# Hopanoid breakdown products (HBPs) and their influence on tetracyclic and tricyclic terpane (cheilanthane) oil-source rock parameters

Tharika Liyanage, Lennart M van Maldegem, Janet Hope, Philippe Schaeffer, Pierre Adam, Hans Peter Nytoft, Dianne Edwards, Jochen Brocks

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Tharika Liyanage, Lennart M van Maldegem, Janet Hope, Philippe Schaeffer, Pierre Adam, et al.. Hopanoid breakdown products (HBPs) and their influence on tetracyclic and tricyclic terpane (cheilanthane) oil-source rock parameters. Organic Geochemistry, 2025, 210, pp.105085. 10.1016/j.orggeochem.2025.105085. hal-05379084

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# Hopanoid breakdown products (HBPs) and their influence on tetracyclic and tricyclic terpane (cheilanthane) oil-source rock parameters

Tharika Liyanage<sup>a,b,*</sup> <sup>D</sup>, Lennart M. van Maldegem<sup>a,c,D</sup>, Janet Hope<sup>a</sup>, Philippe Schaeffer<sup>d</sup>, Pierre Adam<sup>d,‡</sup>, Hans Peter Nytoft<sup>e</sup>, Dianne Edwards<sup>f</sup>, Jochen Brocks<sup>a,*</sup> <sup>D</sup>

## ARTICLE INFO

John Volkman - Associate Editor

**Keywords:**

Hopanoid breakdown products

Tricyclic terpanes

Tetracyclic terpane

Oil-source rock parameters

Terrigenous indicators

Terrestrial indicators

## ABSTRACT

Tricyclic and tetracyclic terpane ratios are used as biomarker proxies for depositional environments and to classify crude oils and source rocks. However, the biological precursors and diagenetic drivers of these proxies are not well understood. This study reports on the discovery of tricyclic and tetracyclic breakdown products of pentacyclic hopanoids that co-elute with 14α-C₁₉ and βα-C₂₀–C₂₂ cheilanthanes and C₂₄ tetracyclic terpane in gas chromatograms. This includes hopanoid breakdown products (HBPs) that are identical to 14α-C₁₉ cheilanthane and C₂₄ tetracyclic terpane, biomarkers that are commonly used as indicators of terrigenous organic matter in source rocks and their derived hydrocarbon products. These HBPs were generated by pyrolysis of C₃₀ diplopterol, a common compound found in the cell membranes of almost all hopanoid-producing bacteria, some lichens and higher plants. HBPs are common in rock-extracted bitumens and crude oils from the Neoproterozoic to the Cenozoic and across a wide range of thermal maturities. Comparison of the relative abundance of HBPs and cheilanthanes against commonly used tricyclic and tetracyclic terpane parameters shows that HBPs can drive or strongly influence terpane-based paleoenvironmental proxies. This study begins to rationalise why some of these parameters indicate specific depositional environments and proposes new ratios that quantify the impact of HBPs on biomarker proxies.

## 1. Introduction

Cheilanthanes are hydrocarbon molecular fossils with no known biological sources and precursor molecules. Found in most sediment-extracted bitumens (referred herein as bitumen) and crude oils, they comprise a C₁₉ to at least C₅₄ pseudo-homologous series of tricyclic terpanes. Cheilanthanes were first discovered in bitumens from the Green River Formation (Anders and Robinson, 1971; Gallegos, 1971) and have been reported widely in a variety of depositional environments from different geological ages (Aquino Neto et al., 1983; Peters et al., 2005a). Stereocentres at C-13 and C-14 lead to four configurational stereoisomers – 13α(H), 14α(H); 13α(H), 14β(H); 13β(H), 14α(H); and 13β(H), 14β(H) (Supplementary Fig. S1). The βα configuration is the

predominant and most commonly reported isomer (Chicarelli et al., 1988; Tuo et al., 1999).

Despite cheilanthanes having unknown biological origins and precursor biomolecules, they are used in oil-source rock correlation studies and paleoenvironmental reconstructions. Early statistical analyses of cheilanthane distributions in global databases of crude oils suggested that the abundance of some pseudohomologues correlates strongly with certain environmental settings (Zumberge, 1981, 1987). These studies revealed trends linking near-shore marine settings with abundant C₁₉ and C₂₀ homologues, high proportions of C₂₁ cheilanthanes in lacustrine crude oils, and deeper marine source rocks were identified by abundant C₂₄ and C₂₅ homologues. Based on these correlations, proxies were developed to differentiate and classify oils (Peters et al., 2005a; Tao

<footnote><sup>a</sup> Research School of Earth Sciences, The Australian National University, 142 Mills Rd, Acton, Canberra, Australia</footnote>

<footnote><sup>b</sup> Doerr School of Sustainability, Stanford University, 367 Panama Mall, Stanford, CA, USA¹</footnote>

<footnote><sup>c</sup> Institute for Alpine and Arctic Research, University of Colorado, 4001 Discovery Dr, Boulder, CO, USA¹</footnote>

<footnote><sup>d</sup> Université de Strasbourg, CNRS, Institut de Chimie de Strasbourg UMR 7177, F-67000 Strasbourg, France</footnote>

<footnote><sup>e</sup> Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark</footnote>

<footnote><sup>f</sup> Geoscience Australia, 101 Jerrabomberra Ave, Symonston, Canberra, Australia</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: tharika1@stanford.edu (T. Liyanage), jochen.brocks@anu.edu.au (J. Brocks).</footnote>

<footnote n="1">¹ Present address.</footnote>
<table><caption>**Table 1**

Lithological description of Aralka Formation samples from drill hole BR05DD01.</caption><thead><tr><td>Sample ID</td><td>Top depth of core (m)</td><td>Lithology description</td><td>Carbonate content (HCl digestion, wt%)</td></tr></thead><tbody><tr><td>LvM-030</td><td>163.7</td><td>Grey carbonate/siltstone</td><td>23</td></tr><tr><td>LvM-032</td><td>179.8</td><td>Grey carbonate/siltstone</td><td>23</td></tr><tr><td>LvM-034</td><td>240</td><td>Grey carbonate/siltstone</td><td>31</td></tr><tr><td>LvM-039</td><td>323.3</td><td>Grey carbonate/siltstone</td><td>41</td></tr><tr><td>LvM-043</td><td>371.6</td><td>Grey carbonate/siltstone</td><td>28</td></tr><tr><td>LvM-051</td><td>440.7</td><td>Dark grey carbonate/siltstone</td><td>21</td></tr><tr><td>LvM-060</td><td>449.4</td><td>Dark grey carbonate/siltstone</td><td>19</td></tr><tr><td>LvM-067</td><td>457.2</td><td>Dark grey carbonate/siltstone</td><td>20</td></tr><tr><td>LvM-071</td><td>461</td><td>Dark grey carbonate/siltstone</td><td>26</td></tr><tr><td>LvM-080</td><td>471</td><td>Dark grey carbonate/siltstone</td><td>20</td></tr><tr><td>LvM-085</td><td>476</td><td>Dark grey carbonate/siltstone</td><td>21</td></tr><tr><td>LvM-086</td><td>476.7</td><td>Dark grey carbonate/siltstone</td><td>25</td></tr><tr><td>LvM-088</td><td>478</td><td>Dark grey carbonate/siltstone</td><td>21</td></tr><tr><td>LvM-094</td><td>481.9</td><td>Dark grey carbonate/siltstone</td><td>61</td></tr></tbody></table>

et al., 2015). However, the biological, environmental, and diagenetic influences on these proxies are not understood.

Tetracyclic terpanes are another group of biomarkers used as proxies for depositional environments, forming a C₂₄–C₂₇ hydrocarbon pseudo-homologous series (Supplementary Fig. S1) (Trendel et al., 1982; Aquino Neto et al., 1983). High relative abundances of C₂₄ tetracyclic terpane have been observed in carbonate and evaporitic source rocks (Clark and Philp, 1989; Connan et al., 1986; Palacas et al., 1984) and coal-rich shales that contain higher plant remains (Philp and Gilbert, 1986) and are therefore used in biomarker parameters to distinguish these environments. Several hypotheses have been put forward to explain their formation, including either catalytic cleavage or microbial oxidation of the E-ring of pentacyclic hopanoids, incomplete cyclisation reactions of squalene, and the occurrence of specific microbial precursors (Trendel et al., 1982; Aquino Neto et al., 1983). Similar to the cheilanthanes, the underlying influences on tetracyclic terpane parameters are not well understood.

To test the hypothesis that some tetracyclic and tricyclic terpanes may form by the degradation of pentacyclic hopanoids, we conducted closed-system pyrolysis experiments on diploptерol, a common C₃₀ biohopanoid found in bacteria and some plants (Rohmer et al., 1984; Inayama et al., 1989). This study explores the pyrolysis products of C₃₀ diploptерol that coelute, or closely elute, with cheilanthanes and tetracyclic terpanes on commonly used GC capillary columns, and their potential impact on proxy ratios.

