Acyclic isoprenoid biomarkers and evolution of biosynthetic pathways in green microalgae of the genus Botryococcus

Acyclic isoprenoids were among the earliest lipids on Earth and today > 30,000 isoprenoid-derived compounds are known, testifying to the remarkable diversification in isoprenoid biosynthetic pathways over time. Many of the isoprenoids have proven to be useful biomarkers in geochemical studies and evidence from sedimentary studies has the potential to provide a timeline for the evolution of different types of isoprenoid biosynthesis. A single green microalgal species termed Botryococcus braunii has been recognised as a major contributor of organic matter to sediments as old as the Precambrian. Modern studies of the taxonomy of B. braunii using molecular biology techniques have shown that the major clades identified from 18S rRNA gene sequences correspond to races A, B, L and S, which are defined on the basis of their hydrocarbon composition. The biosynthetic pathways by which C₃₀–C₃₇ botryococcenes are produced by the B race have now been shown to be due to a duplication of the squalene synthase gene, followed by subsequent changes to the genes, such that one pathway leads to the production of the C₃₀ botryococcene and the other to squalene. Both products are then methylated to produce botryococcenes and methylated squalenes having higher carbon numbers. The mode of biosynthesis of lycopadiene in race L is unclear, but may involve coupling of two C₂₀ phytyl diphosphates. From an examination of the geological record of botryococcenes and lycopadiene it seems likely that these pathways probably evolved early in the Eocene (ca. 55 Ma) and thus are more recently evolved than the genes for highly branched isoprenoid (HBI) alkenes first produced by diatoms about 92 Ma ago. Botryococcane produced from botryococcenes and a monoaromatic hydrocarbon produced from lycopadiene-related lipids, presumably under anoxic conditions, show promise as age diagnostic biomarkers. In view of these results, it seems likely that the Botryococcus species recorded in sediments predating the Eocene lacked the ability to produce botryococcenes or lycopadiene, but nonetheless still contained polymeric non-isoprenoid alkyl chains in the form of an algaenan which on preservation gave rise to a multitude of organic rich rocks.     

Bacterial and algal hydrocarbons in sediments from a saline Antarctic lake, Ace Lake

Capillary gas chromatography-mass spectrometry (C-GC-MS) and Iatroscan thin layer chromatography-flame ionisation detection (TLC-FID) were used to study hydrocarbon distributions in a sediment core from Ace Lake, a saline, meromictic lake in the Vestfold Hills of Antarctica. Hydrocarbons were abundant in most core sections (up to 125/μg/g dry wt), particularly in near-surface samples, and the distributions were very complex. Major constituents were identified as phytane, 2,6,10,15,19-pentamethyleicosane, tetrahydrosqualene, a mixture of phytenes, cholesta-3,5-diene and fern-7-ene. Smaller amounts of sterenes and hopenes were also present. The predominance of the first 3 acyclic isoprenoids in sediments buried less than 30 cm is consistent with high populations of methanogenic bacteria known to be present.Phytenes were abundant in all core sections, and there was no relationship between their abundance and that of phytane which suggests that they were not derived from methanogenic bacteria. Phytadienes were minor constituents at all depths studied. An unusual feature of some distributions was the high concentrations of fern-7-ene which was the major hydrocarbon in the 20–25 cm core section. This alkene was only abundant in sediments which contained high concentrations of methanogen markers suggesting that it may also be indicative of anoxic depositional environments. A possible source might be from purple non-sulphur bacteria. High concentrations of straight-chain C₂₉ and C₃₄ alkenes were also found in these sediments but their origin has not been determined. Major changes in the hydrocarbon distributions with depth indicate that the depositional environment in the lake has altered dramatically since the lake was formed less than 8000 years ago. The present condition of permanent anoxic bottom waters probably developed only in the last 1000 years.     

