Maowen Li

Hydrogen isotopic compositions of individual alkanes as a new approach to petroleum correlation: case studies from the Western Canada Sedimentary Basin

Isotopic compositions of carbon-bound hydrogen in individual n-alkanes and acyclic isoprenoid alkanes, from a number of crude oil samples, were measured using gas chromatography-thermal conversion-isotope ratio mass spectrometry. The precision of this technique is better than 3‰ for most alkanes, compared to the large range of δD variation among the samples (up to 160‰). The oils were selected from major genetic oil families in the Western Canada Sedimentary Basin, with source rocks ranging in age from Ordovician (and possibly Cambrian) to Cretaceous. The hydrogen isotopic composition of alkanes in crude oils is controlled by three factors: isotopic compositions of biosynthetic precursors, source water δD values, and postdepositional processes. The inherited difference in the lipids biosynthetic origins and/or pathways is reflected by a small hydrogen isotopic variability within n-alkanes, but much larger differences in the δD values between n-alkanes and pristane/phytane. The shift toward lighter hydrogen isotopic compositions from Paleozoic to Upper Cretaceous oils in the WCSB reflects a special depositional setting and/or a minor contribution of terrestrial organic matter. The strong influence of source water δD values is demonstrated by the distinctively lower δD values of lacustrine oils than marine oils, and also by the high values for oils with source rocks deposited in evaporative environments. Thermal maturation may alter the δD values of the alkanes in the oil to some extent, but secondary oil migration does not appear to have had any significant impact. The fact that oils derived from source rocks that could be of Cambrian age still retain a strong signature of the hydrogen isotopic compositions of source organic matter, and source water, indicates that δD values are very useful for oil-source correlation and for paleoenvironmental reconstructions.

Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in Duvernay Formation petroleum source rocks in central Alberta, Canada

Abstract A quantitative study of pyrrolic nitrogen compounds was conducted on a series of marine carbonate petroleum source rock extracts taken from the Upper Devonian Duvernay Formation in the central Alberta portion of the Western Canada Sedimentary Basin. With increasing thermal maturity, concentrations of various pyrrolic nitrogen compounds in the rock extracts increase drastically, together with significant compositional variations related to alkyl substitution position. The study provides circumstantial evidence for adsorptive interactions operating between organic nitrogen compounds and solid organic/mineral phases in subsurface sedimentary rocks during petroleum generation and expulsion. The absolute concentrations and relative distributions of pyrrolic nitrogen compounds in the extracts of different maturity ranges provide background information for the calibration of such data in migrated petroleums, as an independent measurement of secondary oil migration range of Duvernaysourced oils in the basin

Bakken/Madison petroleum systems in the Canadian Williston Basin. Part 2: molecular markers diagnostic of Bakken and Lodgepole source rocks

Abstract The uppermost Devonian-Mississippian Bakken Formation black shale and the Mississippian Lodgepole Formation carbonate represent two of the most important source rocks in the Canadian Williston Basin. Quantitative analyses of both saturated and aromatic hydrocarbon fractions reveal significant differences in the relative distributions and absolute concentrations for a wide range of molecular markers between the extracts of the two source units. Among others, the Bakken shales are characterized by their high relative abundance of trimethyl aryl and diaryl isoprenoids likely derived from green sulfur bacteria Chlorobiaceae. In contrast, the Lodgepole carbonates at similar maturity levels display a C35 homohopane prominence and abundant benzohopanes, ring-D monoaromatic 8,14-secohopanes and a tetracyclic monoaromatic hydrocarbon. The distinctive nature of molecular marker “fingerprints” diagnostic of the two source rocks is clearly related to their different organic inputs and depositional environments. Additionally, the large difference in the absolute concentrations of these compounds observed in both source units may potentially lead to biased geochemical interpretations if strictly conventional, saturate-based biomarker approaches were used for oil-oil and oil-source correlation.

