J.M. Moldowan

The Biomarker Guide

About the authors Preface Purpose Acknowledgements Part I. Biomarkers and Isotopes in the Environment and Human History: 1. Origin and preservation of organic matter 2. Organic chemistry 3. Biochemistry of biomarkers 4. Geochemical screening 5. Refinery oil assays 6. Stable isotope ratios 7. Ancillary geochemical methods 8. Biomarker separation and analysis 9. Origin of petroleum 10. Biomarkers in the environment 11. Biomarkers in archaeology Appendix: geologic time charts Glossary References Index.

Effects of source, thermal maturity, and biodegradation on the distribution and isomerization of homohopanes in petroleum☆

Abstract The distributions of C31–C35 17α,21β(H)-homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment. For example, high C35-homohopane indices [C35/(C31–C35)] are typical of petroleums generated from source rocks deposited under anoxic marine conditions, where the C35 backbone of the precursor bacteriohopanetetrol is preferentially preserved. Homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock. Homohopane distributions are altered by secondary processes. For example, the C35-homohopane index decreases with increasing API gravity and thermal maturity of related oils from the Monterey Formation, offshore California. Similar decreases in the homohopane index are observed for expelled oils from Monterey source rock during hydrous pyrolysis. Biodegradation can result in selective loss of high or low molecular weight homologs, apparently depending on the bacterial population in the reservoir. Bacterial demethylation of homohopanes appears necessary to explain 25-norhopanes in at least one biodegraded oil. Hydrous pyrolysis studies of the Phosphatic and Siliceous members in the Monterey Formation show that identical heating conditions result in different homohopane C-22 epimer ratios. Furthermore, a “reversal” in the trend of the epimer ratio at high experimental temperatures is observed for the Phosphatic, but not for the Siliceous lithology. This could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures, possibly mediated by rock mineralogy.

Diamondoid hydrocarbons as indicators of natural oil cracking

Oil cracking—the thermal breakdown of heavy hydrocarbons to smaller ones—takes place within oil-bearing rock formations at depths commonly accessed by commercial oil wells. The process ultimately converts oil into gas and pyrobitumen, and thus limits the occurrence of petroleum and the success of exploration. Thermal cracking of liquid petroleum increases with depth until it reaches completion at the so-called ‘oil deadline’, which is generally placed, at around 5 km depth and at temperatures of 150–175 °C. However, cracking experiments and the discovery of relatively ‘hot’ oil reservoirs, imply that petroleum is thermally more stable than previously assumed; in fact it has been suggested that liquid petroleum might persist at temperatures reaching, or even exceeding, 200 °C. But reliable estimates of the extent of oil cracking and the depth at which it occurs in any given reservoir are difficult to obtain. Here we demonstrate that the relative abundance of diamondoids, a class of petroleum compounds whose unique thermal stability leads to their progressive concentration during cracking, can be used to identify the occurrence and estimate the extent of oil destruction and the oil deadline in a particular basin. We are also able to identify oils consisting of mixtures of high- and low-maturity components, demonstrating that our method yields valuable information on the cracking and mixing processes affecting petroleum systems.

Effects of hydrous pyrolysis on biomarker thermal maturity parameters : Monterey Phosphatic and Siliceous Members

Hydrous pyrolysis of immature Monterey Phosphatic or Siliceous rock at progressively higher temperatures causes systematic changes in biomarker thermal maturity parameters of the generated hydrocarbons. Biomarker ratios based on proposed carbon-carbon cracking or aromatization reactions increase during hydrous pyrolysis along similar pathways for both Siliceous and Phosphatic members. An increase in these biomarker ratios is also observed for oils of increasing thermal maturity from the offshore Santa Maria Basin, although the rates of changes for each parameter differ between the hydrous pyrolysis and natural samples. Changes in some cracking parameters during maturation appear to result from differential thermal stability of the compounds rather than conversion of precursors to products. The behavior of isomerization-based biomarker ratios in these experiments is more complex than ratios based on carbon-carbon cracking or aromatization reactions. During heating, kerogen-bound precursors generate steranes and hopanes showing lower levels of thermal maturity based on isomerization ratios than those extracted from the unheated rock. Asymmetric centers in the kerogen-bound steroids and hopanoids appear to be protected from isomerization compared to those of free steranes or hopanes in the bitumen. The Phosphatic and Siliceous rocks can show different sterane or hopane isomerization ratios when heated under the same time/temperature conditions. Further, these isomerization ratios unexpectedly decrease at high hydrous pyrolysis temperatures (> 330°C) for the Phosphatic, but not for the Siliceous samples. This could be caused by the combined effects of isomerization and differential destruction of epimers, apparently mediated by rock mineralogy. Differences between the biomarker compositions of bitumens and expelled oils in these experiments mimic those caused by natural primary migration. Heavier, more polar compounds are preferentially retained in the bitumen. For example, bitumens are enriched in tri- over monoaromatic steroids, hopanes over tricyclic terpanes, and regular steranes over diasteranes compared to expelled oils. No significant fractionation of stereoisomers was observed, such as 22S vs 22R C32 17α(H),21β(H)-homohopanes or 20S vs 20R and ββ vs αα C29 5α(H)-steranes.