Delineating compositional variabilities among crude oils from Central Montana, USA, using light hydrocarbon and biomarker characteristics

Three compositionally distinctive groups of oils identified in central Montana by biomarker analyses are also recognized by the unique compositions of their light hydrocarbon (gasoline range) fraction. The majority of oils produced from Paleozoic pools (Pennsylvanian Tyler–Amsden interval) group into one broad category based on the distribution of C₂₀–C₄₀ biomarkers. These oils not only have the lowest Paraffin Indices and relative concentrations of normal heptane, but are readily distinguishable from the other compositional groups by using selected “Mango” parameters. However, the biomarker-based subdivision of this group into at least two sub-families is not reflected in the gasoline range fraction, suggesting little effect of source rock host lithology on the distribution of C₅–C₈ hydrocarbons. Oils occurring predominantly in Jurassic–Cretaceous reservoirs display different biomarker and gasoline range characteristics, including Paraffin Indices, K1 parameter and relative concentrations of C₇ compounds, and are classified in two separate compositional categories. In contrast to oils from the Tyler–Amsden interval, the oils produced from the Mesozoic strata are amongst the most mature oils in the study area. The unique biomarker/light hydrocarbon signatures are likely due to different source organic matter. Secondary alteration of oil due to biodegradation and migration, although recognized, appears less significant. The results indicate the overall usefulness of gasoline range compositions in delineating compositional affinities of crude oils in central Montana, clearly suggesting that the oils found in Paleozoic and Mesozoic reservoirs belong to different petroleum systems.     

Insights into oil cracking based on laboratory experiments

The objectives of this pyrolysis investigation were to determine changes in (1) oil composition, (2) gas composition and (3) gas carbon isotope ratios and to compare these results with hydrocarbons in reservoirs. Laboratory cracking of a saturate-rich Devonian oil by confined, dry pyrolysis was performed at T=350–450 °C, P=650 bars and times ranging from 24 h to 33 days. Increasing thermal stress results in the C₁₅₊ hydrocarbon fraction cracking to form C_6–14 and C_1–5 hydrocarbons and pyrobitumen. The C_6–14 fraction continues to crack to C_1–5 gases plus pyrobitumen at higher temperatures and prolonged heating time and the δ ¹³C_ethane–δ¹³C_propane difference becomes greater as oil cracking progresses. There is considerable overlap in product generation and product cracking. Oil cracking products accumulate either because the rate of generation of any product is greater than the rate of removal by cracking of that product or because the product is a stable end member under the experimental conditions. Oil cracking products decrease when the amount of product generated from a reactant is less than the amount of product cracked. If pyrolysis gas compositions are representative of gases generated from oil cracking in nature, then understanding the processes that alter natural gas composition is critical.     

The effects of surface area, grain size and mineralogy on organic matter sedimentation and preservation across the modern Squamish Delta, British Columbia: the potential role of sediment surface area in the formation of petroleum source rocks

Surface sediment samples were collected from the Squamish River Delta, British Columbia, in order to determine the role of sediment surface area in the preservation of organic matter (OM) in a paralic sedimentary environment. The Squamish Delta is an actively prograding delta, located at the head of Howe Sound.Bulk total organic carbon (TOC) values across the Squamish Delta are low, ranging from 0.1 to 1.0 wt.%. The carbon/total nitrogen ratio (C_org/N) ranges from 6 to 17, which is attributed to changes in OM type and facies variations. The <25-μm fraction has TOC concentrations up to 2.0 wt.%, and a C_org/N ratio that ranges from 14 to 16. The 53–106-μm fraction has higher TOC concentrations and C_org/N ratios relative to the 25–53-μm fraction. The C_org/N ratio ranges from 9 to 18 in the 53–106-μm fraction and 5.5–10.5 in the 25–53-μm fraction. Surface area values for bulk sediments are low (0.5–3.0 m²/g) due to the large proportion of silt size material. Good correlation between surface area and TOC in bulk samples suggests that OM is adsorbed to mineral surfaces. Similar relationships between surface area and TOC were observed in size-fractionated samples. Mineralogy and elemental composition did not correlate with TOC concentration.The relationships between surface area, TOC and total nitrogen (TN) can be linked to the hydrodynamic and sedimentological conditions of the Squamish Delta. As a result, the Squamish Delta is a useful modern analogue for the formation of petroleum source rocks in ancient deltaic environments, where TOC concentrations are often significantly lower than those in source rocks formed in other geological settings.     

