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Origin of Chlorite Coating and Its Effect on Reservoir Quality of Permian Upper Urho Sandstone

Summary Chlorite grain coatings play a significant role in reservoir quality and are widely distributed in Meso-Cenozoic clastic reservoir of several petroliferous basins in china. Chlorite coatings can preserve porosity in deeply buried reservoir by inhibiting quartz overgrowths. However the origin of chlorite coatings in onshore litharenite and their impact on reservoir quality are not well known. This study took the late Permian Upper Urho Formation (P3w) at Western Margin of Junggar Basin, China as an example for understanding the grain-coating chlorite origin and the effect on reservoir quality. The Upper Urho sandstones are mostly litharenite and rich in volcanic rock fragments (VRF). The petrographic and chemical study revealed that the chlorite coatings which formed during eodiagenetic stage at temperature lower than 70°C are Fe-rich chlorite and composed of two superimposed layers. The Eodiagenetic I/S coatings are speculated as the precursors of chlorite coatings. The dissolution of the ferromagnesian rock fragment provided cations needed for chlorite growth. The small content of quartz in P3w sandstones limits the importance of chlorite coatings on preservation of porosity by inhibiting quartz cementation. However the occurrence of chlorite coatings in P3w sandstones acts as a barometer of good quality reservoir intervals.