Pierre Albrecht

Simulation of the geochemical transformations of sterols: superacid effect of clay minerals

Abstract The heating of cholestanol 2a with kaolinite (140°C, 16 hr) mainly leads to a mixture of steranes (40%) and sterenes (35%). The major compounds which are formed have been identified as 5α(H)- and 5β(H)-cholestanes, 4a and 4b, the diacholestenes, 5, the diacholestanes, 6, and the D-homodiacholestanes 7. With the exception of the latter, these compounds have all been observed previously in ancient sediments and crude oils. Several of these compounds are identical or bear structural similarities with those obtained from cholestanol at low temperature in acidic or superacidic conditions. The comparison with the results obtained using other minerals (montmorillonite, talc, alumina, polyphos-phate-treated montmorillonite) shows that the observed transformations are catalysed by superacid sites which are present in kaolinite and montmorillonite.

Predictive microbial biochemistry — from molecular fossils to procaryotic membranes

Abstract Many of the complex organic molecules in geological sediments are derived from the membrane lipids of microorganisms. Their identification has led to the recognition of a new class of widespread microbial lipids, the hopanoids, surrogates of sterols in the membranes of many procaryotes. Other sedimentary lipids suggest the existence of other families of membrane lipids in microorganisms yet to be identified.

Rearranged sterenes in a shale: occurrence and simulated formation

The lipids extracted from a bituminous shale contained (20R)- and (20S)-5β,14β-dimethyl-18, 19-dinor-8α,9β,10α-cholest-13(17)-enes [(I) and (II); R = H]. Also present were homologues carrying nuclear methyl groups and variously alkylated side chains.Cholestanol has been treated under chemical and mineralogical conditions to yield these backbone-rearranged products. The value of these reactions as simulators of geological diagenesis, and the utility of these sterenes as biological markers in the geological environment is discussed.

The microbial input in carbonate-anhydrite facies of a sabkha palaeoenvironment from Guatemala: A molecular approach

Abstract Nine representative rock samplesincluding four anhydrites, an oil-stained dolomite and a laminated mudstone (source rock) from a sabkha-type palaeoenvironment (La Felicidad 1 well, Guatemala) have been compared in order to tentatively characterize each facies by a geochemical signature. Each sample has been studied by using basic parameters (TOC, Rock-Eval pyrolysis yields) and analyses of alkanes and aromatics by computerized GC-MS. The following families of compounds have been reviewed: n-alkanes, branched and cyclic alkanes, n-alkylbenzenes, benzohopanes, monoaromatic steroids, D-ring aromatic 8,14-secohopanoids, alkylnaphthalenes, alkylphenanthrenes, alkyldibenzothiophenes and triaromatic steroids. Despite close geochemical relationships of all the sample, a more in-depth investigation taking into account molecular patterns and semi-quantitative data reveals significant differences among the samples. Anhydrites may be differentiated from carbonates which illustrates the efficiency of organic geochemistry in environmental palaeoreconstruction even in organiccarbon-lean sediments (TOC = 0.04–0.08% in anhydrites). In these anoxic, marginally mature sediments the strong predominance of triterpanes over steranes (TT to ST > 20) associated with other molecular features leads to the conclusion that the main contribution to the organic matter of all facies is by bacteria. Several prominent components including novel families (iso-C23 and iso-C27 alkanes, tetracyclic terpanes, D-ring aromatic 8–14-secohopanoids, 1-phenylpentadecane, undecylnaphathalene and their mono- and dimethylated counterparts) have been interpreted as derived from halophilic bacteria. These conclusions may be extended to all crude oils from the same area which may be considered as true bacterially-derived petroleums. The La Felicidad 1 well data are discussed with reference to other palaeosabkha-type sequences from the SW Aquitaine Basin (France) in which the bacterial input is less important. However, in these sabkha sequences 1-phenylpentadecane, undecylnaphthalene and related structures have also been identified.

Tri- and tetraterpenoid hydrocarbons in the Messel oil shale

Abstract The high molecular weight constituents of the branched and cyclic hydrocarbon fraction of the Messel oil shale (Eocene) have been examined by high resolution gas chromatography and combined gas chromatography-mass spectrometry. The following compounds are present: perhydrolycopene (1; lycopane), together with one or more unsaturated analogues with the same skeleton; a series of 4-methylsteranes (2c) in higher abundance than their 4-desmethyl analogues; two series of pentacyclic triterpanes, one series (C27-C32) based on the hopane structure (3a-e), and the other (C27-C31) based on the 17α-H hopane structure (3a-d, 17αH); and an intact triterpene hop-17 (21)-ene [3c, Δ 17(21)]. Only two additional triterpanes were detected in minor concentrations, viz. 30-normoretane (3b, 21αH) and a C31 triterpane based on the hopane/lupane-type skeleton. The presence of these compounds suggests a significant microbial contribution to the forming sediment. Comparison of the tri- and tetraterpenoid hydrocarbons with those of the Green River Shale indicates differences in the organisms contributing to the two sediments.

Polar Lipids of Archaebacteria in Sediments and Petroleums

Glycerol tetraethers with head-to-head isoprenoid 40-carbon chains that are typical of archaebacteria, in particular of methanogens, were identified in the polar lipids of sediments and petroleums. These structures are at least partially preserved in the subsurface beyond the stage of petroleum formation. Their identification provides further evidence that a significant part of geological organic matter derives from the lipids of membranes of microorganisms.

A novel pathway of soil organic matter formation by selective preservation of resistant straight-chain biopolymers: chemical and isotope evidence

A resistant soil organic residue, “humin”, has been analyzed by solid-state 13C-NMR and pyrolysis followed by molecular and 13C determination of the pyrolysate alkane–alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway is additional to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.

Novel families of tetra- and hexacyclic aromatic hopanoids predominant in carbonate rocks and crude oils

Two novel families of monoaromatic geomarkers related to hopanoids have been separated and characterized in sediments and crude oils: the benzohopanes (C32–C35) 1 and the ring D aromatized 8,14-secohopanoids (C27–C35) 4 and 5. Both series are particularly abundant in many carbonate samples, where they amount to approximately 100 p.p.m. per component in some crude oils, but can also be detected in other environments. The benzohopanes have been separated as pure compounds by high performance liquid chromatography and identified by X-ray diffraction. The structure of the 8,14-secohopanoids mostly relies on spectroscopic data (NMR and MS) and comparison with synthetic analogs. Both families must be formed in the subsurface from bacterial C35 hopanoid precursors, but the benzohopanes seem to occur at an earlier stage of sedimentation. The two series appear in varying distributions in the different samples and may therefore be potentially useful as geochemical correlation parameters.

Geochemical studies on the organic matter from the Douala Basin (Cameroon)—I. Evolution of the extractable organic matter and the formation of petroleum☆

Abstract The soluble extracts, as well as the hydrocarbons from cores taken between 700 and 4000 m depth in the thick homogeneous sedimentary series of the Douala Basin (Cameroon), have been analyzed by gas chromatography and mass spectrometry. They display quantitative and qualitative changes due to the effect of diagenesis with burial. The progressive evolution of this detrital organic matter has been followed from the very immature stage to the generation of oil and gas. Thermal maturation experiments in the laboratory have confirmed the most important naturally occurring changes.

Novel tricyclic terpanes (C19, C20) in sediments and petroleums

Abstract Three homologs ( 1 – 3 ; C19,C20) of a tricyclic terpane series (C19 C300 widely occuring in sediments and petroleums have been identified as probable degradation products of tricyclohexaprenane 13 , a yet unknown tricyclic terpane skeleton.