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GOR-Isotopes - A New Tool for the Quantitative Assessment of Gas Generation and Gas Typing in Petroleum Systems |
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Determination of Origin of Gas
• In this case, we have three gas wells with the following CH4 and C2H6 carbon isotopic compositions: |
Well # |
C1 |
C2 |
1 |
-50 |
-34.2 |
2 |
-46 |
-33.5 |
3 |
-40 |
-30.2 |
4 |
-42 |
-33.0 |
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• By plotting d13C1 versus d13C2 we can determine that the gases from wells 1, 2, and 3 mainly belong to the type III (coaly) gases (Figure 3A)
• The gas in well 4 appears to be due to mixing of coal and shale gas
• Model results suggest that this field is dominated by coal gas with a small contribution of shale gas (Figure 3B) |
Figure 3A: Determination of the Source of Gas |
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Figure 3B: Model Results Show Gases of Mixed Origin |
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Barnett Shale, Texas |
Drumheller Coal, Alberta |
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Powder River Basin, Wyoming |
Kimmeridge Clay |
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Green River Formation |
Monterey Shale |
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Determination of Gas Migration
• The difference between the carbon isotopic values of C3 and C2 gases in a well producing from the Trail Ridge formation (Cameo Coal) is1.9‰
• Is this gas indigenous or has it migrated into place from another source?
• GOR Isotope model results show that the gas in this reservoir was generated at a much higher maturity (Ro = 2.15%) (Figure 3C)
• This suggests that the gas in this field migrated from a much deeper source (Figure 3D)
• It can also be infered that the volume of gas in place is likely less than the predicted amount for this source (higher risk) |
Figure 3C: Predicted Maturity (Ro) versus Gas Isotopic Composition |
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Figure 3D: Gas at Trail Ridge has Most Likely Migrated from a Deeper, More Mature Source (Ro = 2.15) |
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Determination of Gas to Oil Ratio
• Gas to oil ratio (GOR) can be predicted from several different parameters (e.g., d13C of C1, C2, C3, C1/C2, C2/C3, etc.)
• Field data from the Duvernay formation in Western Canada primarily fall within the predicted range of GOR values (Figure 3E)
• Data that do not follow the predicted trend indicate that these areas merit further investigation
• Other geologic and geochemical data suggest that elevated GOR values are likely due to oil loss during migration and depressed GOR values may be due to mixing with dry gas from an additional source |
Figure 3E: Model Prediction of Gas to Oil Ratio |
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Determination of Age of Gas Formation
• A given gas field has ethane with a d13C value that ranges from -30 to -24 ‰
• It is extremely important to determine the timing (age) of gas formation
• GOR Isotope model predictions indicate an age of 27 to 40 million years (Figure 3F)
• It is known from other geologic data that the gas reservoir (trap) in this field did not form until 40 million years
• Gases encounted in this reservoir have most likely migrated in from from a deeper source in the basin
• The volume of this gas may not be as great as the volume of indigenous gas |
Figure 3F: GOR Isotope Model Prediction of Gas Generation Age vs Istopic Composition
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Cummulative vs Instantaneous Gas
• The determination of whether gas in a reservoir was generated cummualtively or instantaneously can provide critical information regarding the timing and temperature of gas formation, migration, and seal and reservoir formation
• In the example shown (Figure 3G), the predicted isotopic composition for the cummulative gas up to 190° C, and from 160° C to 190° C, as well as the instananeous gas at 190° C are plotted
• Comparison of the predicted and observed isotopic values suggests that most of the gas trapped in this field is from the cummulative gas generation from 160° C to 190° C
• One well has isotopic values that suggest the presence of a gas that was generated instantaneously at 190° C; however, the low gas yield from this well may produce data that is not representative of the actual reservoir gas |
Figure 3G: Determination of Instantaneous vs Cumulative Gas Based on Isotopic Composition
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Integration with Geologic Data
Results from the GOR Isotope model can easily be integrated with geologic field data and basin model results to help constrain model predictions and formulate exploration stragetgies. The output from GOR Isotopes can easily be exported to any maping program (e.g., ArcGIS, Surfer, etc.) as is shown in these examples from the Ft. Worth Basin (Figures 3H, 3I, 3J). |
Figure 3H: Depth at Which Gas Formed
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Figure 3I: Temperature of Gas Formation (° C) |
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Figure 3J: Age of Gas Formation |
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Conclusions
• Old methods of interpretation of gas geochemistry data based on empirical correlations of field observations have limited forward predictive capability
• A new approach is proposed based on the integration of laboratory experimental data with kinetics theory
• GOR Isotopes is a user friendly software package that easily allows for modelling the isotopic composition of natural gas
• Depending on the level of detail of the input data, GOR Isotopes can predict:
Gas Yield, Condensate Yield, Gas Source, Gas Composition, Thermal Maturity, Gas Migration History, Gas to Oil Ratio (GOR), Reservoir Filling History, Age of Gas Formation |
