Abstract: The Qingshankou Formation in the Qian’an oil field of the south Songliao basin contains abundant dawsonite-bearing sandstone. The clastic composition, cements, authigenic minerals, diagenetic paragenesis succession and fluid evolution are investigated through polarizer microscope, scanning electron microscope, alizarin red-staining, X-ray diffraction etc. The results shows that the sandstone is fine-grained and micro-grained, poorly to moderately sorted silt dawsonite-bearing arkoses and silty dawsonite-bearing lithic arkoses. The dominant cements and authigenic minerals include dawsonite, ankerite, calcite, secondary quartz oergrowths and authigenic clay mineral (kaolinite and illite ). Part of dawsonite occurs as radial, bundle and plate, and some are very special as large massive condensates. The paragenetic sequence are in the order of calcite-kaolinite, secondary quartz overgrowths-illite-dawsonite and ankerite. The authigenic minerals formed before CO₂ charging include multi-stages of calcite assemblages, kaolinite and secondary quartz overgrowths assemblages. While the main cements formed after CO₂ charging are dawsonite and ankerite. Fluid evolution are in a sequence of alkaline-acidic-the transition from acid to alkaline fluids and/or transition from meta-alkalescence-alkaline fluids.

Key words: dawsonite-bearing sandstone, dawsonite, Qingshankou Formation, Qian’an oil field, Songliao basin, oil fields