## 2. Materials and methods

### 2.1. Geological setting of natural samples

#### 2.1.1. Aralka Formation

The Aralka Formation is a Cryogenian, post-Sturtian unit conformably overlying the glacial Areyonga Formation, in the Amadeus Basin, central Australia. Re-Os dating of the base of the Aralka Formation revealed an age of 657 ± 5.4 Ma (Kendall et al., 2006). This age is comparable with Re-Os dating of glacial and interglacial units in Zambia, Canada, and Mongolia, constraining the termination of the Sturtian Snowball Earth period to 659 ± 4.5 Ma (Rooney et al., 2015). The Aralka Formation consists of interbedded siltstones and mudstones deposited in a relatively deep intracratonic marine basin (Ambrose et al., 2010; Grey et al., 2012; Allen et al., 2018). Samples for biomarker analysis (Table 1) come from the drill core BR05DD01 (Northern Territory Geological Survey, 24°26'60" S, 130°22'48" E).

#### 2.1.2. Oils of Australia

Sixty-four whole oils were sub-sampled from the Repository, Geoscience Australia, herein referred to as the Oils of Australia collection (Table 2). Samples were collected from a range of inferred depositional

environments, source ages, maturities, and biodegradation levels, previously determined in The Oils of Eastern Australia and The Oils of Western Australia volumes (Summons et al., 1996, 2002; Edwards and Zumberge, 2005).

### 2.2. Diploptерol and cheilanthane standards synthesis

C₃₀ diploptерol was provided by Hans Peter Nytoft (Geological Survey of Denmark and Greenland, Copenhagen), and its synthesis has been described elsewhere (Nytoft and Bojesen-Koefoed, 2001). Synthetic standards of 13β-C₁₉ and βα-C₂₀-C₃₀ cheilanthanes (Heissler et al. 1984) were obtained from the Laboratory of Molecular Biogeochemistry at the University of Strasbourg. All synthetic standards were stored at 4 °C.

### 2.3. Controls for hydrocarbon contamination

Glassware was baked (300 °C, 9 h) before use. Pre-combusted quartz sand (500 °C, 12 h) was used to perform accumulative procedural system blanks to identify and quantify any systematic sources of contamination in the laboratory. The blanks were processed and analysed in parallel with samples, covering the entire procedure from rock crushing to instrumental analysis.

### 2.4. Pyrolysis experiments

Silica glass pyrolysis tubes (6 mm i.d., 25 cm long, Choice Analytical) were cut to 15 cm in length, flame sealed on one end, then combusted (300 °C, 9 h). An aliquot (1.5 mg) of synthetic diploptерol was transferred in minimal dichloromethane (DCM, solvent grade 99.8 %, Ajax Finechem) to a pyrolysis tube. Once the DCM was evaporated, the tube was flushed with N₂ gas and evacuated 5 times on a vacuum line and then flame sealed. The sealed tube was pyrolysed at 400 °C for 10 h. The pyrolysate was extracted from the tube using 5 mL of DCM:Methanol (1:1, v/v) solution (MeOH, solvent grade 99.9 %, Optima®, Fisher Scientific). Solvents were evaporated under a gentle stream of N₂ gas. Hydrogenation was undertaken by re-dissolving the pyrolysate in 3 mL n-hexane (solvent grade 99.5 % Ajax Finechem) and bubbling H₂ through the solution for 5 min. Next, 0.2 mg of platinum (IV) oxide on charcoal (PtO₂ 10 % on carbon, dry, Alfa Aesar) was added while stirring, followed by another 10 min of H₂ bubbling. The H₂ supply was removed, and the sample was tightly capped, covered in aluminium foil, and stirred for 16 h. The PtO₂ catalyst was removed by filtration over a short silica gel chromatography microcolumn, and the hydrogenated fraction was collected using 3 mL n-hexane and then 3 mL DCM as eluents. The hydrogenated pyrolysate was not fractionated before GC-MS analysis, in accordance with petroleum industry screening methods.
<table><caption>**Table 2**

