Frederick J. Fago

The molecular fossil record of oleanane and its relation to angiosperms.

Oleanane has been reported in Upper Cretaceous and Tertiary source rocks and their related oils and has been suggested as a marker for flowering plants. Correspondence of oleanane concentrations relative to the ubiquitous microbial marker 17α-hopane with angiosperm diversification (Neocomian to Miocene) suggests that oleanane concentrations in migrated petroleum can be used to identify the maximum age of unknown or unavailable source rock. Rare occurrences of pre-Cretaceous oleanane suggest either that a separate lineage leads to the angiosperms well before the Early Cretaceous or that other plant groups have the rarely expressed ability to synthesize oleanane precursors.

Sedimentary 12-n-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae

Certain C30-steranes have been used for identifying sedimentary rocks and crude oils derived from organic matter deposited in marine environments. Analysis of a C30-sterane from Prudhoe Bay oil indicates that these C30-steranes are 24-n-propylcholestanes that apparently are derived from precursor sterols 24-n-propylidene-cholesterols and 24-n-propylcholesterol. These widely occurring sterols are biochemically synthesized in modern oceans by members of an order (Sarcinochrysidales) of chrysophyte algae. These data thus imply that C30-sterane biomarkers in sedimentary rocks and crude oils have a marine origin. Screening of a few organic-rich sedimentary rocks and oils from throughout the Phanerozoic suggests that these C30-steranes first appeared and, therefore, their source algae evolved between Early Ordovician and Devonian.

Paleozoic oil-source rock correlations in the Tarim basin, NW China

We studied a suite of 40 oils and extracts of purported source rocks from the Tarim basin in NW China. The main group of oils comes from Tazhong and Tabei wells, which sample the largest known petroleum accumulations in the basin. These oils can be statistically correlated with extracts of Ordovician rocks based upon high relative concentrations of 24-isopropylcholestanes and low relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. In contrast, extracts from Cambrian rocks have low relative concentrations of 24-isopropylcholestanes with high relative concentrations of dinosteranes, triaromatic dinosteroids, and 24-norcholestanes. Although some Tarim basin Cambrian rocks yield high total organic carbon contents, we see little evidence in the analyzed oil samples to suggest that they came from Cambrian source rocks.

Rearranged hopanes in sediments and petroleum

Abstract Two new rearranged hopanoid hydrocarbons have been isolated from a Prudhoe Bay crude, Alaska. 17α(H)-15α-methyl-27-norhopane was determined by X-ray crystallography. It is the first identified member of a new series of rearranged hopanes we propose to call “17α(H)-diahopanes.” Analysis by gas chromatography-mass spectrometry—mass spectrometry (GC-MS-MS) of the parents of m/z 191 in several crudes suggests that this compound is a member of a C 29 -C 34 series of 17α(H)-diahopanes common to many crude oils and sediments. In addition, a new member of the 18α(H)-neohopane series has also been elucidated. Determination of 18α(H)-17α-methyl-28,30-dinorhopane [18α(H)-30-norneohopane ], which we propose to nickname “C 29 Ts,” hinged upon advanced nuclear magnetic resonance (NMR) techniques (at 500 and 600 MHz) such as proton-detected 1 H- 13 C correlated spectra for the C-skeleton and Rotating-frame Overhauser Enhancement Spectroscopy (ROESY) for stereochemistry, as well as several other two-dimensional (2D) NMR techniques. This compound is the second known pseudohomolog of the neohopane series (together with 18α(H),22,29,30-trisnorneohopane, Ts), but the existence of additional pseudohomologs is still not clear. The structures of these rearranged hopanes are consistent with an origin by catalytic rearrangement from hopenes during early diagenesis. Carbon isotopic data collected on Ts, 17α(H)-diahopane, C 29 Ts, 17α(H)-22,29,30-trisnorhopane (Tm), 17α(H)-30-norhopane, and 17α(H)-hopane isolated from the Prudhoe Bay oil are in the -27 to -28%o δ 13 C range supporting mechanistic arguments based on structures that all are derived from common precursors. These δ 13 C values are slightly more positive than the whole Prudhoe Bay oil (-30.1%), suggesting that these hopanes may have been derived from heterotrophic or cyanobacteria in the paleoecosystem during deposition of its source rock. Molecular mechanics calculations predict relative thermal stabilities in the order 17α(H)-diahopanes > 18α(H)-neohopanes > 17α(H)-hopanes, suggesting new maturity parameters that may be useful into the late oil window.

