Clifford C. Walters

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.

Predicting generation and expulsion of paraffinic oil from vitrinite-rich coals

Rock—Eval HI values for coals vary with rank and do not give a direct measurement of oil potential. However, oils from coals are characteristically paraffinic and can be considered to derive from a polymethylene (PM) component, so the PM content should provide an estimate of the paraffinic oil potential. A trend apparently representing lignin evolution has been identified on the Van Krevelen diagram which permits the relative proportions of carbon in lignin and PM to be determined for coals that approximate a mixture of these two components, such as the members of the New Zealand (NZ) Coal Band. On the basis of this compositional model, HI values can be calibrated to provide an alternative estimate of the paraffinic oil potential. A maximum in HI is generally reached in coals near the onset of oil generation, at Rank(S) 12 (R. ca. 0.7%), from which it is suggested that the PM contri¬bution can be obtained using the formula HIpm = 1.15HIma„-172 for the suite of NZ coals examined. The onset of oil expulsion can be identified from a variety of geochemical measurements, and occurs in the Rank(S) range ca. 12.0 — 14.5 (Ro ca. 0.7-1.1%) for coals with paraffinic oil potentials exceeding ca. 40 mg HC/g TOC. Data from Taranaki Basin coals correlate well with the theoretical relationship between BI/HIPM and HIpm, using bitumen index (BI = S 1/TOC) values of 10 mg HC/g TOC at the start of oil generation (i.e. bitumen inherited from diagenesis) and 40 mg HC/g TOC at the onset of oil expulsion, suggesting the HIpm model is reasonably accurate for members of the NZ Coal Band. Kinetic modelling of paraffinic oil generation from vitrinite-rich coals may be best approximated by consideration of PM degradation alone.

Petroleum systems in the Jiangling-Dangyang area, Jianghan Basin, China

Source-related geochemical data define at least four petroleum systems in the Jiangling-Dangyang area of the Jianghan Basin. Eocene-Paleocene anoxic evaporitic lacustrine source rocks generated most of the crude oils in the area. • • Eocene Qianjiang rock from the Sha 13 well (1322 m) contains fast-reacting, sulfur-rich Type IS organic matter, and its extract is most similar to the Sha 13 oil sand bitumen (Qianjiang reservoir). • • Lower Eocene-Paleocene Xingouzhui rocks from the Xin 73 well (842 and 862 m) contain slow-reacting, low-sulfur Type I organic matter, and their extracts are most similar to the Sha 26 oil sand bitumen (Eocene Jinsha reservoir) and the Ling 2, Sha 24, and Tuo 3 oils (Xingouzhui reservoirs). • • Two unidentified Middle Triassic or older marine carbonate-evaporite source rocks or different facies of the same source rock generated the Daxiakou oil (Triassic Jialingjiang Formation outcrop, Xingshan County) and the moderately biodegraded Tianwan seep oil (Permian Changxing outcrop, Chengxi County), respectively. • • One or more unidentified marine source rocks, which could include the Lower Permian Qixia or the Upper Sinian Doushantuo Formations, generated the Miaoshi and Yanmenkuo seep oils (Permian Qixia outcrops). The Jingshan seep oil (Ordovician Baota outcrop) probably is related to these oils, but could represent another petroleum system. Different kinetics for hydrocarbon generation among Eocene Qianjiang and Lower Eocene-Paleocene Xingouzhui Formation source rocks and chemical differences among the related oils are caused by organic facies variations. High salinity and low Eh enhanced the preservation of oil-prone organic matter in these lacustrine settings and facilitated incorporation of sulfur into the organic matter. Anoxia and the unusual presence of abundant sulfate as gypsum resulted in the microbial reduction of sulfate to sulfide and incorporation of this sulfur into the kerogen. For example, biomarkers show that source rock in the Sha 13 well (1322 m) was deposited under more saline, lower Eh conditions than that in the Ling 80 well (1808 m), although both are from the Qianjiang Formation The Sha 13 rock sample is more organic-rich (6.62 vs. 1.27 wt.% TOC) and has a higher hydrogen index (794 vs. 501 mg HC/g TOC) and faster reaction kinetics than the Ling 80 sample. Kerogen from the Sha 13 sample is Type IS because it has a high hydrogen index and an atomic S/C ratio (0.074) in the range of sulfur-rich, fast-reacting kerogens of the Monterey Formation (S/C > 0.040). Organic-rich Lower Jurassic coaly rocks from outcrops at Daxiakou contain immature to mature gas-prone organic matter that is not related to any oils in the study. Several organic-rich Upper Sinian to Permian samples could have been source rocks in the past, but are now highly mature based on high Tmax (464–540°C) and estimated vitrinite reflectance (Ro) values. Mass balance calculations were used to estimate the original TOC (TOC°C) in these samples prior to maturation. These samples could not be correlated with the oils using biomarkers because of high maturities and low extract yields. However, stable carbon isotope type-curves suggest that the Miaoshi, Yanmenkuo (Permian Qixia Formation) and the Jingshan (Ordovician Baota Formation) seep oils originated from source rocks in the Lower Permian Qixia (3.00 wt.% TOC°) or Upper Sinian Doushantuo Formations (5.96 wt.% TOC°). Lack of triaromatic dinosteroids in the Miaoshi and Yanmenkuo seep oils supports, but does not prove a Permian source rock. Very negative stable carbon isotope ratios for kerogens from the Lower Cambrian Shuijintuo Formation (−33.5 to −33.6‰; 8.85−16.64 wt.% TOC°) show that they are not related to any of the analyzed oils.

