Thermal maturation indicators

Wood D.A. (1988) Relationship between thermal maturity indices calculated using Arrhenius equation and Lopatin... 

Heroux I, Chagnon A, Bertran R (1979) Complication and correlation of major thermal maturation indicators. AAPG Bull 63(i2) 2i28-2i44... 

C4-benzene and C4-naphthalene thermal maturity indicators for pyrolysates, oils and condensates ... 

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. 

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. 

Prior to this study, the C4-benzenes had not been evaluated as potential thermal maturity indicators. The C4-benzenes in Table 4 were identified in the WCSB oU and aU the Fort Worth Basin oils analyzed. The TeMB and MiPB ratios appear to be robust maturity indicators, increasing linearly with increase in TAS for the Fort Worth Basin hydrocarbons. This result corroborates observations from the oil cracking pyrolysis experiments. In general, the DEMB-2 ratio also increases with increase in thermal maturity, although one apparent outlier makes preliminary interpretation of the DMEB-2 data questionable. 

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. 

Experience has shown that crudes from adjacent wells can be different in composition. The siuface, physical, and chemical properties of associated emulsions can be expected to be as diverse and complex as the soiuce crudes and water. The variances in the elemental C, H, and N composition of components such as asphaltenes of crudes from various geologic origins as compiled (10, 12) show no apparent pattern emerging from the data as was the case with coal (5, 7). Sharma et al. (13) have shown the use of bitumen asphaltenes as thermal maturation indicators. 

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