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Occurrence and distribution of unusual tricyclic and tetracyclic terpanes and their geochemical significance in some Paleogene oils from China Hong Xiao, Tie-Guan Wang, Meijun Li, Jian Fu, Youjun Tang, Shengbao Shi, Zhe Yang, and Xiaolin Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01025 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Occurrence and distribution of unusual tricyclic
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and tetracyclic terpanes and their geochemical
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significance in some Paleogene oils from China
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Hong Xiao,† T.-G. Wang,† Meijun Li,*,†,‡ Jian Fu,† Youjun Tang,‡
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Shengbao Shi,† Zhe Yang,† Xiaolin Lu†
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†
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University of Petroleum, Beijing 102249, China
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‡
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College of Resources and Environment, Yangtze University, Wuhan 430100, China
State Key Laboratory of Petroleum Resources and Prospecting, College of Geosciences, China
Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education,
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ABSTRACT: Two unusual tricyclic terpanes (compounds X and Y) and four tetracyclic terpanes
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(compounds X1, Y1, Z and Z1) have been detected in crude oils from the Pearl River Mouth,
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Beibuwan and Liaohe basins in China. Based on their elution order, relative retention times in m/z 191
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mass chromatograms and their diagnostic ion fragments, we have identified two tricyclic terpanes
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(marked by peaks X and Y in previous literature) as C21 and C25 tricyclic terpanes and four tetracyclic
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terpanes (marked by peaks X1, Y1, Z and Z1) as C24-des-A-lupane, C24-des-A-oleanane,
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C24-des-A-ursane and C27 tetracyclic terpane, respectively. These six compounds have similar
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characteristics to oleanane, ursane and lupane in their chemical structure and are considered likely to
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originate from alcohols or ketone precursors present in higher plants. The high abundance of these
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tricyclic and tetracyclic terpanes is probably related to the distinctive contribution of higher plant
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material in the organic matter content of source rocks. In addition, the redox conditions and water depth
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in the depositional environment impact significantly on distribution patterns of the compounds, and it is
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possible that they are readily formed under oxidation conditions. Because of their unique biological
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origin and prominent geochemical significance, they may have potential application in oil-source
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correlation and the classification of crude oil families.
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Keywords: tetracyclic terpane; tricyclic terpane; organic matter input; higher plant; depositional
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environment.
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1. INTRODUCTION
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Tricyclic and tetracyclic terpanes are present ubiquitously in crude oils and source rock extracts in
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various types of basins. They have come to be widely used in oil-oil and oil-source correlation
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studies1-7 and may be useful indicators for providing a variety of geological and geochemical
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information; such as constraining organic matter input,5, 8, 9 depositional paleoenvironment8 and thermal
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maturity.1, 10, 11
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Ourisson et al.(1982) reported that prokaryotic cell membranes could possibly be the source of
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tricyclic terpanes.12 Then, Aquino Neto et al.(1983) further proposed that high-abundance tricyclic
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terpanes compositions in crude oils may be associated with a higher contribution of marine algae.8 A
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series of tricyclic terpanes, including C19-C45 series, were identified in crude oils by gas
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chromatography–mass spectrometry (GC–MS) analysis for the first time13 and their distribution
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characteristics systematically investigated in a variety of depositional environments.8 Several papers
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have suggested that relatively high concentrations of C24 tetracyclic terpane may result from
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terrigenous organic matter input.5,
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abundances of C24 tetracyclic terpane may be indicative of a carbonate-evaporite environment.4, 15
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9, 14
In contrast, other studies have shown that relatively high
Due to their potential geochemical implications in organic geochemistry, more and more tricyclic
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and tetracyclic terpanes have been identified in source rock extracts and crude oils. Woolhouse et
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al.(1992) firmly identified a new series of C24 tetracyclic terpanes by tandem mass spectrometry in a
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number of Tertiary oils,16 which are suggested to originate from terrigenous source material.16-18 More
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recently, C24-des-A-oleanane and C24-des-A-lupane were detected in Holocene19 and Eocene source
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rocks.20 Novel C21 and C25 tricyclic terpanes and a C27 tetracyclic terpane were first discovered in oils
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from the Niger Delta, Assam (India) and Beaufort-Mackenzie (Canada) basins by Samuel et al.(2010),
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all of which were thought to originate from oleanoid precursors in angiosperm plants.21, 22
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Although the occurrence and distribution of conventional tricyclic and tetracyclic terpanes and
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tetracyclic terpanes has been reported in some domestic sedimentary basins, less work on these unusual
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tricyclic and tetracyclic terpanes has been done to date.23 Furthermore, previous studies have mainly
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focused on their structure identification and possible biological origins. The distribution, potential
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applications and other geochemical significant aspects of these unusual compounds in crude oils from
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different sedimentary basins have received less attention. By comparison of the characteristics of
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molecular ion peaks, diagnostic ion fragments, elution order and relative retention time,16, 21, 23 a total of
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six compounds have been detected by GC–MS (C21 tricyclic terpanes, C25 tricyclic terpanes, C27
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tetracyclic terpanes, C24-des-A-oleanane, C24-des-A-lupane and C24-des-A-ursane) in domestic crude
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oils from the Pearl River Mouth Basin, the Beibuwan Basin and the Liaohe Basin, China. Systematic
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geochemical studies of crude oils have sought to compare the distribution of these unusual tricyclic and
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tetracyclic terpanes in the three different basins and analyze the influence of terrestrial organic matter
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input, redox conditions and water depth on their content and distribution patterns.
