Transient-pooled Natural Gas Systems: Moving Beyond the Continuous-type Gas Illusion
GeoX Consulting Inc., Salt Lake City, Utah
The conundrum of conventional vs. unconventional gas in low-permeability "tight" sandstone reservoirs is merely a function of scale in space and time. 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 the conventional gas pool. However, 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. The gas pools exist only because the rates of gas escape through the imperfect local topseal is balanced by the entry of gas from below. In relatively short geologic timeframes, on the order of a few Ma or less, transient-pooled gas systems depend on the continuous generation of gas from an intercalated or deeper source rock. These systems merely inhibit and delay the flow of gas as it migrates from the source rock to the surface, which on a basin-scale creates the illusion of a continuous cloud of gas or a continuous-type basin-centered gas resource. In all instances and regardless of scale, gas migrates by Darcy flow driven by buoyancy forces. The tight gas resource requires (1) continuing generation of natural gas from source units, and (2) sustained flow of gas along established carrier pathways to inhibit diagenetic occlusion and retain high gas saturations in potential reservoirs located along the gas migration routes.