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A comprehensive regional organic geochemical study was performed on outcrop and core samples from the Eagle Ford Group with the aim of determining variations in organic matter source, thermal maturity, and depositional environments. Total organic carbon (TOC) and Rock-Eval pyrolysis parameters show that the Eagle Ford Group has excellent hydrocarbon source rock potential and is dominated by type II kerogen. Distributions of regular steranes, hopanes, and monoaromatic steroids confirm a marine carbonate depositional environment. Aryl isoprenoid ratios suggest the occurrence of intermittent photic zone anoxia. The n-alkane and sterane distributions and the presence of pregnanes, homopregnanes, and gammacerane suggest deposition under hypersaline conditions particularly in west and east Texas. In east Texas, biomarkers indicate that the Eagle Ford Group was partially sourced from terrigenous organic matter and deposited under oxic–suboxic conditions. Thermal maturity parameters including the temperature at which the peak corresponding to the compounds produced from the thermal degradation of kerogen in the Rock-Eval chromatogram maximizes (Tmax), vitrinite reflectance, and biomarker ratios indicate that in west and central Texas, the Eagle Ford Group is immature to marginally mature; in southwest Texas it is in the oil to dry gas window; and in east Texas, the Eagle Ford is in the main stage of oil generation. Geochemical logs of biomarker ratios show that the Lower Eagle Ford Formation has the highest TOC and hydrogen index, particularly in the Lozier Canyon Member; experienced stronger anoxic conditions during deposition; and has lower clay content compared with the Upper Eagle Ford Formation. Geochemical parameters show that the latter may have received an additional siliciclastic and terrigenous organic matter input under oxic–dysoxic conditions. Isotope data indicate a marine organic matter source for the Eagle Ford Group, but surprisingly, δ13C values do not show significant organic facies, depositional environment, or thermal maturity variations.

Understanding the environmental features and processes responsible for the formation of organic-rich, laterally extensive source rocks of appreciable thickness, such as the Late Devonian–Early Mississippian Woodford Shale, is necessary to refine current depositional models. In this study, a detailed assessment of the paleowater column was undertaken via examination of molecular fossil (biomarker) data. Specifically, C40 aromatic carotenoids were used to delineate the nature and severity of photic-zone euxinia concomitant with the deposition of the lower, middle, and upper Woodford. The immature character of the Woodford in the study area ensured that biomarker interpretations were reflective of the depositional environment and not altered by secondary processes. Twenty-nine Woodford Shale samples from the Lawrence uplift of the western Arkoma Basin were subjected to bulk and molecular geochemical analyses, with the objective to enhance the understanding of the paleowater column. A suite of C40 aromatic carotenoids—paleorenieratane, isorenieratane, renieratane, and renierapurpurane—was identified for the first time in the Woodford Shale. Paleorenieratane, the predominant C40 aromatic carotenoid, was found to be isotopically enriched (δ13C e −12‰ ± 1.5‰). This was interpreted as a product of carbon fixation via the reverse tricarboxylic acid cycle, a diagnostic trait of Chlorobiaceae. Additional biomarker parameters sensitive to changes in redox assisted in substantiating interpretations of Chlorobiaceae derivatives, with the severity of photic-zone euxinia inferred by the effects that this environmental state imposed on the overlying microbial community and algal populations. Previously unrecognized inter- and intramember variations in water-column structure and redox state were documented. The lower Woodford was characterized by isolated pulses of photic-zone euxinia, with the middle Woodford deposited during persistent photic-zone euxinia, as evinced by elevated concentrations of the C40 aromatic carotenoids. A deep, persistent chemocline likely typified upper Woodford deposition, but lack of sampling intensity in this member may belie a more episodic nature. Effects of photic-zone euxinia appeared to be most severe in the middle Woodford, where progressively increasing C40 aromatic-carotenoid concentrations coincide with rising gammacerane indices and a systematic decrease in sterane/hopane values. Environmental conditions (e.g., oxygen minimum zone, restriction) that facilitate the development of euxinia in the photic zone were interpreted to reconstruct the paleoenvironment on a local to subregional scale.

Analyses of multiple Mississippian and Pennsylvanian aged source rock samples in the Anadarko Basin, Oklahoma, revealed the presence of a compound that was tentatively identified as 18α(H)-oleanane. The identity of the 18α(H)-oleanane was subsequently confirmed by GCMS, GCMSMS, and coinjection with a sample known to contain 18α(H)-oleanane. The approach to identifying 18α(H)-oleanane was similar to that described in previous publications that have noted the presence of 18α(H)-oleanane in pre-Cretaceous samples. The presence of 18α(H)-oleanane in an Early Mississippian Kinderhookian sample appears to be the earliest reported occurrence to date. 18α(H)-Oleanane has typically been reported to be derived from a Late Cretaceous/Early Tertiary angiosperm source which would appear unlikely in this case since there are no Cretaceous or Tertiary aged source rocks in Oklahoma. This paper discusses possible sources for 18α(H)-oleanane including the suggestion that angiosperms, or angiosperm related species, split off from the gymnosperms far earlier than previously proposed. It is proposed that the data and methods discussed in this paper clearly show the presence of a compound identified as 18α(H)-oleanane in Mississippian and Pennsylvanian age samples. However, the source of the 18α(H)-oleanane still remains unclear.