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2. GEOLOGICAL SETTINGS
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The Pearl River Mouth Basin, developed on a metamorphic basement, is a typical Mesozoic-Cenozoic
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passive continental margin basin in the northern part of the South China Sea (Figure 1).24 It is
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composed principally of five sub-structural units. The Baiyun Sag, with an area of approximately
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20,000 km2, is located in the southern depression zone.25 Sedimentary strata have developed in eight
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formations from Paleogene to Quaternary which consist of the Shenhu, Wenchang, Enping, Zhuhai,
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Zhujiang, Hanjiang, Yuehai, and Wanshan formations.26 The Wenchang and Enping formations are the
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most important and productive source beds in the basin. The sediments of the Eocene Enping
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Formation, which mainly contains type III kerogen, are composed of the shallow lake mudstone and
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sandstone as well as swamp mudstone containing coal seam interlayers (Figure 2).23, 24, 26 The crude
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oils from the Pearl River Mouth Basin used in this study are derived from swamp facies source rock of
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the Enping formation in the Baiyun Sag.
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The Beibuwan Basin, which is located on the northern continental shelf of the South China Sea
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(Figure 1), is a typical Mesozoic-Cenozoic rift basin, divided into six depressions and three uplifts.27-29
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The Cenozoic sedimentary thickness in the basin is over 9,000 m on a basement of Paleozoic groups
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and Yanshanian granite rocks.30 In the Fushan Depression of the Beibuwan Basin, the dark mudstone
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and shale of the Eocene Liushagang Formation was identified in previous publications as the petroleum
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source bed and most of widely occurring crude oils in the sandstone reservoir of the third members of
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this formation are considered to be typical terrestrial-sourced oils.31-34 All selected crude oil samples
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from the basin derived from a single source kitchen in the Liushagang formation which contains type
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II- III kerogen with significant organic matter input from angiosperms (Figure 2).35, 36
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The Liaohe Basin lies in Liaoning province of northeast China and is adjacent to the Bohai Bay
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(Figure 1). It is a rift basin with a Cenozoic sedimentary thickness of more than 4,000 m, and the basin
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is roughly 470 kilometers long and about 65 kilometers wide.37, 38 The Paleogene strata are composed
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of the Fangshenpao, Shahejie and Dongying formations. The Eocene Shahejie Formation is considered
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to be the main source rock for oil and gas generation and is characterized by lacustrine shales with
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interbedded sandstone.38 More geological features of the petroleum system in the Liaohe Basin have
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been described in previously published papers.37, 39-41 All crude oil samples from the Liaohe Basin were
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selected from the Eocene Shahejie Formation which is a typical lacustrine source sock mainly
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containing type II kerogen (Figure 2).38, 42, 43
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Figure 1. Location map of indicating the studied basins in China.
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Figure 2. IH-Tmax diagram indicating the organic matter types of source rocks.
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3. SAMPLES AND EXPERIMENTS
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A total of 37 crude oil samples were collected from different basins for use in this study, including 13
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crude oils from swamp facies source rock in the Baiyun Sag of Pearl River Mouth Basin, 10 crude oils
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from fluvial-deltaic facies source rock in the Fushan depression of Beibuwan Basin, and 14 typical
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lacustrine crude oil samples from the Liaohe Basin.