Description of oil samples from the Oils of Australia collection examined in this study.</caption><thead><tr><th>Sample ID</th><th>Basin/region</th><th>Well</th><th>Well No./Test <sup>a</sup></th><th>Inferred source rock age</th><th>Inferred depositional environment <sup>b</sup></th><th>Maturity assessment <sup>c</sup></th><th>Biodegradation assessment <sup>d</sup></th><th>Ref</th></tr></thead><tbody><tr><td colspan="9"><strong>Paleogene (23–66 Ma) <sup>f</sup></strong></td></tr><tr><td>179</td><td>Gippsland</td><td>Fortescue</td><td>2; RFT 1</td><td>Eocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ <sup>f</sup></td></tr><tr><td>147</td><td>Gippsland</td><td>Cobia</td><td>1; FIT 2</td><td>Eocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>151</td><td>Gippsland</td><td>Fortescue</td><td>3; RFT 1</td><td>Eocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>148</td><td>Gippsland</td><td>Cobia</td><td>2; RFT 4</td><td>Paleocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>146</td><td>Gippsland</td><td>Halibut</td><td>A10; DST</td><td>Paleocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>175</td><td>Gippsland</td><td>Halibut</td><td>A11; DST</td><td>Paleocene/Upper Cretaceous</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td colspan="9"><strong>Cretaceous (66–143 Ma)</strong></td></tr><tr><td>10269</td><td>Bonaparte/ Flamingo High</td><td>Elang</td><td>1; DST 1</td><td>Jurassic/Lower Cretaceous</td><td>Distal Marine Shale</td><td>Higher Maturity</td><td>Nondegraded</td><td>WOZ2 <sup>g</sup></td></tr><tr><td>10270</td><td>Bonaparte/ Flamingo High</td><td>Elang</td><td>2; DST 1B</td><td>Jurassic/Lower Cretaceous</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Higher Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>190</td><td>Perth</td><td>Gage Roads</td><td>1; DST 1A/2A</td><td>Upper Jurassic- Lower Cretaceous</td><td>Lacustrine Fresh</td><td>Lower Maturity</td><td>Nondegraded</td><td>WOZ <sup>h</sup></td></tr><tr><td>224</td><td>Northern Carnarvon</td><td>Rough Range</td><td></td><td>Cretaceous</td><td>Lacustrine Fresh</td><td>Lower Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>20029023</td><td>Northern Carnarvon</td><td>Parrott Hill</td><td>1; DST 1</td><td>Lower Cretaceous</td><td></td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td colspan="9"><strong>Jurassic (145–201 Ma)</strong></td></tr><tr><td>508</td><td>Bonaparte/Vulcan</td><td>Challis</td><td>1; PT</td><td>Upper Jurassic</td><td>Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>507</td><td>Bonaparte/Vulcan</td><td>Jabiru</td><td>1A; PT2</td><td>Upper Jurassic</td><td>Marine Shale</td><td>Higher Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>888</td><td>Bonaparte/Vulcan</td><td>Skua</td><td>9 ST1; Completion 1</td><td>Upper Jurassic</td><td>Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>894</td><td>Carnarvon/ Dampier</td><td>Cossack</td><td>1; PT</td><td>Upper Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>642</td><td>Carnarvon/ Dampier</td><td>Cossack</td><td>1; PT</td><td>Upper Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>399</td><td>Carnarvon/Barrow</td><td>Barrow Island</td><td>15; PT</td><td>Upper Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Polyhistory</td><td>WOZ</td></tr><tr><td>166</td><td>Carnarvon/Barrow</td><td>Barrow Island</td><td>17</td><td>Upper Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Polyhistory</td><td>WOZ</td></tr><tr><td>349</td><td>Carnarvon/Barrow</td><td>North Herald</td><td>3; DST 1</td><td>Upper Jurassic</td><td>Marine Marl</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>89</td><td>Carnarvon/ Dampier</td><td>Legendre</td><td>1; DST 2</td><td>Upper Jurassic</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>524</td><td>Bonaparte/Vulcan</td><td>Puffin</td><td>2; RFT 4</td><td>Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>416</td><td>Bonaparte/Vulcan</td><td>Swan</td><td>1; WFT 1</td><td>Jurassic</td><td>Paralic-Deltaic Marine Shale/Marine Carbonate</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>887</td><td>Bonaparte/Vulcan</td><td>Swan</td><td>3; RFT 3/4</td><td>Jurassic</td><td>Marine Shale</td><td>Moderate maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>457</td><td>Carnarvon/ Dampier</td><td>Angel</td><td>2; DST 1A</td><td>Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>101</td><td>Carnarvon/ Dampier</td><td>Angel</td><td>3; DST 3</td><td>Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>886</td><td>Carnarvon/ Dampier</td><td>Lambert</td><td>1; DST 1A</td><td>Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>19999642</td><td>Carnarvon/Barrow</td><td>Bennet</td><td>1</td><td>Jurassic</td><td>Coal-Resinitic</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>19999650</td><td>Carnarvon/ Peedamullah Shelf</td><td>Tubridgi</td><td>2; PT</td><td>Jurassic?</td><td>Marine Carbonate</td><td>Moderate Maturity</td><td>Heavy Biodegradation</td><td>WOZ2</td></tr><tr><td>20009172</td><td>Carnarvon/ Dampier</td><td>Saffron</td><td>1; MDT</td><td>Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Higher Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>10094</td><td>Cooper/Eromanga</td><td>Tickalara</td><td>7; DST 3</td><td>Jurassic</td><td></td><td></td><td></td><td>EOZ</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 2 (continued)**</caption><thead><tr><th>Sample ID</th><th>Basin/region</th><th>Well</th><th>Well No./Test <sup>a</sup></th><th>Inferred source rock age</th><th>Inferred depositional environment <sup>b</sup></th><th>Maturity assessment <sup>c</sup></th><th>Biodegradation assessment <sup>d</sup></th><th>Ref</th></tr></thead><tbody><tr><td>880</td><td>Carnarvon/Beagle</td><td>Nebo</td><td>1; DST 1</td><td>Middle Jurassic</td><td>Lacustrine Fresh</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>460</td><td>Bonaparte/Sahul Platform</td><td>Troubadour</td><td>1; DST 2</td><td>Lower-Middle Jurassic</td><td>Marine Carbonate</td><td>Higher Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>411</td><td>Perth</td><td>Walyering</td><td>2; PT 4</td><td>Lower-Middle Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>10326</td><td>Bonaparte/Vulcan</td><td>Montara</td><td>2; RFT</td><td>Lower-Middle Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>19999351</td><td>Bonaparte/Sahul Platform</td><td>Sunrise</td><td>2; DST 1</td><td>Lower-Middle Jurassic</td><td></td><td>Condensate/ Light oil</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>19999352</td><td>Bonaparte/Sahul Platform</td><td>Sunrise</td><td>2; DST 2A</td><td>Lower-Middle Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td></td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td colspan="9"><strong>Triassic (201–251 Ma)</strong></td></tr><tr><td>74</td><td>Carnarvon/N. Rankin Platform</td><td>North Rankin</td><td>4; DST 1</td><td>Triassic/Lower-Middle Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Condensate/ Light oil</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>891</td><td>Carnarvon/N. Rankin Platform</td><td>North Rankin</td><td>A; PT</td><td>Triassic/Lower-Middle Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>10059</td><td>Carnarvon/ Dampier</td><td>Venture</td><td>1</td><td>Triassic/Lower-Middle Jurassic</td><td>Distal Marine Shale</td><td>Higher Maturity</td><td>Polyhistory</td><td>WOZ2</td></tr><tr><td>19999818</td><td>Carnarvon/ Dampier</td><td>Mutineer</td><td>1B; MDT Run 1/1</td><td>Triassic/Lower-Middle Jurassic</td><td>Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>20019008</td><td>Carnarvon/Barrow</td><td>Wonnich</td><td>1; DST 1</td><td>Triassic/Lower-Middle Jurassic</td><td>Distal Marine Shale/ Paralic-Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>405</td><td>Perth</td><td>Dongara</td><td>14</td><td>Lower Triassic</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>485</td><td>Perth</td><td>Erregulla</td><td>1; Swab Test</td><td>Lower Triassic</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>409</td><td>Perth</td><td>Mount Horner</td><td>1; DST 4</td><td>Lower Triassic</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>47</td><td>Perth</td><td>Yardarino</td><td>1; DST 3</td><td>Lower Triassic</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ</td></tr><tr><td>19999687</td><td>Perth</td><td>East Lake Logue</td><td>1; DST 1</td><td>Lower Triassic</td><td>Marine Carbonate/Coal-Resinitic</td><td></td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>27</td><td>Cooper/Eromanga</td><td>Coonatie</td><td>1; DST 1</td><td>Triassic</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td colspan="9"><strong>Permian (251–298 Ma)</strong></td></tr><tr><td>10331</td><td>Perth</td><td>Whicher Range</td><td>4;</td><td>Permian</td><td>Distal Marine Shale/ Paralic Deltaic Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td>WOZ2</td></tr><tr><td>26</td><td>Cooper/Eromanga</td><td>Gidgealpa</td><td>3; DST 5</td><td>Permian</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>23</td><td>Cooper/Eromanga</td><td>Kanowana</td><td>1; PT 1</td><td>Permian</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>394</td><td>Cooper/Eromanga</td><td>Gidgealpa</td><td>2; PT</td><td>Permian</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>24</td><td>Cooper/Eromanga</td><td>Chandos</td><td>1; DST 14</td><td>Permian</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td>22</td><td>Cooper/Eromanga</td><td>Kanowana</td><td>1; PT 3</td><td>Permian</td><td></td><td></td><td></td><td>EOZ</td></tr><tr><td colspan="9"><strong>Carboniferous (298–358 Ma)</strong></td></tr><tr><td>19999123</td><td>Canning</td><td>Sundown</td><td>2</td><td></td><td></td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td>20139103</td><td>Canning</td><td>Ungani</td><td>2</td><td>Carboniferous</td><td>Marine + Terrestrial (?)</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td>19999124</td><td>Canning</td><td>West Terrace</td><td>1</td><td>Carboniferous</td><td>Marine + Terrestrial</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td>20149693</td><td>Canning</td><td>West Terrace</td><td>1</td><td>Carboniferous</td><td>Marine + Terrestrial</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td>19999122</td><td>Canning</td><td>Sundown</td><td>2</td><td>Carboniferous</td><td>Marine + Terrestrial</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td colspan="9"><strong>Devonian (358–419 Ma)</strong></td></tr><tr><td>19999118</td><td>Canning</td><td>Blinka</td><td>1</td><td>Upper Devonian</td><td>Marine</td><td>Moderate Maturity</td><td>Nondegraded</td><td></td></tr><tr><td>20009019</td><td>Canning</td><td>Wattle</td><td>ST1</td><td>Upper Devonian</td><td>Distal Marine Shale</td><td>Moderate Maturity</td><td>Nondegraded</td><td></td></tr><tr><td>19999117</td><td>Canning</td><td>Blinka</td><td>1</td><td>Upper Devonian</td><td>Marine</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 2 (continued)**</caption><thead><tr><th>Sample ID</th><th>Basin/region</th><th>Well</th><th>Well No./Test <sup>a</sup></th><th>Inferred source rock age</th><th>Inferred depositional environment <sup>b</sup></th><th>Maturity assessment <sup>c</sup></th><th>Biodegradation assessment <sup>d</sup></th><th>Ref</th></tr></thead><tbody><tr><td>19999119</td><td>Canning</td><td>Blinka</td><td>4</td><td>Devonian</td><td>Marine</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td>20149689</td><td>Canning</td><td>Blinka</td><td></td><td>Devonian</td><td>Marine</td><td></td><td></td><td>(Spaak et al., 2020)</td></tr><tr><td colspan="9"><strong>Ordovician (419–486 Ma)</strong></td></tr><tr><td>678</td><td>Canning</td><td>Pictor</td><td>2: PT</td><td>Ordovician</td><td>Distal Marine Shale</td><td>Condensate/ Light oil</td><td>Nondegraded</td><td>WOZ</td></tr></tbody></table>

<sup>a</sup> DST – Drill Stem Test; FIT – Formation Interval Test; PT – Production Test; RFT – Repeat Formation Test; WFT – Wireline Formation Tester.

<sup>b</sup> Inferred depositional environment based on previous studies referenced.

<sup>c</sup> Maturity. 'Lower maturity' oils were generated within the early oil window (0.6>%VR < 0.7), 'moderate maturity' oils within the main oil window (0.8%VR), and 'higher maturity' oils within the upper oil window (0.9>%VR < 1.2).

<sup>d</sup> Biodegradation levels correspond to the following scale: Mild = 2–4, heavy = 5–7, severe = 8–10. Polyhistory describes reservoirs that have experienced several phases of biodegradation and recharge (Peters and Moldowan, 1993).

<sup>e</sup> Age ranges for geological periods are based on the International Commission on Stratigraphy's International Chronostratigraphic Chart v2024/12 (Cohen et al., 2013).

<sup>f</sup> EOZ – Oils of Eastern Australia (Summons et al., 2002).

<sup>g</sup> WOZ2 – Oils of Western Australia II (Edwards and Zumberge, 2005).

<sup>h</sup> WOZ – Oils of Western Australia (Summons et al., 1996).