Oil-generating coals of the San Juan Basin, New Mexico and Colorado, U.S.A.

Coal beds of the Upper Cretaceous Fruitland Formation in the San Juan Basin of northwestern New Mexico and southwestern Colorado have significant liquid hydrocarbon generation potential as indicated by typical Rock-Eval Hydrogen Indexes in the range of 200–400 mg hydrocarbon/g organic carbon (type II and III organic matter). Small, non-commercial quantities of oil have been produced from the coal beds at several locations. The oils are characterized by high pristane/phytane (ca 4) and pristane/n-C₁₇ ratios (ca 1.2), abundant C₂₁₊ alkanes in the C₁₀₊ fraction with a slight predominance of odd carbon-numbered n-alkanes, abundant branched-chain alkanes in the C₁₅₊ region, and a predominance of methylcyclohexane in the C₄----C₁₀ fraction. The oils are indigenous to the Fruitland Formation coals and probably migrated at thermal maturities corresponding to vitrinite reflectance values in the range 0.7–0.8%. Although the oils found to date are not present in commercial amounts, these findings illustrate the potential of some coals to generate and expel oil under conditions of moderate thermal heating.     

Dinosterol δD values in stratified tropical lakes (Cameroon) are affected by eutrophication

In freshwater settings, dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (δD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested δD values of dinosterol in both particulate organic matter (POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the δD values of dinosterol correlated with lake water δD values, confirming a first order influence of source water δD values. However, we observed that sedimentary dinosterol was D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface (E_h OAI). The observations suggest that paleohydrologic reconstruction from δD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water δD values, with more positive dinosterol δD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C₃₄ botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and ββ-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with E_h OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol δD values to evaluate the effect of lake trophic status on sedimentary dinosterol δD values, as a basis for accurately reconstructing tropical lake water δD values.     

Lipid geochemistry of Lake Kinneret

Lake Kinneret, a relict lake from the Neogene, is characterised by the dominance among its phytoplankton of the dinoflagellate Peridinium cinctum. The lipid geochemistry of Lake Kinneret is discussed herein in terms of the biology, chemistry and hydrology of the lake. Lipids isolated from two sediment sections (surface and 15 cm deep), obtained from the deepest point of Lake Kinneret, include: (1) 4α-methyl-5α (H)-stanols and related derivatives characteristic of P. cinctum, the novel sterol 4a-methylgorgosterol, and peridinosterol and 4α-methylgorgostanol, not previously reported to occur in lacustrine sediments; (2) C₃₀ and C₃₂ alkane-1,15-diols, not previously reported to occur in contemporary lacustrine deposits, and (3) products of early diagenesis. Many similarities were observed with the more widely studied marine dinoflagellates and marine sediments with dinoflagellate input.     

Sediment lipid biomarkers as recorders of the contamination and cultural eutrophication of Lake Erie, 1909–2003

Lake Erie is a large lake that has a well documented history of changes in organic matter (OM) production. We have evaluated the hydrocarbon (HC), fatty acid (FA) and fatty alcohol (OL) contents of a century-long sediment core from the lake as recorders of the changes in biogeochemical processes that accompanied its eutrophication. Although variations in the concentrations of the three groups of biomarkers are closely linked to changes in primary productivity within the lake, their molecular distributions reflect different aspects of the biogeochemical processes affecting OM preservation and accumulation. Even numbered n-alkenes from C₁₆–C₂₂ constitute an important fraction of the HC composition. Even numbered dominance is observed in the n-alkanes < C₂₂, and an odd dominance occurs in higher homologs. The source of this unusual HC distribution is uncertain, but may be microorganism populations sustained by anthropogenic oil contaminants. Changes in the relative inputs from terrigenous and aquatic sources are reflected in variations in the distributions of the OLs, whereas the FAs only reflect aquatic input and a dominance of aquatic organisms. Post-burial diagenetic alteration of OM is especially evident from decreases in the unsaturated and branched FAs with depth.     

