Catalytic Gas in Deltaic Basins

Mango, Frank¹, Daniel Jarvie² (1) Petroleum Habitats, Houston, TX (2) Humble Geochemical Services, Humble, TX

We have analyzed the catalytic activity in twenty one sandstone reservoir rocks from the Gulf Coast geosyncline in southern Louisiana, including Bastian Bay and Midland fields, the subjects of this report. All were active, increasing rates of gas generation by factors ranging from 500 to 140,000, placing many of these reservoirs in the gas window at current depths. We recognize two distinct environments in this basin, one promoting catalytic activity and the other suppressing it: interbedded shales increase activities by factors of about eight, while anhydrite poisons activity altogether.

Catalytic gas is estimated at over 50% in these fields based on reservoir rock activities and depths of burial. Model estimates account for 79% of the gas in Bastian Bay field, 40% of the gas in Midland field, and 87% of the gas in the basin.

The model accounts for the empirical observations that characterize this basin (Paine et al., AAPG Memoir 9, 1968): high gas-probability associated with interbedded shales, high oil-probability associated with salt domes, and a transition from oil to gas habitats at 10,000 ft.

The results show surprisingly high levels of catalytic activity in ordinary sandstone reservoir rocks. Although its origin and nature may be unclear, it is nevertheless quite real, and could be a major force shaping the distribution of oil and gas in these basins. Understanding the dynamics of catalytic gas generation and predicting activity in un-drilled reservoirs could bring unprecedented predictive powers to gas-generation models of deltaic basins.