It has been estimated that technically recoverable reserves from shales in the continental U.S. are believed to be approximately 24 billion barrels of oil (BBO) and 750 trillion cubic feet (Tcf) of gas. In Oklahoma the most important hydrocarbon source rock is the Woodford Shale, proposed to be the source of some 70% of the state's liquid hydrocarbon reserves. Such a regionally widespread source rock provides potential for both academic study and industrial opportunities. In this study representative samples of the Woodford Shale from the McAlister Quarry outcrop have been characterized by a variety of geochemical techniques. TOC values range from 0.07 to 15.6 wt% and, based on Rock-Eval Pyrolysis, most of the samples plot within the Type I/II kerogen range, with several low TOC samples plotting in the Type III kerogen range. TOC and biomarker parameters, suggests that two sections of the Upper Woodford are immature paleosols, having been subaerially exposed due to abrupt episodes of sea level fall during the Late Devonian. Evidence for these zones of paleoweathering includes highly depleted TOC, increased oxygen index (OI) values, loss of n-alkanes, decreases in the aryl isoprenoid ratio (AIR), sesquiterpenoids, cheilanthanes, hopanoids, and decreases in many of the polycyclic aromatic hydrocarbons (PAHs). The values of several paleoenvironmental proxies for samples immediately following the Frasnian-Famennian (F–F) Stage extinction boundary within the Upper Woodford suggests that a large influx of weathered terrigenous material occurred at that time. The presence of pyrogenic compounds implies that paleowildfires were widespread throughout the Middle and Late Devonian in the North American midcontinent. It is possible that these influxes of weathered terrestrial organic material stimulated algal blooms that led to the anoxic water column conditions often attributed to the F–F Stage extinction event. Investigation of the stratigraphic framework of these source rocks assists in evaluating their potential as hydrocarbon producers and/or unconventional reservoirs. Integration of geochemical data adds another powerful dimension to this approach. It has been proposed that the Woodford Shale was deposited during a 2nd order depositional sequence. The lower member represented the transgressive systems tract (TST), the Upper member the highstand systems tract (HST), and the Middle member bridging the transition between the two. Numerous 3rd order parasequences were also identified as being part of the overall 2nd order sequence and could be correlated to the Devonian sea level curve. The major objectives of the study were: While this study focuses locally on the McAlister Quarry samples, it is also of regional importance. The Woodford is a prolific source rock throughout the Anadarko Basin and has been the subject of extensive exploration and production efforts, particularly in the past decade with the development of horizontal drilling and hydrofracturing techniques. Continued investigation of the heterogeneity of the shale will greatly enhance the ability to predict more productive facies within the formation throughout the basin.

Reactive transport modeling of multi-element, compound-specific isotope analysis (CSIA) data has great potential to quantify sequential microbial reductive dechlorination (SRD) and alternative pathways such as oxidation, in support of remediation of chlorinated solvents in groundwater. As a key step towards this goal, a model was developed that simulates simultaneous carbon, chlorine, and hydrogen isotope fractionation during SRD of trichloroethene, via cis-1,2-dichloroethene (and trans-DCE as minor pathway), and vinyl chloride to ethene, following Monod kinetics. A simple correction term for individual isotope/isotopologue rates avoided multi-element isotopologue modeling. The model was successfully validated with data from a mixed culture Dehalococcoides microcosm. Simulation of Cl-CSIA required incorporation of secondary kinetic isotope effects (SKIEs). Assuming a limited degree of intramolecular heterogeneity of δ37Cl in TCE decreased the magnitudes of SKIEs required at the non-reacting Cl positions, without compromising the goodness of model fit, whereas a good fit of a model involving intramolecular CCl bond competition required an unlikely degree of intramolecular heterogeneity. Simulation of H-CSIA required SKIEs in H atoms originally present in the reacting compounds, especially for TCE, together with imprints of strongly depleted δ2H during protonation in the products. Scenario modeling illustrates the potential of H-CSIA for source apportionment.

EDB (1,2-dibromoethane) is frequently detected at sites impacted by leaded gasoline. In reducing environments, EDB is highly susceptible to abiotic degradation. A study was conducted to evaluate the potential of compound-specific isotope analysis (CSIA) in assessing abiotic degradation of EDB in sulfate-reducing environments. Water containing EDB was incubated in sealed vials with various combinations of Na(2)S (0.7 mM) and mackinawite (FeS) (180 mM). Degradation rates in vials containing FeS exceeded those in Na(2)S-only controls. In the presence of FeS, first-order constants ranged from 0.034 ± 0.002 d(-1) at pH 6 to 0.081 ± 0.005 d(-1) at pH 8.5. In the presence of FeS, products from reductive debromination (ethylene) and from S(N)2 substitution with S(II) nucleophiles were detected (1,2-dithioethane, DTA). Relatively high yields of DTA suggested that the S(N)2 reactions were not mediated by HS(-) only but likely also included reactions mediated by FeS surface. Significant carbon isotope effects were observed for nucleophilic substitution by HS(-) (ε = -31.6 ± 3.7‰) and for a combination of reductive and substitution pathways in the presence of FeS (-30.9 ± 0.7‰), indicating good site assessment potential of CSIA. The isotope effects (KIEs) observed in the presence of FeS corroborated the predominance of S(N)2 substitution by nucleophiles combined with two-electron transfer reductive debromination.

