Fort Worth Basin

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. 

Fort Worth Basin oils and condensates were collected in Jack County and Wise County, Texas, USA with permission from Republic Energy (Fig. 1) The source of the oils and condensates is most likely the marine Mississippian Barnett Shale. API gravity for the oils ranges from 40 to 59° and sulfur contents are less than 0.5%. 

The Fort Worth Basin crude oils and condensates were diluted 1 75 oil to methylene chloride and then 1 /a1 of diluted sample was injected directly into a Varian 3400 GC connected to a... 

Fig. 1. Map showing location of oil and condensate samples collected in the Fort Worth Basin, Texas, USA.

The C4-benzene and C4-naphthalene compounds detected in the original WCBS oil, the oil pyrolysates and the Fort Worth Basin oils and condensates are listed in Table 1. The compounds listed in Table 1 are shown in the chromatograms in Fig. 2. The distributions of C4-benzene and C4-naphthalene compounds in the WCSB oil pyrolysates and the Fort Worth Basin oils... 

Table 1. C4-benzene and C4-naphthalene compounds detected by GC/MS in oil pyrolysates and Fort Worth Basin oils...

Table 3. C4-benzene and C4-naphthalene distributions in Fort Worth Basin oils and compound ratios useful in thermal maturity determination... 

Thermal maturity for the Fort Worth Basin oils in this study was assessed using the TAS ratio (Mackenzie et aL, 1981). In this study, the (C21 C22)A 2i h 22 h C26 F C27 C28)... 

All the C4-benzene and C4-naphthalene ratios increase linearly with increase in calculated TAS for Fort Worth Basin oils with the possible exception of MiPB. Correlation of each C4-benzene and C4-naphthalene ratio with TAS is good with values exceeding 0.68 for all parameters except MiPB (R = 0.16). One datum point which could easily reflect an analytical error that falls off the otherwise Unear trend, is the reason the MiPB correlation is low. 

The initial distribution of C4-benzene and C4-naphthalene compounds is different for the original WCSB oil and the Fort Worth Basin oils (Tables 2 and 3). The 1,2,3,5-tetramethylbenzene isomer is most abundant relative to the other two isomers in the Fort Worth Basin oils whereas the 1,2,3,4-tetramethylbenzene isomer is most abundant in WCSB oil. The l,3-dimethyl-5-ethylbenzene and the 1,2-dimethyl-4-ethylbenzene isomers are most abundant and next most abundant in the WCSB and Fort Worth Basin oils when measured as a fraction of the total dimethylethylbenzenes. The 1-methy 1-4-isopropylbenzene isomer is most abundant in the WCSB and Fort Worth Basin oils relative to the other two isomers. However, 1-methy 1-2-isopropylbenzene, the next most abundant isomer in both oils is more abundant relatively in the WCSB oil than in the Fort Worth Basin oils. The 1,3,6,7-TeMN isomer is most... 

Fig. 5. Changes in C4-benzene isomer ratios in Fort Worth Basin oils and condensates as a function of triaromatic steroid maturity parameter.

The C4-naphthalene and C4-benzene data presented in this paper were collected as part of a larger study addressing the influence of pressure, temperature and time on hydrocarbon generation (Hill et al, 1994, 1996, 2(X)3). The goal of the study was to evaluate whether maturity parameters defined during oil pyrolysis could be extended to natural systems. Thus, the Devonian sourced oil from the WSCB was used to define C4-naphthalene and C4-benzene maturity parameters in the oil pyrolysis experiments and Mississippian oils from the Fort Worth Basin were used to evaluate C4-naphthalene and C4-benzene maturity parameters in natural systems. It was not the goal of this study to determine whether results from pyrolysis of a Devonian oil will compare exactly with oils from a Mississippian source. Due to facies differences between Devonian and Mississippian marine shales, the initial distribution of C4-naphthalene and C4-benzene isomers is different as discussed earlier. For this reason, we would also not expect maturity ratios from pyrolysis of a Devonian oil to exactly match maturity ratios from Mississippian Barnett Shale oils. However, if the maturity trends observed from oil pyrolysis results are valid, the C4-naphthalene and C4-benzene ratios should correlate with a well established thermal maturity parameter such as TAS in Mississippian Barnett Shale oils. 

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. 

To assess applicability of the pyrolysis results to natural systems, C4-benzenes were quantified and thermal maturity ratios calculated for oils and condensates from the Fort Worth Basin (Fig. 5). The Fort Worth Basin hydrocarbons in this study were generated from a single source, the Mississippian Barnett Shale. The only molecular maturity indicator available for use for these oils and condensates was TAS. The steranes and hopanes had reached equilibrium and even TAS was no longer useful for the highest maturity condensates (Table 2). 

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. 

Acknowledgements— The authors thank Republic Energy for permission to sample oils from the Fort Worth Basin and puhUsh the results. We also thank Dave King and Mike Prihil of the USGS for assistance in analysis of the Fort Worth Basin Oils. This paper was significantly improved hy reviews from Mike Lewan and Ken Peters. 

The Barnett Shale formation is located in the Fort Worth Basin of north-central Texas. It is a Mississippian-age black shale occurring at a depth of 6500 to 8500 feet (Table 5.2) and is bounded by limestone formations above (Marble Falls Limestone) and below (Chappel Limestone) (Figure 5.2). With 15,306 wells drilled or pending, the Barnett Shale was the most prominent shale gas play in the United States, but the Flaynesville Shale gas play has surpassed Barnett in shale gas production (Railroad Commission of Texas, 2009 ... 

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