CLINOFORM GROWTH IN DELTA OF YANCHANG FORMATION IN LONGDONG AREA,ORDOS BASIN,CHINA

The understanding of how lake basin fill is approached according to the analysis of clinothem characteristics and palaeoenvironment,such as the seismic reflection, clinothem superposition, based on the high-resolution 2D seismic and drilling data from the Yanchang Formation, southwestern Ordos Basin, China. The study subdivides the observed clinothems to identify six types: 1. Oblique wedge 2. Tangential oblique plate 3. Oblique divergent 4. Parallel oblique wedge 5. Sigmoida wedge 6. Sigmoida lenticular . In addition to the pristine terrain, sediment influx and accommodation are dominant control on clinothem development and lake basin fill, whereby increasing sediment influx and limited accommodation promotes clinothems with oblique shape and low trajectory angles, while increasing accommodation and limited sediment influx promotes higher trajectory angles and Sigmoida clinothems.The shoreline migration trajectory delineate the development of lacustrine basin. And the longitudinal superposition and lateral extension of clinothems reveal the filling process of the lacustrine basin.Sigmoida lenticular clinothems are generally characterised by flat trajectories, which tend to show a strong relationship between their overall slop and toe advance, suggesting a strong link between slopes with topset and delta plain subfacies . This reveals that the lacustrine basin is under the early stage of filling, with slightly higher sediment supply rate and higher accommodation.The Sigmoida wedge clinothems with descending trajectories and thick bottomsets are generally characterised. The seismic profiles of Sigmoida wedge clinothems show deep gully reflections in river channels or canyons in the bottomsets, suggesting the possible development of deep water deposits.Parallel oblique wedge clinothems have topset and bottomset development, with a slight descent trajectory and larger overall slope. Gamma-ray log curve from wells through the topset are mostly box type and finger type, indicating that delta to deep-water sedimentary patterns may have developed.Tangential oblique plate clinothems are in lack of topsets and bottomsets, which reveals that the lacustrine basin was in the late stage with lower sediment influx rate and reduced accommodation.