Transient-Pooled Natural Gas Systems in the Rocky Mountain Basins

Schamel, Steven¹ (1) GeoX Consulting Inc, Salt Lake City, UT

The conundrum of conventional versus unconventional natural gas in “tight” sandstone reservoirs is merely a function of scale in time and space. Where the reservoir unit is relatively continuous spatially and sealed by a high-quality, and equally continuous, cap rock, we recognize a clear gas-water contact and other features of a conventional gas pool. However, in the Rocky Mountain and other basins where the stratigraphic succession is heterogeneous and the reservoir sandstones are discontinuous, as in fluviodeltaic successions, the gas pools are small, disconnected and highly transient. Such gas pools exist only because the rate of gas escape through the imperfect local topseal is balanced by the entry of gas from below. In relatively short timeframes, on the order of a few millions of years or less, transient-pooled natural gas systems depend on the continuous generation and/or release of gas from an intercalated or deeper source. These systems merely inhibit and delay the flow of gas as it migrates from its source to the surface, which on a basin-scale creates the impression of a continuous cloud of gas, a continuous-type basin-centered gas resource. The Rocky Mountain basins have lost as much as 10,000 ft of sediment cover due to late Neogene-Recent erosion. Whereas the unloading has slowed the rates of deep gas generation, the reduction in ambient pressure has released very large volumes of free natural gas through dissolution from connate waters and desorption from coals and dispersed kerogen.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana