Anpeng deep strata series are usually buried deeper than 3 000 m

and belong to the lower part of the third member of Hetaoyuan Formation fan delta sedimentary system.The difference of sedimentary environment resulted in serious heterogeneity of the sandstone reservoirs.Core descriptions

conventional thin section

injecting thin sections

SEM observations

X-ray analysis of clay minerals

mercury injection data and pattern analysis of pore structures show that the major rock types are lithic sandstone

feldspathic litharenite and lithic arkose.Also it is found that

in the sandstone

the subaqueous distributary channel sandbodies and the mouth bar sandbodies have the best physical properties.Rapid accumulation

miscellaneous grain size

low maturity of texture and composition are the predetermination condition of low permeability.The main type of pores is the secondary porosity.The reservois are poor in pore throat distribution

complex in structure

small in throat and their evident character is heterogeneity.They are the typical compacted low permeable reservoirs.The diagenesis

especially the compaction and the later carbonate cementation are the key factors affecting the microheterogeneity of the reservoirs and leading the reservoirs to be compacted and low permeable.