STUDY ON THE DIAGENESIS OF CARBONIFEROUS SANDSTONE IN THE NORTHWEST REGION OF TARIM BASIN

The sedimentary environments of Early- Middle Carboniferous period in the northwest region of Tarim Basin, the sequence is(From bottom to top) river, shore to shallow sea environments. Late Carboniferous period in the district stepped to a model near shore carbonate flat. The author is the first one who studied the diagenesis of Carboniferous sandstone in the northwest region of the Tarim Basin. Based on the microxopic analysis, cathodoluminesceae. SEM, X-diffraction oxygen and carbon isotopes are adopted on the research of major aspects of diagenesis such as compaction. presolution. cementation. In addition, nemorous approaches on features, content, sequence and environment of forming and their impacts on the pores in sandstone have been made to the aspects of siliceous dioxide, carbonate minerals and caly minerals cement which were drawn from the sandstone of the Carboniferous. Depend on the analysis results of reflectance of vitrinite (Ro)，spore-pollen colour and illite opening index, it is deduced that the bight diagenetic temperature of the sandstone. The purpose of this paper is to determine the diagenetic stage of the sandstone and to analyze its diagenetic history and the charateristic of pore evolution. Through research. the following main conclusions are proposed in the present paper: 1 .Carbonifbrous is in the middle-late postdiagenetic stage, have undergone deep compaction and presolution, quartz overgrowth is wide-spread, illite bas high crystallinity, ferrocalcite, dolomite and ferrodolomite often appear sandstone cements. Diagenetic temperature is 120D to 2001r. 2. During the long term diagenesis. the primary pores of Carboniferous sandstone nearly disappeared because of compaction. presolution. cementation of silica. self-precipitation and recrystallization of clay minerals. replacement of carbonate; the most effective in fluence factor of proposity is the cementation of silica represented by quartz overgrowth. which filled up more than 50 percent of residual pruaary pores. 3. During the postdiagenetic stage few secondary pores can be formed in siliceous cemented sandstone, so it can not be a valuable reservoir. But for calcareous cemented sandstone. under the reaction of acidic pore water with soluble components such as carbonate minerals. some secondary pores can be formed. promoted the quality of servoir bed.