Chemostratigraphic reconstruction of biofacies: Molecular evidence linking cyst-forming dinoflagellates with pre-Triassic ancestors

New data from numerous detailed mass-spectrometric studies have detected triaromatic dinosteroids in Precambrian to Cenozoic rock samples. Triaromatic dinosteroids are organic geochemicals derived from dinosterols, compounds known in modern organisms to be the nearly exclusive widely occurring products of dinoflagellates. We observed the ubiquitous occurrence of these dinosteroids in 49 Late Triassic through Cretaceous marine source rocks and the absence of them in 13 Permian-Carboniferous source rocks synergistic with the dinoflagellate cyst record. However, finding dinosteroids in lower Paleozoic and Precambrian strata presents challenging results for molecular paleontologists, evolutionary biologists, palynologists, and especially for those concerned with the food web at various times of biological crisis. Other than the few species known as parasites and symbionts, many other dinoflagellate species are important as primary producers. The presence of Precambrian to Devonian triaromatic dinosteroids gives chemostratigraphic evidence of dinoflagellates (or other organisms with similar chemosynthetic capabilities) in rocks significantly older than the oldest undisputed dinoflagellate fossils (dinoflagellate cysts from the Middle Triassic, ∼ 240 Ma), and older than the putative Silurian ∼ 420 Ma) dinocyst, Arpylorus antiquus (Calandra) Sargent, from Tunisia. This systematic chemostratigraphic approach can shed light not only on lineages of dinoflagellates and their precursors, but potentially on many other lineages, especially bacteria, algae, plants, and possibly some metazoans.

CARBON ISOTOPIC COMPOSITIONS OF 28,30-BISNORHOPANES AND OTHER BIOLOGICAL MARKERS IN A MONTEREY CRUDE OIL

Abstract An immature, tar-like oil (API~3°, δ 13 C = −23.6‰ vs. PDB) from the Miocene Monterey Formation offshore California was selected for a study of carbon isotopic signatures of individual biomarkers. The three principal stereoisomers of 28,30-bisnorhopane (C 28 ) have, within analytical precision, identical carbon isotopic compositions (average δ 13 C = −32.3 ± 0.4‰ ) and are considerably depleted in 13 C compared to the whole oil. These 28,30-bisnorhopanes (BNH) differ isotopically from C 29 and C 30 17α(H)-hopanes (−25.8%. and −26.1%.) and C 31 –C 35 extended hopanes ( δ 13 C = −27.7‰ ) and suggest different precursors for the C 28 hopanes than for C 29 –C 35 hopanes. The relative depletion of BNH of almost 9‰ compared to the isotopic composition of the whole oil suggests that these hopanes derive from chemoautotrophic bacteria, possibly not yet identified H 2 S oxidizers, which utilize 13 C-depleted substrates. The C 29 and C 30 hopanes are, within analytical precision, isotopically identical (~ −26‰) and similar to algal-derived compounds, e.g., C 27 steranes (~ −25.9‰), which is consistent with a cyanobacterial source for these hopanes. An archaebacterial biomarker, 1,1′-biphytane ( δ 13 C = −25.5‰ ), likely derived from methanogens, is also isotopically similar to C 27 sterane. Norpristane, pristane, and phytane, liberated by desulfurization of the aromatic and polar maltene fractions, show isotopic compositions similar to the same isoprenoids in the free lipids of the bitumen (total range from −24.5 to −27.5‰). This isotopic similarity supports a common origin for the free and sulfur-bound forms of these isoprenoids. This origin could be algal and/or archaebacterial lipids, which both show isotopic compositions within the range of the C 18 –C 20 isoprenoids. Like other marine-derived organic matter, this Monterey oil does not show the strong 13 C depletion typical for methylotroph-derived compounds characteristically found in organic matter of lacustrine origin. This may indicate fundamental differences of methane recycling processes in marine (sulfate-dominated) as compared to lacustrine environments.

