Tight sand gas is an important unconventional
gas resource occurring widely in different petroleum
basins. In coal-bearing formations of the Upper Triassic in
the Sichuan Basin and the Carboniferous and Permian in
the Ordos Basin, coal measure strata and tight sandstone
constitute widely distributed source–reservoir assemblages
and form the basic conditions for the formation of large
tight sand gas fields. Similar to most tight gas basins in
North America, the Sichuan, and Ordos Basins, all experienced
overall moderate uplift and denudation in Meso-
Cenozoic after earlier deep burial. Coal seam adsorption
principles and actual coal sample simulation experiment
results show that in the course of strata uplift, pressure
drops and desorption occurs in coal measure strata, resulting
in the discharge of substantial free gas. This accounts
for 28 %–42 % of total gas expulsion from source
rocks. At the same time, the free gases formerly stored in
the pores of coal measure source rocks were also discharged
at a large scale due to volumetric expansion resulting
from strata uplift and pressure drop. Based on
experimental data, the gas totally discharged in the uplift
period of Upper Paleozoic in the Ordos Basin, and Upper
Triassic Xujiahe Formation in the Sichuan Basin is calculated
as (3–6) 9 108 m3/km2. Geological evidence for gas
accumulation in the uplift period is found in the gas
reservoir analysis of the above two basins. Firstly, natural
gas discharged in the uplift period has a lighter carbon
isotope ratio and lower maturity than that formed in the
burial period, belonging to that generated at the early stage
of source rock maturity, and is absorbed and stored in coal
measure strata. Secondly, physical simulation experiment
results at high-temperature and high-salinity inclusions,
and almost actual geologic conditions confirm that substantial
gas charging and accumulation occurred in the
uplift period of the coal measure strata of the two basins.
Diffusive flow is the main mode for gas accumulation in
the uplift period, which probably reached 56 9 1012 m3 in
the uplift period of the Xujiahe Formation of the Sichuan
Basin, compensating for the diffusive loss of gas in the gas
reservoirs, and has an important contribution to the formation
of large gas fields. The above insight has promoted
the gas resource extent and potential of the coal measure
tight sand uplift area; therefore, we need to reassess the
areas formerly believed unfavorable where the uplift scale
is large, so as to get better resource potential and exploration
prospects.