Based on the systematic study of aromatic hydrocarbons in over 100 crude oil samples collected from the Tabei and Tazhong uplifts in the Tarim Basin, the western depression area in the Qaidam Basin and the Tabei depression in the Turpan Basin, the geochemical characteristics of the marine (Tarim Basin), saline lacustrine (Qaidam Basin), and swamp (Turpan Basin) oils were investigated. The marine oils from the Tarim basin are characterized by relatively low abundance of diaromatic hydrocarbons such as biphenyl and naphthalene, and relatively high abundance of triaromatic hydrocarbons including phenanthrene, dibenzothiophene and fluorene. In contrast, the swamp oils from the Turpan Basin are dominated by the highest relative abundance of diaromatic hydrocarbons and the lowest relative abundance of triaromatic hydrocarbons in all the oil samples in this study. The relative abundance of diaromatic and triaromatic hydrocarbons in the saline lacustrine oils from Qaidam Basin is between that in Tarim oils and Turpan oils. Aromatic parameters based on the isomer distributions of dimethylnaphthalenes (DMN), trimethylnaphthalenes (TMN), tetramethylnaphthalenes (TeMN) and methylphenanthrenes (MP), i.e., 1,2,5-trimethylnaphthalene(TMN)/1,3,6-TMN ratio, 1,2,7-TMN/1,3,7-TMN ratio, (2,6-+2,7-)-dimethylnaphthalenes (DMN)/1,6-DMN ratio, 1,3,7-TMN/(1,2,5- +1,3,7-)-TMN, 1,3,6,7-TeMN/(1,3,6,7- +1,2,5,6- +1,2,3,5-)-TeMN ratio and MP index, may reflect the diversity of organic source input, thermal maturity and depositional environments. In addition, the dibenzothiophenes (DBTs)/fluorenes(Fs) and dibenzofurans (DBFs)/Fs ratios were found to the very useful and effective in determining genetic types of crude oils for the marine, saline lacustrine, and swamp depositional environments, and for oil-oil correlations.

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.

Significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). Bulk enrichment factors (E(bulk)) for PCE were -30.2 +/- 4.3 per thousand (pH 7), -29.54 +/- 0.83 per thousand (pH 8), and -24.6 +/- 1.1 per thousand (pH 9). For TCE, E(bulk) values were -33.4 +/- 1.5 per thousand (pH 8) and -27.9 +/- 1.3 per thousand (pH 9). A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 (BB1) and Sulfurospirillum multivorans (Sm)) and a bacterial consortium (BioDechlor INOCULUM (BDI)). The E(bulk) values for biological PCE microbial dechlorination were -1.39 +/- 0.21 per thousand (BB1), -1.33 +/- 0.13 per thousand (Sm), and -7.12 +/- 0.72 per thousand (BDI), while those for TCE were -4.07 +/- 0.48 per thousand (BB1), -12.8 +/- 1.6 per thousand (Sm), and -15.27 +/- 0.79 per thousand (BDI). Reactions were investigated by calculation of the apparent kinetic isotope effect for carbon (AKIEc), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from the differences in rate-limiting steps during the dechlorination reaction. Measurement of more negative E(bulk) values at sites contaminated with PCE and TCE may suggest the occurrence of abiotic reductive dechlorination by FeS.

Currently it is unclear if natural attenuation is an appropriate remedial approach for groundwater impacted by methyl tertiary butyl ether (MTBE). Site-characterization data at most gasoline release sites are adequate to evaluate attenuation in MTBE concentrations over time or distance. But, demonstrating natural biodegradation of MTBE requires laboratory microcosm studies, which could be expensive and time-consuming. Recently, compound-specific carbon isotope ratio analyses (13C/12C expressed in delta13C notation) have been used to demonstrate aerobic biodegradation of MTBE in laboratory incubations. This study explored the potential of this approach to distinguish MTBE biodegradation from other abiotic processes in an anaerobic groundwater plume that showed extensive decrease in MTBE concentrations. To our knowledge, this is the first study to use delta13C of MTBE data in groundwater and laboratory microcosms to demonstrate anaerobic biodegradation of MTBE. The delta13C of MTBE in monitoring wells increased by up to 31 per thousand (-25.5 per thousand to +5.5 per thousand) along with a 40-fold decrease in MTBE concentrations. Anaerobic incubations in laboratory microcosms indicated up to 20-fold reduction in MTBE concentrations with a corresponding increase in delta13C of MTBE of up to 33.4 per thousand (-28.7 per thousand to +4.7 per thousand) in live microcosms. Little enrichment was observed in autoclaved controls. These results demonstrate that anaerobic biodegradation was the dominant natural attenuation mechanism for MTBE at this site. The estimated isotopic enrichment factors (epsilon(field) = -8.10 per thousand and epsilon(lab) = -9.16 per thousand) were considerably larger than the range (-1.4 per thousand to -2.4 per thousand) previously reported for aerobic biodegradation of MTBE in laboratory incubations. These observations strongly suggest that delta13C of MTBE could be potentially useful as an "indicator" of in-situ MTBE biodegradation.

