Biomarkers maturity indicators

Maturity. Conventional biomarker maturity indicators suggest that source rock maturity has... 

Fig. 10. Saturate (Ts/Ts-f Tm) and aromatic (MPI-1) biomarker maturity indicators for Magnolia oil and condensate samples. The shallowest saturated gas-condensate sample is omitted due to low yields. For ratio definitions see Table 2.

Calculating an equivalent %Rq value for pyrolysis experiments based on experimental conditions is a convenient way to compare the level of thermal stress achieved in experiments performed at various temperatures and times and eliminates uncertainty in biomarker maturity indicators that arise during pyrolysis. In this way, the level of thermal stress achieved for an experiment performed at 360°C and 12 days can be easily compared with an experiment performed at 400°C and 1 day, for instance. The positive correlation between various C4-naphthalene and C4-benzene ratios with increased thermal stress (calculated %/ o) in the oil pyrolysis experiments motivated the evaluation of these ratios as maturity parameters in oils from the Fort Worth Basin. The Fort Worth Basin oils were analyzed as part of this study because the Barnett Shale is the only petroleum system in the basin and also because samples were readily available from various stratigraphic horizons. The TAS ratio is an accepted thermal maturity indicator for low API gravity oils (Mackenzie et al, 1981) and was used in this study to evaluate C4-naphthalene and C4-benzene as potential maturity indicators for high API gravity oils. Thus, correlation of C4-naphthalene or C4-benzene ratios with the TAS maturity ratio is viewed as a confirmation of the effectiveness of these parameters to estimate the thermal maturity of a light crude oil. 

Fig. 7. Cross plots of calculated vitrinite reflectance equivalent biomarker maturity (Ro Ts) and light hydrocarbon maturity (Schaefer Ro) effectively indicates that almost all oils are mixtures in the true sense with a range of maturities of charge being present together in the trap. Only the flagged oils (red dots) may represent single maturity point charges, but even here that is doubtful. A mass fraction maturity concept better describes the maturity of an oil.

Fig. 8. The distribution of fraction-specific maturity for the oils and condensates in the data set. Mean maturities for the data set based on representative biomarker, aromatic hydrocarbon and light hydrocarbon parameters are indicated, based on parameters proposed by Radke (1988) and Schaefer Littke (1988). Biomarker maturities are based on proprietary source rock correlations. The cyclic biomarkers are generated earlier than the aromatic hydrocarbons with the quantitatively most abundant light hydrocarbons being generated, on average at higher maturity than the more structurally exotic molecules.

Oil biomarker compositions were measured in samples from three wells in the central part of the Ross Field (Fig. 11). Wells 13/28-2 and 13/28a-5 had similar compositions, but the oil in 13/29a-3 has a different composition, related to a higher thermal maturity. If diffusion was the only mechanism leading to fluid mixing, then in 40 Ma the oil would be able to mix over a distance of about 520 m, illustrated by dashed circles around the wells in Figure 11. It is thus possible that the biomarker differences were inherited from the reservoir filling history, and subsequently have not had time to mix completely. This would indicate that the oil chemistry could not be interpreted to indicate compartmentalization. However, it is possible that the biomarker maturity parameters reflect oils of different density (unfortunately, density was not measured in oils from the crucial location). Indeed, there are density differences between the oils in wells 13/29a-l and 13.29a-3. The density-mixing model (equation (10)) would indicate that density overturn would mix oil densities over the whole central part of the field... 

Determining the thermal maturity of light oils and condensates can be difficult. Biomarker concentrations in crude oils are low and biomarker maturity parameters have limited applicability at high levels of thermal maturity (Peters and Moldowan, 1993). Light hydrocarbons (C6-C7) are volatile, susceptible to biodegradation and maturity parameters derived from these compounds may be unreliable. In this chapter, we report on the correlation of C4-benzene and C4-naphthalene compounds with thermal maturity in oil cracking pyrolysis products of a Western Canada Sedimentary Basin (WCSB) oil. The use of C4-benzene and C4-naphthalene compound ratios as thermal maturity indicators in natural systems was evaluated using crude oils from the Fort Worth Basin, Texas, USA. 