Free and bound lipids in recent (1835–1987) sediments from Santa Barbara Basin

In order to study the first steps of incorporation of lipids in recent organic-rich sediments into free and bound fractions, we have selected the Santa Barbara Basin, off California. This basin with a maximum of 590 m water depth is characterized by high phytoplankton production in surface waters and a low oxygen content in bottom waters. Sediments show the following features: high sedimentation rate ≈ 4 mm/yr, no bioturbation, and development of a bacterial mat community at the surface with predominance of sulphur-oxidizing bacteria trapping particles, thus preventing re-distribution of sediment, which permit a unique observations of organic sedimentation on a new few years basis.A sediment core has been divided into 2 cm thick slices corresponding to a time resolution of ≈5 years, from 1835 up to 1987. Samples have been analyzed using a multi-parameter approach, such as for plankton species identification, carbon stable isotope ratios and, as reported here, for lipid organic tracers. Organic tracers have been analyzed in the sterol and fatty series for both free and bound compounds by gas chromatography and gas chromatography/mass spectrometry.The incorporation processes of sterols with depth appear different for free and bound compounds. Total free sterol concentrations show high values in surface sediments (≈ 100 μg/g dry sediment), rapidly decrease up to ≈ 10 cm depth and remain at a constant value of 30–40 μg/g. Total bound sterol concentrations show low values in surface sediment (13.6 μg/g), and vary irregularly with depth up to a value of 55 μg/g at 7.3 m, and then remain constant at 25–26 μg/g.Profiles of evolution with depth of free C₂₇, C₂₈ sterols show a regular decrease, whereas C₂₉ sterols show an irregular decrease with anomalies at 7.3 cm (approximately age: 1977–1978) and at 14.1 cm (1962–1964). Profiles of bound compounds are rather different, very regular for C₂₈ sterols, irregular with oscillations for C₂₇ and C₂₉ sterols at 7.3, 12.2 and 18.0 cm (1954–1956), suggesting a different mode of incorporation and probably different inputs for C₂₇, C₂₉ and C₂₈ sterols.The study of the kinetics of degradation of sterols between surface (1987) and 10.5 cm (1968–1972) shows that C₂₇ compounds are degraded at a slight higher rate (0.53 μg/μg of initial C₂₇ concentration/ year) than are C₂₈ and C₂₉ compounds (0.047 μg/μg of initial concentration/year). An intermediate value is found for brassicasterol: 0.049 μg/μg/year.     

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America

Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C₂₁-C₃₁) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C₂₁n-alkane, and the C₂₅ and C₂₇ alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C₃₀ 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U₃₇^K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U₃₇^K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.     

Lignin is preserved in the fine silt fraction of an arable Luvisol

Knowledge about the fate of individual biomolecules during the decomposition process in soil is limited. We used the natural isotopic label introduced by 23 years of continuous maize cropping, together with compound specific ¹³C isotope analysis, to study lignin monomers in particle size fractions of a Luvisol. Isotope data indicated apparent decadal turnover times for lignin. A kinetic model suggests the existence of a fast and a slow decomposing lignin pool in the soil, reconciling a low stock-to-input ratio with decadal turnover times. We found new, maize-derived lignin primarily in the 63–2000 μm fraction, whereas old, C₃-derived lignin from the pre-maize vegetation had accumulated mainly in the silt (2–20 μm) fraction. This distribution of lignin differed from that of total organic carbon, which was concentrated in the <2 μm fraction. Old, C₃-derived carbon in all the soil fractions was depleted in lignin compared to new, maize-derived carbon. The observation that the 2–20 μm fraction was less depleted than the others indicates that lignin preservation is particle size specific, but the underlying mechanism controlling its preservation is not clear.     

