Research On Sedimentary Facies Of Mesozoic In North Depression Of South Yellow Sea Basin

The South Yellow Sea Basin, major part of the lower Yangtze, is the seawardlongation of the Yangtze block that located between China and Korean peninsula.South Yellow Sea Basin is a multicycle superimposed basin formed on the pre-Sinianmetamorphic rock basement of the Lower Yangtze Platform. North Depression is thesecondary structural unit of South Yellow Sea Basin, and on the whole it downfaultsto north and overlaps to the south, with a total area of about51,000km2. According tofaulting tectonics, North Depression can be divided into8third-level structural units,including Northeast Sag, East Salient, North Sag, West Sag, East Salient, East Sag,Middle Salient and South Sag with the same tectonic trends of North East East.The oil and gas exploration of South Yellow Sea Basin began from1968. Chinahas found liquid crude oil in the crack of Taizhou Formation in North Sag, and Koreahas found gas show in its2wells, but both countries didn’t gets commercializable gasand oil flows so far.On the basis of former studies of South Yellow Sea Basin, seismic facies areanalyzed in sequences through fully utilization of the geological, geophysical,geochemical data of the study area. Characteristics of single well facies andsedimentary environment are analyzed, and the sedimentary facies model of singlewell is established with the drill cores and logging data of8wells. This thesisdetermined the plane distribution of the sedimentary facies of Mesozoic in thenorthern depression by integrated analysis of single well facies and seismic facies,analyzed its sedimentary model, and then evaluated the oil and gas potentiality ofMesozoic in the basin.Based on the single well facies of well Huang7, well Zhucheng7-2-1,wellZhucheng1-2-1and combination of color, lithology, primary structure of core,paleontology and logging curves,4sedimentary facies and8subfacies could berecognized. Lacustrine facies, meandering fluvial facies, braided fluvial facies and fan-delta facies, which can be further refined as subfacies of the offshore lake,shallow lake, channel, flood-plain, diara, fan delta plain and fan delta front, are themain precipitation facies in the northern depression of South Yellow Sea Basin ofMesozoic.Seismic facies is the comprehensive reflection of different characteristics of theseismic reflectional section of depositional body. In this thesis four seismicparameters such as inner reflection structure, frequency, amplitude and continuity areselected and complex nomenclative principle of seismic facies, namely figure+configuration+amplitude+frequency+continuity, is adopted. By the analyses of theseismic profile of the South Yellow Sea Basin, the core and well-logging facies，eightkinds of seismic facies unit have been identified and five kinds of sedimentary facieshave been generalized, for example, lacustrine facies, fluvial facies, fan delta, alluvialfan, underwater fan in the northern depression of South Yellow Sea Basin ofMesozoic. The author has made out plane maps of seismic facies and sedimentaryfacies of Gecun formation of lower Cretaceous, Chishanpukou formation of middleCretaceous and Taizhou formation of early Cretaceous, obtained sedimentary patternof Cretaceous.The sedimentary evolution characteristic of North Depression of South YellowSea Basin of Cretaceous can be summarized as fluvial facies–lacustrine facies. SouthYellow Sea Basin began deposition of continental facies extensional faulted basinfrom early Cretaceous, according to explanation of seismic section, the range ofsediment was small at that time, mainly developed at the east of South Sag and thewest of Northeast Sag, and only a few developed at the west of North Sag. Majorsedimentary facies include braided fluvial facies, meandering fluvial facies,shore-shallow lacustrine, alluvial fan and a few semi lacustrine facies during the earlyCretaceous, sediment of most areas was fluvial facies. Owing to the development oftensional faults, the lake basin began to descend fleetly and, the range of sediment hadbeen enlarged a little until middle Cretaceous. Fluvial facies was developed widely inthe study area, and there are littoral-shallow lacustrine-semi lacustrine facies at theNortheast Sag and the east of North Sag and alluvial fan facies at the southern edge of North Sag. The range of sediment had been shrunk obviously at late Cretaceous, onlyscattered at the North Sag and South Sag, the sediment were such kinds: fluvial facies,lacustrine facies and a few fan delta facies. It mainly deposited fluvial facies in theLower Cretaceous and lacustrine facies in the Upper Cretaceous. Taken together, thefluvial facies are the main types of sedimentary facies zone in Cretaceous.There are Qinglong formation (Triassic) and Taizhou-Chishan formation(Cretaceous) source rocks of Mesozoic in the North Depression of South Yellow SeaBasin. Porous dolomites of Qinglong formation, sandstone from shore-shallowlacustrine facies and fluvial facies of Cretaceous can be used as reservoir rock. Thecap rocks are mudstone of Taizhou formation and Cenozoic. In summary, there aregood prospect for oil and gas in Cretaceous of Mesozoic in the North Depression ofSouth Yellow Sea Basin.