Geophysical Features And Geological Interpretation Of Cenozoic Sedimentary Basin Basement In The Northeast Area Of South China Sea

Passing through complicated geological process, the northeast area of SouthChina Sea locates at the common border of Eurasia Plate and PacificPlate. It includesthe Pearl River Mouth Basin having abundant oil & gas resources and the SouthwestBasin of Taiwan who have important research prospects. Therefore, aimed for furtheroil & gas exploration in the north continental margins and the studies of theformation & evolution of the South China Sea, it is quite important and meaningfulto study the features of Cenozoic sedimentary basement of this area. Stemming fromthe projects of national major research program of 973 'The key problems of formation& evolutionand resources of Chinese marginal seas' and its 2nd level project withthe series No. G2000046702-02 'Studies of basement features and tectonic patternsof the Cenozoic sedimentary basin in the northeast area of South China Sea', thedissertation firstly emphasizes on the studies of the basement features of theCenozoic sedimentary basin in the northeast area of South China Sea which includesthe basement depth and lithology, the igneous activity, the fault layout and thedivision of tectonic units, then makes discussion on the control roles on theCenozoic sediments played by basement tectonics, the distribution, the thickness,the lithology and the tectonic control factors of the Mesozoic sediments especiallythe sea-facies Mesozoic sediments in the northeast area of South China Sea, alsothe relationship between the Mesozoic sea-facies sediments and the extensiontowards east of Tethys.Related finished achievements of geology and geophysics about the studied areawere extensively collected and analyzed in the dissertation. Guided by the tectonicconception of history proposed by Academician Liu Guangding, the overall researchroute for integrated geophysical & geological interpretation which fulfills thecombinations of geophysics & geology, qualitative & quantitative analyses, forwardmodeling & inversion calculation by multiple feedbacks on the basis of rocks physicalproperties and physical & geological models, is determined. A set of researchtechniques and methods concerning the basement depth and lithology, faults and deepstructure of the Cenozoic sedimentary basin is put forward in accordance with thegeological & geophysical conditions and exploration situations of the northeast areaof South China Sea especially the Pearl River Mouth Basin. The specific researchessentials are as follows, the Cenozoic basement and the Mesozoic thickness arecalculated by inversion using gravity & magnetic stripped data from the known strataconstrained by seismic and drills; the basement lithologies are studied by thecalculations of apparent density & apparent magnetic intensity of the basement usinggravity & magnetic and seismic data; the basement faults are determined by thecombinations of conventional processing methods, the wavelet analyses and graphicprocessing methods using gravity & magnetic data and seismic profiles; the deepgeological structure is revealed mainly by regional gravity & magnetic dataintegrated with the sonar floating buoy materials.In the dissertation, the methods for forward modeling & inversion calculationrelated to the separation of potential fields and the integration with seismic dataare studied in accordance with the research route mentioned above. The gravity &magnetic data processing in the northeast of South China Sea represents theprinciples of the combinations of conventional processing, the wavelet analyses andgraphic processing. Furthermore, the roles that the wavelet multi-scale analyses playing in the separation of gravity anomalies, different wavelet basic functionsin the decomposition of gravity anomalies, are particularly studied. Theregularities of the wavelet analyses and the features of the wavelet generatingfunction in gravity field separation are summarized consequently. Then appropriatewavelet generating function is chosen to fulfill the wavelet transformation andquite good application results are yielded. In addition, the characteristic forwardmodeling& inversion methods developed by the discipline group, including gravity& magnetic iterative inversion method with changing physical properties, basementapparent density & magnetic susceptibility inversion method with changing depth,the joint inversion method of gravity and seismic data usingstripping process, areall put into use in the dissertation. The effective applications of these methodsgive full play to the accuracy of separated field calculation and forward modeling& inversion in the course of integrated geophysical interpretation.Overall speaking, using integrated geophysical methods, the Pre-Cenozoicbasement depth map of the whole studied area in the northeast of South china Sea.is obtained for the first time, the basement in this area. is inferred by lithologystudies as the composition of Precambrian folded basement, Paleozoic metamorphicrock, Mesozoic middle acidic intrusion rock, Mesozoic sedimentary rock and middlebasic igneous rock. Igneous rocks widely distribute in the studied area.Particularly an important Mesozoic sediments distribution zone in Chaoshan Sag andin the eastern of Zhuyi Sag is deduced and verified by later borehole drilling.Moreover, 35 basement faults in the basin are determined through integratedgeophysical interpretation, which results in the tectonic pattern with stripe stylein north-south direction and block style in west-east direction in the basin andkeeps close relationship with the basement property and oil & gas accumulation.Totally 7 tectonic units in the northeast area of South China Sea are divided mainlyin accordance with the basement features and Mesozoic distribution, which are theMesozoic sedimentary unit along the south China coast in the north of the studiedarea, the Mesozoic missing deposit area from the Pearl River Mouth Basin toQiondongnan Basin, the Mesozoic arc basin unit in the northeast area of South ChinaSea, the Southwest Basin of Taiwan, the Cenozoic sea basin area, the land block unitin Xisha-Zhongsha area and the tectonic zone in Taiwan-Lusong area. Based on theachievements finished above, the distribution of the Mesozoic especially thesea-facies Mesozoic sediments, the lithologiesand the tectonic control factors inthe northeast area of South China Sea, are studied. The tectonic evolut-ion ofMesozoic is focally discussed. With a view to the point that Chaoshan Sag ever locatedat the joint position of middle Tethys and the transformed continental margin ofmiddle Tethys and Pacific ocean in geology history and was controlled by middleTethys and Mesozoic arc basins in the geological tectonic evolution history, anewviewpoint that Chaoshan Sag is a Mesozoic forearc basin is brought forward in thedissertation, then the distribution of the system of Mesozoic subduction zone andthe trough arc basin in the northeast of South China Sea is preliminarily determined.Finally, the control impacts for the formation of Cenozoic sedimentary basin in thestudied area resulting from this system are also discussed. The Cenozoic tectonicevolution is regarded mainly controlled by the convergence of Eurasia Plate andPhilippine Plate and influenced by the collision of India Plate and Eurasia Plate,the uplift of Qingzang Plateau and the subsidence resulting from the expansion ofSouth China Sea.