Tight sandstone reservoir of the Xujiahe Formation in Guanyinchang area mainly consists of delta plain distribu-tary channels sandbodies.Based on analyses of reservoir diagenesis and tightening

detailed study shows that the hydrodynamic conditions during sedimentation determine the storage capacity of reservoirs.Strong compaction is the major factor causing the decrease of inter-particle primary porosity

and quartz overgrowth in diagenetic stage is the key factor causing the tightening of reservoir.The temperatures of fluid inclusions in quartz overgrowth is between 82.5-125.1 ℃

accor-ding to which the tightening depth is estimated to be between 1 725-2 864 m.This depth interval corresponds to the period of Yanshan movement.According to analysis of the tightening process and hydrocarbon accumulation stages in the Xujiahe Formation sandstone reservoir

the following understandings were obtained.The sandstone reservoir was not tightened at the initial phase of gas accumulation

so gas could migrate long distance laterally.Before the peak gas accumulation

the sandstone reservoir had become tight

thus the gas could not migrate laterally for a long distance

instead

they migrated along the unconformity of channel and microfractures opened by high pressure.of the composite structural and lithologic gas reservoir got finalized during Himalayan epoch

and the gas reservoir was controlled by both lithology and structures

so their heterogeneity was strong.