H.L. ten Haven

Tetrahymanol, the most likely precursor of gammacerane, occurs ubiquitously in marine sediments

Abstract Tetrahymanol has been identified in several sediment samples from different depositional environments by gas chromatography-mass spectrometry and by coinjections with an authentic standard. Together with literature data this shows that tetrahymanol is likely to be widespread, which is in accordance with the ubiquitous occurrence of its presumed diagenetic product, gammacerane, in more mature sediments and crude oils. The diagenetic conversion of tetrahymanol to gammacerane most likely proceeds via dehydration and subsequent hydrogenation. The intermediate in this conversion, gammacer-2-ene, has been synthesized, and its presence in one sample confirmed by coinjections. The identification of tetrahymanol in marine sediments indicates either that protozoa of the genus Tetrahymena are widely distributed or that tetrahymanol is also a natural product of organisms other than Tetrahymena.

Origin and diagenetic transformations of C25 and C30 highly branched isoprenoid sulphur compounds : further evidence for the formation of organic sulphur compounds during early diagenesis

Abstract A number of C25 and C30 highly branched isoprenoid (HBI) sulphur compounds (E.G., thiolanes, 1-oxo-thiolanes, thiophenes, and benzo[ b ]thiophenes) with 2,6,10,14-tetramethyl-7-(3-methylpentyl)pentadecane and 2,6,10,14,18-pentamethyl-7-(3-methylpentyl)nonadecane carbon skeletons were identified in sediments, ranging from Holocene to Upper Cretaceous. These identifications are based on mass spectral characterisation, desulphurisation, and, in some cases, by comparison of mass spectral and relative retention time data with those of authentic standards. The presence of unsaturated C25 and C30 HBI thiolanes in a Recent sediment from the Black Sea (age 3–6 × 10 3 a) strongly supports their formation during early diagenesis. The co-occurrence of HBI polyenes (C25 and C30) and unsaturated HBI thiolanes (C25 and C30) possessing two double bonds less than the corresponding HBI polyenes, in this Recent sediment, testifies to the formation of unsaturated HBI thiolanes by a reaction of inorganic sulphur species with double bonds of the HBI polyenes. Furthermore, a diagenetic scheme for HBI sulphur compounds is proposed based on the identification of HBI sulphur compounds in sediment samples with different maturity levels.

Early diagenetic transformation of higher-plant triterpenoids in deep-sea sediments from Baffin Bay☆

Abstract A sediment sequence of early Miocene to Holocene from Ocean Drilling Program Site 645 in central Baffin Bay was found to contain abundant triterpenoids of higher-plant origin including monodesmethyl derivatives and other oxidative degradation products. 24-nor-triterpenoids of the oleanene, ursene, and lupane series were found in the nonaromatic hydrocarbon and alcohol fractions of the sediment extracts. Other diagenetic transformation products included 24,28- dinor -olean-17-ene and 24- nor -urs-12-en-1 1-one as well as their tentatively identified des -A counterparts. The identification of these novel degradation products was confirmed by synthesis of a suite of reference compounds. The simple reaction sequence applied to obtain 24- nor -urs-12-ene from 3α-acetoxyurs-12-en-24-oic acid is likely to happen in a similar manner during early diagenesis of organic matter in nature. These results have implications for the understanding of the diagenetic reaction sequence leading to the formation of other demethylated triterpenoids of terrigenous origin (e.g., 24,28- dinor -lupanes). 28,30- dinor -17α-hopane is sometimes the dominant hydrocarbon in the most deeply buried samples. Its co-occurrence with higher plant triterpenoids and the low abundance of other regular hopanoids is not considered an indication, however, that 28,30 dinor -17α-hopane is derived from a higher plant precursor molecule. It is suggested that two independent sedimentary processes are responsible for the presence of the two types of demethylated triterpenoids observed in Baffin Bay sediments.

The diagenetic fate of taraxer-14-ene and oleanene isomers

Abstract The extractable organic matter in Holocene to early Miocene deep-sea sediments from Site 645 (ODP Leg 105) in Baffin Bay contains almost exclusively biological markers of terrigenous origin. Pentacyclic triterpenoids of the α-(ursene) and β-amyrin (oleanene) type often occur as the most abundant compounds in the aliphatic hydrocarbon, ketone and alcohol fractions of the sediment extract. A specific diagenetic reaction involves the conversion of taraxer-14-ene into olean-12-ene in the upper 700 metres of the sedimentary sequence. The taraxerene-to-oleanene conversion is complete before the onset of isomerisation of diasterenes at C-20. In the sediment as well as in laboratory simulation experiments, olean-12-ene further isomerises to olean-13(18)-ene and olean-18-ene, the latter of which may be the direct precursor of 18β(H)- and 18α(H)-oleanane found in sediments containing more mature organic matter and in crude oils. The subsurface interconversion of these triterpenoid skeletons indicate that oleanane does not necessarily start life as an oleanoid.

Applications and limitations of Mango's light hydrocarbon parameters in petroleum correlation studies

Mango (1987) reported on the remarkable invariance of four isoheptanes in petroleums: regardless of the absolute concentration, the ratio of sums of concentration of (2-methylhexane + 2,3-dimethylpentane)/(3-methylhexane +2,4-dimethylpentane) remains constant, the K1 ratio value being around unity. Gas chromatographic analyses of an additional 500 oils contradict this invariance, as a suite of oils from the Cretaceous Northwest Basin (Argentina) deviate significantly by having a ratio of 1.67. Still, oils derived from the same structural kerogen have invariant ratios that can vary from a lowest value of 0.79 found for the Midland oil set (Mango, 1990) up to a highest value of 1.67 found for the Cretaceous Northwest Basin. n nMango (1990) postulated so-called parent-daughter schemes having an invariance in the ratio of isoheptanes and dimethylcyclopentanes, an invariance unique for a homologous set of oils, but possibly distinctly different from another suite of homologous oils. It will be shown here that this difference in invariance can be used in oil/oil correlation studies, and that this technique is especially powerful for oil/condensate correlation, condensates in which the concentration of biological markers is quite often below the limit of detection. Further, the concentration of light hydrocarbons, needed for this approach, can be determined from whole-oil gas chromatography without any pre-treatment, thus providing a rapid and cheap tool for correlation studies.

