ON SOME BASIC CONDITIONS OF COAL AND OIL FORMATION TAKING THREE TAKES IN YUNNAN AS EXAMPLES

In the earlier stage of 1980's,almost a handred of scientists from several institutes organized by the Earth Sciences Division of Academia Sinica synthetically studied the enviroments and sediments of Dianchi, Erhai and Fuxian Lakes, what dealt with in this paper is one of these research achievements. Stress was put on the basic conditions of coal and oil formation,through the study of the quantity and quan-ylity of aquatic organisms of the three lakes, the relationship between sedimentary environments and organic materials, evolution tendency of organic materials during early-burial and early diagenesis. Relating to the formation conditions of Cenozoic coal and oil in China,the paper puts forward six viewpoints as follows: 1. Both depression and fault subsidence are necessary tectonic background conditions for oil and coal formation. 2. If the climate zone in which the lakes stand can ensure the water salinity neceessary for the multiplication of aquatic organisms and continental organic materials, the favorable conditions for the preservation of organic materials can be got. particularly,when the climatic zone is relatively dry and water salinity is not higher than that in which gypsum can deposit, the essential water mediu conditions for aquatic plants multiplication could be satisfied. Furthermore,if the rainfall of the lake is less than evapration the permanent layered water with high salinity is much more profitable to form in deep lake so that the organic materials can better preserved 3. The nutrient type of lakes is also an important factor for the multiplication of aguatic organisms. Both Dianchi lake of rich nutient type and Erhai Lake of middle nutrient type are favourable for the multiplication of auatic organisms. So aquatic plants communities are wealthy and the producive ability of aeuatic organisms is high.On the contrary,in the poorly nutrient Fuxian Lake, there is less community and the yield is much lower. 4. As to the depth of lake water, coal formation reguires less than 10 meters, which can provide sufficient sunshine for aquatic plant growing.But for the oil for mation, the depth of lake water must be at least deeper than the wavebases,so that organic materials cannot be easily oxidated after their deposition.From the investi- gation of three lakes in Yunnan, the depth of wavebases is less than 10 meters.If the lake water is deeper than the thermal layer, that is in the sedimentary regions with the water depth of 20-30 meters,the deposited organic materials will be affected less easily by the vertical convection of lake water,so to be perserved better. 5. Research data on the lake-water physics collected from all the world have suggested that lakes with permanent layered water,as the specific gravity of the bottom water is larger and the salinity higher and the content of dissolved oxygen lower, are suitable for the preservation of deposited organic materials. Therefore,it s one of the profitbale conditions for oil formation in deep lakes. Although Fuxia Lake is a deep lake, it has only seasonal layered water, so the dissolved oxygen in 100m water depth can reach to about 4mg/L, and there are aquatic earthworms in the bottom of the lake at 150m.It shows that the reducing condition has not been formed yet at the bottom of Fuxian Lake.That is another reason why Fuxian Lake is not suitable for oil formation. 6. As to the conditions of coal formation,besides the sufficient mass and preservation of organic materials, small input of continental clastic materials is also the ssential factor.