Formation Of Lowstand Fans And Their Hydrocarbon Accumulation Since Middle Miocene In The Yinggehai And Qiongdongnan Basins

In the recent years, with the development of the theory and technology of hydrocarbon exploration, there have great progresses in deepwater areas in the world, such as America, Norway, Brazil, and the hydrocarbon exploration in deepwater lowstand fans has become a leading edge and hotspot field. The Yinggehai and Qiongdongnan basins, Cenozoic sedimentary basins in the northern margin of South China Sea, have the similar tectonic background as these famous deepwater hydrocarbon basins, and the prospect of hydrocarbon exploration in the two basins has been proved by several decades' exploration. On the basis of analysis from the data of wells, seismics, logs and cores, firstly, this thesis expatiates the four histories of filling basins, tectonic evolution, organic matter mature and diagenesis, and then, discusses the effects of high geothermal and overpressure on reservoirs diagenesis, identification and characteristics of fourth order sequences, evolution of shelf-slope systems and prediction of sedimentary types and reservoir features of lowstand fans by the hydrodynamics modeling. Finally, the whole process from the formation of lowstand fans to their diagenesis and the accumulation of hydrocarbon, the models of reservoir and the prediction of possible exploration areas have been dissertated specifically and deeply. The results could be used for the hydrocarbon exploration in lowstand fans in the two basins. The main results in the thesis is as follows:1. "Four histories" include basin filling, tectonic evolution, organic matter mature and diagenesis histories. It is the basement of the evaluation for the hydrocarbon exploration targets, and it is important to analysis the specific basin. After the researches of "four histories" for the Yinggehai and Qiongdongnan basins, it is found that since mid-Miocene, though the tectonic background is para-passive margin in the two basins, the Yinggehai basin is transform-stretch type, and the Qiongdongnan basin is discrete one. The differences of their tectonic background cause the evolvution of various shelf-slope systems and the formation of distinct lowstand fans. As for basin filling, although rapid subsidence occurs in both of the two basins, sediments supply is different. More deficient sediment supply causes steep slopes and more slump structures in the Qiongdongnan basin. As for organic matter mature, source rocks in both of the two basins have low organic richness, and it is difficult for produce and reservoir by itself. So it must need good migration pathways for hydrocarbon accumulation. This is difference between many of overseas lowstand fans reservoirs and those in the two basins, and it is also the reason of many drills' failure. In addition, the characteristics of high geothermal and overpressure and their influences has been summarized which affects not only in cementation diagenesis of reservoir, but also in accelerating compaction and overpressure restrains diagenesis and organism mature.