Enrichment, resolution, and identification of nickel porphyrins in petroleum asphaltene by cyclograph separation and atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry.

We report here the first high resolution mass spectrometric evidence of nickel porphyrins in petroleum. A petroleum asphaltene sample is fractionated by a silica-gel cyclograph. Nickel content is enriched by approximately 3 fold in one of the cyclograph fractions. The fraction is subsequently analyzed by atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) with an average mass resolving power of over 500 K (M/DeltaM(fwhm)). Similar to vanadyl porphyrins, monocylcoalkano-type (presumed to be deocophylerythro-etioporphyrin DPEP) Ni porphyrins are found to be the most abundant family followed by etio, bicycloalkano-type, and rhodo-monocylcoalkano-type Ni porphyrins. A Z number ranging from -28 to -44 and a carbon number ranging from 26 to 41 were observed. A significant amount of nickel and vanadyl geoporphyrins are in more condensed tetrapyrrolic cores than just chlorophyll-derived DPEP- and etioporphyrins. Ni has a higher etio/DPEP ratio and rhodo-etio/rhodo-DPEP ratio than does VO.

Evaluation of kinetic uncertainty in numerical models of petroleum generation

Oil-prone marine petroleum source rocks contain type I or type II kerogen having Rock-Eval pyrolysis hydrogen indices greater than 600 or 300–600 mg hydrocarbon/g total organic carbon (HI, mg HC/g TOC), respectively. Samples from 29 marine source rocks worldwide that contain mainly type II kerogen (HI = 230–786 mg HC/g TOC) were subjected to open-system programmed pyrolysis to determine the activation energy distributions for petroleum generation. Assuming a burial heating rate of 1C/m.y. for each measured activation energy distribution, the calculated average temperature for 50% fractional conversion of the kerogen in the samples to petroleum is approximately 136 7C, but the range spans about 30C (121–151C). Fifty-two outcrop samples of thermally immature Jurassic Oxford Clay Formation were collected from five locations in the United Kingdom to determine the variations of kinetic response for one source rock unit. The samples contain mainly type I or type II kerogens (HI = 230–774 mg HC/g TOC). At a heating rate of 1C/m.y., the calculated temperatures for 50% fractional conversion of the Oxford Clay kerogens to petroleum differ by as much as 23C (127–150C). The data indicate that kerogen type, as defined by hydrogen index, is not systematically linked to kinetic response, and that default kinetics for the thermal decomposition of type I or type II kerogen can introduce unacceptable errors into numerical simulations. Furthermore, custom kinetics based on one or a few samples may be inadequate to account for variations in organofacies within a source rock. We propose three methods to evaluate the uncertainty contributed by kerogen kinetics to numerical simulations: (1) use the average kinetic distribution for multiple samples of source rock and the standard deviation for each activation energy in that distribution; (2) use source rock kinetics determined at several locations to describe different parts of the study area; and (3) use a weighted-average method that combines kinetics for samples from different locations in the source rock unit by giving the activation energy distribution for each sample a weight proportional to its Rock-Eval pyrolysis S2 yield (hydrocarbons generated by pyrolytic degradation of organic matter).

Mixed signals of the source and thermal maturity for petroleum accumulations from light hydrocarbons: an example of the Beryl field

Abstract We have investigated compositional variations of light hydrocarbons in crude oils from the Beryl and adjacent fields of the North Sea in combination with carbon isotopic ratios of light hydrocarbons and biomarker properties of oils. Although there appears to be a strong source influence on various light hydrocarbon maturity parameters, this is a consequence of the mixing of hydrocarbons derived from the Kimmeridge Clay Formation as well as from the Heather Formation and Brent coal at varying thermal maturity levels. Light hydrocarbon parameters based on Mangos reaction scheme involving 3-ring and 5-ring intermediates are supported by carbon isotopic ratios of light hydrocarbons. However, there also is evidence that iso- and cyclo-heptane precursors exist that contribute C 7 light hydrocarbons independent of Mangos model. The ratio of 2,4-dimethylpentane to 2,3-dimethylpentane (2,4-DMP/2,3-DMP) and other related parameters appear to be reliable indicators of thermal stress, but must be interpreted within a complete understanding of the petroleum system under study.

