Mark A. McCaffrey

Paleoenvironmental implications of novel C30 steranes in Precambrian to Cenozoic Age petroleum and bitumen

Petroleums and bitumens from Early Proterozoic (≈ 1800 Ma) to Miocene (≈ 15 Ma) age marine strata contain 24-isopropylcholestanes, a novel group of C30 steroids. The abundance of these compounds, relative to 24-n-propylcholestanes, varies with source rock age. Late Proterozoic (Vendian) and Early Cambrian oils and/or bitumens from Siberia, the Urals, Oman, Australia, and India have a high ratio of 24-isopropylcholestanes to 24-n-propylcholestanes (≥1), while younger and older samples have a lower ratio (≤0.4). Temporal changes in this parameter may reflect the relative abundance of certain Porifera (sponges) and certain marine algae through time. Geochemical indicators such as this, which can constrain the source rock age of a migrated oil, are useful in source rock identification during petroleum exploration.

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.

A novel microbial hydrocarbon degradation pathway revealed by hopane demethylation in a petroleum reservoir

Abstract The origin of 25-norhopanes is controversial, despite their abundance in many biodegraded oils. In a biodegraded oil reservoir, we found concomitant and directionally opposite changes with depth in the concentrations of certain 17α-hopanes and their 17α,25-norhopane counterparts, demonstrating microbially induced demethylation of hopanes. These observations suggest a hitherto unknown biodegradation mechanism and provide insight into the poorly understood process of natural petroleum biodegradation.

Using Biomarkers to Improve Heavy Oil Reservoir Management: An Example From the Cymric Field, Kern County, California

For biodegraded oil accumulations, field development can be optimized by using geochemical indicators of variations in the extent of bacterial alteration. Biodegradation typically reduces oil producibility by increasing oil viscosity. In the Cymric field (Kern County, California), sidewall core extracts reveal that the extent of oil biodegradation changes substantially over extremely short vertical distances in a shallow, low-permeability reservoir. Zones of more degraded oil can extend laterally for more than a mile. The relationships between oil viscosity and biomarker biodegradation parameters in this field were calibrated from analyses of produced oils, and these relationships were used to convert sidewall core biomarker analyses into quantitative predictions of lateral and vertical changes in oil viscosity and gravity. Compositional variations were also used to allocate production to discrete zones. Viscosity prediction and production allocation can be used to optimize (1) the placement of new wells, (2) the placement of completion intervals, (3) the thickness of steam injection intervals, and (4) the spacing between injection intervals in the same well.

Extended 3β-alkyl steranes and 3-alkyl triaromatic steroids in crude oils and rock extracts

Abstract In oils and Precambrian- to Miocene-age source rocks from varying depositional environments, we have conclusively identified several novel 3-alkyl sterane and triaromatic steroid series, including (1) 3β-n-pentyl steranes, (2) 3β-isopentyl steranes, (3) 3β-n-hexyl steranes, (4) 3β-n-heptyl steranes, (5) 3,4-dimethyl steranes, (6) 3β-butyl,4-methyl steranes, (7) triaromatic 3-n-pentyl steroids, and (8) triaromatic 3-isopentyl steroids. We have also tentatively identified additional homologs with 3-alkyl substituents as large as C11. The relative abundances of these compounds vary substantially between samples, as indicated by (1) the ratio of 3β-n-pentyl steranes to 3β-isopentyl steranes and (2) the ratio of 3-n-pentyl triaromatic steroids to 3-isopentyl triaromatic steroids. These data suggest possible utility for these parameters as tools for oil-source rock correlations and reconstruction of depositional environments. Although no 3-alkyl steroid natural products are currently known, several lines of evidence suggest that 3β-alkyl steroids result from bacterial side-chain additions to diagenetic Δ2-sterenes.

Source Rock Quality Determination from Oil Biomarkers II--A Case Study Using Tertiary-Reservoired Beaufort Sea Oils

Biomarkers (molecular fossils) in 26 Beaufort Sea Mackenzie delta oils reveal three genetic oil groups and suggest descriptions of their probable sources. Group 1 (21 Tertiary-reservoired oils) was soured from Tertiary deltaic sediments. Group 2 (three Cretaceous- and one Devonian-reservoired oil) and group 3 (one Lower Cretaceous-reservoired oil) derive from two high-quality sources deposited in open-marine environments. Geographical variations in the geochemistry of the 21 group 1 oils suggest that their deltaic source was substantially more oil-prone in the distal deltaic portions than in the region closer to the paleoshoreline. Although these data do not address source rock volumes, they do indicate that poor source quality is not a cause of underfilled traps in the o fshore. The map pable lateral variations in group 1 oil compositions illustrate how, in basins with vertically drained sources, lateral source rock facies changes can bc inferred from regional variations in the geochemistry of the overlying oils. Furthermore, the data suggest that geochemical studies attempting to deduce genetic relationships between geographically separated oils should carefully consider the potential effects of source rock facies changes on the oil compositions.

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.