C4-benzene ratios provide a method to determine the relative maturity of light oils and condensates. The range of usefiilness for C4-benzene parameters appears to extend beyond the thermal maturity limits for all the biomarker parameters. Provided the C4-benzene parameters can be calibrated to another maturity parameter that extends beyond TAS, such as vitrinite reflectance, the C4-benzenes hold great potential as a thermal maturity indicator. 

Subsequent sections of this chapter (Sections 2.7, 2.8, and 2.10) discuss the available information on biomarkers, interactions, and methods for reducing toxic effects. Most of the available information is from adults and mature animals no child-specific information was identified, with the possible exception of biomarker data. However, there are some data to suggest that interactions with PCBs and CDFs may influence the developmental toxicity of 2,3,7,8-TCDD. Data from children living in Seveso suggest that serum 2,3,7,8-TCDD levels are reflective of exposure levels and are a sensitive indicator of past exposure. Likewise, it is likely that the available information in adults on interactions and methods for reducing toxic effects will also be applicable to children. 

Figure 3 shows the effect of maturity (assessed here as Ro% equivalent by the light hydrocarbon parameters defined by Schaeffer Littke 1988) on the biomarker concentrations in these fluids, indicated here by the hopanes. The figure shows how, with increased fluid maturity, the absolute concentrations of hopanes drop over 3 orders of magnitude through the oil window. Thus, hopane distributions from low maturity fluids in a mixture will invariably dominate in mixed oils sourced even from a single source rock. 

These results indicated that the Draupne formation in Kitchen 1 west and northwest of Snorre has not reached the level of transformation required by the asphaltene kinetic data from the Snorre field. Kitchen 1 was modelled to only have reached a transformation ratio of less than 5% (Fig. 6) at present. In addition the limited source rock volume in this area cannot account for the petroleum volumes in the Snorre field. Thus, we would suggest on the basis of our maturity modelling, that the proposed kitchen area west and northwest of the Snorre (Kitchen 1) is immature. Shows in the 33/6-2 well, classified as in situ generated, not representative of significant petroleum migration, support this hypothesis. Additionally the fact that the main intra-field maturity trend known in Snorre (Horstad et al. 1995) is the increase in biomarker and aromatic maturity parameters from south to north, suggests clearly that Kitchen 1 did not contribute to Snorre as any influx of lower maturity petroleum from... 

The 30/7a-P12 (Jocelyn) condensate is the most mature of all the Pre-Cretaceous petroleums analysed (Fig. 7). Because of its high maturity, many of the biomarkers have been destroyed and so some parameters cannot be quoted. GC-MS fingerprints of the triterpanes (Fig. 10) indicate that only the most thermally resistant molecules Ts, C29TS and C30 diahopane (tt) remain in any abundance. The 30/7a-l Iz and 30/7a-P5z samples from the northernmost part of the field are also highly mature, being more mature than those from the crestal part of the Judy structure, i.e. 30/7a-7, 30/7a-P3 and 30/7a-P9. The 30/7a-P3 fluid appears less mature than that from 30/7a-P9, despite its location within the same crestal fault block, suggesting the possibility of compartmentalization. The decrease in oil maturity from 30/7a-P12 to 30/7a-P5z to 30/ 7a-P9 to 30/7a-P3 is shown in the triterpane... 

One of the main uncertainties related to the application of petroleum inclusions as indicators of earlier reservoir crude oil concerns the possibility that the growing authigenic phase has trapped not only oil but also immature kerogen which may mature in-situ in the progressively buried structure. This material could potentially contribute to the oil inclusion biomarker composition, making it less mature. 

The total hydrocarbon yield exhibits a maximum at 330-360°C and decreases above those temperatures due to severe cracking (gas formation). The Pr/Ph ratio increases with temperature to a maximum at 330°C and above that temperature the isoprenoid hydrocarbons are destroyed. The 17a-hopane biomarkers are still relatively immature in the unheated samples. They show no further maturation and are destroyed above 360°C. Alkanoic acids have a maximum yield at 330-360°C and are present over the complete temperature range. This indicates continual formation from kerogen and cracking of the products. PAHs increase in concentration with increasing temperature and the alkyl PAHs concomitantly decrease. At 500°C essentially only the parent PAHs remain in the extractable bitumen. 

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