R. Paul Philp

Geochemical characterization of gases from the Mississippian Barnett Shale, Fort Worth Basin, Texas

Molecular and isotopic compositions of 51 gas and 7 condensate samples produced from the Barnett Shale were determined by gas chromatography–isotope ratio mass spectrometry to investigate their origin and maturity at the time of generation. Additionally, two hydrous pyrolysis experiments were performed to calibrate maturity values predicted for gas generation. Molecular and carbon and hydrogen isotopic compositions of the gases indicate that gas produced from the Barnett is thermogenic. To estimate generation maturity, additional analyses were performed on the condensates and available core samples. Diamondoid indices combined with vitrinite reflectance (Ro) measurements, isotopic and molecular composition, and hydrous pyrolysis experiments suggest that gas produced from the Barnett Shale was generated within the condensate–wet gas window (1.3–2.0% Ro) at a later stage than hydrocarbons accumulated in shallower reservoirs. The origin of the gas is thermogenic, probably derived from both kerogen cracking and secondary cracking of previously generated nonmigrated hydrocarbons. Additionally, the occurrence of a methane isotope reversal in group 1 gases corroborates that in-situ oil or gas cracking is occurring in the easternmost part of the study area.

Stable carbon isotopic composition of gasolines determined by isotope ratio monitoring gas chromatography mass spectrometry

Abstract A large number of underground gasoline storage facilities in the United States continuously leak gasoline into the subsurface, which makes gasoline a major groundwater contaminant. Gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) are used currently to characterize contaminated groundwater and soils. Correlations of contaminants with suspected source(s) are extremely difficult by these techniques because many gasolines have similar hydrocarbon distributions. The present study applied the technique of isotope ratio monitoring gas chromatography–mass spectrometry (irmGC–MS) to 19 gasoline samples from different areas of the USA. This allows a much better correlation of gasoline contaminants to source. Data obtained indicate a wide range of δ 13 C values for 16 ubiquitous compounds in the gasolines The majority of samples could be distinguished from each other on the basis of δ 13 C hydrocarbon composition. The oxygenated additive methyl tertiary butyl ether (MTBE) was present in ten of the gasolines analyzed, and had a relatively narrow range of δ 13 C values (−30.4 to −28.3‰). Preliminary investigations were also made to determine the extent of carbon isotopic fractionation after simple water washing and evaporation experiments. Results indicate that the majority of compounds did not undergo significant carbon isotopic fractionation as a result of these processes.

Organic geochemistry of the Woodford Shale, southeastern Oklahoma: How variable can shales be?

Woodford Shale samples, obtained from a cored outcrop in southeastern Oklahoma, were geochemically analyzed to determine vertical variations of organic facies, thermal maturity, and an evaluation of their depositional environments. Total organic carbon values ranged from 5.01 to 14.81%, indicating a good source rock potential. In this area, the Woodford Shale is marginally mature, as indicated by vitrinite reflectance values. Rock-Eval data revealed that the samples are dominated by type II kerogen. Biomarker ratios, based on pristane, phytane, steranes, and hopanes, show a mix of marine and terrigenous organic matter. High-salinity conditions and water density stratification also prevailed during deposition of this formation, as indicated by the presence of gammacerane. The Woodford Shale was subdivided into lower, middle, and upper members based on the integration of geochemical and geologic data. Moreover, the presence and extent of photic zone anoxia (PZA) were determined by the presence of aryl isoprenoids. The lower and upper Woodford Shale members were deposited under dysoxic to suboxic conditions and episodic periods of PZA. The middle member was deposited under anoxic conditions and persistent PZA. In addition, aryl isoprenoids helped infer the position of the chemocline during deposition of the different members. The relative hydrocarbon potential parameter was used in determining transgressive and regressive cycles within the Woodford Shale. This study undoubtedly demonstrates the significant lithologic and chemical variability that occurs within shales. The application of this workflow to regional studies can have a direct influence on exploration and production activities in shale-gas systems.

