SEDIMENTARY DIFFERENCE AND LOW-PERMEABILITY RESPONSE OF THE UPPER TRIASSIC YANCHANG FORMATION IN THE LONGDONG AREA, GANSU-EXAMPLES FROM THE CHANG 6 RESERVOIR IN HESHUI AND CHANG 8 RESERVOIR IN BAIMA

The Chang 8 reservoir at Baima in the Longdong area consists of delta-front underwater distributary channel deposits formed in the process of lake basin expansion,while the Heshui Chang 6 reservoir was mainly delta-front turbidity sand.They are low-and very low-permeability reservoirs respectively,both belonging to the southwest provenance sedimentary system of the Ordos basin,and the very low-permeability reservoir formed in the area due to different hydrodynamic conditions,transport distances,depositional topographic conditions and depositional water depths,as well as the influence of the moderate compaction strength.The fine grain size,lacking of chlorite and high contents of carbonate cements are the main controlling factors that make the physical properties of the Heshui Chang 6₃ reservoir poorer than those of the Baima Chang 8₁ reservoir.