## 2.5. Biomarker analyses of geological samples

A total of 14 samples from the Aralka Formation (Table 1) were analysed from the BR05DD01 drill core. Exterior/interior (E/I) experiments were carried out as described in Jarrett et al. (2013) and Brocks et al. (2017) to assess biomarker syngeneity. Briefly, a diamond wafering saw (Buehler Isomet 1000; blade thickness 340 or 460 µm) was used to remove at least 1 mm of the rock exterior. The blade was thoroughly cleaned with MeOH and DCM before use and between samples to prevent cross-contamination. Deionised water (DI) was used to lubricate the blade while cutting. The exterior and interior rock portions were crushed to >240 mesh powder using a stainless-steel puck mill (Rock-labs Ltd, New Zealand). Rock powders were extracted with DCM: MeOH (9:1, v/v) using a Dionex Accelerated Solvent Extractor 200. Total lipid extracts (TLEs) were fractionated using microcolumn chromatography over activated (290 °C, 4 h), dry-packed silica gel (Silica Gel 60; 230–600 mesh; EM Science). The saturated hydrocarbon fraction was collected with 1.5 dead volume (DV) of n-hexane.

Sample details, previous results, and interpretation for the Oils of Australia collection are described elsewhere (Summons et al., 1996, 2002; Edwards and Zumberge, 2005). In this study, whole oil sub-samples were fractionated using microcolumn chromatography, and the saturated hydrocarbon fraction was collected with 1.5 DV of n-hexane.

## 2.6. Gas Chromatography–Mass Spectrometry (GC–MS)

Gas chromatography–mass spectrometry (GC–MS) analyses were carried out on an Agilent 6890 GC coupled to a Micromass Autospec Premier double sector MS (Waters Corporation). The gas chromatograph was equipped with either a 60 m DB-5 MS or DB-1 MS capillary column (both 0.25 mm inner diameter, 0.25 µm film thickness, Agilent JW Scientific, USA), and helium was used as the carrier gas at a constant flow rate of 1 mLmin⁻¹. Samples were injected in splitless mode into a Gerstel PTV injector at 60 °C (held for 0.1 min) and heated at 260 °C min⁻¹ to 300 °C. The MS source was operated at 260 °C in electron ionisation mode at 70 eV ionisation energy and 8000 V acceleration voltage. For full-scan and metastable reaction monitoring (MRM) analyses, the GC oven was programmed from 60 °C (held for 4 min) to 315 °C at 4 °C min⁻¹, with a total run time of 100 min. All samples were injected in n-hexane to avoid deterioration of chromatographic signals by FeCl₂ build-up in the MS ion source through the use of halogenated

solvents (Brocks and Hope, 2014).

Cheilanthanes were identified based on their retention times and mass spectra in comparison to synthesised cheilanthane standards (Heissler et al., 1984) and the Australian Geological Survey Organisation (AGSO) Oil standard, a mixture of oils used for biomarker identification (Norgate and Murray, 2017). The relative proportions of cheilanthanes and hopanoid breakdown products (HBPs) were quantified in MRM transitions M⁺>191 and 262 > 123. Biomarkers were quantified relative to an internal standard (25 ng D₄-C₂₉-ααα-ethylcholestane; Chiron Laboratories AS) without correcting for differential response factors. Co-injection experiments were carried out with the diplopterol pyrolysate and synthetic standards of 13β-C₁₉ and βα-C₂₀ to C₃₀ cheilanthanes on DB-5 MS and DB-1 MS GC columns.

# 3. Results

## 3.1. Detection of Hopanoid Breakdown Products (HBPs)

C₂₇ to C₃₀ hopanes were detected in the diplopterol pyrolysate (Supplementary Fig. S2), demonstrating that C–C cleavage occurred during pyrolysis. Multiple isomers were observed, including αβ-, βα-, and ββ-hopanes. C₃₀ hop-17(21)-ene was identified based on the diagnostic ions *m/z* 410, 367, 231, 191, 161, and 135 (e.g., Sessions et al., 2013). Several saturated and unsaturated cleavage products of diplopterol, subsequently referred to as hopanoid breakdown products (HBPs), were detected in the diplopterol pyrolysate and had similar retention times to βα-cheilanthanes on DB-5 MS and DB-1 MS GC columns.

HBP-I (Fig. 1C,D) was identified in the MRM 262 > 191 transition. It has an identical elution time to 14α-C₁₉ cheilanthane (Fig. 1A,B) on both DB-5 MS and DB-1 MS GC columns, and the recorded mass spectra are almost identical (Fig. 1E,F). Based on these results, and the assumption that HBP-I was generated by D-ring cleavage of diplopterol, we posit that the structure of HBP-I is identical to the compound commonly identified as ‘14α-C₁₉ cheilanthane’ (Fig. 1E,F: inset molecules).

HBP-II coelutes with 13β-C₁₉ cheilanthane (Fig. 2A,C,E) on a DB-5 MS GC column. However, HBP-II elutes later than 13β-C₁₉ cheilanthane on a DB-1 MS GC column (Fig. 2B,D,F). The mass spectrum of 13β-C₁₉ cheilanthane (Fig. 2G) is distinct from HBP-II (Fig. 2H). HBP-II has a molecular ion at *m/z* 278 and a base peak ion at *m/z* 123. The mass spectrum of HBP-II indicates that the pyrolysis product is most likely a bicyclic C₂₀ compound. We speculate that HBP-II forms from diplopterol
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (A–D) MRM transition 262 > 191. (A) Elution times of 14α-C₁₉ cheilanthane in the AGSO standard on DB-5 MS and (B) DB-1 MS GC columns. (C) Elution times of HBP-I in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) Mass spectrum of 14α-C₁₉ cheilanthane from a synthetic standard and inset molecular structure. (F) Mass spectrum of HBP-I generated in 10 h, 400 °C diploptерol pyrolysate. Based on mass spectra and elution times, 14α-C₁₉ cheilanthane and HBP-I are tentatively identified as identical (see text).</figcaption></figure>

either by cleavage of the C and D rings (Fig. 2H, inset molecule) or from
the cleavage of the 8–14 bond of C₂₀ HBP-III.

Under the chosen GC conditions on a DB-5 MS GC column, HBP-III (Fig. 3C) elutes 0.05 min (3 s) after βα-C20 cheilanthane (Fig. 3A). This was confirmed with a co-injection experiment (Fig. 3E). However, the two compounds coelute on a DB-1 MS GC column (Fig. 3B,D,F). Comparing the mass spectra of the two compounds (Fig. 3G,H), HBP-III has most fragments in common with βα-C20 cheilanthane, but the

relative intensities notably differ. βα-C20 cheilanthane has m/z 81/82 and 163/165 fragment ratios >1, whereas HBP-III shows the opposite. The intensity of the m/z 191 fragment is high relative to m/z 123 and m/z 163 in HBP-III, suggesting the absence of a methyl group at C-13 and two methyl groups at C-14. The proposed structure for HBP-III (Fig. 3H) likely forms by cleavage of the D-ring of diploptерol.

Fig. 4 shows the elution behaviour of HBPs IV-VIII compared to βα-C21 cheilanthane in MRM transition 290 > 191. The mass spectra of
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (A–F) MRM transition 262 > 123. (A) Elution times of 13β-C₁₉ cheilanthane synthetic standard on DB-5 MS and (B) DB-1 MS GC columns. (C) Elution times of HBP-II in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) Co-injection experiments on DB-5 MS and (F) DB-1 MS GC columns show that 13β-C₁₉ cheilanthane and HBP-II coelute only on a DB-5 MS GC column. (G) Mass spectrum and molecular structure of 13β-C₁₉ cheilanthane synthetic standard. (H) Mass spectrum of HBP-II in 10 h, 400 °C diploptерol pyrolysate. The proposed molecular structure (H, inset molecule) is based on the mass spectrum and distinct elution time to 13β-C₁₉ cheilanthane on a DB-1 MS GC column.</figcaption></figure>

the co-injection experiment on a DB-5 MS GC column reveal that two
compounds coelute with βα-C21 cheilanthane, identified as HBP-IV and
HBP-V (Fig. 4G,H). Using the DB-1 MS GC column, HBP-IV and HBP-V
are chromatographically resolved (Fig. 4D), and only HBP-V coelutes

with βα-C21 cheilanthane (Fig. 4F). The structure of HBP-V is likely close
to that of βα-C21 cheilanthane, given its coelution on two columns and
their similar mass spectra (Fig. 5A, B). HBP-IV has an m/z 252 molecular
ion and a base peak at m/z 237 (Fig. 4G), similar to simonellite, a BC-
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (A–F) MRM transition 276 > 191. (A) Elution times of βα-C₂₀ cheilanthane synthetic standard on DB-5 MS and (B) DB-1 MS GC columns. (C) Elution times of HBP-III in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) βα-C₂₀ cheilanthane and HBP-III in co-injection experiments on DB-5 MS and (F) DB-1 MS GC columns. (G) Mass spectrum and molecular structure of βα-C₂₀ cheilanthane synthetic standard. (H) Mass spectrum of HBP-III in 10 h, 400 °C diploptерol pyrolysate. The proposed molecular structure (H, inset) is based on mass spectra and distinct elution times to βα-C₂₀ cheilanthane on a DB-5 GC column.</figcaption></figure>

ring diaromatic tricyclic compound (Philp, 1985). HBP-V to VIII all have an *m/z* 290 molecular ion and *m/z* 191 base peak (Fig. 5B–E), reflective of a saturated C21 tricyclic structure (Fig. 5). HBP-VI, VII, and VIII did not coelute with βα-C21 cheilanthane on either column. The proposed structure of HBP-V (Fig. 5B) likely forms from cleavage of the D-ring of diplopterol.

