Thermal simulation experiments with lowering temperature and extending heating time have been performed on 3 coal samples with low maturity (R

0

=0.29%)

normal maturity (R

0

=0.66%) and high maturity (R

0

=1.71%)

respectively

to study the change of maturity and gas generating yield of source rocks led by extending heating time while lowering the temperature. The vitrinite reflectance (R

0

) of source rocks still can increase if the heating time is extended with lowering temperature to a certain point after heating. The increment is in the range of 0.15%-0.40%. The rates of increase of R

0

value vary at different time interval at the same temperature. It is relatively large at the initial stage

but then gradually lowers with extending heating time. Gas generating yield of source rocks measured through extending heating time with lowering temperature is relatively low at the initial stage

but the cumulative gas generating yield increases gradually along with the extension of heating time. Just like R

0

value

the rate of gas generation is larger at the initial stage but then lowers with extending heating time. The experiment results show that hydrocarbon generation can go on after the temperature has been lowered to a certain point after heating if the heating time is extended

but the rate of gas generation would decrease gradually.