Diagenetic facies of the Chang 8 oil-bearing layer of the Upper Triassic Yanchang Formation in the Jiyuan area,Ordos Basin

Based on comprehensive testing data,this paper discussed diagenetic types,timing and fluid properties of the Chang 8 oil-bearing layer in the Jiyuan area,Ordos Basin.Several lines of novel cognition were made as follows: ①The Chang 8 oil-bearing layer in the study area is at Phase A of the middle diagenesis that consists mostly of various diagenetic processes,such as compaction-pressolution,cementation,metasomatism,corrosion,filling of autogenous minerals and fracture.②Sedimentary facies significantly dominate types of diagenesis,the early rim cementation of chlorites controlled by the location of paleo-lakeline is of importance to the preservation of primary intergranular pores.③Compaction-pressolution leads to the most reduction of pores that lasts through the whole diagenesis.④The filling of autogenous minerals and cementation of carbonates continue from Phase B of the early diagenesis to Phase A1 of the middle diagenesis and may destroy reservoirs in various degrees.⑤Corrosion and disruption occur mostly at Phase B of the early diagenesis and Phase A1 of the middle diagenesis,significantly improving physical properties of reservoirs.On the basis of the above study,the Chang 8 oil-bearing layer of the study area was divided into four diagenetic types of facies combination,including dense-mudstone compaction,rim cementation-corrosion of chlorites,corrosion and carbonate cementation,and poikilitic cementation of calcites.The time-space distribution of reservoirs was obviously controlled by sedimentary and diagenetic facies,and chlorite-rim cementation-corrosion belts distributed in subaqueous distributary channels of the middle study area were proposed to be the most favorable places for reservoir development.