HBP-IX coelutes with βα-C22 cheilanthane on DB-5 MS and DB-1 MS GC columns (Fig. 6). The similar elution times of HBP-IX and βα-C22 cheilanthane suggest that HBP-IX is a saturate C22 tricyclic hydrocarbon with a cheilanthane-like structure. However, we were unable to obtain a

“clean” mass spectrum as HBP-IX was a minor pyrolysis product of diploptерol compared to other HBPs. It presumably forms during pyrolysis through a series of reactions.

Several tricyclic C23 HBPs were detected in MRM 318 > 191 transition of the diploptерol pyrolysate (Supplementary Fig. 3C,D). However, these did not coelute with βα-C23 cheilanthane (Supplementary Fig. 3E,F).

HBP-X (= C24 tetracyclic terpane; Fig. 7C,D) has the same retention time as C24 tetracyclic terpane on DB-5 MS and DB-1 MS GC columns (Fig. 7A,B), and the mass spectra are similar (Fig. 7E,F). However, in the
<figure><img src="image_6.png" /><figcaption>Fig. 4. (A–F) MRM 290 > 191 chromatograms. (A) The elution time of βα-C₂₁ cheilanthane synthetic standard on DB-5 MS and (B) DB-1 MS GC columns. (C) The elution time of HBP-IV to HBP-VIII in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) Relative retention time of the βα-C₂₁ cheilanthane and HBPs IV-VIII in co-injection experiments on DB-5 MS and (F) DB-1 MS GC columns. (G) Mass spectrum of HBP-IV, and (H) of coeluting HBP-IV + HBP-V in 10 h, 400 °C diploptерol pyrolysate.</figcaption></figure>

HBP-X mass spectrum, additional ions occur at *m/z* 145, 157, 171, 295, and 310. The MS fragmentation pattern of the latter shows strong similarities to a C₂₃ tetracyclic monoaromatic hydrocarbon derived from the cyclization of regular polyprenoids (compound 3 in Schaeffer et al., 1994). We suggest that this minor coeluting compound could be a C₂₃ D-ring monoaromatic tetracyclic terpane derived from diploptерol (Fig. 7F, inset molecules).

## 3.2. Widespread occurrence of HBPs in crude oils and bitumens

Sterane, hopane, and cheilanthane biomarkers in the Aralka

Formation samples exhibited E/I ratios ≈ 1 and were thus considered syngenetic. All Aralka Formation samples discussed here are from clean rock interiors.

14α-C₁₉ to βα-C₂₆ cheilanthanes were detected in bitumens from the Neoproterozoic Aralka Formation and the Oils of Australia collection on a DB-5 MS GC column. HBP-III, HBP-VII, and HBP-VIII were resolved in the MRM chromatograms of all samples, indicating the general occurrence of HBPs, thus potentially also including HBPs that coelute with cheilanthanes. Representative summed MRM M⁺>191 chromatograms from three Aralka Formation bitumens exhibit cheilanthanes (blue) and HBPs (red), and coeluting compounds (purple) in changing relative
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (A) Mass spectrum and molecular structure of βα-C₂₁ cheilanthane synthetic standard. (B-E) Mass spectra of HBP-V to –VIII in 10 h, 400 °C diplopterol pyrolysate. (B) Note that the mass spectrum from HBP-V is background subtracted. Inset in (B), proposed molecular structure of HBP-V.</figcaption></figure>

abundances (Fig. 8). βα-C20 cheilanthane is chromatographically well-resolved from HBP-III on the DB-5 MS GC column, enabling direct observation of HBP relative abundances in these samples. As HBP-III increases in abundance relative to βα-C20 cheilanthanes from the lower to upper chromatogram in Fig. 8, HBP-I (= ‘14α-C19 cheilanthane’), HBP-VII, and HBP-VIII also increase in abundance relative to βα-C23 cheilanthane. HBP-X (= C24 tetracyclic terpane) also increases with an increasing relative abundance of other HBPs.

### 3.3. A tool to identify the relative abundance of HBPs

We tested two new proxies to gauge the relative abundances of HBPs and cheilanthanes: the ratio C₂₀ HBP-III/βα-C₂₀ cheilanthane and, in case these two compounds are not chromatographically resolved under chosen GC conditions, the ratio C₂₁ HBP-VIII/βα-C₂₃ cheilanthane. The latter compounds do not coelute with other related compounds on DB-5 MS and DB-1 MS GC columns. The relationships were tested using Spearman correlation coefficients (ρ) since the data were not normally

distributed, and outliers were not excluded. In both the Aralka Formation and the Oils of Australia collection samples, the two ratios show statistically significant (p < 0.05), strong positive correlations with each other (Fig. 9A,B), confirming them as suitable proxies for the relative abundance of tricyclic HBPs.

### 3.4. Are HBPs and other degraded and rearranged hopanes related?

To test to what extent hopane diagenesis and catagenesis are related to the generation of HBPs, we compared ratios C₂₀ HBP-III/βα-C₂₀ cheilanthane against C₂₉/C₃₀ hopane (αβ + βα C₂₉/αβ + βα C₃₀, Fig. 9C) and C₂₇/C₃₀ hopane (C₂₇ Tₛ + Tₘ/αβ + βα C₃₀, Fig. 9E) in the coherent set of marginally mature bitumens from the Aralka Formation. There is a negative correlation with C₂₉/C₃₀ hopane ratio (ρ = -0.69, p = < 0.05, n = 14) and C₂₇/C₃₀ hopane ratio (ρ = -0.76, p = < 0.05, n = 14). However, note that the correlations in Fig. 9C are driven by two data clusters. In the Oils of Australia collection (Fig. 9D,F), representing a wide spectrum of ages, maturities and environments, there is a weak
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (A–F) MRM transition 304 > 191. (A) Elution times of βα-C₂₂ cheilanthane synthetic standard on DB-5 MS and (B) DB-1 MS GC columns. (C) HBP-IX in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) βα-C₂₂ cheilanthane and HBP-IX in co-injection experiments on DB-5 MS and (F) DB-1 MS GC columns.</figcaption></figure>

correlation with C₂₉/C₃₀ hopane ratio (ρ = 0.27, p = < 0.05, n = 64) and no significant correlation with C₂₇/C₃₀ hopane ratio (ρ = -0.16, p = 0.21, n = 64).

To control for the influences of precursor sources and hopane dia/
catagenesis, we also compared 14α-C₁₉ cheilanthane/C₃₀ hopane (αβ +
βα C₃₀), C₂₀ HBP-III/C₃₀ hopane (αβ + βα C₃₀) and C₂₁ HBP-VIII/C₃₀
hopane (αβ + βα C₃₀) ratios against C₂₉/C₃₀ hopane and C₂₇/C₃₀ hopane
ratios (Supplementary Figs. S4 and S5). In the Aralka Formation, C₂₁
HBP-VIII/C₃₀ hopane ratio exhibited strong negative correlations with
C₂₉/C₃₀ hopane (ρ = -0.72, p = < 0.05, n = 14) and C₂₇/C₃₀ hopane (ρ =
-0.77, p = < 0.05, n = 14) ratios. The Oils of Australia collection only
exhibited weak to moderate positive correlations (Supplementary
Figs. S4 and S5).

The Aralka Formation bears a subset of samples that have unusually
high abundances of 17α(H)-diahopanes and early-eluting (EE) rear-
ranged hopanes. To ascertain whether their occurrence was related to
the abundance of HBPs, the C20 HBP-III/βα-C20 cheilanthane ratio was
compared to C30 diahopane/regular C30 hopanes (αβ + βα C30) (Fig. 9G)
and C30 EE-hopane/regular C30 hopanes (αβ + βα C30) ratios (Fig. 9I).
Among samples where diahopanes were detectable, four samples also
had high proportions of hopanoid breakdown products (ρ = 0.72, p = <
0.05, n = 14). The same four samples had high C30 EE-hopane/C30
hopanes values (ρ = 0.76, p = < 0.05, n = 14). In the Oils of Australia
collection (Fig. 9H,J) there was a moderate positive correlation of C20
HBP-III/βα-C20 cheilanthane ratio with C30 EE-hopane/C30 hopanes
ratio (ρ = 0.39, < 0.05, n = 64) and a weak positive correlation with C30
diahopane/regular C30 hopane ratio (ρ = 0.27, p = < 0.05, n = 64).