N-alkanes in extracts from possible carbonate source rocks of the Lower Ordovician Majiagou Formation in the central gas field of the Shanganning Basin and the Upper Sinian Dengying Formation in the Weiyuan gas field of the Sichuan Basin, are characterized by bimodal distributions with the dominant carbon numbers in the range C17-C21 and C25-C29. In most samples, the lower carbon number components are present in greater abundance than the higher carbon number ones. Most samples contain significant concentrations of waxy hydrocarbons (C22+) with C21-/C22+ ratios between 0.50 to 3. 16, and an average value of 1. 34. The n-alkanes in extracts of outcrops and shallow core samples of Upper Proterozoic and Lower Palaeozoic carbonate rocks in the western and southern parts of the North China Basin are of unimodal distributions dominated by n-alkanes maximizing in the C25-C29 range. These extracts have very high concentrations of waxy hydrocarbons with C21-/C22+ ratios all < 1. 0, ranging from 0. 14 to 0. 90 and averaging 0. 36. All of the extracts have a marked odd/even predominance in the high molecular weight n-alkane range regardless of whether they are from shallow or deep cores or out-crop samples. Simulation experiments were performed using typical sapropelic-type kerogens from the immature Sinian Lower Xiamalin Formation carbonate-rich shales collected at Jixian, Hebei Province, North China Basin, and the contemporary microplanktonic blue-green algaeSpirulina subsala. Results indicate that the unusual distribution of n-alkanes in the extracts of Upper Proterozoic and Lower Palaeozoic carbonate rocks possibly originated from algae in the source rocks at high levels of maturity and overmaturity.

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.

The natural attenuation of polyaromatic hydrocarbons (PAHs) in the vadose zone of a naturally revegetated former industrial sludge basin (0.45 ha) was examined. This was accomplished by comparing the concentration of 16 PAH contaminants present in sludge collected below the root zone of plants with contaminants present at 3 shallower depths within the root zone. Chemical analysis of 240 samples from 60 cores showed the average concentration of total and individual PAHs in the 0-30 cm, 30-60 cm, and bottom of the root zone strata were approximately 10, 20, and 50%, respectively, of the 16, 800 ppm average total PAH concentration in deep non-rooted sludge. Statistically significant differences in average PAH concentrations were observed between each strata studied and the non-rooted sludge except for the concentrations of acenaphthene and chrysene present at the bottom of the root zone in comparison to sludge values. The rooting depth of the vegetation growing in the basin was dependent on both vegetation type and plant age. Average rooting depths for trees, forbs (herbaceous non-grasses), and grasses were 90, 60, and 50 cm, respectively. The deepest root systems observed (100-120 cm) were associated with the oldest (12-14 year-old) mulberry trees. Examination of root systems and PAH concentrations at numerous locations and depths within the basin indicated that plant roots and their microbially active rhizospheres fostered PAH disappearance; including water insoluble, low volatility compounds, i.e. benzo(a)pyrene and benzo(ghi)perylene. The reduced concentration of PAHs in the upper strata of this revegetated former sludge basin indicated that natural attenuation had occurred. This observation supports the concept that through appropriate planting and management practices (phytoremediation) it will be possible to accelerate, maximize, and sustain natural processes, whereby even the most recalcitrant PAH contaminants (i.e. benzo(a)pyrene) can be remediated over time.

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 δ 13 C values were observed although the samples had isotopically distinct δ-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.

High-temperature gas chromatography (HTGC) has enhanced our ability to characterize hydrocarbons extending to C120 in crude oils. As a result, hydrocarbons in waxes (> C20) have been observed to vary significantly between crude oils, even those presumed to originate from the same source. Prior to this development, microcrystalline waxes containing hydrocarbons above C40 were not characterized on a molecular level due to the analytical limitations of conventional gas chromatography. Routine screenings of high pour-point crude oils by high-temperature gas chromatography has revealed that high molecular weight hydrocarbons (> C40) are very common in most oils and may represent 2% of the crude oil. Precise structures, origins, and significance of these high molecular weight compounds remain elusive. As a preliminary step to expand our knowledge of these compounds their general molecular structures and formulas have been investigated in this study. Initial results suggest that the major high molecular weight compounds include a homologous series of n-alkanes, methylbranched alkanes, alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and alkylcycloalkanes. # 2000 Elsevier Science Ltd. All rights reserved.

Diamantanes were identified in extracts from the Lower Ordovician Majiagou Formations (O1m5) of the central gas field, Shanganning Basin, China. At 3100‐3800 m, corresponding to Ro 1.9‐3.9%, the MDI [methyl diamantane index=4-MD/(1-MD+3-MD+4-MD)] ranges from 40 to 65% for the source rock extracts. Changes in the MDI index in the very mature sections (Ro>2.0%) are relatively small and no linear correlation between MDI and Ro ,o r MDI and depth, is noted, as previously reported, suggesting the possibility that the MDI may have limited applicability in maturity determinations. # 2000 Elsevier Science Ltd. All rights reserved.

Results of extraction experiments performed on immature Green River Formation source rock samples from the Black Shale Facies (Uinta Basin, Utah) demonstrate the potential of extracting high molecular weight hydrocarbons (HMWHC) with sequential dichloromethane:methanol/soxhlet and p -xylene/sonication techniques. The first soxhlet extraction step with DCM:MeOH removes lower molecular weight alkanes from the source rock while the HMWHC remain in the rock residue. Sonication of the rock residue with p -xylene recovers these HMWHC, showing distinct hydrocarbon distributions and preferences in high temperature gas chromatograms. Extracts obtained by using p -xylene/soxhlet with original source rocks and sequential soxhlet extraction instead of sonication have relatively lower concentrations of HMWHC. Sequential extraction of two other source rock types (Mahogany Shale and Black Shale Facies coal) did not result in detection of similar HMWHC in the extract. This indicates that the presence of these compounds in the Black Shale Facies of the Green River Formation is a primary source signal and a specific characteristic of this source rock.

