GSB, family of Chlorobiaceae) in the water body during deposition. The GSB and PSB contributions are also evidenced by the presence of the monoaromatic carotenoid derivatives chlorobactane and okenane as well as C40 diaromatic carotenoid derivatives, including isorenieratane and renierapurpurane. The presence of related biomarkers of gammacerane, phytane and methyl trimethyltridecyl chromans (MTTCs) in the oil samples implies that these prolific photosynthetic sulfur bacteria grew in a mesosaline condition, which is consistent with their niches above and below the oxic−anoxic chemocline in a stratified water column. Thus, the general presence of 2,3,6-AIPs, chlorobactane, and isorenieratane sourced from GSB is consistent with sulfidic (euxinic) conditions in the photic zone of the water column, while their concurrence with 2,3,4-AIPs and okenane further indicates a permanent stratified water column with sulfidic (euxinic) conditions. In such a paleowater-column redox state with ideal growing conditions for the photosynthetic bacteria, PSB likely lived beneath the GSB layer and/or co-existed in the same layer with a symbiotic relationship and/or even as a “Chlorochromatium” aggregate with an endosymbiont relationship. This ecosystem model helps further explain the concurrence of similar amounts of 2,3,6- and 2,3,4-AIPs in the Jianghan Basin oils.

1. INTRODUCTION Aryl isoprenoids (AIPs) have a 1-isoalkyl-trimethyl substitution of a tail-to-tail linked isoprenoid chain on a benzene ring.1,2 Ostroukhov et al.3 first reported a homologous series of C10− C30 1-alkyl-2,3,6-trimethyl AIPs in oils from the Soviet Union. Summons and Powell1,2 found the same compounds in midDevonian and Silurian oils and source rocks of Western Canada and Michigan basins and confirmed the AIP structures having 2,3,6-trimethyl substitution by comparing to synthesized standards. AIPs with 2,3,6 substitution were also reported later in oils and organic-rich carbonates of the Sunniland limestone of the South Florida Basin,4 the Kupferschiefer of the Lower Rhine Basin,5 and selected oils/source rocks from the Western Canada and Williston Basins.6 Our previous work found that 2,3,6-AIPs with a predominance of C13−C23 homologues occurred in crude oils from Paleozoic petroleum systems in Tarim Basin, northwestern China. 7 These compounds were also reported in the oil samples from the Sergipe−Alagoas Basin, northeastern Brazil.8 Besides the oil © XXXX American Chemical Society

samples, the 2,3,6-AIPs have been shown to occur in a wide range of organic-rich sediments,9−12 exhibiting their huge application potential in paleoenvironmental interpretation. In most reports, the AIP distributions were limited to 2,3,6trimethyl structures, which were commonly interpreted as biomarkers for green sulfur bacteria (GSB, family of Chlorobiaceae), indicating anoxic and euxinic conditions in the photic zone of the water column.1,2,13 In a few cases, two series of AIPs were reported. The 1-alkyl-3,4,5-trimethyl isomers were initially assigned with nuclear magnetic resonance (NMR) evidence to a compound series with slightly shorter gas chromatography (GC) retention times and lower concentrations than the corresponding 2,3,6 isomers.14−17 However, their precursor carotenoids, 2,3,6-/3,4,5-trimethyl diaryl isoprenoids, were not found in organisms.17 Hence, they were Received: January 19, 2015 Revised: July 2, 2015

A

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Figure 1. Geological map of Jianghan Basin and oilfield locations.

associated mainly with a semi-isolated sulfate salt lake environment, while the latter is dominated mainly by a hypersaline environment. About 50% of the individual beds in the Eq Formation are >10 m thick. They represent predominantly dark gypsiferous mudstones, indicative of evaporative conditions. Gypcrete, oil shales, and salt rocks mixed with sandstones and salt, occurring as rhythmic layers. The multiple rhythmic layers of salt rocks and gypsum provided good sealing conditions. During the long course of deposition, more than 220 salt layers developed in the source rocks of Eq and Ex as a result of the frequent alternation of dry and moist climates. Salt and gypsum were precipitated with increased salinity during arid intervals, while mudstones and sandstones with more freshwater influx were deposited during more humid intervals. These cycles included chemical evaporite rocks (non-water kainitite, glaserite, and syngenite rocks), evaporite clastic rocks (glauberite mudstones and anhydrite), and clastic rocks. Salinity in the formation water was on the order of 250−330 g/L in the Eq Formation and in a range of 120−230 g/L in the Ex Formation.23 The formation water is rich in trace elements, such as I, Br, B, Li, K, Rb, Cs, Sr, Mn, Ni, Mo, Co, and Zn, characteristic of high halide water rich in Na2SO4.23