New aromatic biomarkers in sulfur-rich coal

A molecular study of linear, branched and isoprenyl alkylbenzene skeletons and alkenylbenzenes in the soluble fraction extracted from a sulfur-rich Utrillas coal was carried out using gas chromatography–mass spectrometry (GC–MS). The presence of C₂₄–C₂₈ diaromatic compounds, not previously reported in coals, suggests that photosynthetic green sulfur bacteria may have made an input of organic matter to these coals. The unsaturated linear alkenylbenzenes and isoprenyl (C₁₅ and C₂₀) alkylbenzene skeletons are also described for the first time in the soluble fraction of geological samples.     

Vertical distribution and source identification of polycyclic aromatic hydrocarbons in anoxic sediment cores of Chini Lake, Malaysia: Perylene as indicator of land plant-derived hydrocarbons

Four anoxic sediment cores were collected from Chini Lake, Malaysia in order to investigate the variability of polycyclic aromatic hydrocarbon (PAH) and perylene concentrations. The study also determined significant differences of perylene concentrations in different sediment layers. Total PAH concentrations ranged from 248 to 8098 ng g⁻¹ in the samples. Diagnostic PAH ratios such as methylphenanthrenes/phenanthrene (MP/P), phenanthrene/anthracene (P/A) and fluoranthene/(fluoranthene + pyrene) (Fl/(Fl + Py) revealed a dominance of pyrogenic influences and partial petrogenic inputs to the top sediment layers. Perylene concentrations were high in the top layers (<12 cm) and increased with increasing depth. There is a significant positive correlation (r = 0.705, p = 0.01) between perylene concentrations and TOC. Analysis of variance (ANOVA) and LSD revealed significant differences (p < 0.05) in TOC-normalized perylene concentrations between the upper (<12 cm) and bottom layers (>12 cm). The average perylene concentrations accounted for 26–50% (0–12 cm) and 50–77% (12–36 cm) of pentacyclic-aromatic hydrocarbon isomers (PAI) present whereas it made up 10–34% (0–12 cm) and 46–66% (12–36 cm) of the total PAH. The average pyrene concentrations decreased with increasing depth and accounted for 62% (0–3 cm), 20–23% (3–12 cm) and 3–1.4% (12–36 cm) of perylene present. The results of hierarchical cluster analysis based on these ratios suggested different input sources for the top and bottom layers. It is concluded that the activity of termites on woody plants produced perylene which is supplied to the lake by run-off from the heavy and frequent rains in this Asian tropical climate. In addition, there was also in situ formation of perylene in the bottom layers due to diagenetic processes.     

Gas accumulation from oil cracking in the eastern Tarim Basin: A case study of the YN2 gas field