Results are presented on changes in the content and composition of waxes and asphaltenes of oil produced during the transition from primary to secondary recovery following water flooding. Oil samples have been collected for a period of about three years from one well in Prairie Gem field in central Oklahoma. Wax and asphaltene fractions have been isolated from the produced oils and analyzed, qualitatively and quantitatively, by high temperature gas chromatography. The results demonstrate significant variations in the asphaltene and wax content of the oil produced during the transition period from primary to secondary water flood recovery, ranging from 0.7 to 5 wt%, and 11 to 33 wt%, respectively. Moreover, these variations are found to be accompanied by changes in wax composition and to precede specific production events. The oil produced a month before the response of the well to water flooding indicated an enhanced concentration of nC 22 –C 35 alkanes in the wax. The following episode of wax plugging in well equipment was preceded by increased amounts of asphaltenes and high molecular weight (nC 35 –C 50 ) alkanes in the wax of produced oils. Changes in the asphaltene and wax compositions are discussed in terms of processes displacing weakly adsorbed organic material from the pore spaces of the reservoir under increased reservoir pressure and flowing fluid volumes related to water flooding.

Cyclical sedimentation patterns are common in unconventional resources shales, whether they be carbonate- or siliciclastic-dominated. In many resource shales the cyclical patterns have been related to eustatic sea level fluctuations, even though these fluctuations may sometimes be obscured by tectonic overprint. The cyclicity is reflected in repetitive sedimentation patterns which represent alternating relatively shallow/oxic- and deeper/anoxicwater deposition. Typical cyclicity might occur in the form of alternating clay/organic-rich and quartz/calcite-rich strata. In more carbonate-rich sequences, organic-rich marls might alternate with organic-poor limestones. This cyclicity can occur at a variety of stratigraphic scales and provides a sequence stratigraphic framework for mapping, correlation, and interpretations. Incised valley fill may provide a localized, thicker, more organic-rich stratigraphic section (‘sweet spot’), than adjacent areas. Within the context of geomechanics, these cyclical strata are termed ‘brittle-ductile couplets’. Using the Barnett and Woodford shales as examples, clay/organic-rich intervals tend to be relatively ductile (relatively low Young’s Modulus and high Poisson’s ratio) and cleaner quartz/calcite-rich intervals tend to be relatively brittle (relatively high Young’s Modulus and low Poisson’s ratio. In carbonate-rich deposits such as the Eagle Ford Formation, recrystallized, TOC-poor limestones tend to be stronger and more brittle than TOC-rich marls. Within the context of microseismic, microseisms may be vertically stratified or layered, with some horizons containing more events than other horizons. This stratification has been related to brittle-ductile couplets in at least one area, and placed within a sequence stratigraphic framework. Within the context of geochemistry, ductile strata tend to contain more TOC, and are thus better potential HC source rocks, than brittle strata, which contain less TOC. Biomarkers (geochemical fingerprints) are useful for interpreting sources and environments of deposition of organic matter, and ultimately used for environmental zonation of shales. Using these principles and observations, it is possible to build a sequence stratigraphic framework from multiple data sets to map and correlate brittle and ductile strata, organic-/hydrocarbon-rich zones, and more fracturable stratigraphic intervals. A suggested horizontal landing zone is the brittle strata within a brittle-ductile couplet. It is hypothesized that when hydraulically fractured, both brittle and ductile strata become fractured, and hydrocarbons move from the ductile to the brittle zone, whose fractures remain open after proppant emplacement. With time, ductile strata may close around the proppant and become sealed. Sequence stratigraphy Sequence stratigraphy provides a basis for subdivision of stratigraphic intervals into geologically realistic and cyclical patterns. These cyclical patterns are common in unconventional resource ‘shales’, whether they be carbonate- or siliciclastic-dominated. Shales with these patterns include Barnett Shale (Slatt et al, 2012), Woodford

Although polycyclic aromatic hydrocarbons (PAHs) have usually been found to persist under strict anaerobic conditions, in a previous study an unusual site was found in San Diego Bay in which two PAHs, naphthalene and phenanthrene, were oxidized to carbon dioxide under sulfate-reducing conditions. Further investigations with these sediments revealed that methylnaphthalene, fluorene, and fluoranthene were also anaerobically oxidized to carbon dioxide in these sediments, while pyrene and benzo[a]pyrene were not. Studies with naphthalene indicated that PAH oxidation was sulfate dependent. Incubating the sediments with additional naphthalene for 1 month resulted in a significant increase in the oxidation of [14C]naphthalene. In sediments from a less heavily contaminated site in San diego Bay where PAHs were not readily degraded, naphthalene degradation could be stimulated through inoculation with active PAH-degrading sediments from the most heavily contaminated site. Sediments from the less heavily contaminated site that had been adapted for rapid anaerobic degradation of high concentrations of benzene did not oxidize naphthalene, suggesting that the benzene- and naphthalene-degrading populations were different. When fuels containing complex mixtures of alkanes were added to sediments from the two sites, there was significant degradation in the alkanes. [14C]hexadecane was also anaerobically oxidized to 14CO2 in these sediments. Molybdate, a specific inhibitor of sulfate reduction, inhibited hexadecane oxidation. These results demonstrate that a wide variety of hydrocarbon contaminants can be degraded under sulfate-reducing conditions in hydrocarbon-contaminated sediments, and they suggest that it may be possible to use sulfate reduction rather than aerobic respiration as a treatment strategy for hydrocarbon-contaminated dredged sediments.

the macromolecular structure of the pyrobitumen wurtzilite has been examined by a variety of chemical and analytical techniques, including desulfurization with lithium in ethylamine. The results of these analyses provide strong evidence for the presence of intermolecular sulfur linkages in the wurtzilite geopolymer matrix, which are thought to have formed via a process of low temperature “natural vulcanization”. The polymerized hydrocarbon components appear to be a complex mixture of n -alkanes, isoprenoids, and heterocompounds, offering further insight toward a bulk structural model for wurtzilite.

