Keywords: 
• authigenic chlorite /
• Xujiahe Formation /
• origin of materials /
• diagenesis /
• reservoir quality

Abstract: The authigenic chlorite graincoatings or porelinings play an important role in preserving primary porosities effectively by resisting the compaction of the overlying rock and decreasing the number of nucleation of authigenic quartz on the surfaces of grains in reservoir sandstones of Xujiahe Formation, Sichuan basin, China. The early materials forming the graincoatings of chlorite are involving the solution of the syndepositional Fe rich sediments, flocculation of the soluble fluvial Fe and the intrusion of the compacting fluids in the adjacent shale bed. And moreover, the graincoatings mainly formed as news neomorphic precipitations between syngenesis and early eodiagenesis. Due to the Continuous intrusion of the Mgrich fluids through the fracture system ,the authigenic chlorites in Xujiahe Formation II are in possession of the enough materials to regrow in mesodiagenesis, furthermore, the contents of MgO are increasing with the increasing depths distinctly. However, in Xujiahe Formation IV sandstones, the regrowth of chlorite is not obvious. The impact of the chlorite graincoatings to the reservoir properties are well evaluated by the relation of the chlorite contents versus the porosity values and the porosity distribution figure in study area. As for the influence to reservoir quality, there is an "optimum thickness" for chlorite graincoatings to effectively preserve the porosities, but without radically decreasing permeability and increasing the microporosity in reservoirs. Within Xujiahe Formation sandstones, the optimum thickness of chlorite grims seems to between 5 μm and 10 μm.