Physical properties of tight sands in the 2

nd

member of Xujiahe Formation in northwestern Sichuan Basin are controlled by structural fractures(SF)formed at different stages.The study on characteristics

timing and controlling factors of the structural fractures can provide useful information for reservoir evaluation and prediction.Identification of cutting relations of the SF on the basis of fracture observation on outcrops

cores and thin sections reveals that the SF can be grouped into NW-SE-trending

NE-trending

NNW-trending

near EW-trending and SEE-trending

of them NW-SE-trending is predominant.At least four stages of SF were identified based on the analysis of the fluid inclusion homogenization temperature of fillings in fractures with different cutting relation and ESR

as well as palaeo-stress data.The first stage is the Late Indosinian

and the SF formed at this stage feature in calcite filling and fluid inclusion homogenization temperature(FIHT)of 84-120℃.The second stage is the Late Yanshanian

and the SF formed at this stage are characterized by quartz filling and FIHT of 72-137℃.The third stage is the Early-middle Himalayan Period and the SF formed at this stage feature in quartz filling

cutting relations with the SF of previous two stags

and FIHT of 153-179℃.The fourth stage is the Late Himalayan Period

and the SF formed in this stage are characterized by coarser calcite fillings with some dolomites and FIHT of 165-190℃.Because of the active tectonic movement during the Indosinian and Himalayan Period

the SF are well developed.The formation and distribution of SF are jointly controlled by tectonic stress and the mechanical property of rocks.Lithic sandstone with high porosity

fine particle and thin layer are favorable for SF development.