Isotope ratios of volatile organic compounds (VOCs) in the environment are often of interest in contaminant fate studies. Adsorbent preconcentration-thermal desorption of VOCs can be used to collect environmental vapor samples for compound-specific isotope analysis (CSIA). While active adsorbent samplers offer logistic benefits in handling large volumes of air, their performance in preserving VOCs isotope ratios was not previously tested under sampling conditions corresponding to typical indoor air sampling conditions. In this study, the performance of selected adsorbents was tested for preconcentration of TCE (for determination of C and Cl isotope ratios), PCE (C and Cl) and benzene (C and H). The key objective of the study was to identify the adsorbent(s) permitting preconcentration of the target VOCs present in air at low μg/m3 concentrations, without significant alteration of their isotope ratios. Carboxen 1016 was found to perform well for the full range of tested parameters. Carboxen 1016 can be recommended for sampling of TCE, PCE and benzene, for CSIA, from air volumes up to 100 L. Variable extent of isotope ratio alteration was observed in the preconcentration of the target VOCs on Carbopack B and Carbopack X, resulting from partial analyte loss via adsorbent bed breakthrough and (possibly) via incomplete desorption. The results from testing the Carbopack B and Carbopack X highlight the need of adsorbent performance validation at conditions fully representative of actual sample collection conditions, and caution against extrapolation of performance data toward more challenging sampling conditions.

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.

Isotope fractionation has been used with increasing frequency as a tool to quantify degradation of chlorinated aliphatic pollutants in the environment. The objective of this research was to determine if the electron donor present in enrichment cultures prepared from uncontaminated sediments influenced the extent of isotope fractionation of tetrachloroethylene (PCE), either directly, or through its influence on microbial community composition. Two PCE-degrading enrichment cultures were prepared from Duck Pond (DP) sediment and were incubated with formate (DPF) or H(2) (DPH) as electron donor. DPF and DPH were significantly different in both product distribution and extent of isotope fractionation. Chemical and isotope analyses indicated that electron donors did not directly affect the product distribution or the extent of isotope fractionation for PCE reductive dechlorination. Instead, restriction fragment length polymorphism (RFLP) and sequence analysis of the 16S rRNA clone libraries of DPF and DPH identified distinct microbial communities in each enrichment culture, suggesting that differences in microbial communities were responsible for distinct product distributions and isotope fractionation between the two cultures. A dominant species identified only in DPH was closely related to known dehalogenating species (Sulfurospirillum multivorans and Sulfurospirillum halorespirans) and may be responsible for PCE degradation in DPH. Our study suggests that different dechlorinators exist at the same site and can be preferentially stimulated by different electron donors, especially over the long-term (i.e., years), typical of in-situ ground water remediation.

The development of tandem mass spectrometry (MS-MS) and its combination with gas chromatography (GC-MS-MS) has greatly enhanced our ability to analyze exceedingly complex mixtures from a wide variety of sources, including fossil fuels. This paper describes a comparison of the conventional electron impact (El) mass spectral fragmentation patterns, obtained in the MS-MS precursor – product mode, of three oleanane isomers and one ursene, all of which are used as indicators of higher plant input to the sedimentary environment Results show that El and collision-activated dissociation (CAD) spectra are most similar at collision energies between −15 and −20 eV.The presence of methyl substituents in these pentacyclic terpenes plays an important role in the process of ring fragmentation: the daughter spectra of the M – 15 fragments can be used to obtain structural information, such as the positions of the double bonds.

Reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) was studied in well-defined microcosms prepared with aquifer materials from three locations. Electron donors and terminal electron acceptors were added to both stimulate microbial activity and generate reactive minerals via microbial iron and sulfate reduction. The relative importance of abiotic and microbial PCE and TCE reductive dechlorination was then assessed by analysis of reaction products and kinetics and, in some cases, by stable carbon isotope fractionation. The predominant PCE and TCE transformation pathway in most microcosms was microbial reductive dechlorination. Rates of abiotic transformation were similar in magnitude to those for microbial reductive dechlorination in only a few cases where the activity of dechlorinating bacteria was low. Comparison of geochemical conditions with abiotic product recoveries showed thatthe greatest extent of abiotic reductive dechlorination occurred under iron- and sulfate-reducing conditions. Under these two geochemical conditions, high concentrations of Fe(II) and S(-II) solid species were present, suggesting the involvement of Fe(II) and S(-II) minerals in abiotic reductive dechlorination. Both abiotic and microbial dechlorination of PCE and TCE took place under almost all microcosm conditions; the relative rates of the two processes under field conditions will depend on factors such as the abundance of dechlorinating bacteria, soil properties, and the mass loading of reactive minerals.

