Wolfgang K. Seifert

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.

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.

Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks

Abstract Comparison of biological marker alkanes in the kerogen pyrolyzate and bitumen from a sediment is a useful test for the indigenous nature of sediment extracts. For the pyrolysis conditions used, the bulk of the hydrocarbons is released from the kerogen matrix between 375° and 550°C; and its steriochemistry is almost the same as that observed in the extractable bitumen in a genuine source rock. Examples are given to demonstrate that, during pyrolysis, the sterane/terpane ratio decreases and secondary terpanes are generated at the expense of primary ones. The mechanism of artificial petroleum generation by pyrolysis differs from ‘natural’ diagenesis during geological time and is reflected in the composition of certain C27-C29 steranes, as demonstrated by simulation experiments and C29-C30 moretanes and hopanes. The 5α 5β -sterane ratios, jointly with 17α(H)-hopane 17β(H)-moretane ratios, tricyclic terpane concentrations and 17α(H) 17β(H) -trisnorhopane ratios, allow the differentiation of kerogens from adjacent stratigraphies.

Identification of an extended series of tricyclic terpanes in petroleum

A series of tricyclic terpanes from C19 to C45 has been identified in petroleum by gas chromatographic mass spectroscopic (GCMS) analysis. This discovery extends the previously known homologous series reported from C19 to C30. A method of preparation of a tricyclic terpane concentrate is described which facilitates tricyclic terpane analysis by the GCMS m/z 191 fragment. Metastable scanning GCMS is described as an additional method for characterization of the tricyclic terpanes.

First discovery of botryococcane in petroleum

The fossil biological marker botryococcane has been discovered in high concentration in a crude oil.

Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Abstract Two C28H48-pentacyclic triterpanes were isolated from Monterey shale. X-ray crystallography of a crystal containing both compounds proved their structures as 17β,18α,21α(H)-28,30-bisnorhopane and 17β,18α,21β(H)-28,30-bisnorhopane. Several differences are found between 28,30-bisnorhopanes and the regular hopanes. Unlike the regular hopane epimers, for practical purposes the three epimeric 28,30-bisnorhopanes [17α,21β(H)-, 17β,21α(H)-, and 17β,21β(H)-]cannot be distinguished by their mass spectra. Special conditions are needed to separate them by gas chromatography. The diagenetically first-formed epimer is thought to be 17α,21β(H)- because it predominates in immature shales. The order of thermodynamic stability is 17β,2lα(H) 17α,21β(H) > 17β,21β(H), and all three epimers are present in petroleum. 25,28,30-Trisnorhopanes can be analyzed in similar fashion and are found to have similar thermodynamic characteristics. The percent of the ring D/E cis epimer of 28,30-bisnorhopane and/or 25,28,30-trisnorhopane is a useful maturation parameter similar to the 20S/20R sterane ratio. Evidence indicates 25-demethylation of 28,30-bisnorhopane to 25,28,30-trisnorhopane during advanced stages of biodegradation. Hence, percent ring D E cis 25,28,30-trisnorhopane has an application to maturation assessment in heavily biodegraded oils.