Selective biodegradation of extended hopanes to 25-norhopanes in petroleum reservoirs. Insights from molecular mechanics

Abstract In-reservoir microbial removal of the C-25 methyl group from the extended 17α,21β(H)-hopanes (hopanes) generates 25-norhopanes in crude oils from the West Siberia and San Joaquin basins. This C-25 demethylation occurs preferentially among low molecular-weight hopanes (e.g. C31), while higher homologs are progressively more resistant. Conversion of each hopane to its corresponding 25-norhopane occurred without significant side products and was incomplete at the time of sampling. This is indicated by the match between reconstructed C31C35, hopane distributions for heavily biodegraded oils (sum of hopane parent and 25-norhopane product for each homolog) and those of related, nonbiodegraded oils. C-25 demethylation favors 22S epimers of the C31, and C32, hopanes compared to 22R, while the opposite applies to the C34, and C35, hopanes, because molecular shapes, dimensions, and volumes vary with stereochemistry at C-22. For example, geometry-optimized C31, and C32, 22S hopanes from molecular mechanics force field calculations are more voluminous, while the C34, and C35, 22S hopanes are less voluminous than their 22R counterparts. The C31, to C35, hopane 22S and 22R epimers show distinct “scorpion-” vs. “rail-shaped” conformations respectively, controlled by different 21-22-29-31 and 17-21-22-30 torsion angles. Because 22S epimers of the extended hopanes tend to favor the scorpion conformation, which folds the side chain back toward position C-25, longer side chains may increasingly hinder C-25 from enzymatic attack. Biodegradation can adversely affect the use of %22S (22S + 22R) ratios for hopanes to assess thermal maturity. The 25-norhopane ratio improves our ability to distinguish different levels of biodegradation among heavily degraded oils where C-25 demethylation has occurred.

Structure and significance of a novel rearranged monoaromatic steroid hydrocarbon in petroleum

Abstract A monoaromatic steroid hydrocarbon isolated from a catalytic dehydrogenation/isomerization mixture derived from 5α-cholestane was determined to be a (10β→ 5β) CH 3 -rearranged monoaromatic steroid (2a) by 1 HNMR analysis. Gas chromatography-mass spectrometry (GCMS) coinjection with a crude oil monoaromatic fraction and GCMS elution patterns suggest that this 5β-Me rearranged monoaromatic steroid is part of a series of C 27 -C 29 homologues in 20S and 20R pairs (2a-f) which are found in nearly all crude oils. Evidence from a study of Toarcian sediments from West Germany shows that the relative amounts of rearranged (2a-f) compared to regular (1a-1) C-ring monoaromatic steroids depends upon the strength of the anoxic environment during sedimentation. In addition, observation of rearranged/regular MA-steroid ratios in a suite of Jurassic oils demonstrates the differential effects of maturation on the preservation of the two series of biomarker compounds.

Affinities of Early Cambrian acritarchs studied by using microscopy, fluorescence flow cytometry and biomarkers

Examination and chemical analysis of extremely well-preserved microfossils from the Lower Cambrian Lukati Formation in Estonia suggests that acritarchs from among the genera Globosphaeridium, Skiagia, Comasphaeridium and Lophosphaeridium have dinoflagellate affinities. The investigation presents a combination of transmitted light microscopy, fluorescence microscopy and flow cytometry, and biomarker analysis that demonstrates a new method for the investigation of problematic organic-walled microfossils. For the chemical analysis, Lukati Formation acritarchs were separated from prasinophycean tasmanitids by size and then divided into two fractions in accordance with the intensity of their autofluorescence signal. Biomarker molecules were generated by pyrolysis directly from isolated acritarch organic walls and studied using gas chromatography–mass spectrometry–mass spectrometry (GC-MS-MS) and metastable reaction monitoring (MRM)-GC-MS. The analysis supported previously made suggestions that acritarchs include microorganisms of different biological affinities. All acritarch fractions contain the common steranes (cholestane, 24-methylcholestane and 24-ethylcholestane) that are characteristic molecules for eukaryotes. However, the dinoflagellate-related biomarkers, dinosterane and 4α–methyl–24–ethylcholestane, were concentrated only in the fraction containing highly autoflourescent acritarchs. Additional chemical analyses of microfossils from the Lower Cambrian Buen Formation of North Greenland confirmed the presence of the dinoflagellate-related biomarkers at a second Early Cambrian locality.