Molecular fossils and oil-source rock correlations in Tarim Basin, NW China

The distribution of “molecular fossils” (biomarkers) of steroid compounds in extracts from some specific geologic age in the Tarim Basin have been analyzed and are used as the fingerprints for the oil-source rock correlation. Having been affected by maturation, migration, phase fractionation and biodegradation, not any molecular fossils related to source and environment can be used as the fingerprints for oil-source rock correlation. Some special biomarkers widely existed in the extracts from Cambrian and Ordovician rocks in the Tarim Basin and showed obvious difference in each stratum, including dinosteranes (C30), 4-methyl-24-ethyl-cholestanes (C30) and their aromatized steroids, C24-norcholestanes and C28 steranes originated from dinoflagellates and diatom. Few oils such as the heavy oil drilled in the Cambrian reservoir from Tadong 2 well (TD2) correlated well with the extracts from the Cambrian. The amazing similarity of the relative contents of these compounds between the marine oils produced in Tazhong and Tabei uplifts and the extracts from the Upper Ordovician suggests that the Middle-Upper Ordovician is the very likely main source for the marine oils.

Geochemical constraints on models for secondary petroleum migration along the Upper Devonian Rimbey-Meadowbrook reef trend in central Alberta, Canada

Abstract The Upper Devonian Rimbey-Meadowbrook reef trend of central Alberta, Canada, is one of the classical examples that was originally used to support Gussows model of differential hydrocarbon entrapment. However, the clear discrepancies existing between the ideal hydrocarbon distribution predicted by this model and the one actually observed led to several alternative geological models, most of which have not been constrained by organic geochemical data. The present study uses a wide range of bulk and molecular parameters to determine the oil source characteristics and thermal maturity along the reef trend. The concentrations and isomeric distributions of alkylcarbazoles and non-alkylated benzocarbazoles in oils are employed to study petroleum migration pathways and to constrain the existing secondary petroleum migration models. The data indicates that Gussows principle is generally applicable, as the oils in the up-dip direction generally have longer implied migration distances although this is not always the case. Other factors not recognized previously, such as the presence of two subfamilies of the Duvernay Formation sourced oils, may also have contributed significantly to the discrepancies in the oil and gas distributions between the model predictions and the actual observational data.

Geochemical characterisation of Middle Devonian oils in NW Alberta, Canada: possible source and maturity effect on pyrrolic nitrogen compounds

Abstract Molecular geochemical compositions of a suite of Middle Devonian sourced and reservoired oils from the Rainbow–Zama–Shekilie sub-basins in N.W. Alberta, Canada, reveal the presence of at least two oil families in the study area. The distribution of each oil family is geographically restricted to a single sub-basin, consistent with the oils being sourced locally within a series of closed generation/migration/trapping systems. The diversity in the biomarker distributions of the oils indicates the wide range of depositional environments and source materials existing in each sub-basin, rather than mixing of end member oils across different sub-basins. Clear maturity differences are observed between the oils from the Rainbow and Zama subbasins. Pre-Cretaceous thermal anomalies along the reactivated regional Precambrian basement faults are proposed as one of the major causes for the relatively high maturity levels for the Middle Devonian source rocks in the study area. We infer that depositional environment and thermal maturity have had a strong impact on the geochemical characteristics of the saturated and aromatic hydrocarbons in the Rainbow–Shekilie–Zama oils, but may not influence the pyrrolic nitrogen compounds to a significant extent. However, recognition of possible source and maturity effects on pyrrolic nitrogen compounds in other studies suggests that these factors should be considered before the pyrrolic nitrogen compounds are used to characterize petroleum migration.

Marine transgressional events in the gigantic freshwater lake Songliao: paleontological and geochemical evidence

Abstract The fossil remains of euryhaline organisms in the Mesozoic–Cenozoic, non-marine sedimentary records of eastern China may have resulted from either marine transgressions or inland hypersaline lacustrine environments. Paleontological and organic geochemical results of this study have provided new evidence for marine transgressional events in the gigantic freshwater lake Songliao during the late Cretaceous.

Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay basin: Part 1: source rock characterization

Detailed mapping of the Tertiary strata of the Dongying Depression, Bohai Bay Basin reveals that oil shales and dark mudstones in the Es4 and Es3 members of the Eocene–Oligocene Shahejie Formation have excellent potential as petroleum source rocks. Regional distributions of total organic carbon content, kerogen type and thermal maturation indicate that abundant mature source rocks are present in the Niuzhuang Sag, whereas source rocks along the Niuzhuang South Slope are currently immature or marginally mature for the generation of conventional oils. This study examined the molecular characteristics for a number of biomarker compound classes, and related these to either source facies change or different burial and thermal maturation. An important observation made during this study is the significant variation in the relative distributions and absolute concentrations of a wide range of aliphatic and aromatic hydrocarbon markers in the rock extracts as a function of burial depth. This variation may potentially explain the uncertainty associated with the maturity assessment and genetic modeling for so-called “immature oils” that are usually defined on the basis of alkane biomarker isomerization ratios and the presence of biomarkers with “biological” configurations.

Geochemistry of petroleum systems in the eastern Pearl River Mouth Basin: evidence for mixed oils

Abstract Detailed analyses of crude oils and source rock extracts indicate the presence of two effective petroleum source rocks in the eastern Pearl River Mouth Basin of the South China Sea region. Oils derived from the lacustrine source rocks in the Eocene Wenchang Formation are characterized by abundant C 30 4-methyl steranes with relatively few C 19 tricyclic terpane and bicadinanes. In contrast, oils from the Eocene-Oligocene Enping Formation source typically contain significant amounts of C 19 tricyclic terpane and bicadinanes, with relatively few 4-methyl steranes and high pristane/phytane ratios. However, the chemical compositions of a large number of oils do not conform to those of the unknown source rocks, consistent with mixing of the two sources. Mixed oils occur mainly in the Huizhou Sag and along its south margin, and the highest proportion of the Enping Formation source in the reservoired mixed oils is estimated to be around 80%. Laboratory mixing experiments using selected end-member oils indicate that, even with 50–80% contribution from the Enping Formation source, the mixtures still display sterane biomarker ratio signatures diagnostic of the Wenchang Formation source. Therefore, the presence of abundant 4-methyl steranes is a necessary but not sufficient indicator for the Wenchang Formation source in the eastern Pearl River Mouth Basin. Although our results support the Wenchang Formation as a major source to the known oil discoveries in the study area, the contribution of the Enping Formation source to traps in the Huizhou Sag and surrounding area cannot be ignored. There is clear evidence for the recharge of Enping Formation derived oils into early-biodegraded oil accumulations to form light oil accumulations.

Organic geochemistry of oils and condensates in the Kekeya Field, Southwest Depression of the Tarim Basin (China)

This study shows that the oils and condensates in the Kekeya Field in the Tarim Basin, NW China, belong to a single family, most likely derived from marine shale source rocks deposited under oxic–suboxic conditions with mixed terrigenous and algal–bacterial organic inputs. The maturity data clearly indicate that the paraffinic condensates were not formed by thermal cracking of oil during late catagenesis (Ro>1.2%). Both the oils and condensates were generated within the normal oil window, whereas addition of gaseous hydrocarbons from a separate source resulted in migration fractionation and hence spuriously high heptane indices. Age specific biomarkers show that the oils and condensates were not generated from the Mesozoic–Cenozoic strata, but from the Carboniferous-Permian sections. 1D basin modeling results are consistent with this interpretation, suggesting that the Mesozoic–Eocene strata are currently immature with respect to hydrocarbon generation and expulsion. Deep-seated faults may have provided routes for upward fluid migration at the time of active deformation during several pulses of the Himalayan orogeny. The favored targets for further petroleum exploration in front of the Kunlun Mountains include the deep structures within the Carboniferous–Permian strata for indigenous petroleum accumulations and anticlines and stratigraphic traps within the Mesozoic–Cenozoic sections that are cut by deep-seated thrust faults where secondary petroleum accumulations most likely occurred.