Gas Plumes and Their Importance in Exploration of the Cretaceous Rocks of the Rocky Mountain Region

Forster, John R.
The Wetterhorn Company, Centennial, CO

A distinct relationship exists between depth and source rocks, verses gas and water recovered from drill stem and production tests in the Cretaceous rocks of the Rocky Mountain Region. Water decreases with increasing depth, while gas recoveries increase. This relationship varies by stratigraphic interval from basin to basin as a function of source rock, reservoir quality, and reservoir seals. This is a characteristic of Cretaceous rocks while pre-Cretaceous intervals do not uniformly exhibit this relationship. These phenomena of gas rich, water depleted deep basins are the foundation of the basin-centered gas concepts. 

However, basin-centered gas regions are not water free regions of gas accumulation. They are better viewed as hydrocarbon plumes migrating out of the source area into the adjacent rock. The plume can be defined as the migration of one phase (gas) away from a source area, into and displacing a second phase (water). This plume and the relative saturation of gas and water are a function of pressure, temperature, and gas source as well as the porosity and permeability of the surrounding rocks. This low energy source and migration mechanism leaves significant quantities of water, even in the “gas-saturated” areas. Production disrupts the saturation stability resulting in the production of both phases; water and gas. The petroleum geology of reservoirs, seals, and source rocks as well as the engineering principles of the reservoir are all applicable to economic production inside a hydrocarbon plume.