considered to occur in an extinct strain of photosynthetic GSB. Later, Brocks and Schaeffer18 used several synthetic C14 trimethyl AIP standards to confirm that the other series of AIPs actually had a 2,3,4-trimethyl substitution pattern that occurred in the carbonaceous dolomites of the 1640 Ma Barney Creek Formation (BCF) in the McArthur Group of northern Australia.18,19 2,3,4-Trimethyl-substituted AIPs were also reported with apparently lower concentrations than 2,3,6AIPs in extracts of 25−65 Ma saline lacustrine formation rocks in the western Qaidam Basin, northwestern China.20 In this paper, we report for the first time the co-presence of similar amounts of 2,3,6- and 2,3,4-substituted AIPs in Chinese Jianghan Basin crude oils. Their concurrence in a hypersaline lacustrine environment may need to be explored further for geochemical significance, including their source organisms and corresponding depositional environment in the sedimentary source rocks for hydrocarbon exploration.

2. GEOLOGICAL SETTING The Jianghan Basin (28 000 km2) is located in Hubei Province, eastern China (see the map in Figure 1). The Eocene sediments are divided into the Lower Eocene Xingouzui Formation (Ex), the Middle Eocene Jingsha Formation, the Upper Eocene Qianjiang Formation (Eq), and the Oligocene Jinghezhen Formation (Figure 1). Two important structural cycles because of thermal uplift and extension controlled the development of lacustrine sediments21 that were followed by two depressional stages/events, leading to deposition of the salt deposits of the Xingouzhui Formation and the Qianjiang Formation. The two stages of salt deposition occurred with a maximum thickness of 468 m in the Xingouzui Formation (Ex) and with a total thickness of >1000 m in the Qianjiang Formation (Eq), constituting two sets of source rocks for the petroleum hydrocarbons in the Jianghan Basin.22 The former is

3. EXPERIMENTAL SECTION 3.1. Samples. A total of 18 oils from the main oilfields, including Zhouji, Guanghua, Haokou, Haoxi, Wangchang, Guangbei, Zhongshi, Tankou, and Yajiao were analyzed, with 8 of them selected for the illustration of AIP distributions, shown as Figure 2, from their summed m/z 133 and 134 mass chromatograms. In these oils, asphaltenes were removed by precipitation with n-hexane followed by filtration. The deasphalted oils were then separated into saturated, aromatic, and polar (NSO) fractions using silica column chromatography with nhexane, benzene, and ethanol (EtOH) as eluting solvents sequentially. It should be noted that the target AIPs have a relatively low polarity B

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Figure 2. Summed m/z 133 + 134 mass chromatograms of the aromatic fractions of selected oil samples from different oilfields in Chinese Jianghan Basin, previously analyzed on the Micromass Platform II: (■) 2,3,6-AIPs and (▲) 2,3,4-AIPs. conditions of GC−MS analyses on Micromass Platform II and Thermo DSQ-II are detailed as follows: The Micromass Platform II mass spectrometer was coupled with a Hewlett-Packard 6890 gas chromatograph. Chromatographic separation was achieved with a 50 m × 0.32 mm inner diameter fused silica capillary column coated with a 0.25 mm film thickness of Chrompack DB-5 phase. The oven temperature was held at 85 °C for 1 min, then programmed from 85 to 290 °C at 3 °C/min, and held at 290 °C for

and may elute off in the saturated hydrocarbon fraction if the column is overloaded.24 The saturated and aromatic hydrocarbon fractions were subjected to gas chromatography−mass spectrometry (GC−MS) analysis. 3.2. GC−MS Analysis. The GC−MS analysis was originally performed on the Micromass Platform II. However, because of decommissioning of the original instrument, the repeated analysis was performed on the newly acquired Thermo DSQ-II. The experimental C

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Figure 3. Summed m/z 133 + 134 mass chromatogram of the aromatic fraction of the oil sample from Zhou 9 well (2589−2596 m, Eq4), recently analyzed on the Thermal DSQ-II, showing the distribution of mono- and diaromatic carotenoid derivatives: (■) 2,3,6-AIPs, (▲) 2,3,4-AIPs, and (◇) long-chain alkyl benzenes. The broad peak with M+ • at m/z 546 and characteristic m/z 134 ion showed possible co-elution of isorenieratane and renierapurpurane. 30 min. Helium was used as the carrier gas, with a flow rate of 1.0 mL/ min. The GC−MS transfer line temperature was at 250 °C. The ion source temperature was at 200 °C, which was operated in the electron impact (EI) mode with electron beam energy at 70 eV.

