Aromatic Maturity Research Articles

Variability of petroleum inclusions in vein, fossil and vug cements—a geochemical study in the Barrandian Basin (Lower Palaeozoic, Czech Republic)

Although no commercial oil or gas occurrences have been found in the Barrandian Basin, residual oils and petroleum inclusions give evidence about the petroleum history of this Lower Palaeozoic Basin. Petroleum inclusions are hosted in six generations of calcite and quartz cements that can be attributed to different stages of a basinal cycle and associated diagenetic events. They were analysed using an on-line crushing and an off-line crush-and-leach approach. Five different groups (PI-A to PI-E) and intermediate mixtures (PI-M) could be differentiated based on the relative distribution of n-alkanes. All oils had a calculated aromatic maturity Rc ranging between 0.9 and 1.6%. PI-A shows a molecular weight maximum in the range of n-C8–11 with a constant attenuation towards higher carbon numbered n-alkanes. This distribution is interpreted as the pristine precursor oil for other petroleum inclusion groups. PI-B has a maximum weight abundance at n-C15–20. The genetic relationship between PI-B and PI-A can tentatively be explained by mixing effects or by gas stripping. PI-C is bimodal and characterised by a molecular weight maximum at n-C32–34, and also a molecular weight maximum at n-C15–20 similar to that of group PI-B. PI-D shows a normally distributed molecular weight maximum in the range n-C25–28 and is interpreted as a wax precipitate from ascending gas saturated in n-alkanes. PI-E contains predominantly gaseous compounds with only a few higher hydrocarbons. Thompson's aromaticity values are elevated for condensates of group PI-E, and also for group PI-C that is dominated by long-chained n-alkanes. This gives evidence that fault-bound petroleum migration in the Barrandian Basin was associated with evaporative fractionation for group PI-C and PI-E. Samples of group PI-E yielded gas-range compounds only, and all come from a last generation of vein-filling whitish calcite that was formed in a late uplift phase of the basin. For other petroleum inclusion groups, only a vague preferential occurrence in specific mineralisation phases or stratigraphic intervals of the host rock has been found. This reflects the predominance of fracture-bound migration in the Barrandian Basin. Aromatic maturity values also showed no correlation between either crystal type or PI-group. In six of the investigated petroleum inclusion samples alk-1-enes are present. These terminal olefins are interpreted to be the result of the natural pyrolysis of petroleum due to the intrusion of volcanic dykes or hydrothermal processes. An artificial generation of olefins during sample work-up and analysis is unlikely. The preservation of alk-1-enes from Palaeozoic times was possible due to the protecting environment of petroleum inclusions.

Source rock palaeoenvironments and controls on the distribution of dibenzothiophenes in lacustrine crude oils, Bohai Bay Basin, eastern China

The applicability of sulphur aromatic parameters, based on ratios of dibenzothiophene (DBT) and its alkylated homologues, as indicators of oil maturity has been investigated in 27 oils from the Damintun and Zhuanhua Depressions, Bohai Bay Basin. The oils were characterised by bulk composition and environmentally-sensitive aliphatic biomarkers and were derived from a wide range of lacustrine source depositional environments ranging from highly reducing hypersaline (sulphur-rich oils) to terrestrial input-dominated freshwater (very high wax oils) with maturities ranging from immature to probable mid-mature. Dibenzothiophene-based maturity parameters are not well-correlated with sterane isomerization ratios, and immature oils, in particular, could not be discriminated by these aromatic maturity indicators. In contrast, DBT-based parameters are strongly-correlated with pristane to phytane and Ts/Tm ratios. High methyldibenzothiophene ratios (MDR) are associated with fresh water source deposition whilst low MDRs are found in oils of hypersaline source affinity. Unusually high relative concentrations of 2+3-MDBT were observed in some of the immature oils inferred to be sourced by lacustrine organic laminites. The data provide further evidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils and that source depositional environments can exert a dominant influence when their range of variation is large. Absence of DBT in some biodegraded oils is tentatively attributed to its aqueous solubility during associated water washing.

