Dinoflagellates are a major component of the marine microplankton and, from fossil evidence, appear to have been so for the past 200 million years. In contrast, the pre-Triassic record contains only equivocal occurrences of dinoflagellates, despite the fact that comparative ultrastructural and molecular phylogenetic evidence indicates a Precambrian origin for the lineage. Thus, it has often been assumed that the dearth of Paleozoic fossil dinoflagellates was due to a lack of preservation or recognition and that the relatively sudden appearance of dinoflagellates in the Mesozoic is an artifact of the record. However, new evidence from a detailed analysis of the fossil record and from the biogeochemical record indicates that dinoflagellates did indeed undergo a major evolutionary radiation in the early Mesozoic.

Thiadiamondoids and diamondoidthiols are orders of magnitude more abundant in oil altered by thermochemical sulfate reduction (TSR) than they are in nonaltered oil. This suggests that thiadiamondoids and diamondoidthiols form during TSR. In order to prove this hypothesis, we perform laboratory TSR experiments on diverse organic compounds using sodium sulfate as an oxidant in the presence of elemental sulfur and deionized water at 200 and 350 °C for 48 and 96 h under acidic conditions (pH ) 4). Our results show that thiadiamondoids and diamondoidthiols can be created from non-sulfur-containing diamondoids by TSR. It seems likely that diamondoid species are organic precursors of thiadiamondoids and diamondoidthiols. In addition, thiocholesterol yields trace quantities of dimethyl-2-thiaadamantanes when heated with montmorillonite at 200 °C, suggesting that these diamondoid derivatives may partly originate by molecular rearrangement of polycyclic sulfides and thiols in the presence of acidic clay minerals since they also exist in crude oil that has not undergone TSR. The present study of these heteroatomic cage compounds improves understanding of TSR and can be used to reduce risk in petroleum exploration.

Compounds significant to the petroleum chemist concerned with petroleum exploration are those hydrocarbons possessing biological marker characteristics, that is, possessing intact steroid, terpenoid, and isoprenoid skeletons. These hydrocarbons are so closely related to the compounds occurring in the living organism from which petroleum was formed that they are capable of yielding very specific information regarding source, maturation, migration, and biodegradation of petroleum. Examples how source shales can be related to petroleum reservoirs using computerized gas chromatography/mass spectrometry, (GC/MS) are shown. The terpane GC/MS multiple array processor (MAP) approach can be used to differentiate source shales of different degrees of maturity and, thus, to determine source rock quality. Heavily biodegraded oils can be characterized and correlated by GC/MS fragmentograms of steranes and terpanes. Ratios of specific biomarkers obtained by quantitation from GC/MS data are used to differentiate oils of different degrees of migration. Examples from the exploration arena worldwide are given to illustrate these applications.

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Concentrations of biomarkers remaining in straight-run refinery products of crude oil feedstock are mainly controlled by relative volatility. Thermal cracking exerts a second-order control on biomarker compositions of these products. Factors controlling biomarker concentrations and distributions in processed materials are more complex and include volatility, thermal stability, generation from heavier precursors, and the effects of catalysts and hydrogen pressure. Differential volatility of compounds within each biomarker class and sharp temperature gradients defining each distillation cut complicate interpretation of source-and maturation-dependent biomarker parameters used by petroleum exploration geochemists. For example, conventional biomarker parameters could be interpreted to indicate that the residuum is unrelated to and less mature than the feedstock. Partitioning of biomarkers by volatility among distillation cuts affects the compound ratios used for assessment of thermal maturity where the numerator and denominator consist of early-and lateeluting biomarkers, respectively. For example, the residuum, which is enriched in the least volatile biomarkers, shows diasterane/sterane, tricyclic terpanell7a(H)-hopane, and triaromatic steroid TA-(I)/TA(I+II) ratios indicating lower maturity than the feedstock. The residuum also shows lower sterane isomerization ratios than the feedstock, possibly due to release of epimers showing the immature stereochemical configuration (e.g., 20R) from heavier precursors during cracking. Hydrocracked products lack mono-and triaromatic steroids due to their destruction at high temperatures and hydrogen pressures. Increased sterane concentrations in the hydrocracker product could be due to their generation from bound precursors in the feedstock. Source-and maturity-related biomarker parameters for hydrocracker product and TKN feed (gas oil feed for the hydrocracker) are nearly identical. Hydrofining of vacuum gas oil does not significantly alter the concentrations or distributions of terpanes, except Tm [17a(H)-22,29,30-trisnorhopanel. Tm appears less stable to hydrofining than other terpanes. Sterane and aromatic steroid concentrations generally decrease during hydrofining without changes in distribution, except the diamonoaromatic steroids. Fluid catalytic cracking reduces the amounts of most biomarkers without changing their distributions significantly. Fluid catalytic cracking product lacks monoaromatic steroids. Medium coker gas oil is highly volatile and lacks biomarkers, except C19 and CZO tricyclic terpanes. Coking of residuum severely reduces and alters distributions of biomarkers. High concentrations of Tm, 5a,14a,17a(H)-27-norcholestane 20R, and other compounds indicate generation from heavier precursors. Heavy coker gas oil lacks monoaromatic steroids, probably due to low concentrations in the residuum and subsequent destruction during coking.

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