To test whether diagenetic conditions favouring rearrangement

reactions also led to the formation of HBPs, we compared 14α-C₁₉
cheilanthane/C₃₀ hopane, C₂₀ HBP-III/C₃₀ hopane, and C₂₁ HBP-VIII/
C₃₀ hopane ratios with C₃₀ diahopane/regular C₃₀ hopanes
(Supplementary Fig. S6) and C₃₀ EE-hopane/regular C₃₀ hopanes ratios
(Supplementary Fig. S7). In the Aralka Formation, there was a strong
positive correlation between C₂₁ HBP-VIII/C₃₀ hopane ratio with C₃₀
diahopane/regular C₃₀ hopanes (ρ = 0.74, p = < 0.05, n = 14) and with
C₃₀ EE-hopane/C₃₀ hopanes (ρ = 0.74, p = < 0.05, n = 14) ratios. The
Oils of Australia collection showed a strong positive correlation between
C₂₀ HBP-III/C₃₀ hopanes ratio and C₃₀ EE-hopane/C₃₀ hopanes ratio (ρ
= 0.62, p = < 0.05, n = 64).

3.5. *The impact of HBPs on commonly used cheilanthane parameters*

To gauge the impact of HBPs on environmental reconstructions and
oil-source rock correlations, the C20 HBP III/βα-C20 cheilanthane ratio
was compared to commonly used tricyclic and tetracyclic terpane
abundance ratios. The cheilanthane parameter C19/C23 (= 14α-C19/
βα-C23; Fig. 9K,L) shows a statistically significant positive correlation
with C20 HBP III/βα-C20 cheilanthane in the Aralka Formation (ρ = 0.81,
p < 0.05, n = 14) and the Oils of Australia collection (ρ = 0.79, p < 0.05,
n = 64). The C22/C21 cheilanthanes ratio (βα-C22/βα-C21; Fig. 9M,N)
increases with C20 HBP III/βα-C20 cheilanthane in the Aralka Formation
(ρ = 0.59, p < 0.05, n = 14) but shows no statistically significant cor-
relation for the Oils of Australia (ρ = 0.17, p = 0.17, n = 64). Parameters
involving C24 tetracyclic terpane (C24 Tet) such as C24 Tet/βα-C20-26
cheilanthanes ratio (ρ = 0.59, p < 0.05, n = 14) and C24 Tet/βα-C26
cheilanthane ratio (ρ = 0.65, p < 0.05, n = 14), show moderate to strong
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** (A–D) MRM transition 330 > 191. (A) Elution times of C₂₄ tetracyclic terpane in the AGSO standard (Norgate and Murray, 2017) on DB-5 MS and (B) DB-1 MS GC columns. (C) HBP-X in the 10 h, 400 °C diploptерol pyrolysate on DB-5 MS and (D) DB-1 MS GC columns. (E) Mass spectrum of C₂₄ tetracyclic terpane from the AGSO standard. (F) Mass spectra of HBP-X in 10 h, 400 °C diploptерol pyrolysate. The proposed molecular structures (F, inset molecules) are based on mass spectra and elution time.</figcaption></figure>

positive correlations with C20 HBP III/βα-C20 cheilanthane ratio in the Aralka Formation (Fig. 9O,Q). In the Oils of Australia samples, these parameters also show strong positive correlations for low C20 HBP III/βα-C20 cheilanthane ratio but start to deviate strongly for values > 2 (Fig. 9P,R). The abundance of total hopanes (αβ-C27-C35 hopanes) compared to cheilanthanes (C19, βα-C20-C45 cheilanthanes + coeluting HBPs) shows a moderate but significant positive correlation with C20 HBP III/βα-C20 cheilanthane ratio in the Aralka Formation (ρ = 0.57, p < 0.05, n = 14) but not in the Oils of Australia collection (ρ = 0.29, p = < 0.05, n = 64) (Fig. 9S,T).

**4. Discussion**

**4.1. Speculation on HBP formation in geological samples**

Diploptерol was pyrolysed to obtain tricyclic and tetracyclic degrada-
dation products that could be compared with cheilanthanes occurring in
bitumens and crude oils. The pyrolysis conditions were chosen with the
intent of breaking C–C bonds and to form hopanoid-related tricyclic and

tetracyclic compounds and not specifically to investigate the mecha-
nisms of thermal geochemical processes affecting hopanoids. The
mechanisms of thermal cleavage during pyrolysis and their geochemical
relevance are still not well understood (e.g. Love et al., 2005, Green-
wood et al., 2006, Berwick et al., 2010). However, the presence of
functionalities and, notably, double bonds in the hopanoid skeleton
might favour thermal carbon bond cleavage reactions at allylic positions
(e.g. Liu and Farooq, 2024). In our experiments, unsaturated hopanoids
can be easily formed by elimination of the tertiary hydroxy group of
diploptерol, and the resulting double bond(s) can migrate into the ring
system.

During pyrolysis, tricyclic and tetracyclic degradation products
likely formed by C–C bond cleavage reactions in the D- and E-rings of C₃₀
hopenes resulting from dehydration of diplopterol. Co-injection exper-
iments of synthetic 13β-C19 and βα-C20–C30 cheilanthanes with the
diplopterol pyrolysate products demonstrated that some HBPs coelute
with C19–C22 cheilanthanes on DB-5 MS and/or DB-1 MS GC columns.
The HBPs formed were identified in natural geological samples. HBP
abundance can significantly influence commonly used tri- and
<figure><img src="image_10.png" /><figcaption>Fig. 8. Summed MRM M⁺>191 chromatograms on a DB-5 MS GC column of Aralka Formation bitumens. (A) LvM-085, (B) LvM-094 and (C) LvM-080 comparing distributions of 14α-C₁₉ to βα-C₂₀-C₂₆ cheilanthanes and HBPs. Blue peaks represent βα-cheilanthanes, red peaks are HBPs, and purple peaks indicate coeluting cheilanthanes and HBPs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

tetracyclic terpane parameters for paleo-depositional reconstructions.

While we cannot unambiguously deduce the formation pathways
from functionalised hopanoids to saturated HBPs in natural systems
based on the performed pyrolysis experiments, we can examine corre-
lations with other degraded and rearranged hopanes and speculate on
possible influences on their formation.

4.1.1. HBPs form during diagenesis

In our study of the Aralka Formation, the strong negative correla-
tions between HBPs and the abundances of C<sub>27</sub> and C<sub>29</sub> hopanes (relative
to C<sub>30</sub> hopanes) suggest that the conditions and processes of side-chain
cleavage differ from those leading to ring cleavage. When using ratios
that normalise to hopane abundances to control for different precursor
lipid sources (Supplementary Figs. S4 and S5), the Aralka Formation
exhibits a strong negative correlation between the abundance of C<sub>21</sub>
HBP-VIII and increasing C<sub>27</sub> and C<sub>29</sub> hopane abundance. However, the
opposite trend is observed in the heterogeneous Oils of Australia
collection (Supplementary Fig. S5F). Although the correlations are
moderate, this suggests that there are likely varied pathways and con-
trols for ring and side-chain cleavage.

Previous work has suggested that tricyclic terpanes do not seem to
form during thermal degradation of saturate hopanes in natural systems.
Xiao et al., (2024) measured the relative abundances of C₁₉ to C₂₃ tri-
cyclic terpanes and hopanes in 17 rock samples from a single well,
spanning a depth of 2826 to 4733 m and covering a thermal maturity
window of %R₀ = 0.58 to 1.47. The study used a HP-5 MS GC column
equivalent to the DB-5 MS GC column used in this study. Thus, the in-
tegrated values of C₁₉ to C₂₃ tricyclic terpanes likely included C₁₉ to C₂₁
HBPs. While the ratio of measured tricyclic terpanes to hopanes (TT/
hops) increased by nearly one order of magnitude down-core with
increasing thermal maturity, the relative abundances of C₁₉ + C₂₀, C₂₁
and C₂₃ tricyclic terpanes relative to each other remained relatively
constant. This indicates that the substantial thermal degradation of

hopanes down-core did not produce significant amounts of C₁₉ to C₂₁
HBPs. This is consistent with findings from Farrimond et al., (1999), who
quantified the absolute abundances of C₂₀–C₂₆ tricyclic terpanes and
C₂₇–C₃₅ hopanes in a >1.6 m carbonate concretion thermally altered by
a proximal dyke. Their quantitative data suggest that the increase in TT/
hops with maturity is driven by the thermal degradation and decrease of
C₂₇–C₃₅ hopanes rather than an increase in tricyclics from the degra-
dation of hopanes. Additionally, Alexander et al. (1984) reported the
widespread occurrence of bicyclic drimanes in crude oils and bitumens.
They proposed that the drimanes could form from similarly widespread
pentacyclic hopanoid precursors via “diagenetic transformation as
opposed to a thermocatalytic one, since drimanes are present in ancient
sediments of low maturity” where catagenetic cleavage of hydrocarbons
was not observed. Collectively, our data and previous studies suggest
that the abundance patterns of HBPs are thus likely set during
diagenesis.