Biodegradation of low-molecular-weight alkanes under mesophilic, sulfate-reducing conditions: metabolic intermediates and community patterns

We evaluated the ability of the native microbiota in a low-temperature, sulfidic natural hydrocarbon seep (Zodletone) to metabolize short-chain hydrocarbons. n-Propane and n-pentane were metabolized under sulfate-reducing conditions in initial enrichments and in sediment-free subcultures. Carbon isotope analysis of residual propane in active enrichments showed that propane became enriched in (13)C by 6.7 (+/-2.0) per thousand, indicating a biological mechanism for propane loss. The detection of n-propylsuccinic and isopropylsuccinic acids in active propane-degrading enrichments provided evidence for anaerobic biodegradation via a fumarate addition pathway. A eubacterial 16S rRNA gene survey of sediment-free enrichments showed that the majority of the sequenced clones were phylogenetically affiliated within the Deltaproteobacteria. Such sequences were most closely affiliated with clones retrieved from hydrocarbon-impacted marine ecosystems, volatile fatty acid metabolizers, hydrogen users, and with a novel Deltaproteobacterial lineage. Other cloned sequences were affiliated with the Firmicutes and Chloroflexi phyla. The sequenced clones were only distantly (<95%) related to other reported low-molecular-weight alkane-degrading sulfate-reducing populations. This work documents the potential for anaerobic short-chain n-alkane metabolism for the first time in a terrestrial environment, provides evidence for a fumarate addition mechanism for n-propane activation under these conditions, and reveals microbial community members present in such enrichments.

The distribution and quantitation of organonitrogen compounds in crude oils and rock pyrolysates

Abstract Organonitrogen compounds are present in varying concentrations in aromatic fractions of various organic-rich geologic samples. Quinolines, benzoquinolines and carbazoles are the most important families of organonitrogen compounds in the aromatic fractions of crude oils and Phosphoria Formation pyrolysates. Aromatic fractions of oils generated from phosphatic shales contain higher concentrations of organonitrogen compounds than those from carbonate source rocks. Artificial maturation experiments on Phosphoria rocks at about 300°C yield high concentrations of quinolines relative to benzoquinolines and carbazoles, while higher temperature experiments yield high concentrations of benzoquinolines and carbazoles relative to quinolines. Organonitrogen compounds are potentially useful as indicators of the depositional environment of source rocks and maturity of crude oils.

Sesquiterpanes, diterpanes and other higher terpanes in oils from the Taranaki basin of New Zealand

Abstract The bi-, tri- and tetracyclic terpenoids in crude oils from the Taranaki basin of New Zealand have been studied. The distribution of C14, C15 and C16 bicyclanes was essentially invariant amongst the various fields. A suggestion is made that the proportion of rearranged to total drimanes may be related to maturity and the ratio of homodrimane to drimane may be related to depositional environment. Mass spectra are reported for a number of unidentified sesquiterpanes, including possibly norbisabolane and bisabolane. The distribution of bi-, tri- and tetracyclic diterpanes was very similar to that of crude oils from the Gippsland basin of south-east Australia. The occurrence of rimuane was confirmed, 8α(H)-labdane was identified and 4α(H)-18-norisopimarane was tentatively identified. Isopimarane dominated the diterpane distribution of oils from the Maui field in the south-west of the basin and its abundance gradually diminished in a north-westerly direction until it became a minor component of oil from the New Plymouth field. This trend coincided with an increase in the complexity of the triterpane distribution and with the oleanane content of the oils. These northwards trends also followed the evolution of coal deposits in the region. The diterpanes in these oils had the same structure as those which occur in extant trees of the Araucariaceae and Podocarpaceae which flourish in New Zealand today and did so even more extensively in the past. Coal measures, formed from forests of such trees, are probably the source of the diterpanes, a situation analogous to that in the Gippsland basin of south-eastern Australia. Tricyclanes of the cheilanthane series were not detected in any of the Taranaki oils, lending support to the view that these tricyclanes occur most abundantly in oils derived from marine sources. C24-Tetracyclanes (secotriterpanes) occurred abundantly in all the oils as they do in terrigenous oils from other parts of the world.

The Use of Stable Isotopes to Differentiate Specific Source Markers for MTBE

The recent controversy over the use of MTBE within gasoline to boost oxygen content and decrease carbon monoxide emissions to the atmosphere has led to a proposed phase-out of this compound by 2002. This paper is a preliminary investigation into the use of gas chromatography isotope-ratio mass spectrometry (GCIRMS) to determine both carbon and hydrogen isotopic compositions of MTBE as a means of differentiating sources of MTBE. Three pure MTBE samples were purchased from chemical distributors. Little variation of the i 13 C values were observed although the samples had isotopically distinct i -D values. Four different methods of obtaining carbon isotope ratios of neat MTBE, MTBE in gasoline, and MTBE in water are described, and the precision and accuracy of each is discussed. The carbon isotopic compositions of MTBE within 10 gasoline samples from three different areas of the United States show a wide range of carbon isotope compositions. This novel method of MTBE analysis could be valuable in forensic investigations.