Twelve oils from the Sirikit field of the Phisanulok Basin, Thailand, have been examined to determine the nature of their source materials, depositional environment, relative maturity and extent of biodegradation. The geochemical analyses performed in this study suggested that all of the oils were derived from similar types of organic source matter consisting of a mixture of bacterial, algal and higher plant material. The sterane contents of the oils were relatively low and conventional maturity parameters based on sterane ratios suggested the oils were relatively immature. Extensive strike/slip faults in the Phisanulok Basin may have led to rapid burial of the sediments during deposition preventing these ratios from having attained their equilibrium ratios.

Thirty oils from the Pauls Valley area of the Anadarko Basin, Oklahoma, have been analyzed for n-alkane, terpane and sterane distributions in an effort to classify genetically related oils. Ten rock samples from the area were also analyzed in an attempt to relate oil types to their possible source rocks. The Pauls Valley area, located near the southeast corner of the Anadarko Basin, has producing formations ranging in age from Middle Ordovician to Late Pennsylvanian. Trapping mechanisms responsible for the accumulation of hydrocarbons vary from purely structural traps, to fractured reservoirs, to up-dip truncations of reservoirs and up-dip pinchouts of sandstones. Results show that 85% of oils in the area probably have a common source, the Woodford Shale. A group of oils reservoired in the Viola Group exhibit distinct geochemical characteristics and are thought to have a different source. The differences between the oils from these two sources include: enhanced C24 tetracyclic terpane concentrations relative to C26 tricyclic terpanes; diminished C28–30 tricyclic terpanes relative to Ts and Tm; diminished C30-steranes relative to their C29 counterparts; predominance of C35 over C34 extended hopanes and an even/odd predominance of the normal alkane distribution. Extracts from the Viola Group generally exhibit the same qualities as the associated oils and the limestone is proposed as their probable source rock. The predominance of C29-steranes for oil samples which appear to have a marine source (inferred by their n-alkane distribution, the presence of C30-steranes and high levels of tricyclic terpanes) suggests that caution be used in the interpretation of terrigenous sources based on the high abundance of C29-steranes. The Deese Group, the Springer Formation, the Sylvan Shale, and the Arbuckle Group are not likely sources for the oils analyzed from the Pauls Valley area. Differences in API gravity and color, previously used to classify oils in this region, are not source-related but are related to maturation and migration effects.

The formation of melanoidins from d -lysine in the presence of H 2 S and N 2 was examined to clarify the effect of H 2 S on the condensation of sugars and amino acids. Significant amounts of dark brown-colored precipitates formed in a mixture of glucose and lysine heated at 80°C for 20 days under N 2 . However, the same solution heated at 80°C for 30 days under H 2 S showed no major change apart from a slight change in color to light brown. This observation indicates that the condensation of glucose and lysine proceeds very slowly in the presence of H 2 S compared to their condensation in the presence of N 2 . Instead, significant amounts of organosulphur compounds formed from the glucose-lysine solution and H 2 S as confirmed by py-GC (pyrolysis gas chromatography) using both FID (flame ionization detector) and FPD (flame photometric detector). The rapid reaction of H 2 S with reactive carbonyl groups of reducing sugars is proposed to hinder the progess of the condensation between reducing sugars and amino acids. The incorporation of sulphur from H 2 S into sedimentary organic matter in the very early stage of diagenesis could be one of important controlling factors governing the formation of melanoidin-like geopolymers in Recent sediments. The organic geochemical characteristics of kerogens in carbonate rocks (relatively low abundance and high organic sulphur contents) could be interpreted by the rapid reaction between H 2 S and reducing sugars in the very early stage of diagenesis.

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.

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW-SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH 4, thermogenic C2+ gases, and associated condensates were evaluated separately. Methane appears to have 3 origins: Group-1 CH4 is bacteriogenic (Calculated ?13CC1-C = -61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH4. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (?13CC1-C = -35.80‰; Hamitabat Field) and Group-3 (?13C 1-C = -49.10‰; Degirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C 2+ gases. The Group-2 C2+ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (?13C = -21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C2+ gases, produced from one field, were generated from isotopically light marine kerogen (?13C = -29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases. The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C2+ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases. Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (?13C = -21‰; -29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (Req = 0.55-0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin. © 2005 Elsevier Ltd. All rights reserved.

The Tertiary Green River petroleum system in the Uinta basin generated about 500 million bbl of recoverable, high pour-point, paraffinic crude oil from lacustrine source rocks. A prolific complex of marginal and open-lacustrine source rocks, dominated by carbonate oil shales containing up to 60 wt. % type I kerogen, occur within distinct stratigraphic units in the basin. Petroleum generation is interpreted to originate from source pods in the basal Green River Formation buried to depths greater than 3000 m along the steeply dipping northern margin of the basin. Producing fields in the Altamont-Bluebell trend have elevated pore-fluid pressures approaching 80% of lithostatic pressure and are completed in strata where open fractures provide permeability. Active hydrocarbon generation is one explanation for the origin of the overpressured reservoirs. In this study, experiments were undertaken to examine the mechanisms of hydrocarbon generation and accumulation in the Uinta basin. We combined analyses of representative source rocks from the entire Green River stratigraphic section with detailed laboratory simulation experiments using both open- and closed-system pyrolysis. This information provides new insights on lacustrine source rock lithofacies, gas-oil-source rock correlations, hydrocarbon generation kinetics, and basin modeling. The results show that the basal Green River Formation contains a unique type I source facies responsible for generation of paraffinic crude oils. The classic type I oil shales in the upper Green River Formation correlate well with low-maturity aromatic-asphaltic samples. We determined kinetic parameters for the source rocks and used them to develop basin models for hydrocarbon generation. The models show that hydrous pyrolysis kinetic parameters are more consistent with the natural data in terms of predicted timing and extent of oil generation as compared to models using Rock-Eval kinetics. (Begin page 1334)

