Unravelling the impacts of palaeoecology, palaeoenvironment and lithofacies on sedimentary organic NSO compounds
The organic matter in sedimentary rocks that is soluble in common organic solvents comprises a complex mixture of hydrocarbons and NSO compounds (nitrogen, sulfur and oxygen bearing). The N, S, O atoms can be derived from both biogenic sources and abiogenic processes, thus organic NSO compounds are intrinsically well suited to record precursor biotic and palaeoenvironmental signatures. In addition, their physical properties are such that they possess the capacity to interact specifically with minerals of distinct surface chemistry, so they can provide precise indications for elucidating lithofacies-influenced fractionation during petroleum expulsion and migration. Until relatively recently, their molecular-level characterization was limited to the low-molecular-weight fraction, but now this range has been extended, thanks to ultra-high resolution mass spectrometry. This thesis utilises that technique to provide information on biomass input, depositional conditions and lithofacies-influenced fractionation using heavy NSO compounds, thus complementing and supplementing information contained in lower molecular weight biomarkers. The impact of biological sources (marine algae, terrestrial plants and lacustrine Botryococcus braunii) on organooxygen and organonitrogen compounds was revealed through the investigations on the solvent extracts of immature–early mature rock samples from the marine Dynow, Schöneck, Posidonia formations, the lacustrine Wealden Formation, and the terrestrial Waikato and Brunner coal measures. Coals, being the in-situ deposits of terrestrial plant remains, primarily consist of aromatic polyoxygenated Ox and N1Ox compounds, representing degradation products of lignin and tannin such as phenolic ketones and phenolic carboxylic acids as well as their condensation products with the proteinaceous degradation intermediates. Aliphatic Ox moieties derived from the plant protective substances (mainly waxes and cutan) show a pronounced even or odd carbon number predominance among the C23–C33 range with C26, 28, 30 or C27, 29, 31 as the major homologs. In contrast, marine and lake microbial communities contribute abundant middle-chain C22, C24 or C23, C25 Ox compounds. The marine rock extracts are furthermore characterized by abundant organonitrogen compounds, especially the N2 and N2Ox classes, interpreted as signatures of protein-rich marine algae. The highly aliphatic algaenan of Botryococcus braunii sterically protects its oxygen-bearing groups leading to a great abundance of Ox compounds, furthermore, it characterizes the Botryococcus braunii source by substantial heteroatomic compounds containing more than 40 carbon atoms. Organosulfur compounds, as inorganic-organic incorporation products during early diagenesis, were characterized in extracts of the aforementioned lacustrine and marine samples to obtain both palaeoecological and palaeoenvironmental information. The iron-deficient sulfidic depositional settings of the Posidonia and Schöneck formations are reflected by abundant organosulfur compounds bearing up to three sulfur atoms. The high ratios of reduced relative to oxidized forms (Sz versus SzOx) further illustrate the restricted presence of oxidants at the oxic-anoxic interfaces. The observed prominent enrichment of organosulfur compounds containing 40, 35, 30, 25 carbon atoms are associated with the selective preservation of polyfunctionalized biomolecules via sulfurization, such as C40 carotenoids, C35 bacteriohopanepolyols, C30 unsaturated tetracyclic polyprenoid alcohols, C30 or C25 highly branched isoprenoid (HBI) polyenes. The strong enrichment of sulfurized C35 bacteriohopanepolyols can be developed as an indicator of the low levels of oxygen exposure prior to sulfurization, which occur only in the Dynow and Schöneck formations. The prominent enrichment of sulfurized carotenoids is typically associated with high primary productivity. While the strongly enriched sulfurized HBI polyenes are indicative for diatom blooms, the precursors of C30 pentacyclic polyprenoid organosulfur compounds are more abundant in fresh/brackish water algae. For petroleum systems having undergone expulsion and migration, the composition of NSO compounds retained in rocks is determined not only by their origin but also by their interactions with different mineral surfaces. The controls of lithofacies on NSO inventories were investigated using examples of unconventional systems with the three globally most significant lithofacies, namely the biogenic carbonate-rich Niobrara Shale, the biogenic quartz-rich Barnett Shale and the detrital clay-rich Posidonia Shale. Extracts of the siliciclastic Barnett and Posidonia samples reveal high fractions of NSO compounds confirming their generally higher retention capacities for the polar compounds. While biogenic quartz preferentially preserves and retains organonitrogen compounds, the more polar acidic organooxygen compounds are preferably retained by clay minerals. Within the Niobrara and the Barnett systems, lithofacies variations respectively in carbonate and biogenic quartz content lead to intra-formation migration. The low-polarity organonitrogen compounds are preferentially retained in both Niobrara (to a less extent) and Barnett source rock units. While the highly alkylated small acidic NSO molecules preferably migrate out of the Niobrara carbonate source rock units, acidic NSO compounds irrespective of molecule size and alkylation degree are all strongly retained in the biogenic quartz-rich Barnett source showing no fractionation.
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