The Thermo DSQ-II mass spectrometer was coupled with a Trace Ultra GC. Separation was achieved using a 30 m × 0.32 mm inner diameter fused silica column coated with a 0.25 μm film thickness of DB-5 (Chrompack). The oven temperature was held at 65 °C for 1 D

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Figure 4. Mass spectra of the diaryl carotenoid precursors in the oil samples. min, then programmed to 295 °C at 3 °C/min, and held at 295 °C for 30 min. Helium was used as the carrier gas at 1.2 mL/min. The GC− MS transfer line temperature was at 290 °C. The ion source temperature was at 200 °C, which was operated at 70 eV EI mode to acquire full-scan mass spectra with a scan range of m/z 50−600.

small amounts of C24−C26 seen in a few samples. The most abundant member in each series is either C19 or C20. In comparison to the results from the Micromass Platform II in Figure 2, the AIPs with lower carbon numbers (C13−C17) appear to show a higher response in the new Thermo DSQ-II shown in Figure 3. This could be caused by the instrument response and column separation effect as previously pointed out.24 In addition, the reseparated saturate, aromatic, resin, and asphaltene (SARA) fractions were reanalyzed because the C40 precursors only appeared in a few samples. 4.2. C40 Aromatic Hydrocarbons. In the high retention time region of the m/z 133 + 134 mass chromatograms (Figure 3), β-isorenieratane (M+ • m/z 552), chlorobactane (M+ • m/z 554), and okenane (M+ • m/z 554) were found (Figure 4) and assigned on the basis of published spectra.18,25 In addition, an intense broad peak with M+ • at m/z 546, corresponding to C40 diaryl carotenoid derivatives and comprising several components, was also present. According to published reference spectra,14,15,26 these co-eluting components can be assigned as any of the following: isorenieratane, renieratane, renierapurpurane, or a 3,4,5-/2,3,6-diaryl isoprenoid. In comparison to the literature,18,25 our GC−MS results showed a reversed elution order for monocyclic (chlorobactane and okenane) and diaryl carotenoids (renieratane, isorenieratane, and renierapurpurane). Hence, we used different GC conditions and found that the difference in retention order can be resolved by increasing the flow rate of carrier gas from 1.2 to 1.9 mL/min, with the final column temperature at 305 °C. Also, a longer DB-5 column (60 m × 0.32 mm × 0.25 μm) was used on the DSQ-II for the repeated analysis. The shift of retention order (Figure 3) was confirmed by the corresponding mass spectra having distinct features and different molecular ions, as shown in Figure 4.

4. RESULTS The aromatic hydrocarbon fractions were analyzed by GC−MS originally using a Micromass Platform II coupled with HewlettPackard 6890 GC, with the results shown in Figure 2. Recently, we repeated the GC−MS analysis for specifically looking into the AIP precursors (C40 carotenoids) that yielded poor responses in previous GC−MS analysis by employing a new Thermo DSQ-II mass spectrometer coupled with Trace Ultra GC, with the results exhibited in Figure 3. 4.1. Distributions of AIPs. Upon 70 eV EI, all of the compounds of interest, i.e., containing trimethyl alkyl aryl groups, yield characteristic fragment ions at either m/z 133 or 134. Therefore, the summed mass chromatograms of m/z 133 and 134 are used to display the distributions of AIPs. The summed m/z 133 + 134 mass chromatograms of the aromatic fractions of 8 selected oils shown in Figure 2 reveal two series of AIPs in similar amounts in most of the oil samples. Their mass spectra are characterized by a base peak at m/z 133 or 134 and molecular ion peak (M+ •) at m/z 176 +14n, where n = 0−13, consistent with the general formula of CnH2n − 6 as a trimethylbenzene series. The absence or low abundance of the C17 and C23 members in the series are consistent with the branching points of irregular isoprenoid side chain with a tailto-tail linkage. According to previous reports,2,18,19 these compounds are assigned as 2,3,6- and 2,3,4-AIPs. Unlike most of the results reported in the literature, 2,3,6and 2,3,4-AIPs in our samples have similar abundances, with a slight predominance of 2,3,4-AIPs in most of the samples (Figure 2). The two series fell in the range of C13−C26, with E

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Figure 5. Palaeoenvironment salinity condition defined by Pr/Ph versus MTTCI (after Schwark et al.46 and Wang et al.61) and gammcerane index versus Pr/Ph (after Peng et al.52) for Jianghan oils.