95/01248 New onshore production in Yorkshire

Concentration ratios of aromatic compounds, such as naphthalenes, phenanthrenes, and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils. Generally, these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl-aromatic homologues towards thermally more stable isomers. A combination of both concepts in the Methylphenanthrene Index (MPI) resulted in an excellent maturity parameter, as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance (Rm) between 0.65 and 1.4%, i.e., in the zone of oil formation. In cases that Rm measurements were ambiguous, this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance (Rc) values from the MPI. On the other hand, for potential petroleum source rocks reaching maturity at relatively low temperatures, the influence of the organic matter type on the aromatic distributions cannot be neglected. Consequently, organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils. Since the impact of the organic facies is strongly reduced with increasing maturity, aromatic maturity parameters are particularly useful in the maturity evaluation of post-mature crude oils and condensates. In contrast to biological markers, substantially higher concentrations of the key alkyl-aromatics persist at elevated maturation levels. Thus, aromatic maturity parameters should be considered as versatile tools in organic maturation studies, and can complement established maturity indicators based on steranes and terpanes.

Alkyl-substituted cis-bicyclo[3.3.0]octanes, potentially informative crude oil constituents

A number of 2- and 3-alkylbicyclo[3.3.0]octanes have been detected in seven oil samples which have been generated in different depositional environments and which exhibit varying maturity as revealed by biomarker and aromatic maturity indicator analyses. The presence of these hydrocarbons was established by performing SIM GC-MS and GC coelution on the basis of the mass spectra and the GC properties of synthetic standards, alkyl = methyl, ethyl, propyl, butyl. The methylbicyclo[3.3.0]octanes were much more abundant than their homologs. As a rule the amount of a 2-alkylbicyclo[3.3.0]octane was larger than the amount of the 3-substituted analog.

Application of aromatic compounds as maturity indicators in source rocks and crude oils

Concentration ratios of aromatic compounds, such as naphthalenes, phenanthrenes, and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils. Generally, these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl-aromatic homologues towards thermally more stable isomers. A combination of both concepts in the Methylphenanthrene Index (MPI) resulted in an excellent maturity parameter, as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance ( R m) between 0.65 and 1.4%, i.e., in the zone of oil formation. In cases that R m measurements were ambiguous, this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance ( R c) values from the MPI. On the other hand, for potential petroleum source rocks reaching maturity at relatively low temperatures, the influence of the organic matter type on the aromatic distributions cannot be neglected. Consequently, organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils. Since the impact of the organic facies is strongly reduced with increasing maturity, aromatic maturity parameters are particularly useful in the maturity evaluation of post-mature crude oils and condensates. In contrast to biological markers, substantially higher concentrations of the key alkyl-aromatics persist at elevated maturation levels. Thus, aromatic maturity parameters should be considered as versatile tools in organic maturation studies, and can complement established maturity indicators based on steranes and terpanes.

New aromatic biomarkers and possible maturity indicators found in New Albany Shale extracts

New aromatic biomarkers and possible maturity indicators found in New Albany Shale extracts

Heating rate effects on aromatic maturity indicators

Aromatic maturity indicators based upon dimethylnaphthalenes, trimethylnaphthalenes and methylphenanthrenes have been determined for two sedimentary sequences from northwest Western Australia. The rate of change of these indicators with depth has been modelled using a kinetic expression which accounts for the effects of continuously varying time and temperature. Activation energies and entropies of activation have been measured. Maturity indicators with similar and with very different kinetic parameters have been used to construct plots to illustrate their behaviours under a variety of thermal regimes. Maturity indicators with similar activation energies behave almost identically over a wide range of heating rates while those with substantial differences display more diverse behaviour. The former may be used to relate kerogen-based maturity indicators with those determined from crude oils, whereas the latter can provide information about the maximum palaeotemperatures and heating rates of sedimentary hydrocarbons. It was also observed that the kinetic parameters for particular aromatic maturity indicators differed between the two sedimentary sequences, suggesting that source and mineral matrix effects can influence their behaviour.