Early developments in petroleum geochemistry

Petroleum geochemistry is the outgrowth of the application of the principles and methods of organic chemistry to petroleum refining and petroleum geology. This paper reviews 120 years of petroleum geochemistry, from about 1860 to 1980, and includes a discussion of the formal recognition of petroleum geochemistry as an earth-science discipline starting in 1959 when a general petroleum geochemistry symposium was first organized at Fordham University, New York. A chronology of significant events, including concepts, techniques, and textbook publications, is presented. Because petroleum geochemistry has been a tool for petroleum exploration from the beginning, the early developments of surface prospecting, source-rock identification, and oil/oil and oil/source correlation are discussed, along with the application of geochemistry to petroleum migration, accumulation, and alteration. In addition the paper deals with the biomarker revolution, which began in earnest about 1964, and with early models of geothermal history. Concepts in petroleum geochemistry have continually evolved, enhanced by the development of new analytical techniques, leading to new discoveries concerning the origin and occurrence of petroleum.

Kinetic and isotope analyses of tetrachloroethylene and trichloroethylene degradation by model Fe(II)-bearing minerals

The kinetics and in some cases stable carbon isotope fractionation associated with abiotic reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) by model Fe(II)-bearing minerals present in anaerobic soils were measured. The minerals studied were chloride green rust (GR-Cl), sulfate green rust (GR-SO(4)), pyrite, magnetite, and adsorbed Fe(II) or FeS formed at the surface of goethite by treatment with dissolved Fe(II) or S(-II). We observed some abiotic transformation of PCE and TCE in every system studied, as evidenced by the presence of abiotic reaction products. Bulk enrichment factors (epsilon(bulk) values) for TCE transformation by GR-Cl and pyrite were -23.0+/-1.8 per thousand and -21.7+/-1.0 per thousand, respectively, which are more negative than reported values for microbial TCE dechlorination and could provide one means for distinguishing microbial from abiotic dechlorination of TCE in the environment. Considering the time scale of subsurface remediation technologies, including natural attenuation, minerals such as green rusts, pyrite, and magnetite have the potential to contribute to the transformation of PCE and TCE at contaminated sites.

Compound specific isotopic variability in Uinta Basin native bitumens: paleoenvironmental implications

Abstract We have analyzed the solvent extracts from three different types of native bitumens from the Uinta Basin in northeastern Utah by a variety of analytical techniques, including GC-IRMS, to evaluate variations in the paleodepositional environment during two periods of Lake Uinta deposition. The gilsonite and tabbyite bitumens are associated with Parachute Creek Member sediments deposited during a major expansion of ancient Lake Uinta. Compound specific isotopic analyses of s-carotane and phytane ( δ 13 C = −32.6 to −32.1% 0 ) from these bitumens reflect input from primary photosynthetic producers such as cyanobacteria. Sterane δ 13C values (−34.5 to −29.2%0) refflect contributions from lacustrine algae, while extremely depleted δ 13C values for methylhopanes (−58.1 to −61.5%0) suggest input from methanotrophic bacteria. Variations in the δ 13C values of the αβ-hopanes (−51.4 to 37.7%0) imply additional input from other bacterial sources. The wurtzilite bitumen was generated from the Saline Facies of the Green River Formation deposited during a later regression of Lake Uinta. Compound specific isotopic analyses of phytane ( δ 13 C = −30.1% 0 ) and steranes δ 13 C = −29.6 to −26.7% 0 ) from this bitumen indicate continued input from primary producers and eukaryotes. The higher relative concentrations of gammacerane ( δ 13 C = −26.9% 0 ) indicate increasing input from aerobic protozoa. We observed a slight enrichment in δ 13C in the wurtzilite extract (and several biomarkers) and suggest that this is a result of sulfate-reducing bacteria outcompeting methanogens, thereby, eliminating the influence of methanotrophs in this later saline stage of deposition.