Diagenetic Response Caused By Hydrocarbon Charging Of Chang-8 Member In Yanchang Formation,Ordos Basin

This paper carried out the sedimentary and reservoir study in Chang 81 oilbearing formation,which in Zhenjing region with high-degree exploration,southwest Ordos basin,by outcrop measurement,core observation,slice observation and logging interpretation method.Studies have shown that Chang 81 oil-bearing formation is delta front sub phase;lithology is mainly feldspathic lithic sandstone and debris-feldspar sandstone with low maturity of composition and structure;Reservoir space is dominated by intergranular solution pores and residual primary remanent intergranular pores;the main diagenesis types are compaction,cementation,dissolution and metasomatism and fracturing.By comparing the evolution of the diagenetic processes of late K1 in different oil level,it shows that the effect of hydrocarbon charging on reservoir physical properties and diagenetic autogenetic minerals is significant.Based on burial history and hydrocarbon thermal evolution history,it is found that it experienced 3 hydrocarbon charging period:late J3,late K1,mid K2.Diagenesis may stop or suppressed with higher oil saturation,on the contray,fluid in pore will continue to occur chemical reaction and siliceous or calcareous cementation will form in pore,which exist in some parts that not filled with oil and gas,eventually leading to reservoir rocks are tight and become invalid reservoir.Study on Chang 8 reservoir group has 3 types of "dessert" : Primary type,secondary type and mixed type.The main factors that affect the "dessert" are the time and extent of hydrocarbon charging and the main diagenetic events.