Paleozoic source rocks and crude oils from the Anadarko basin, Oklahoma, have been characterized by means of a variety of organic geochemical techniques. The potential source rocks investigated include the Viola Group, Sylvan Shale, Lower Mississippian limestone, Chester group, Springer Formation, and Morrow group. The Woodford Shale, which is an important source rock in the basin and has been discussed extensively in the literature, was used as a laboratory standard in this study. The crude oils examined in this study were obtained from reservoir formations of various ages from throughout the basin. The source rock screening analysis indicates that the lower and middle subfacies of the Viola, especially from the southeastern part of the basin, have substantial oil-generation potential. The Sylvan is not a source rock, although the black shales in the Springer and Morrow have substantial oil- and especially gas-generation potential. The results of this study support the proposal for multiple sources of oil and gas generation in the Anadarko basin. Selected source rock and oil samples were characterized in more detail, and the distributions and concentrations (both relative and absolute concentrations) of biomarkers in different source rocks and oils were determined. The biomarker characteristics were combined with other geochemical and geological data to interpret the sources, depositional environments, diagenesis and catagenesis processes, migration, and weathering. Oil­source rock correlations were made between source rocks and a number of crude oils based on quantitative and qualitative biomarker distributions. Petrographic descriptions and graptolite reflectance measurements were used to evaluate ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received November 1, 1995; revised manuscript received April 8, 1996; final acceptance September 13, 1996. 2ARCO International Oil and Gas Company, 2300 W. Plano Parkway, Plano, Texas 75057. 3School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019.

Biomarkers are compounds, generally hydrocarbons, present in crude oils and source rocks whose carbon skeletons can be readily related to a precursor molecule occurring in the original source material. The use of biomarkers in source rock extracts and crude oils for the purpose of determining source materials, thermal maturity, extent of biodegradation and migration and for making oil/oil and oil/source rock correlation studies has been well documented. One of the major problems with this approach is the possibility of contamination either naturally, through crude oil migration, or artificially during sample collection or work-up procedures. A solution to this potential problem can be found in the use of biomarkers produced by kerogen pyrolysis. The actual isolation of the kerogen eliminates any potential contaminants and pyrolysis of the kerogen produces indigenous biomarkers. In this paper it is intended to review progress that has been made in the study of biomarkers produced by kerogen pyrolysis. The various techniques that have been used in this approach will be discussed and examples given to show, in particular, the distributions of triterpanes and steranes produced by kerogen pyrolysis. In addition, results obtained from a combination of hydrous pyrolysis to artificially mature kerogens, and anhydrous pyrolysis to characterize the artificially matured kerogens, will be discussed. Certain biomarkers such as the C 28 -bisnorhopane and gammacerane are not produced during anhydrous pyrolysis and possible reasons for these observations will be discussed. Finally, the production of sulphur-containing compounds during pyrolysis reactions will be briefly discussed with reference to their role as possible indicators of maturity and depositional environments.

The average stable carbon isotope ratio of 21 oils from the Taranaki Basin of New Zealand suggests that the source rocks date from the Cretaceous period, when sedimentation in this basin began. Stable carbon isotope ratios also confirm the grouping of these oils into the genetic families which were previously suggested by the correlation of source-related biomarkers. Stahl isotope type-curves for the oils also reflect the correctness of these relationships. A plot of the saturate versus aromatic δ13C values for the New Zealand oils is particularly valuable for oil correlation in this basin and clearly demonstrates that the Ahuroa and Stratford oils, while separated from the McKee field by some 30 km, probably originate from the same oil-source formation as do the oils of the McKee field. The plot also indicates, that the Kaimiro, Mangahewa, Pukearuhe and Waihapa oils are discrete in situ accumulations of petroleum which do not have the characteristics of oils from adjacent fields. These conclusions are further supported by the values of the Sofer canonical variable for all of the oils. Additionally, the Sofer plot provides clear evidence of the predominantly terrigenous origin of the oils in the Taranaki basin. Consideration of the data for these oils, leads to a hypothesis that three independent sources of organic material, microbes, gymnosperms and angiosperms, gave rise to the oils in this region.

The hexane-soluble fractions of hydroliquefied products from three Yallourn brown coal lithotypes have been separated into five fractions by combined silica-alumina packed column chromatography. Analyses of various fractions by g.c.-m.s. permitted the identification of ≈50 components in the saturate fraction and 40 components in the diaromatic fraction, together with 30 components in the monoaromatics. The components identified were quite similar among hexane-soluble portions of all three lithotypes. A marked predominance of even carbon number alkane (C23-C29) was observed in the hydrocarbon fractions from pale lithotype over ZnCl 2 KCl , and medium light lithotype over both pure ZnCl2 and ZnCl 2 KCl . However, medium dark lithotype over both melt catalysts produced a saturate fraction with an odd carbon number(C22-C28) preference. Based on spectral methods, Soxhlet extracts obtained from untreated lithotypes (hexane and benzene solubles) were characterized as complex mixtures of higher molecular weight(300–1000) aliphatic hydrocarbons, which were supposed to be a precursor of the saturates produced from the corresponding lithotype in the catalytic hydroliquefaction.

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.