AAPG Hedberg Conference
Vail, Colorado
April 24-29, 2005
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.
Copyright ©2005. The American Association of Petroleum Geologists. All Rights Reserved.