Open- vs closed-system generation modeling
Hydrocarbon generation results from computerized 1-D basin modeling software are typically presented as either open- or closed-system yields. Open-system models are based on the assumption that hydrocarbons have been expelled from the source rock, have been transported away, and are no longer subject to the thermal conditions of the 1-D model. Closed-system models assume the hydrocarbons have not been expelled from the source rock and that all generated hydrocarbon phases are subject to further cracking to lighter phases.
Implications for yield predictions
Figure 1 shows examples of yield curves for the same well, based on both open- and closed-system modeling conditions. The open-system model is run with an expulsion efficiency of 70%, meaning 70% of the generated hydrocarbons were expelled before subsequent maturation and were not subjected to secondary cracking to lighter phases. The closed-system model does not incorporate an expulsion component; therefore, the hydrocarbons are subject to cracking to lighter phases. The closed-system model (right) predicts significantly greater quantities of gas at depth than the open-system model (left).
Note: In general, open-system models overestimate the quantities of liquids present in a system. Closed-system models overestimate the amount of gas present in the system and are pessimistic in terms of predicting quantities of liquids. Closed-system models typically are not run any more. However, when evaluating older models, consider this effect.
See also
External links
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