Oil and gas exploration in eastern Tarim Basin, NW China has been successful in recent years, with several commercial gas accumulations being discovered in a thermally mature to over-mature region. The Yingnan2 (YN2) gas field, situated in the Yingnan structure of the Yingjisu Depression, produces gases that are relatively enriched in nitrogen and C₂₊ alkanes. The δ¹³C₁ (−38.6‰ to −36.2‰) and δ¹³C₂ values (−30.9‰ to −34.7‰) of these gases are characteristic of marine sourced gases with relatively high maturity levels. The distributions of biomarkers in the associated condensates suggest close affinities with the Cambrian–Lower Ordovician source rocks which, in the Yingjisu Sag, are currently over-mature (with 3–4%Ro). Burial and thermal maturity modeling results indicate that paleo-temperatures of the Cambrian–Lower Ordovician source rocks had increased from 90 to 210 °C during the late Caledonian orogeny (458–438 Ma), due to rapid subsidence and sediment loading. By the end of Ordovician, hydrocarbon potential in these source rocks had been largely exhausted. The homogenization temperatures of hydrocarbon fluid inclusions identified from the Jurassic reservoirs of the YN2 gas field suggest a hydrocarbon emplacement time as recent as about 10 Ma, when the maturity levels of Middle–Lower Jurassic source rocks in the study area were too low (<0.7%Ro) to form a large quantity of oil and gas. The presence of abundant diamondoid hydrocarbons in the associated condensates and the relatively heavy isotopic values of the oils indicate that the gases were derived from thermal cracking of early-formed oils. Estimation from the stable carbon isotope ratios of gaseous alkanes suggests that the gases may have been formed at temperatures well above 190 °C. Thus, the oil and gas accumulation history in the study area can be reconstructed as follows: (1) during the late Caledonian orogeny, the Cambrian–Lower Ordovician marine source rocks had gone through the peak oil, wet gas and dry gas generation stages, with the generated oil and gas migrating upwards along faults and fractures to form early oil and gas accumulations in the Middle–Upper Ordovician and Silurian sandstone reservoirs; (2) since the late Yanshanian orogeny, the early oil accumulations have been buried deeper and oil has undergone thermal cracking to form gas; (3) during the late Himalayan orogeny, the seals for the deep reservoirs were breached; and the gas and condensates migrated upward and eventually accumulating in the relatively shallow Jurassic reservoirs.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

The occurrence of norlupanes and bisnorlupanes in oils of Tertiary deltaic basins

Lupanoid hydrocarbons are known to occur in several petroleum systems, and lupane (C₃₀) has recently been confirmed to exist in several crude oils. In contrast, norlupanes (C₂₉) and bisnorlupanes (C₂₈) are rarely observed in oil. All of these compounds are considered to derive from natural products of angiosperms, and numerous examples of their functionalized analogs are known. The occurrence of C₂₈ and C₂₉ lupanoids in biochemical and geochemical systems is reviewed here, and the presence and origin of their hydrocarbon analogs in crude oils are examined in detail. Although direct biochemical precursors for the lupane of crude oil are evident, such precursors for norlupane and bisnorlupane are not obvious. Nor is it clear if the C₂₈ and C₂₉ analogs are diagenetic descendants from the lupane structure. Adding additional confusion is the occurrence of these analogs in oils which show numerous indications of post-source molecular addition during migration and entrapment, making it unclear if they originate from a conventional source rock or from carrier or seal rock. Despite these uncertainties, there is extensive potential – some of which has already been realized – to use these compounds in oil–oil and oil-source rock correlations, particularly in instances where extensive biodegradation has occurred. Deconvolution of the time(s) of introduction of norlupane and bisnorlupane into the fluid – as well as various other hydrocarbons, including olefins – also provides great potential as a tool for mapping the migration history of an oil.     

Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin

Natural gas from the Zhaolanzhuang field of the Jizhong Superdepression, Bohai Bay Basin contains the highest proportions of H₂S (40–92%) among the sour gases encountered in China. The gas payzones include the Eocene–Oligocene Kongdian Formation (Ek) and the Es4 member of the Shahejie Formation. The sedimentary sequence consists of halite, anhydrite, carbonate, sandstone and shale interbeds deposited in the evaporative brackish water lacustrine – salt lake setting. In the deepest part of the Jinxian sag, the total thickness of evaporites is more than 1000 m, of which halite accounts for over 40%. Various organic-rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window (with a depth of 2500–3500 m), and likely the source for the oil and sour gas in the Zhaolanzhuang field. The temperatures of the gas reservoirs range from 75 to 100 °C, too low for significant thermochemical sulfate reduction. The co-occurrence of abundant elemental sulfur with the sour gas and the δ³⁴S values of the various sulfur-containing compounds indicate that the H₂S gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred.     

