DIAGENETIC EVOLUTION AND DISTRIBUTION OF HIGH QUALITY RESERVOIR OF CHANG 2 SANDSTONES(UPPER TRIASSIC)IN YANCHI-JIYUAN AREA,ORDOS BASIN

Chang 2 sandstones of Yanchang Formation (upper Triassic) in Yanchi-Jiyuan area,southwest Ordos basin,are a set of distributary channel and mouth bar sandstone bodies formed in delta front facies,and mainly composed of lithic feldspathic sandstone and feldspathic sandstone,of which the average content of quartz,feldspar and lithic fragments accounts for 32.6%,38.9% and 8.0%,respectively.Metamorphic and igneous rock fragments are main lithic constitutes,accounting for 87.5% of the total fragments.At present,these sandstones are at the late A period of the early diagenetic stage(R_O=0.8%,T=70 ℃~90 ℃),and their diagentic fabrics are controlled by the acidic diagenetic environment,resulting in widespread dissolution of feldspathic grains and well developed quartz overgrowth.Primary porosity is the dominant pore types,accounting for 30%～45% of the total porosity,and interganular and intragranular porosity for 40～55%.On the basis of the statistics on the sandstone compositions,the quantitative calculation shows that mechanic compaction is the main cause for porosity loss and cementation comes second,whereas the increase of secondary porosity resulted from the dissolution only accounts for less than 10% of the initial porosity.In combination with the results from sedimentary facies analysis,five types of the favorable sedimentary-diagenetic facies zone can be recognized,and the most favorable zone with porosity 14% and permeability 5×10 -3 μm2 is mainly composed of the frontier part of the delta plain distributary channel-secondary porosity facies,the delta plain distributary channel-chlorite rim cementation+ secondary porosity facies,and the platform delta front-chlorite rim cementation+primary porosity facies as well as the main part of the slope delta front-secondary porosity facies.Thus,the exploration strategy in the near future should focus on the above-mentioned favorable sedimentary-diagenetic facies zone.