| Northern Tibet Baqing County and Ando area northern part belongs to Qiangtang strata district, southern belongs the Gangdese-nyainqentanglha strata District. Reseach of domestic Jurassic reefs were Less and low level. Mid-Late Jurassic reefs found in research areas by oil and gas geoligical exploration. The study of northern Tibet of reefs is significant, opened up new areas of reseach Mid-Late Jurassic reefs of northern Tibet, complement the types of Mid-Late Jurassic reefs of northern Tibet, improve the evolutionary phase of Paleogeography, paleoenvironment and paleoclimate in Mid-Late Jurassic of northern Tibet.This thesis devotes to analyze the organic reef systematically and comprehensively, and establish a set of scheme for classifying the organic reef more normative and integrated. Meanwhile, the lithology of containing reef formation, reef building organism, reef types and characteristics are studied in this paper, analyzing the relationship between the organic reef development and palaeogeographic patterns, and it between the tectonic activity and the eustatic sea level change. It also classifies the sedimentary facies of containing reef and the paleogeography and palaeoenvironment of Jurassic in this region. Besides, by using the method of palaeosynecology and geochemistry, this paper also analyze the condition and environment for the development, growth and decline of organic reef. Though above researches, the details of this thesis is as follow:According to the reefs in palaeogeography, development environment, growth form and architectural approach to a comprehensive classification, we divided reef into three-classification:the first biological reef classification, the second biological reef classification, the third biological reef classification. The first biological reef classification is to be divided according to the geographical location of reef growth. Different location determines the size of its causes, so it plays a decisive role. Other classification should be subordinate to the first biological reef classification. In the first classification, the reef can be divided into fringing reef, platform inner reef, platform margin reef, slope reef and basin reef. In the second biological reef classification, according to the scope of the reef, the external shape and size of the area is divided into layer reef, patch reef, block reef, barrier reef, pinnacle reef, circular reef. In the third biological reef classification, framework reef, barrier reef, bonding reef, plaster reef could be divided into according to the reef way. In this region, the main reef building organism in Mid-Jurassic is hexacoralla, stromatoporoid and bivalve. The reef building organism in Late-Jurassic is seldom plants and blue-green algae followed by hexacoralla, stromatoporoid. Among those, the aboundant content is stromatoporoid with many forms and types, columnar shape. However, the hexacoralla are main of massive colony and Plexiform group. The bivalve monomers are all belong to the fixing ecotype. All though these three kinds of organic reef are reef building organism without exception, but their ecological characteristics are all different.According to research of paleoecology, the organic reef could be divided into8reef-forming communities. They are Milleporidium-Cladocoropsis communities, Cladocoropsis-illeporidium-Milleporella communities, Milleporidium-Actinatraea communities, Schizosmilia-Parastromatopora communities, Parastromatopora communities, Schizosmilia communities, Liostrea communities, Cyanobacteria-Liostrea communities, and etc.The Jurassic organic reef in this region distributed in the Dongqiao, Suoxian, Baqing and etc. in the Ando County of north Tibet. The reef-forming organisms’ community in the Dongqiao region experienced4evolution stages. The reef-forming organisms’community in the Suoxian and Baqing regions experienced3evolution stages respectively.In accordance with the classification scheme proposed in this paper, the first classification of local Jurassic reefs including fringing reef, units within the reef and platform margin reef; the second classification including layer of reef, block reef, patch reefs and barrier reef; the third classification could be subdivided into framework reefs, barrier product reef, bonding reef and so on. Specific division as follows:(1)Fringing reef:①Layer reef:Cyanobacteria-Liostrea bond-barrier layer reef(), Liostrea barrier layer reef;(2) platform reef:①Block reef:Milleporidium-Cladocoropsis block reef, Milleporidium-Milleporella barrier-skeleton block reef, Milleporidium-Actinatraea barrier-skeleton block reef;②Patch reef:Cladocoropsis barrier patch reef, Milleporidium styliferum barrier patch reef.(3)Platform margin reef:①Barrier reef: Schizosmilia-Parastromatopora barrier-skeleton barrier reef, Schizosmilia skeleton barrier reef. Above all, there are nine kinds of reef types. As in the study area was at the end of latitudes, the warm and humid environment is the basis of the formation of reefs. Therefore, various types of reefs have great amount of development. The reef base of biological reef composed of sparry calcarenite or Nuclear-shaped limestone. It indicates that the reefs in the region is no exception that the development is on the basis of hard-ground.The divide of sedimentary facies and sedimentary microfacies are based on analyzing the containing organic reef sequence formation. Through the research the lithostratigraphy and biostratum of containing reef sediment group, there are abundant organic fossils in the formations of this region, including the bivalve, brachiopoda, stromatoporoid, ammonoid, sporopollen and darwinulid ostracod. The sedimentary facies distribution of Shamuluo Formation in Upper-Jurassic and the Sangkalayong and Buqu Formation in Mid-Jurassic have nothing in common with each other. The main sedimentary facies of Upper-Jurassic Shamuluo Formation in Dongqiao of Ando County include platform edge facies, open platform facies and tidal flat facies. The sedimentary facies of Mid-Jurassic Sangkalayong Formation in the eastern of Suoxian County are open platform facies and platform edge facies. The sedimentary facies of Mid-Jurassic Buqu Formation in Maru area of Baqing County could be divided into platform edge facies and open platform facies. The existence of shoal subfacies is the foundation of many organic reef developments.The temporal evolutions of lithofacies association constitute the lithofacies combination sequence, which also called lithofacies sequence. According the distribution characteristics in section of lithofacies association, it could be divided into Platform facies sequences of Terrestrial detritus and carbonate mixture and the Carbonate platform facies sequences.Through the analysis of content of carbon isotope、oxygen isotope and resce element in the study area. The divide of paleoenvironment, paleoclimate and fluctuation of sea level. The the overall trend of change from value of δ13C and δ18O in the samples are similar. Found a large number of reef in Shamuluo Formation in Upper-Jurassic, biological agent is strong, so12C is consumed, the reserve of organic carbon in biological in the reducing environment increase,13C is rich in sea water,813C values increased. The change from value of813C and δ18O in the samples were increased'reduced by three times, last increased again. Therefore sea level has gone through rise'dropped by three times. The change from value of Sr、P and Ti element are similar, and are good indicative to paleoenvironment and paleosalinity. Through the analysis of content of S、P and Ti element, variation tendency is were increased'reduced by three times, can get to know that paleoenvironment and paleosalinity. are three times undulate in Dongqiao area.Shamuluo Formation consist of gray sandshale and bioherm limestone, pertain to typical land sources clast and carbonate composit deposit, bioherm developing. In this text, by analyse the feature of source rock and reservoir, research the oiliness of Jurassic bioherm in Ando area. The analyse of resource rock feature shows that, Shamuluo Formation is not resource rock, the type of organic material is containing sapropel, and become mature condition. The research of reservoir characteristic indicate that, Shamuluo Formation reservoir is Ultra-low porosities and permeability reservoirs, and could regard as fissure reservoir. |
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