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New Insights on Timing of Oil and Gas Generation in the Jurassic Petroleum System of the Central Gulf Coast Interior Zone Based on Hydrous Pyrolysis Kinetic Parameters

By

LEWAN, MICHAEL D.

U. S. Geological Survey, Denver, CO

 

Hydrous-pyrolysis kinetic parameters have been shown to give more reasonable predictions of the timing of oil generation from Type-IIS kerogen and gas generation from oil cracking than previously employed anhydrous-pyrolysis kinetic parameters. Applying hydrous-pyrolysis kinetic parameters to Cretaceous and Jurassic source-rock intervals in 12 one-dimensional burial-history curves in the central Gulf Coast Interior Zone (i.e., East Texas , Louisiana, and Mississippi interior salt basins) predicts that only Late Jurassic source-rock intervals with Type-IIS kerogen completed oil generation between 80 and 120 Ma. Conversely, Late Cretaceous source-rock intervals with Type-II kerogen remain immature within the same area. This difference in Cretaceous and Jurassic source-rock maturities does not require postulating extreme variations in thermal gradient (i.e., 60 to 20 oC/km) over the last 160 Ma, as required with the use of anhydrous-pyrolysis kinetic parameters. Under this simpler thermal regime, hydrous-pyrolysis kinetic parameters predict cracking of Jurassic oil to gas started 30 to 90 Ma and ended as early as 56 Ma with some areas is still generating gas today. The 30 to 50 m.y. between the end of oil generation and the start of oil cracking to gas is coincidence with major regional uplifts, which can explain the distribution of oil and gas accumulations associated with the Sabine uplift and the scarcity of oil associated with the Monroe uplift. The entire central Interior Zone can be considered the active source pod for oil generation, while the individual successor salt basins are the active source pods for oil cracking to gas.