| The Qiongdongnan Basin experienced a series of tectonic activities from faulting, through fault-depression and post-faulting thermal subsidence to post-faulting accelerated subsidence, had a sedimentary environment from coast in Eocene to deep-sea in present, formed a set of strata combination including littoral sediments, offshore sediments, shelf-slope sediments and bathyal sediments, and possessed favorable source-reservoir-cap conditions for hydrocarbon accumulation. In our study, a combination of geological, geophysical, paleontological geochronology and hydrocarbon accumulation methods are used to unravel the basins’tectonic evolution in greater detail. We studied the structure and evolution of the basin, analyzed of hydrocarbon accumulation and established of regional stratigraphic framework, which will enrich our knowledge about regional stratigraphic analysis and petroleum geology in the Qiongdongnan basin.The highlights of this thesis are shown as follows:(1) Since the Oligocene, the sedimentary rate of the Qiongdongnan Basin was mutated in 5.5Ma B.P.,11.6Ma B.P.,16.0Ma B.P.,23.0Ma B.P. and 31.5Ma B.P.. Each formation (from new to old chronological order) the relationship between the deposition thickness and ages presents a "V" pattern, which is Huangliu and Meishan formation thinner than the others.(2) There are some similarities and differences in the change of sedimentary rate at shelf area and slope area in the Qiongdongnan Basin of, mainly as follows:1) In the period about 16.0Ma B.P., the sedimentary rate of shelf area was slowing, thus no significant change in slope area; 2) In the period about 16.0Ma B.P., the sedimentary rate of shelf area was faster than slope area; 3) Both of their relationship between stratigraphic thickness and ages are show a "V" pattern. The stratigraphic thickness variation of the Qiongdongnan Basin has regional differences, which the main factors are regional tectonics, sedimentation and different terrain. Analysis of regional differences shows:the transition of the expansion axis of the South China Sea in Miocene (about 23.03Ma B.P.) impacts the Qiongdongnan Basin regionally; the migration of subsidence center in Pliocene has affection on the stratigraphic thickness of the Qiongdongnan Basin.(3)Thus it is considered as an important region of hydrocarbon exploration and development. In this study, the key strata interfaces (T20, T30, T40, T50, T60 and T70) in the wells-through seismic profiles were identified, then were tracked and correlated combined with the regional wells-tie seismic profiles in the basin. On this basis, the regional stratigraphic framework of the Qiongdongnan Basin was established associated with the age of fossil assemblages. As a boundary of T60 seismic reflection interface (23.03Ma B.P.), the tectonic evolution stage of the Qiongdongnan basin is divided into two stages:faulting stage and post-faulting depression stage. According to the faults and the affecting of tectonic events, the faulting stage is divided into faulting phase (Eocene-early Oligocene) and fault-depression phase (late Oligocene), the post-faulting depression stage is divided into thermal subsidence phase (early middle Miocene) and accelerated subsidence phase (late Miocene-Pliocene).(4)The Neogene strata (T60-T20) are primary in the shallow water area, in which the faults are rarely distributed, the stratigraphic thickness has little variation, and onlaps and truncations can be rarely found. Delta sedimentary system, which is beneficial to hydrocarbon reservoir, develops highly in local area. The evidence mentioned above indicates that tectonism exerted trivial influence on the Qiongdongnan Basin during the Neogene. The characteristics of the Neogene strata (T60-T20) in the deep water area are similar to that in the shallow water except seismic reflection characteristics. The internal sequence structure of the Paleogene strata (T100-T60) is dominantly sphenoid or subparallel with obvious onlaps and truncations. The thickness of the strata is large, and obvious faults result in strata fractures. These characteristics imply that the deep water area in the Qiongdongnan Basin was controlled mainly by tectonism, accompanying with the strong extension and rapid subsidence, and being in neritic environment during the Paleogene.(5)Three stages of sand bodies are identified in the Central Canyon from east to west:the first stage (depth of 3528-3336m, thickness of-192m) formed during 11.6-5.5Ma B.P. (T40-T30) distributes in the Lingshui-Songnan-Baodao segment of the Central Canyon, which includes turbidite channel deposit, turbidite sheet sand, mass flow deposit, deep-sea politic deposit, natural levee and overflow deposit; the second stage (depth of 4100-3900m, thickness of ～200m) formed during 5.5～ 4.2Ma B.P. (T30-T29), spans Ledong-Lingshui segment of the Central Canyon, being primarily composed of gravity flow deposits; the third stage(depth of 3630-3400m, thickness of ～230m) formed during 4.2-3.6Ma B.P. (T29-T28), presents in the Ledong-Yingdong segment of the Central Canyon, consisting mainly of turbidite channel deposits. The three stages of sand bodies from oldest in the east to youngest in the west, constituted the favorable reservoirs in the Central Depression Belt of the Qiongdongnan Basin.(6) Based on the analysis of TOC and S2-, Yacheng and Lingshui formation are the best at the Qiongdongnan Basin in generating hydrocarbon, Meishan formation is the second best. In the Qiongdongnan Basin there are several source-reservoir assemblages, Lingshui delta and sandstone is reservoir and its shallow sea mudstone is seal. They locate at Yacheng, Songtao uplift, Yabei and Songxi sag, Songdong northern slope, middle part of Baodao, northern Changchang sag and Lingnan lower uplift, these are the target stratum of prospecting.(7) The Qiongdongnan Basin’s different structures control different method to transport oil and gas, which build three different modes of hydrocarbon accumulation including fault block hydrocarbon accumulation, uplift hydrocarbon accumulation and valley hydrocarbon accumulation. In different stage of basin evolution, I entrapment, which is buried hill and structure entrapment and ～ entrapment, which is anticline and stratum were built. In Palaeogene, fault activity induced the transport of oil and gas to the lateral side at rift stage. In Nogene, fault activity induced the transport of petroleum and gas in the vertical direction. |
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