Tight sandstone gas serves as an important
unconventional hydrocarbon resource, and outstanding
results have been obtained through its discovery both in
China and abroad given its great resource potential.
However, heated debates and gaps still remain regarding
classification standards of tight sandstone gas, and critical
controlling factors, accumulation mechanisms, and development
modes of tight sandstone reservoirs are not determined.
Tight sandstone gas reservoirs in China are
generally characterized by tight strata, widespread distribution
areas, coal strata supplying gas, complex gas–water
relations, and abnormally low gas reservoir pressure. Water
and gas reversal patterns have been detected via glass tube
and quartz sand modeling, and the presence of critical
geological conditions without buoyancy-driven mechanisms
can thus be assumed. According to the timing of gas
charging and reservoir tightening phases, the following
three tight sandstone gas reservoir types have been
identified: (a) ‘‘accumulation–densification’’ (AD), or the
conventional tight type, (b) ‘‘densification–accumulation’’
(DA), or the deep tight type, and (c) the composite tight
type. For the AD type, gas charging occurs prior to reservoir
densification, accumulating in higher positions under
buoyancy-controlled mechanisms with critical controlling
factors such as source kitchens (S), regional overlaying cap
rocks (C), gas reservoirs, (D) and low fluid potential areas
(P). For the DA type, reservoir densification prior to the gas
charging period (GCP) leads to accumulation in depressions
and slopes largely due to hydrocarbon expansive
forces without buoyancy, and critical controlling factors
are effective source rocks (S), widely distributed reservoirs
(D), stable tectonic settings (W) and universal densification
of reservoirs (L). The composite type includes features of
the AD type and DA type, and before and after reservoir
densification period (RDP), gas charging and accumulation
is controlled by early buoyancy and later molecular
expansive force respectively. It is widely distributed in
anticlinal zones, deep sag areas and slopes, and is controlled
by source kitchens (S), reservoirs (D), cap rocks
(C), stable tectonic settings (W), low fluid potential areas
(P), and universal reservoir densification (L). Tight gas
resources with great resource potential are widely distributed
worldwide, and tight gas in China that presents
advantageous reservoir-forming conditions is primarily
found in the Ordos, Sichuan, Tarim, Junggar, and Turpan-
Hami basins of central-western China. Tight gas has served
as the primary impetus for global unconventional natural
gas exploration and production under existing technical
conditions.