4.1.2. Unusual correlations with rearranged hopanes and the role of redox

The Aralka Formation exhibits strong positive correlations of dia-
hopanes and EE-hopanes with C20 HBP-III/βα-C20 cheilanthane (Fig. 9G,
I) and C21 HBP-VIII/C30 hopanes (Supplementary Figs. S6C and S7C),
pointing to possible links in methyl-group migration and ring cleavage
reactions. The weak, but significant positive correlations of diahopanes
and EE hopanes with HBPs in the Oils of Australia dataset also support
this. Hopanoids with rearranged hopane skeletons have not been re-
ported in living bacteria, making it likely that they are diagenetic
products of regular hopanoids. Although there are several hypotheses
about the formation of rearranged hopanes, the mechanisms remain
unresolved (Moldowan et al., 1991; Farrimond and Telnæs, 1996).

The main controversy around the formation of rearranged hopanes is
about the role of redox conditions. The oxidation of C-16 to form a Δ¹⁵
double bond could lead to the methyl group shifting from C-14 to C-15,
which is acid-catalysed, similar to conditions used in laboratory settings
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Testing the relationship between HBPs and cheilanthanes abundances in the Aralka Formation (left panels, orange, n = 14) and (B) the Oils of Australia collection (right panels blue, n = 64). Ratios are defined in Supplementary Table S1. Note that correlation statistics in the Oils of Australia dataset do not include the Aralka Formation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
to synthesise diahopanes (Nytoft et al., 2007). A similar mechanism is proposed for the formation of the EE-hopanes, since both rearranged molecules require methyl shifts, and their abundances often covary (Moldowan et al., 1991; Farrimond and Telnæs, 1996; Wang et al., 2018). Rearranged hopanes have also been observed in Australian oil samples thought to have high terrigenous input and possibly elevated oxygen exposure (Philp and Gilbert, 1986; Preston and Edwards, 2000). However, more recently, Philp et al. (2022) compared the abundance of rearranged hopanes (C₃₀EE and C₃₀dia) and rearranged tricyclics (structures unknown) with the pristane/phytane (Pr/Ph) ratio, which they used as a measure of oxicity, in oils from the Anadarko Basin, Oklahoma, USA. They found no relationships and argued against the role of redox conditions in the formation of rearranged hopanes. They suggest that exposure to oxygen instead plays a role in whether the double bonds in the functionalised precursors are preserved, which then leads to rearrangement reactions. Similar to the Anadarko Basin oils, we did not observe any correlations between the abundance of diahopanes or EE-hopanes with Pr/Ph in the Oils of Australia collection (Supplementary Fig. S8).

With our current dataset, it is not possible to deconvolute the roles of redox conditions and acid catalysis in the formation of EE-hopanes, diahopanes, and HBPs. However, it is plausible that unsaturated hopa-noid intermediates bearing double bond(s) in the ring system originating from diagenetic processes might favour rearrangement and C–C bond cleavage reactions in the polycyclic skeleton and the subsequent for-mation of HBPs. In our pyrolysis experiments, this could occur on hopenes formed from the dehydration of diplopterol. The role of the double bond could be further explored with pyrolysis experiments using hopane hydrocarbons and hopanoids with a functionality in the ring system.

In summary, HBPs presumably form from unsaturated hopanoid precursors during diagenesis but not by direct thermal degradation of hopanes during catagenesis. The discovery of HBPs and their relation-ship to rearranged hopanes offers new opportunities to evaluate the precise diagenetic mechanisms and formation conditions.

## 4.2. Paleoenvironmental parameters impacted by HBPs

### 4.2.1. C₂₄ Tet and “14α-C₁₉ cheilanthane” are HBPs

Our pyrolysis experiments show that C₂₄ HBP-X has a similar mass spectrum to C₂₄ tetracyclic terpane (C₂₄ Tet) and coelutes on two different GC columns, suggesting that the two compounds are identical (Fig. 7). Commonly used source parameters include C₂₄ Tet/βα-C₂₆ cheilanthane and C₂₄ Tet/βα-C₂₀–C₂₆ cheilanthanes. The latter param-eter was also proposed as a thermal maturity parameter, which was thought to reflect greater generation of C₂₄ Tet compared to tricyclic terpanes towards the oil window (Farrimond et al., 1999).

Our study supports the hypothesis that C₂₄ Tet is sourced from pentacyclic hopanoid precursors. Statistically significant positive cor-relations are observed between HBP abundances and parameters involving C₂₄ Tet in both the Aralka Formation and Oils of Australia dataset (Fig. 9Q–T), suggesting that proxies involving C₂₄ Tet could predominantly be driven by HBP input. This is consistent with the suggestion from Philp et al. (2022), who revised previous ideas about C₂₄ Tet being related to higher plant input (Philp and Gilbert, 1986) and, instead, asserted that C₂₄ Tet is a product of E-ring cleavage of hopa-noids. As naturally occurring products with the C₂₄ Tet carbon skeleton are unknown, it is plausible that this biomarker solely occurs as a breakdown product of hopanoids and other structurally related poly-cyclic terpenoids.

In our study, coelution experiments on DB-5 MS and DB-1 MS GC columns combined with the mass spectral analyses suggest that the structures of C₂₀ HBP-III and βα-C₂₀ cheilanthane differ by the position of one methyl group (Fig. 2). Thus, the two compounds could coelute, or closely elute, under various GC conditions. In published partial *m/z* 191 mass chromatograms, the two peaks are rarely resolved, and it seems

likely that most published concentration data and ratios combine both compounds. The coelution on two GC columns with different stationary phases and the identical mass spectra of HBP-I and ‘14α-C₁₉ cheilanthane’ indicate that they are the same compound. It is likely that publications referring to ‘C₁₉ tricyclic terpane’ or ‘14α-C₁₉ cheilanthane’ usually identify HBP-I. Our correlation analyses also highlight that the abundances of ‘14α-C₁₉ cheilanthane’ = C₁₉ HBP-I and βα-C₂₀ cheilanthane and any ratios that include them are either partly or largely driven by the abundance of HBPs and therefore the degradation of hopanoids (and presumably some other terpenoids).

### 4.2.2. Historical use of 14α-C₁₉ and C₂₀ cheilanthanes as terrigenous indicators

High abundances of 14α-C₁₉ and C₂₀ cheilanthanes are thought to characterise high terrigenous input to sediments (Simoneit, 1977; Zumberge 1981; Richardson and Miller, 1982; Barnes and Barnes, 1983; Walters and Cassa 1985; Noble et al., 1986). However, the distinction between plant-derived C₁₉ and C₂₀ tricyclic terpanes and C₁₉ and C₂₀ cheilanthanes was not clear in these early papers. It is possible to form precursors to C₁₉ and C₂₀ cheilanthanes by acid-catalysed cyclisation of labdanoids (Arîcu, 2021), which are widely distributed in conifer and some angiosperm resins (Lambert et al., 2008). This may be the reasoning behind the subsequent use of the ratio 14α-C₁₉/C₂₃ cheilanthane, which could distinguish between terrigenous and marine organic matter in paleoenvironmental reconstructions and oil-source rock studies. High values have been observed to co-occur in sedimentary rocks with terrigenous-sourced clay and high abundances of plant-derived microfossils (e.g., pollen) and other biomarkers, including high molecular weight *n*-alkanes indicative of plant waxes, oleanane, or high C₂₉ steranes (Simoneit, 1977; Richardson and Miller, 1982; Philp and Gilbert, 1986; Preston and Edwards, 2000; Peters et al., 2005b).