A natural pyrolysis experiment — hopanes from hopanoic acids?

The formation or generation of hopanes are important processes during both the natural heating of organic-rich sediments and laboratory pyrolysis experiments. Molecular maturity parameters as well as the amounts (ng/g rock) of the C₃₁ hopanes and C₃₀–C₃₂ hopanoic acids were quantified in a Jurassic silty shale horizon (Isle of Skye, Scotland) as a function of distance from an igneous intrusion. The maturity profiles of the homohopanes and the hopanoic acids are comparable. There is also a correlation between the decreasing amounts of C₃₀–C₃₂ hopanoic acids and concomitant increases in C₂₉–C₃₁ hopanes suggesting that free hopanoic acids could be one potential source of hopanes in this particular horizon. Other possible sources could include hopanoic acids that are bound into the macromolecular fraction.     

The effect of biodegradation on polycyclic aromatic hydrocarbons in reservoired oils from the Liaohe basin, NE China

The occurrence and distribution of polycyclic aromatic hydrocarbons (PAHs) has been studied in oil columns from the Liaohe basin, NE China, characterized by varied degrees of biodegradation. The Es3 oil column has undergone light to moderate biodegradation – ranging from levels 2 to 5 on the [Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ, p. 363] scale (abbreviated as ‘PM level’) – while the shallower Es1 column has undergone more severe biodegradation, ranging from PM level 5 to 8. Both columns show excellent vertical biodegradation gradients, with degree of biodegradation increasing with increasing depth toward the oil–water contact (OWC). The compositional gradients in the oil columns imply mass transport control on degradation rates, with degradation occurring primarily at the OWC. The diffusion of hydrocarbons to the OWC zone will be the ultimate control on the maximum degradation rate. The chemical composition and physical properties of the reservoired oils, and the ‘degradation sequence’ of chemical components are determined by mixing of fresh oil with biodegraded oil.The PAH concentrations and molecular distributions in the reservoired oils from these biodegraded columns show systematic changes with increasing degree of biodegradation. The C₃₊-alkylbenzenes are the first compounds to be depleted in the aromatic fraction. Concentrations of the C_0–5-alkylnaphthalenes and the C_0–3-alkylphenanthrenes decrease markedly during PM levels 3–5, while significant isomer variations occur at more advanced stages of biodegradation (>PM level 4).The degree of alkylation is a critical factor controlling the rate of biodegradation; in most cases the rate decreases with increasing number of alkyl substituents. However, we have observed that C₃-naphthalenes concentrations decrease faster than those of C₂-naphthalenes, and methylphenanthrenes concentrations decrease faster than that of phenanthrene. Demethylation of a substituted compound is inferred as a possible reaction in the biodegradation process.Differential degradation of specific alkylated isomers was observed in our sample set. The relative susceptibility of the individual dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, pentamethylnaphthalene, methylphenanthrene, dimethylphenanthrene and trimethylphenanthrene isomers to biodegradation was determined. The C₂₀ and C₂₁ short side-chained triaromatic steroid hydrocarbons are degraded more readily than their C_26–28 long side-chained counterparts. The C_21–22-monoaromatic steroid hydrocarbons (MAS) appear to be more resistant to biodegradation than the C_27–29-MAS.Interestingly, the most thermally stable PAH isomers are more susceptible to biodegradation than less thermally stable isomers, suggesting that selectivity during biodegradation is not solely controlled by thermodynamic stability and that susceptibility to biodegradation may be related to stereochemical structure. Many commonly used aromatic hydrocarbon maturity parameters are no longer valid after biodegradation to PM level 4 although some ratios change later than others. The distribution of PAHs coupled with knowledge of their biodegradation characteristics constitutes a useful probe for the study of biodegradation processes and can provide insight into the mechanisms of biodegradation of reservoired oil.