Through the detailed core observation and lithologic interpretation of logging data

rock association characteristics

sedimentary architectures

sedimentary facies and sequence stratigraphy of Chang 8 reservoir in Zhenyuan area were studied.The results show that a shallow braided-river delta sediments with two slope breaks was developed during the deposition Chang 8

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and the paleo-shoreline moved frequently within tens of kilometers above the slope breaks.These slope breaks were mainly identified by logging and sedimentary architecture features.Slope breaks of Chang 8

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sand group in Zhenyuan area were mainly controlled by basement faults affected by violent volcanic and tectonic activity in the southern and western Ordos Basin.During deposition of Chang 8

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the study area can be divided into three geomorphology units

including platform-like plain (front) above the upper slope break

central-slope between the two slope breaks and lower-slope below the lower slope break.The distribution of sand body in transgression and regression stages was controlled by the ancient geomorphological features.The platform-like plain which was close to the provenance received more sandstone during transgression stage

and mudstone and siltstone deposited during regression stage.The central-slope that was farther from the provenance developed thicker sand bodies during lake transgression stage

while sands preferentially accumulated around the first slope during the lake regression stage.The lower-slope that was far away from the provenance developed small amount of sand bodies in anytime.The shoreline moved to the first slope in low water stand

and sand bodies around the slope break may feature "coastal sand bar" because of the transportation and transformation of waves and coastal current.