Abstract: Fine-grained sedimentary rocks have become an important field of oil and gas exploration in the depression lake basin. The study of fine-grained sedimentary petrography and sedimentary microfacies can provide a theoretical and practical basis for exploration and target selection. Taking the Chang 7 Member of Triassic Yanchang Formation in Ganquan area of Ordos basin as an example, the types and characteristics of lithofacies and sedimentary facies of Chang 7 Member were clarified by core observation, logging curve, grain size analysis, and oil test and production test. The distribution law of sedimentary facies was predicted, and its control effect on reservoir distribution was discussed. The results show that the sedimentary structures such as massive bedding, horizontal bedding, slump deformation, ball pillow structure, water discharge structure, and flame structure are developed in the Chang 7 Member of the study area. Plant fossils are enriched in Chang 7₁ and 7₂ Submembers, but not found in Chang 7₃ Submember. The probability curve of grain size shows three types: Single-stage suspension, two-stage gradual change, and wide and gentle arching. It is comprehensively reflected that the sedimentary environment of Chang 7 Member is semi-deep lake-deep lake and delta front. According to lithology and sedimentary structure, 10 lithofacies types and 4 lithofacies combination modes are divided, and 6 lithofacies are mainly developed, such as massive bedding fine sandstone facies, flow cross bedding fine sandstone facies, wave cross bedding siltstone facies, deformed bedding argillaceous siltstone facies, rhythmic bedding silty mudstone facies, and massive mudstone facies. Six sedimentary microfacies are identified, including underwater distributary channel, interdistributary bay, semi-deep lake-deep lake mud, turbidity current, sandy debris flow, and slip-slump. Underwater distributary channel sand body, turbidite sand body and sandy debris flow sand body can act as high-quality reservoirs, and the underwater distributary channel sand body is the main production layer in the study area.

Key words: fine-grained sedimentary rocks, sedimentary characteristics, phase mark, sedimentary facies, lithofacies, Chang 7 Member, reservoir distribution, Ordos basin