More recently, parameters involving C₁₉–C₂₆ tricyclic terpanes, C₂₄ Tet, and C₃₀ hopane have been proposed from analyses of large datasets of oils and source-rock extracts from petroleum provinces in China (e.g., Tao et al., 2015; Peng et al., 2024; Wang et al., 2024; Xiao et al., 2024). Of note, Wang et al. (2023) showed that the relative abundances of C₂₀, C₂₁, and C₂₃ tricyclic terpanes in 271 samples from 32 basins and 18 countries varied with relative terrigenous and marine algal input. Low ratios of C₂₁/C₂₀ and C₂₃/C₂₁ tricyclic terpanes (TT) correlated with terrigenous source organic matter. The authors attributed this correla-tion to “the control of depositional environments on the input pro-portions of plankton and terrigenous plants, which act as two end-member precursors for the TTs in a depositional system”. Scrutiny of an *m/z* 191 mass chromatogram with representative abundance patterns in a high terrigenous input sample shows elevated abundances of HBPs. The C₁₉ HBP-I signal is prominent (labelled “C₁₉TT”), and C₂₁ HBP-VII and VIII, eluting between C₂₁TT and C₂₂TT, are clearly present (c.f. Fig. 3C in Wang et al., 2023 with Fig. 8A in this manuscript). In the same Figure, the C₂₀ tricyclic terpane (“C₂₀TT”) is resolved as a double peak, suggesting the presence of βα-C₂₀ cheilanthane as well as C₂₀ HBP-III.

### 4.2.3. Rationalising the use of C₂₄ tetracyclic terpane, C₁₉ and C₂₀ tricyclic terpanes as biomarkers for terrigenous input

The existence of HBPs can rationalise why abundant C₁₉ and C₂₀ tricyclic terpanes and C₂₄ Tet are linked with terrigenous source input. A wide range of bacteria synthesise polyfunctionalised biohopanoids (e.g. Talbot and Farrimond 2007; Belin et al., 2018). Biohopanoids can be abundant and diverse in soils (Ries-Kautt and Albrecht, 1989; Kusch and Rush, 2022) and peats (Inglis et al., 2018, 2025) when compared to marine surface waters and sediments (Pearson et al., 2009; Pearson and Rush 2009; Sáenz et al., 2011a; Zhu et al., 2011). This could explain why sediments with high terrigenous input are more likely to have elevated HBP concentrations and high proportions of C₁₉, C₂₀, and C₂₁ tricyclic terpanes and C₂₄ Tet. Moreover, many soils and peats contain substantial concentrations of other higher plant triterpenoids, including compounds with lupane, oleanane and ursane skeletons. These may also
break down to form C₁₉ HBP-I and C₂₀ HBP-III. Compound-specific isotope studies could disentangle mixed source inputs in samples from the late Paleozoic and younger. However, other sources and influences must explain elevated HBP abundances in pre-Devonian environments, including the Neoproterozoic Aralka Formation (present study), the Mesoproterozoic Xiamaling Formation (Luo et al., 2015), and the Ordovician oils from the Williston Basin (Zumberge, 1981), since these predate the first appearance of land plants. Recent work has shown that biohopanoid relative abundance and structural diversity increase in suboxic and anoxic zones in pelagic marine settings (Blumenberg et al., 2007; Wakeham et al., 2007; Sáenz et al. 2011b; Kharbush et al. 2013, 2015, 2018), highlighting that microaerophilic or anaerobic bacteria could be notable sources.

Furthermore, we propose to abandon the term "14α-C₁₉ cheilanthane" in favour of 'C₁₉ HBP-I' to reflect the predominant hopanoid precursor sources. Contributions to the HBP-I peak from cheilanthane precursor biomolecules are likely minimal since it would require side-chain cleavage as well as loss of the methyl group from C-13. We show that HBP-I can be generated from hopanoids (and presumably some plant terpenoids) by cleavage of the D-ring. Based on our data, the abundance of HBP-I correlates well with the abundance of other HBPs. Therefore, C₁₉ HBP-I should no longer be regarded as a member of the cheilanthane pseudohomologous series.

### 4.2.4. *High C₂₂/C₂₁ cheilanthane values are not driven by HBPs in this study*

The ratio of βα-C₂₂/C₂₁ cheilanthanes in crude oils is commonly used to identify carbonate source rocks. A cross-plot of βα-C₂₂/C₂₁ and βα-C₂₄/C₂₃ appears to distinguish oils from carbonate source rocks, with C₂₂/C₂₁ values ranging from 0.025 to 2.5, while other source rocks have values <1 (Peters et al., 2005b). This cross-plot was initially generated by GeoMark Research (https://www.geomarkresearch.com) using a proprietary global database of characterised oils, although it remains unclear exactly how each oil was classified as carbonate-derived. C₂₂/C₂₁ > 1 values are unusual, and in our study, they only occur in one of 64 oils. This is presumably because the generation of the C₂₂ pseudohomologue from higher molecular weight cheilanthane precursors requires the cleavage of two C–C bonds in the side-chain, while the generation of the C₂₁ pseudohomologue requires only one (Aquino Neto et al., 1982).

As C₂₂ HBP-IX coelutes with βα-C₂₂ cheilanthane on DB-5 MS and DB-1 MS GC columns (Fig. 6), we tested whether this HBP contributes to unusually elevated C₂₂/C₂₁ cheilanthane ratios. The Aralka Formation samples show a statistically significant positive relationship between C₂₂/C₂₁ and C₂₀ HBP-III/βα-C₂₀ cheilanthane ratios (Fig. 9M). However, there is no statistically significant correlation between βα-C₂₂/C₂₁ cheilanthanes and carbonate content (Supplementary Fig. S9). Thus, in this sample set, carbonate content is likely not a driver of increased βα-C₂₂/C₂₁ cheilanthane values, and the elevated values are more likely due to GC coelution with C₂₂ HBP-IX. Conversely, in the single analysed sample (GA19999650, Tubridgi 2, Table 2) from the Oils of Australia collection with βα-C₂₂/C₂₁ cheilanthane >0.90, HBPs are not unusually high (Fig. 9N). This sample was sourced from the Tubridgi gas field and was categorised as heavily biodegraded, which explains the high abundances of cheilanthanes. Thus, a detailed search for other coeluting compounds is required to test whether extremely elevated βα-C₂₂ signals are caused by coelution, or whether distinct diagenetic or biological sources of βα-C₂₂ cheilanthane exist.

## 5. Conclusions

This study explored hopanoid breakdown products (HBPs) generated from the pyrolysis of C₃₀ diplopterol, a functionalised C₃₀ biohopanoid. We discovered compounds that are identical to, or coelute with, C₁₉–C₂₂ tricyclic and C₂₄ tetracyclic terpanes. These HBPs are present in all bitumens and oils analysed in this study (n = 78). We also observed statistically significant correlations between the abundances of HBPs and

rearranged hopanes (C₃₀ diahopane and C₃₀ early eluting hopane), pointing to similar diagenetic conditions that lead to their formation. This result suggests that migration of double bond(s) into the ring system might favour rearrangement and C–C bond cleavage reactions of the hopanoid polycyclic ring system. We show that co-eluting HBPs can make up a significant proportion of the apparent C₁₉–C₂₂ tricyclic and C₂₄ tetracyclic terpanes gas chromatogram signals and can have a major influence on parameters involving these biomarkers. We demonstrated that C₁₉ HBP-I (14α-C₁₉ cheilanthane) and C₂₄ tetracyclic terpane are generated through degradation of hopanoids (and/or related terpenoids). Furthermore, we propose that "14α-C₁₉ cheilanthane" is not a true member of the cheilanthane family and henceforth should be identified as "C₁₉ HBP-I". The presence of HBPs rationalises abundance patterns of some tricyclic and tetracyclic terpanes and their relationships to terrigenous source inputs, which can be influenced by high relative abundance of HBPs, likely sourced from hopanoids in soils.

## CRediT authorship contribution statement

**Tharika Liyanage:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Conceptualization. **Lennart M. van Maldegem:** Writing – review & editing, Investigation, Formal analysis. **Janet Hope:** Writing – review & editing, Methodology. **Philippe Schaeffer:** Writing – review & editing, Resources, Formal analysis. **Pierre Adam:** Writing – review & editing, Resources, Formal analysis. **Hans Peter Nytoft:** Writing – review & editing, Resources. **Dianne Edwards:** Writing – review & editing, Resources. **Jochen Brocks:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We are grateful to Clifford Walters and John Volkman for their editorial handling, and we thank Gordon Inglis and one anonymous reviewer for their constructive comments that improved this manuscript. T.L. would like to thank the three anonymous examiners of her PhD thesis who contributed thoughtful feedback to an earlier version of this manuscript. T.L. acknowledges support from an Australian Government Research Training Program (RTP) Scholarship and the Australian National University Research School of Earth Sciences' John Conrad Jaegar Scholarship. This research was financially supported by Australian Research Council grant DP170100556 (to J.B.). D.E. publishes with the permission of the CEO, Geoscience Australia.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.orggeochem.2025.105085.

## Data availability

The calculated values of biomarker proxies exhibited in Fig. 9 and Supplementary Figs. S4–8 are available in Supplementary Table S1. GC–MS data are available upon request.
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