Petroleum generation kinetics: Single versus multiple heating-ramp open-system pyrolysis

Some recent publications promote one-run, open-system pyrolysis experiments using a single heating rate (ramp) and fixed frequency factor to determine the petroleum generation kinetics of source-rock samples because, compared to multiple-ramp experiments, the method is faster, less expensive, and presumably yields similar results. Some one-ramp pyrolysis experiments yield kinetic results similar to those from multiple-ramp experiments. However, our data for 52 worldwide source rocks containing types I, II, IIS, II/III, and III kerogen illustrate that one-ramp kinetics introduce the potential for significant error that can be avoided by using high-quality kinetic measurements and multiple-ramp experiments in which the frequency factor is optimized by the kinetic software rather than fixed at some universal value. The data show that kinetic modeling based on a discrete activation energy distribution and three different pyrolysis temperature ramps closely approximates that determined from additional runs, provided the three ramps span an appropriate range of heating rates. For some source rocks containing well-preserved kerogen and having narrow activation energy distributions, both single- and multiple-ramp discrete models are insufficient, and nucleation-growth models are necessary. Instrument design, thermocouple size or orientation, and sample weight likely influence the acceptable upper limit of pyrolysis heating rate. Caution is needed for ramps of 30–50°C/min, which can cause temperature errors due to impaired heat transfer between the oven, sample, and thermocouple. Compound volatility may inhibit pyrolyzate yield at the lowest heating rates, depending on the effectiveness of the gas sweep. We recommend at least three pyrolysis ramps that span at least a 20-fold variation of comparatively lower rates, such as 1, 5, and 25°C/min. The product of heating rate and sample size should not exceed ∼100 mg °C/min. Our results do not address the more fundamental questions of whether kinetic models based on multiple-ramp open-system pyrolysis are mechanistically appropriate for use in basin simulators or whether petroleum migration through the kerogen network, rather than cracking of organic matter, represents the rate-limiting step in expulsion.

Biodegradation of oil hydrocarbons and its implications for source identification

Biodegradation puts a major impact on the composition of petroleum products and crude oils. This chapter discusses the fundamentals of hydrocarbon biodegradation, especially of liquid fossil fuels, and attempts to bring together the conclusions from two rather disparate areas of research. One is from the community of microbiologists and environmental scientists studying biodegradation in the laboratory and the field. The other is from the community of geochemists studying petroleum in reservoirs. Significant routes for hydrocarbons to leave the biosphere are combustion and photochemical oxidation. Photo oxidation is a very important process in the atmosphere, and while it does convert aromatic compounds in slicks to oxygenated species, it accounts for relatively little loss of nonvolatile hydrocarbons. Oxygen is both an essential reactant in the initial activation of hydrocarbons under aerobic conditions and the terminal electron acceptor for microbial growth. Most natural petroleum contains only trace amounts of alkenes. A few crude oils contain minor amounts from radiolysis, but alkenes can be quite abundant in refined products such as gasoline.

The Origin of Petroleum

and proposed that bitumen, like other minerals, condensed from sulfur. Andreas Libavius, another German physician, theorized in his 1597 text Alchemia that bitumen formed from the resins of ancient trees. These early discussions mark the beginnings of one of the longest running scientific debates: whether petroleum is formed by abiogenic processes that occur deep within the Earth, or from sedimentary organic matter that was once living organisms. As fossil evidence emerged during the 18 century that coals were derived from plant remains, many scientists proposed similar origins to

Geochemical variations in sedimentary organic matter within a “homogeneous” shale core (Tuscaloosa Formation, upper Cretaceous, Mississippi, U.S.A)

Abstract Geochemical analyses were conducted on samples collected from a ∼100 m core of the middle Tuscaloosa Formation from southwestern Mississippi. By studying a section that has uniform lithology and negligible differences in thermal maturity, we can focus on parameters that reflect changes in depositional environment. Heterogeneity in the organic facies is observed at various intervals with different sampling density throughout the core. A lithological transition between deep water prodeltaic shales and nearshore siltstones is marked by major perturbations in all measured geochemical parameters. Variations in the distribution of organic matter within the prodeltaic laminated shales are believed to reflect a shallowing event not obvious from the “homogeneous” lithology. Major heterogeneities in molecular distributions and concentrations are restricted to the tetra- and pentacyclic-terpanes, while molecular parameters that are controlled or predominantly influenced by marine eukaryotic input show little variation. These observations suggest that the chemistry and biota of the water column remained relatively constant during deposition of the laminated shales; and, although conditions at the sediment-water interface remained dysaerobic, the chemical environment appears to have varied enough to perturb communities and the diagenetic reactions of hopanoid hydrocarbons. Maturity-dependent parameters based on the marine contribution exhibit little variation. Parameters based on pentacyclic triterpanes show a high degree of source-dependency.