Diagenetic evolution and high-quality reservoir in Chang 6 sandstone in the western Ordos Basin

The diagenetic characteristics of Chang 6 sandstone of the Upper Triassic in Yanchi-Jiyuan area of the western Ordos Basin show that this set of sandstones are in the early A period of the late diagenetic stage at present and controlled by the acidic diagenetic environment,where the abundant diagenetic phenomena including widespread dissolution of feldspar and lithoclast,secondary enlargement of quartz and well-developed early chlorite film can be observed.The quantitative statistics and calculation show that the mechanic compaction is the main cause resulting in the disappearance of primary porosity,and the next one is cementation.The increasing porosity by dissolution accounts only a little proportion of the initial porosity(lower than 10 percent).According to the sedimentary facies analysis,three types of the typical sedimentary-diagenetic facies zones could be recognized,including platform delta front-chlorite cementation-late ferrous calcite replacement facies,slope delta front-chlorite cementation-late ferrous calcite cementation facies,delta front end-chlorite cementation-late ferrous calcite replacement facies.The high-quality reservoir is controlled by chlorite cementation facies zone distributed in an irregular area from Anbian to Tiebiancheng region.