THE DIAGENESIS AND RESERVOIR PROPERTIES OF SILURIAN CLASTIC ROCKS IN THE SOUTHERN MARGIN OF YANGTZE PARA-PLATFORM

The diagenesis on the sandstones bitumen bearing in certain regions and sections of the southern margin,Yangtze Para-platform is discussed in this paper. It is suggested that the diagenesis in Majiang Anticline Belt is dominated dy early stage dolomite cementation and by the corrosion and replacement of dolomite in clasts. The late Guangxi movement caused the formation and then the successively upliftings of the anticline in the area. The axis of the anticline was eroded and eluviated by atmosphere fresh water, resulted the development of secondary pores in dolomites, which would be favourable for oil/gas accumulations. The maturities of sandstone minerals in the eastern flank of Majiang Anticline, Wuxi, Changshu and Huangqiao areas are rather high and the diagenesis are mainly quartz overgrowth and kaolinites filling the primary pores. The bitumen was filled into the residual primary pores of quartz overgrowth,so that the accumulation of oil/gas is unfavourable and poor. The Middle-Lower Silurian sandstones in a large area of Anjikang Mountains and lower section area of Yangtze River are observed as mosaicism of clasts and clay matrix fillings which are resulted by severe mechanical compaction, since the content of metamorphic debris is quite high and the heavy load-press of overlain strata. That is why the oil/gas accumulation in such a sandstone area is poor and only vein bitumen controlled by fractures can be formed.