Discussion of Depositional Filling and Sequence Stratigraphy Model of Continental Foreland Basin:Taking the Kuqa Foreland Basin as an Example

Taking the Kuqa continental foreland basin as an example,the key topics of the continental foreland basins have been discussed,such as the formation of the basin,depositional filling and sequence pattern,unconformity and sequence boundary,sequence constitution and models,source-reservoir-seal assemblages and lithostratigraphic traps.It is considered that the sequence of continental foreland basin is response to the tectogenesis of the margin.The foreland sequences are composed of lowstand systems tract,transgressive systems tract and highstand systems tract.Sequence boundary exhibits tectonic or depositional unconformity.Sequence boundary of the foreland basin frequently corresponds to a tectonic episode.Due to the flexible tectonic thrusting adjacent to the orogenic belt,the sequence pattern is response to different stages evolution of the basin.It is reflecting the interval change of the orogenesis from strong to weak.Sequence boundary of the foreland basin often represents the huge change of the depositional architecture.There are the lacustrine facies mudstones and gypseous mudstones of the tectonically stable below the sequence boundary.It developed very thick alluvial fan and fan-delta systems upon the sequence boundary,exhibiting the Molasse filling.There are different depositional fillings and stratigraphic patterns on the ambilateral sides of the foreland basin during the stage of tectonic activity and stability.There are major alluvial lowstand systems tracts adjacent to the orogenic belt during the tectonic activity.It is composed of very thick alluvial fan,fan-delta and braided delta systems.During the tectonic stability,there occurred transgressive systems tract,represented as mainly extensive river and lacustrine facies.The deposition thickness gradually decrease from the adjacent thrust belt to the basin.The matches of source-reservoir-seal assemblage of the continental foreland basin are good.The reservoirs are extensively distributed in the lowstand systems tract,transgressive systems tract and highstand systems tract.They were mostly deposited in braided delta and lacustrine shoreline enviornments.The lithostratigraphic traps mainly distributed in the foreslope area,including stratigraphic onlap unconformity traps and stratigraphic erosion unconformity traps along the margin of ancient bulge.They will become the new fields of petroleum exploration.