J. Michael Moldowan

Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils

Abstract Novel biological marker parameters are applied to problems of geochemical correlation of crude oils in the McKittrick Field, California. An attempt is described to distinguish four diagenetic parameters; namely, source input, source maturation, migration and ‘in reservoir’ maturation. The tools include the absolute concentration of steranes, terpanes and paraffins (n + iso) in combination with internal ratios of individual biomarkers such as primary/secondary terpanes, 17α(H)-trisnorhopane/18α(H)-trisnorhopane II (both maturation specific), 5β/5α-steranes, 5β-steranes/17α(H)-hopanes and rearranged steranes/5α-steranes (all migration oriented), 5α/5α-steranes and a number of terpane ratios of partially unknown chemical structure (source input specific). Among other new correlation parameters are: two series of mass chromatograms (m/e 253 and 239), signaling monoaromatized steranes, a series of presumably rearranged steranes (m/e 259), and a series of methylhopanes (m/e 205). The results obtained on the molecular level exceed the degree of information obtainable from organic geochemical ‘bulk’ parameters such as yields of saturates, aromatics, sulfur compounds and C13/C12 ratios by far; however, both types of parameters are mutually supporting. All conclusions are consistent with subtle stratigraphie and overall geologic prerequisites.

Relationship Between Petroleum Composition and Depositional Environment of Petroleum Source Rocks

Crude oils of nonmarine source can be distinguished from those of marine shale source and from oils originating in marine carbonate sequences by using a battery of geochemical parameters, as demonstrated with a sample suite of nearly 40 oils. A novel parameter based on the presence of C30 steranes in the oil was found to be a definitive indication of a contribution to the source from marine-derived organic matter. A second novel parameter based on monoaromatized steroid distributions was effective in helping to distinguish nonmarine from marine crudes and can be used to gauge relative amounts of higher plant input to oils within a given basin. Sterane distributions were similarly useful for detecting higher plant input but were less effective than monoaromatize steroid distributions for making marine versus nonmarine distinctions. Concentrations of high molecular-weight paraffin can also be effective nonmarine indicators but are influenced by maturation and biodegradation processes. Certain algal-derived nonmarine oils may show little high molecular-weight paraffin response. Oils from carbonate sources (with a few exceptions) can be distinguished by having low pristane-phytane ratios, low carbon preference indexes, and high sulfur contents. Gammacerane indexes and carbon isotope ratios of the whole crude are not effective in distinguishing these types of environmental differences on a global basis.

The effect of biodegradation on steranes and terpanes in crude oils

Abstract Steranes and terpanes biodegrade at a slower rate than isoprenoids and survive moderate biodegradation. Heavy biodegradation results in destruction of regular steranes, survival of diasteranes (20R better than 20S) and tricyclic terpanes and transformation of hopanes to Ring A/B demethylated hopanes. These survivors can be used as source fingerprints in biodegraded crudes. The structure of predominant steranes in undegraded to moderately degraded fossil fuels was proven to be 14β, 17β(H) (20R + S) by molecular spectroscopy. These compounds plus the 20S epimers of regular 5α-steranes (20R) were identified as major constituents and their 5β-counterparts as minor components in a cholestane isomerizate (300°C, Pt on C), allowing assessment of relative thermodynamic stabilities. An observed increase of optical activity in heavily degraded crudes from three different basins is interpreted to be the result of bacterial transformation of terpanes and steranes to new optically active species plus enrichment of the latter by n + isoparaffin depletion rather than total bacterial synthesis. Diagnostic ion profiling by GC-MS-C is a convenient tool for surveying the relative abundance of individual diasteranes and regular steranes plus distinguishing epimeric and ring skeletal isomeric series in complex fossil fuel mixtures. A new practical method of determining the absolute quantities of individual steranes by spiking with 5β-cholane and integration of mass chromatograms is described.

Paleoreconstruction by Biological Markers

Abstract During diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons, the following four basic events needed for paleoreconstruction can be monitored by biological markets: (1) sourcing, (2) maturation, (3) migration, and (4) biodegradation. Actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated. Cretaceous – and Phosphoria-sourced oils in the Wyoming Thrust Belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry. Identification of recently discovered biological markers, head-to-head isoprenoids, allows subtle source differentiation between oils from Sumatra. The degree of crude oil maturation in basins from California, Alaska, Russia, Wyoming, and Louisiana can be assessed by specific novel biomarker ratios (20S/20R sterane epimers). Field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock. An example of immature Sumatra rock is given. Extensive migration is documented by biomarkers in several oils. The effect of biodegradation on biomarker steranes and terpanes is exemlified in a California basin. All biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo-reconstruction.

The effect of thermal stress on source-rock quality as measured by hopane stereochemistry

Abstract Whether or not a suspected source-rock has experienced sufficient thermal stress to have generated petroleum can be determined by GCMS analysis of the terpanes using the following criteria in both the rock extract and the pyrolyzate: (1) 22S 22R ratio of C31C33 hopanes. (2) Concentration of C29C32 β,β hopanes. (3) Ratio of 17βH 17αH trisnorhopanes. (4) Ratio of moretanes/hopanes. (5) Yield of saturates in the pyrolyzates. The use of these parameters in combination is demonstrated with a sequence of suspected source rocks of different stratigraphic horizons from the same basin, ranging from immature/poor to mature/excellent. Of specific value is the subtle graduation, particularly in the pyrolyzates, of these parameters throughout the samples of different degrees of maturity. Cross-supplementing between parameters is demonstrated.

Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany

Abstract A sequence of Lower Toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil-to-source correlation. Lower Liassic marls are representative of a well-aerated shallow-water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions. Various properties have been found to vary considerably within these two units. Amongst the most important are carbon isotopic composition of the kerogen, the pristane/phytane ratios, nickel as opposed to vanadyl porphyrins, and the C27 dia-/regular steranes. Although maturation within the profile does not change, some of the maturation-dependent biomarker properties such as the monoaromatic steroid side-chain cracking and the Tm/Ts ratio exhibit large changes which can be assigned to diagenetic processes. Another maturation-dependent property, the 20S/20R epimerization of C29 steranes, exhibits smaller changes which could also be due to early diagenetic processes. The study suggests that reducing and oxidizing conditions, i.e. Eh and pH in the sediment, exert an influence on several biomarker precursor-product pathways. In maturation studies initial variations due to depositional conditions have therefore to be taken into account.

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.

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.

Source correlation of biodegraded oils

Abstract Heavy biodegradation of crude oils has been shown previously (Seifert and Moldowan (1979) Geochim. Cosmochim. Acta 43 , 111–126) to result in partial destruction of steranes and terpanes. This paper addresses the question of whether or not the surviving biomarker remnants can be utilized to accomplish source correlations. The case study involves a number of heavily biodegraded oil seeps from Greece. Steranes with the “biological” side chain configuration are selectively destroyed by the bacteria, while their “geological” epimers survive. Relying on surviving aromatized steranes and carbon isotopes, it is demonstrated that the Greek oil seeps all belong to two source groups with some subclassing. In addition, a condensate is shown to be identical source as one group of oil seeps. While steranes cannot be used, because of biodegradation, surviving terpanes serve as maturation parameter. The biomarker techniques, presently the only known approach to this type of a problem, allow a clear source distinction of the biodegraded seeps plus condensate from an undegraded commercial oil field in Greece. This kind of distinction is based on a multiparameter approach such as monoaromatized steranes, gammacerane, terpanes, etc. Carbon isotopes alone would have led to erroneous conclusions. The biomarker-based conclusions are consistent with and are enhancing geological hypotheses.