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GC–MS analysis of the saturate hydrocarbon fractions of the crude oils was conducted on an
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Agilent 5975i GC–MS system equipped with an HP-5 MS fused-silica capillary column (60 m×0.25
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mm i.d. with a 0.25 µm film coating). The initial temperature of the GC–MS oven was 50℃, held for 1
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min, then stepped up to 120℃ at a rate of 20℃/min, then increased from 120℃ to 310℃ at a rate of
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3℃/min, and finally held at constant temperature for 25 min. The carrier gas was helium, with injector
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temperature set at a constant 300℃. The MS was operated by electron impact ionization at 70 eV, in
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full-scan mode with a scanning range of 50-600 Da.
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4. RESULTS AND DISCUSSION
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4.1. Detection and distribution. In this study, in addition to the C19 to C29 conventional tricyclic
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terpanes and a C24 tetracyclic terpane, a series of unusual tricyclic and tetracyclic terpanes, including
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compounds X, Y, Z, X1, Y1 and Z1, were identified by comparison of their relative retention times and
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elution order in m/z 191 mass chromatograms with those published in the literature (Figure 3).16, 21, 23
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Background subtracted mass spectrum of all six of these unusual components are shown in Figure 4. The
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compounds’ peaks X1, Y1 and Z, marked in Figure 3, are assigned as C24 tetracyclic terpanes with a
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molecular ion peak of M+. 330 (Figure 4). Both compounds Y1 and Z have similar mass spectrum which
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are composed of predominantly ion fragments at m/z 191, 177, 315, 109, 123, and relatively low intensity
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ion fragments at m/z 206, 136, 149, etc.(Figure 4). According to the relative retention times in m/z 191
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mass chromatograms and many characteristic fragments reported,16, 21, 23 compounds Y1 and Z have been
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identified as C24-des-A-oleanane and C24-des-A-ursane, respectively. However, the compound X1 as
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another C24 tetracyclic terpane, has a base peak at m/z 163 with a unique and intense ion at m/z 287. It was
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identified as C24-des-A-lupane, consistent with previously published literatures.16, 44
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In addition, mass spectrum of compound X shows a base peak at m/z 275 with a molecular ion
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peak at M+ 290 and diagnostic ion fragments at m/z 191, 177, 206, 139, 95, etc. which corresponds to
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C21 tricyclic terpane.21 Mass spectrum of both compound Y and Z1 show a base peak at m/z 191 with
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molecular ion peaks at M+. 346 and M+. 372, respectively, which have been identified as C25 tricyclic
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terpane and C27 tetracyclic terpane.21
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Figure 3. Representative partial m/z 191 mass chromatograms showing the distribution of the conventional and
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unusual tricyclic and tetracyclic terpanes (a-c) and pentacyclic triterpanes (d-f) in typical oils from Pearl River
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Mouth Basin, Beibuwan Basin, and Liaohe Basin, China.
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Figure 4. Mass spectra and molecular structures of (X1) 10β(H)-des-A-lupane, (Y1) 10β(H)-des-A-oleanane, (Z1)
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C27 tetracyclic terpane, (X) C21 tricyclic terpane, (Y) C25 tricyclic terpane and (Z) 10β(H)-des-A-ursane.
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4.2. Biological origin. In this study, 37 crude oil samples were analyzed by GC–MS. The unusual
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tricyclic and tetracyclic terpanes herein described were all detected. It is interesting to note that the
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relative concentration of these compounds vary in crude oils among the three basins (Table 1).
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Oleanane in crude oils or source rock extracts is thought to originate from angiosperms.3, 45 There
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is a similar trending change of the values of selected parameters using these unusual compounds and
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oleanane/[oleanane+C3017α(H), 21β(H) hopane] ratio among oils from these three basin oils (Figures 3
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and 5). Specifically, all selected parameters of these unusual compounds show the largest values in
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crude oil samples from the Pearl River Mouth Basin containing significant amounts of higher plant
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input. The Beibuwan Basin oil samples have a moderate value where the source organic matter
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contains a mixture of aquatic and terrigenous organic material.35, 36 The Liaohe Basin crude oils have
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the lowest values, characterized by predominance of aquatic (algal and bacterial) organic matter
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input.42, 43 Based on the analytical data, it can be assumed that differences in their distribution may
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depend on the proportion of terrestrial plant material input to total organic matter.
