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Application of Seismic Sedimentology in the Predication of Sand-Body Distribution in Deep Sag Zone of Rifted Lacustrine Basin: A Case Study of the Palaeogene Dainan Formation in Gaoyou Depression, Subei Basin, China


Seismic sedimentology is a new branch of geoscience after seismic stratigraphy and sequence stratigraphy, which is regarded as a very effective tool for mapping high-frequency sequence and depositional systems. This study is aimed at predicting the distribution of turbidite sandbodies in the deep sag of rifted lacustrine basin by applying a seismic-sedimentologic study. Five types of depositional systems were identified in Dainan Formation of Gaoyou depression: fan delta and nearshore subaqueous fan which are developed in the south steep slope belt, river delta composed of strike-oriented and lobate deltaic sandbodies which is developed in the north gentle slope belt, and turbidite fan developed in the semi-deep or deep lake in the deep sag zone. Almost 80 wells were drilled both in the steep slope belt and the gentle slope belt of Gaoyou depression, but only two exploration wells were drilled in the deep sag zone; and the wire-line log data from these two wells proved the existence of oil-bearing sandstone reservoir in the deep sag. By using the conventional exploration method, the distribution and the scale of the sandstone reservoir in the deep sag zone are very difficult to be identified. However, there is about 240 km2 high-quality 3D seismic data in the deep sag zone where faults are not developed. Guided by third-order sequence-stratigraphic correlations from seismic and wire-line log data, in this study, stratal slices were prepared from a three-dimensional seismic volume to reveal sand-body distribution in the deep sag zone and to map the associated depositional systems distributions in slopes of the depression. It was found that trubidite fan development was controlled by the interaction of relative lake level change, sediment supply, fault activity, and gravity tectonics. In this work, the symbiotic relationship between trubidite fan sand and the sand of fan delta, nearshore subaqueous fan and river delta in the slopes, was also investigated. Sandstone thickness and dispersal patterns can be predicted by integrating wire-line-log measurements and seismic amplitude patterns.