Timing of Gas Formation and Migration in Petroleum Systems: New Insights from Gas Isotope Modeling

Martin Schoell¹ and Yongchun Tang^2
1 Gas Consult International Inc, Berkeley, CA
² California Institute of Technology, Covina, CA

The timing of gas migration has been one of the enigmas in the evaluation of petroleum systems. Empiric models were so far mostly based on geologic data and were unable to put geologic time spans on the process of gas formation and migration. Through temperature controlled pyrolysis experiments we can relate isotope concentrations in natural gas components methane, ethane and propane to the geologic temperature at which they formed in the source rock. With the time-temperature relationship from basin modeling we can, therefore relate gas isotopes to geologic time, which is literally a new method for age dating of gas formation and migration. With this capability we can now re-interpret isotope variations in one gas or oil field and translate these into migration events. In one case history from the North Sea, we find that gas formation and migration from one gas kitchen to the same reservoir is a process that spans up to 25 Million years. The first charge was between 75 and 65 my, contemporaneously with oil formation. The second event was between 55 and 45 my, following the oil migration. The pools with the early gas in this field are geographically more remote from the kitchen and the younger pools are close to the kitchen, illustrating the filling history. Gases in a neighboring field from the same kitchen formed and migrated 40 and 15 my ago, respectively, illustrating the fact, that gas kitchens are active over 60 my releasing in episodic events gases into reservoirs.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005