Δ2-Triterpenes: Early intermediates in the diagenesis of terrigenous triterpenoids

A series of Δ2-triterpenoids (oleana-2,12-diene, oleana-2,13(18)-diene, oleana-2,18-diene, ursa-2, 12-diene, and taraxera-2,14-diene) has been identified in Holocene buried mangrove sediments (Florida, USA) by comparison of the mass spectra of and by co-injections with authentic standards. In the Oligocene Brandon lignite (Vermont, USA) oleana-2,12-diene and oleana-2,13(18)-diene were also identified, along with two A-ring contracted amyrin derivatives. By comparison with the sedimentary occurrence of related analogues of oxyallobetulin and betulin and the demonstrated diagenetic formation of lup-2-ene from lupan-3-one, these novel terrigenous biological markers are considered diagenetic products formed by dehydration of precursor molecules, such as α- and β-amyrin, and taraxerol. The identification of these early intermediates brings us one step closer to the full understanding of the diagenetic fate of terrigenous triterpenoids.

Oil/oil and oil/source rock correlations in the Carpathian Foredeep and Overthrust, south-east Poland☆

Abstract The chemical composition of 35 crude oils and the extracts of potential source rock samples from south-east Poland were studied in detail by geochemical methods in order to understand their genetic relationships. Twenty-six oils, including three seep samples, were collected from Cretaceous to Oligocene reservoirs within the Carpathian Overthrust. Nine oils originated from Carboniferous to Miocene reservoirs within the Carpathian Foredeep. The rock samples studied were selected after Rock-Eval pyrolysis screening of a large suite of samples and consist of one Lower Cretaceous sample and five Oligocene Menilite shale samples (from three different outcrop locations all within the Carpathian Overthrust). Geochemical fingerprinting indicates at least two families of crude oils: the first, represented only by one oil, has no specific characteristics except for a light carbon isotope composition; the second family is characterized by the presence of 28,30-dinor-hopane, oleanane and a highly-branched isoprenoid C25 alkane. Within this latter family four subfamilies were recognized: (A) 19 oils having the aforementioned characteristics, (B) seven oils similar to subfamily A, but with a relatively high sulphur content, (C) one oil with high abundance of higher-plant derived triterpanes, and (D) seven oils with abundant 28,30-dinor-hopane. Although maturity influences the relative abundance of 28,30-dinor-hopane, it suggested that subfamilies A, B, and C had an initial low 28,30-dinor-hopane content, whereas subfamily D an initial high 28,30-dinor-hopane content. Only one oil of subfamily A could potentially be a high maturity counterpart of subfamily D. A Palaeozoic source rock is suggested for the first family, and the second family, including the majority of the pils from the Carpathian Foredeep, is most likely derived from the Oligocene Menilite Shales. An oil/source rock relationship with the Menilite Shale is supported by the presence (although not in all samples studied) of 28,30-dinor-hopane, oleanane or its precursors, and the highly-branched isoprenoid alkane. The strong facies variations observed within this shale gave rise to the different types of crude oils in the second family.

Identification and geochemical significance of cyclic di- and trisulphides with linear and acyclic isoprenoid carbon skeletons in immature sediments

Abstract Homologous series (C 15 -C 24 ) of novel 3- n -alkyl-1,2-dithianes and 3- n -alkyl-6-methyl-1,2-di-thianes have been identified in immature sediments. The identification of these compounds was based on comparison of mass spectra and Chromatographie data with those of synthesized 3-methyl-6-tridecyll, 2-dithiane. In addition, 4-methyl-3-(3,7,11-trimethyldodecyl)-1,2-dithiane, 4-(4,8,12-trimethyltridecyl)-1,2-dithiane, 5-methyl-4-(3,7,11-trimethyldodecyl)-1,2,3-trithiepane, and a 1,2-dithiane possessing a pentakishomohopane carbon skeleton were tentatively assigned on the basis of mass spectral characteristics, selective chemolysis, and desulphurisation. The occurrence of these cyclic di-and trisulphides with linear, acyclic isoprenoid and hopanoid carbon skeletons in thermally immature sediments indicates that inorganic polysulphides are incorporated into functionalised lipids during the early stages of diagenesis.

Occurrence of dammar-13(17)-enes in sediments: Indications for a yet unrecognized microbial constituent?

A pair of isomeric dammarenes has been identified in numerous Pleistocene to Jurassic sediment samples, most of them from continental margins. Their structures have been determined to be (20R)- and (20S)-dammar-13(17)-ene based on comparison of mass spectra and Chromatographic data with those of synthetic standards. Their saturated counterparts have also been found in a few samples. Based on rigorous nuclear magnetic resonance (NMR) studies with one synthetic standard and molecular mechanics calculations, it is suggested that geologically occurring dammaranes possess 13β,17α(H) stereochemistry. n nCompounds with a dammarane skeleton are well known natural products of several families of land plants, but the widespread occurrence of dammarenes in marine sediments suggests an alternative origin, supported by their carbon isotopic signature. Dammarenes can be envisaged as direct proton-induced cyclisation products from squalene and may represent a contribution from microorganisms. Although we are in favour of this hypothesis, an origin from geologically induced rearrangements of other compounds cannot, at present, be excluded.