Abstract: Reservoirs in the 8th and 9th members of Jurassic Yan'an Fm in southern Ansai oilfield are terrigenous clastic sediments composed mainly of arkose, lithic arkose, lithic quartzose sandstone and feldspar quartzose sandstone of delta front facies. They have been deposited in eogenetic stage and both their compositional maturity and textural maturity are low. The pore types include primary intergranular pores and secondary pores, with the latter being mainly of intergranular dissolved pores. Diagenesis analysis shows that the sources of sediments and palaeo-grographic environment have controlled reservoir qualities, which were improved by dissolution but worsened by diagenesis.