4.3. Trimethyltridecyl Chromans (MTTCs). MTTCs can be found in the m/z 121 + 135 + 149 chromatograms of most samples. They are β-MTTC, γ-MTTC, δ-MTTC, and αMTTC, as reported in the literature.27 The major components are δ-MTTC and γ-MTTC, shown in Figure 3. 4.4. Long-Chain Alkybenzenes. Long-chain C26−C37 alkyl benzenes are present in several samples, as shown in Figure 3), with peaks of open diamonds (between 69 and 102 min), on the basis of their characteristic mass spectra of base peaks at m/z 92, together with lower concentrations of methyl alkylbenzenes (or alkyltoluenes with base peak at m/z 105) in the same carbon range.

Appendix in the Supporting Information). This means that okenone (χ ring) and C40 carotenoids, such as renieratene (χ,φcarotene) and renierapurpurin (χ,χ-carotene) with a χ ring as the end group, are likely sources for the 2,3,4-AIPs. However, the candidate precursors of diaryl C40 carotenoids with 2,3,4-trimethyl aryl structures, renieratene and renierapurpurin, were only reported in marine sponges.28,30,31 Because sponges are not capable of de novo biosynthesis of carotenoids, these two 2,3,4-trimethyl aryl compounds were, therefore, considered to be either derived from unknown sponge bacterial symbionts or generated by the sponge through modification of dietary carotenoids.18,28 Moreover, there are no reports of sponge fossils in source rocks of the Xingouzui and Qianjiang Formations in the Jianghan Basin.22 Thus, sponges cannot be responsible for the occurrences of 2,3,4-AIPs in the Jianghan oils. Although the detected co-eluting peak with M+ • = 546 (Figure 3) cannot provide direct evidence for the precursor and source organism of the 2,3,4-AIPs, the monoaromatic carotenoid compound of okenane, the final diagenetic product of okenone retaining the carbon skeleton, has been shown to be a molecular fossil unique to PSB (family of Chromatiaceae) and a precursor of 2,3,4-AIPs.18,19 The 2,3,4-AIPs are considered as the diagenetic product of okenone or, in a way, by C−C cleavage of renieratene and renierapurpurin.18,32 Thus, on the basis of detected chlorobactane and okenane, we attribute microbial source organisms of the GSB (family of Chlorobiaceae) and PSB (family of Chromatiaceae) to account for the co-occurrences of 2,3,6- and 2,3,4-AIPs in the Jianghan oils. In our samples, however, these two series are in similar

5. DISCUSSION 5.1. Precursors and Source Organisms of 2,3,6- and 2,3,4-AIPs. Chlorobactene, β-isorenieratene, and isorenieratene sourced from the GSB (family of Chlorobiaceae) have been widely accepted as the carotenoid precursors of 2,3,6AIPs.1,2,4−7,9,10,14−17 The 2,3,6-AIPs are thought to be formed from C−C bond cleavage at the isoprenoid side chains of chlorobactene, isorenieratene, and β-isorenieratene28 bound in high-molecular-weight fractions (e.g., kerogen) of the sediment.6,15 The 2,3,6-AIPs can be produced from β-carotene by the shift of a single methyl group upon aromatization.29 However, the diagenetic formation of 2,3,4-AIPs from β-carotene is highly unlikely because it would require a specific shift of two methyl groups.18 Thus, the precursor−product relationship of 2,3,4AIPs needs to be established from the specific methylation pattern of the χ ring, φ ring, and β ring, which is related to the corresponding end groups in their biogenic precursors (see the F

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Figure 6. Typical distributions of abundant phytane and gammacerane in the saturated fraction of Jianghan Basin crude oils and source rock extracts exhibited by the characteristic m/z 191 ion as a bottom trace, along with total ion chromatogram (TIC) for retention time reference. H, hopanes; G, gammacerane.

column and result in further stratification of anaerobic photosynthetic sulfur bacteria.40 This pattern of GSB existing beneath PSB is common in stratified lakes37,41 and prevalent in some stratified lakes, such as the Wintergreen and Burke Lakes in Michigan. It is worth noting that, besides the above symbiotic relationship of co-existing and/or beneath, in the photic zone, euxinia within the water column, GSB and PSB can be present with an endosymbiont relationship42 as “Chlorochromatium” aggregate.35,43,44 All of these scenarios help to explain the concurrence of 2,3,6- and 2,3,4-AIPs with similar amounts in the Jianghan oil samples. 5.3. Depositional Environmental Constraint. 5.3.1. Environmental Indicator Constraint. The Pr/Ph ratio has been widely used as an indicator of depositional redox conditions,45 with a value of >3 indicative of oxic to suboxic depositional condition and a value of