Petroleum isotopic and biomarker composition related to source rock organic matter and depositional environment

Abstract A multivariate approach has been developed that separates oils derived from organic matter in nonmarine shales, marine shales, and marine carbonates. Some or all of the following analyses were completed on 43 oils from known source rocks: (1) C30-sterane and monoaromatic steroid biomarker ratios, and (2) stable carbon and hydrogen isotope compositions of whole oil, saturates, and aromatics. Three-group discriminant analysis of the 28 oils whereall biomarker and isotopic data are available results in no misclassifications at a significance level of 99%. Using only carbon and hydroogen isotopic data for whole oils, saturates, and aromatics results in correct classifications of 82% for oils derived from marine shale organic matter and 100% for oils from the other two sources. Triangular plots probabilities can be used to distinguish oils derived from the three source rock organic matter end-members. Several publications describe classifications of “marine” vs “nonmarine” oils using stable carbon isotope ratios of the whole oil or the saturate and aromatic fractions. Our results confirm that these methods do not completely differentiate oils by source.

Oil composition variation and reservoir continuity: Unity field, Sudan

Abstract A suite of oils from stacked reservoirs in the Unity Field in Sudan has been analyzed by various geochemical techniques for molecular information to elucidate the geological processes which cause variations in oil composition and their resulting oil fingerprints in different reservoir units. Analyses of these highly paraffinic oils indicate that the chromatographic fingerprint variations are due to differences in the abundances of saturated compounds, including branched and cyclic alkanes. Neither aromatics nor NSO compounds have any significant effect on the observed fingerprint variations. This association of saturates, instead of aromatics and NSO compounds, with the fingerprint variations precludes rock-fluid interactions as a cause of the variations. Biomarker analyses show that variations in thermal maturity and organic facies of the source rock are responsible for the fingerprint variations. Thermal maturity increases with the depth of the reservoir, suggesting a multiple-charge process for the oils to fill these reservoirs over an extended period of time. Apparently the source rock generated and expelled progressively more mature oils and little mixing occurred during migration. Thus, knowledge of oil compositional variations from one reservoir to another, organic facies variation and source rock maturity combined with tectonic history may help explain charging and timing of oil emplacement.

Multiple Oil Families in the West Siberian Basin

Two major oil families are identified in the West Siberian basin. Twenty-six of 32 analyzed oils occur in Jurassic and Cretaceous reservoirs and are derived from anoxic marine Upper Jurassic Bazhenov source rock, based on geochemical comparison of oils and source rock extracts. These oils are widely distributed both north and immediately south of the Ob River, and their biomarker ratios indicate a wide range of source rock thermal maturity from early to middle oil window (Van-Egan, Russkoye, Samotlor, Sovninsko-Sovyet, Olyenye, Ozynornoye, and Kogolym), to peak oil window (Srednekhulym, Yem-Yegov, Vostochno-Surgut, Khokhryakov, Fedorov, and Urengoi), to late oil window (Salym). Some of these oils have been mildly (e.g., Fedorov 75) to heavily (e.g., Russkoye) biodegraded n the reservoir. The Bazhenov-sourced oils show different compositions that support regional variations of organic facies in the source rock. For example, the anoxic marine Bazhenov facies, which generated the oils in the Samotlor, Fedorov, and nearby fields, was particularly clay poor and sulfur rich. Six nonbiodegraded, highly mature oils show geochemical characteristics that suggest they were derived from clastic-rich lacustrine or nearshore marine source rocks dominated by terrigenous higher plant input like those in the Lower to Middle Jurassic Tyumen Formation, although no correlation was observed between the oils and a single rock sample (Yem-Yegov 15) from the formation. The six oils occur in the Tyumen (Taitym, Geologiche, and Cheremshan) and fractured basement/Paleozoic (Gerasimov, Yagyl Yakh, and Verchnekombar) reservoirs in positions readily accessible to any oil migrating from the Tyumen source rock. For example, at the Gerasimov location, the Tyumen Formation lies unconformably on weathered basement-Paleozoic reservoir rocks. Most of the probable Tyumen-sourced oils ar from south of the Ob River, but the occurrence of Geologiche oil to the north suggests that related oils may be widespread in the basin.

Functionalized biological precursors of tricyclic terpanes: information from sulfur-bound biomarkers in a Permian tasmanite

Desulfurization of the aromatic and polar maltene fractions of a tasmanite oil shale extract with Raney Ni yielded a series of tricyclic terpanes extending to C40. We suggest that the C40 homologs are derived from tetraterpenoid biological precursor(s) in the tasmanite, and were formed by biochemical cyclization of a C40 polyprenol. It is likely that many of the lower carbon number tricyclic terpanes in these samples are early diagenetic alteration products of these C40 precursors. Tricyclic terpane series in other samples that terminate at C30 or C45 (or higher) could be derived from biochemical cyclization of other polyprenols of different lengths in related microbiota. The desulfurization products indicate that tricyclic terpanes in the tasmanite extract derive from biological precursor(s) with functionalities in both the ring system and the side chain. Novel desulfurization products included substantial amounts of mono-unsaturated tricyclic terpenes (tentatively identified as 8,13-dimethyl-14-alkylpodocarp-13-enes, primarily C39) and a C21 acyclic isoprenoid, probably derived from the side chain of a C40 tricyclic terpenoid. These data suggest that phytane, and possibly other isoprenoids in these samples, may partially derive from the side chain of lower-carbon-number tricyclic terpenoids.