Abstract: In order to investigate the genesis of high quality reservoir, we compare the diagenesis characteristic and the genesis of tight sandstone within sequence stratigraphical framework based on sequence stratigraphical framework division in the Shahezi Formation by making full use of thin-section, SEM, physical properties, X-ray and trace elements analysis. We divide the Shahezi Formation into four 3rd-order sequences and fourteen system tracts. The distinguishing characteristic of diagenesis within the sequence framework is made up of sequence boundary and sequence unit. Cementation and dissolution are under the control of sequence boundaries. The cementation content and compaction strength in sandbody above the sequence boundary are greater than below. There are many more dissolutions in sandbody below the sequence boundary than above. Diagenetic events within different system tracts correspond to periodic evolution of formation fluid and medium environment. The diagenesis is mainly featured by chlorite and calcite cementation in LST, intense feldspar dissolution and illite cementation in TST, mechanical compaction and authigenic quartz in HST, feldspar and rock debris dissolution and kaolinite cementation in RST. The sedimentary environment differences in various positions within the sequence framework influence the sedimentary characteristics and control the physical fundament of diagenesis and primary porosity. Because of the periodic evolution of water properties and environment, the early diagenesis is differentiated in LST, TST, HST, and RST, which makes the sandbody in RST and TST the best reservoir for gas accumulation.

Key words: diagenesis, pore evolution, Shahezi Formation, Xujiaweizi fault depression