Sedimentation during the transgression period in Late Triassic Yanchang Formation, Ordos Basin

The Late Triassic Yanchang Formation in the Ordos Basin experienced three transgression periods of Chang 9, Chang 7 and Chang 4+5. Based on the drilling and logging data, the types of sedimentary facies and the evolvement of sediments are analyzed in the transgression periods. It is concluded that the reservoir sandstones are mainly semi-deep to deep lacustrine turbidites in the west of the lake basin, and are deep-water delta front distributary channel sands in the east. Distribution characteristics of sandstones are distinct in every transgression period. In particular, the Chang 9 sandstones are wide, intensive, and thick in monolayer; the Chang 7 sandstones are small, with turbidite sandstones developed, and stacked and connected locally; the Chang 4+5 sandstones in the northeastern delta of the basin are far extended, with good horizontal connection, thin monolayer of deep-water delta front distributary channel sands, and multiplayer superimposition vertically. The study also shows that in the transgression periods, good source rocks were developed, sandstones had good reservoir space, and the source, reservoir and seal rocks had good relationships, prone to form primary hydrocarbon reservoirs, and they are targets for exploration.