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Some triterpenoids are originated from 3-hydroxy components of higher plants, which has
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been identified in several sediments and crude oils as the indicators of higher plants input.17, 21 A
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series of 3-ketones are detected in a deltaic recent sediment with a large river, which are affected
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by continental largely.46 The 3-ketones (e.g. α-amyrenone, β-amyrenone, and lupeone) are
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common constituents of many higher plants, and also can be formed by oxidation of their
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corresponding alcohols (e.g. α-amyrin, β-amyrin, and betulin).17 The reaction processes of
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forming the 3,4-seco-triterpenoid acids may have occurred before deposition in the precursor plant
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material, since this type of acids has been detected in some higher plants.47 In addition, it also has
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been confirmed that the formation occurred through A-ring microbial degradation under oxidation
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condition as another possible pathway.48,
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previously known were oxidation products of the 3-ketones with an isopropyl in the position of
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C-5. Moreover, the 3,4-seco-olean-12-en-3-oic acid has been detected in immature sediments of
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Clarkia.50 Therefore, it could be proposed that the cleavage of ring A of triterpenes should undergo
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the evolution process from 3-alcohols (1, 8, 12) to 3-ketones (2, 9, 14), and to 3-acids (3, 10, 15)
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(Figure 6). The process may occur primarily in depositional stage or at an early stage of diagenesis
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in recent sediments under oxidizing conditions. The formation of des-A-triterpanes are attributed
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All naturally occurring 3.4-seco triterpene acids
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to further deacidification of 3-acid compounds (3, 10, 15), which may occur in a deep burial with
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reducing or anaerobic conditions. The reaction process of 3,4 cleavage also proposed to be based
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on the photochemical alteration of 3-oxygenated triterpenoids in natural environments.51 Figure 6
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shows our proposed reaction schemes for the studied compounds. For example, 10β(H)-des-A-oleanane
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(5), 10β(H)-des-A-lupane (16), 10β(H)-des-A-ursane (11) and C27 tricyclic terpane (4) could be
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transformed from alcohol and ketone precursors (Figure 6).17, 21, 52 Further degradation of ring B of C27
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tricyclic terpane (4) could yield C25 tricyclic terpane (6) and C21 tricyclic terpane (7). Therefore, based
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on their distribution patterns, their distinct structures characterization and the products of the
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transformation processes of biological molecules, it has been clearly shown that all these unusual
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tetracyclic terpanes originate from A-ring degradation of corresponding higher plants precursors
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(Figure 6). Selected parameters of the studied compounds in Table 1 can be used as biomarkers to
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indicate the inputs of terrestrial plants.
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It should be noted that the Z1/(Z1+C24TT) ratio cannot unambiguously distinguish crude oils
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from the Beibuwan Basin from those of the Pearl River Mouth Basin. The Z1/(Z1+C24TT) ratios for
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both of these oil families mainly range from 0.60 to 0.80 (Table 1). In m/z 191 mass chromatograms
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(Figure 3), the compound Z1 has a shoulder peak, indicating some other unknown compound may
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co-elute with it. According to previous studies, the two-dimensional gas chromatography/time-of-flight
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mass spectrometry (GCxGC/ToFMS) or GC-MS/MS techniques could applied to solve this kind of
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problem of compounds co-elution.53-55
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Figure 5. Cross plots to demonstrate the correlation between the relative abundance of compound Y1, X, Y and Z1
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and oleanane/[oleanane+C30 17α(H), 21β(H) hopane] ratio.
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Figure 6. Proposed biosynthetic reaction schemes for the formation of the studied tricyclic and tetracyclic
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terpanes.
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4.3. Depositional environment. Three types of crude oils derived from source rocks with
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different sedimentary facies in the Pearl River Mouth Basin, Beibuwan Basin and Liaohe Basin, were
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collected and geochemically analyzed. A series of biomarker parameters of analyzed oils are shown in
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Table 1. Typically, the Pr/Ph ratio (relative abundance of pristane to phytane) is widely used as an
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indicator of a paleo-depositional environment for redox conditions during sedimentation and
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diagenesis.56-58 Usually, high Pr/Ph ratios (>3.0) represent oxic environments (eg. coal and peat swamps)
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with organic matter originating predominantly from higher plants. Low Pr/Ph ratios (
