DEPOSITIONAL CHARACTERISTICS OF THE EARLY OLIGOCENE LOWSTAND DELTA IN XIEYANG SLOPE ZONE OF WEIXINAN DEPRESSION, BEIBUWAN BASIN AND THEIR PETROLEUM GEOLOGICAL SIGNIFICANCE

The W2 Formation of Lower Oligocene in the Weixinan Depression of the Beibuwan Basin, South China Sea, is dominated by a set of muddy deposits formed during the period of lake level rising, in which the distribution of thin sand bodies is rather complicated. Petroleum exploration has so far remained in a relatively low stage due to lack of understanding of the origin and distribution of these sand bodies. This time, started from the study of tectonics and infilling history, we reconstructed the cyclic changes of base levels. And three 4th-level sequences, including the upper, the middle, and the lower sequences, are established. Sandstones are mainly concentrated in a positive rhythmic manner in the early stage of the middle sequence. They are the superimposed deposits of traction load formed under stronger water dynamic, and prograded from north to south. Based on the analysis of the tectonic and landform framework, we can find out that it was a very stable large-scale lowstand deltaic system no matter in the sag or on the slope. The deltaic bodies were affected severely by faulted terraces along the edge of the basin. It is believed that the specific depositional background and the types and characteristics of the reservoir and cover match well with the formation of source rock and the migration and accumulation of oil and gas in this area. A lowstand and transgressive system tract is then put forward for the first time for the deposits formed in the early period of the middle sequence of the W2 Formation. It should be a very important exploration target in the future.