Abstract:  Effective reservoir rocks and their genesis in a heterogeneous petroleum reservoir are a critical geological issue of deep exploration. The Lower Jurassic Sangonghe Formation in the Fukang sag, central Junggar basin has been used as a case study to examine the dynamic relationship between diagenetic evolution and oil chargeing in sandstones, and discuss effective reservoir rocks and their formation using an integrated approach of petrography, SEM, stable carbon and oxygen isotopes, and fluid inclusion analyses. The results show the sandstones in the Sangonghe Formation consist mainly of ductile lithic-rich, very fine-to fine-grained, and medium- to coarse-grained petrofacies. The two defined petrofacies are interbeded, and represent demonstrable differences in diagenetic evolution and oil charge. Ductile lithic-rich, very fine-to fine-grained sandstones had a limited diagenetic process in which compaction and neomorphism of easily deformed, clay-rich lithic grains predominated, resulting in a very rapid loss of porosity during burial. They have become relatively nonporous before the first oil arrival and have never been charged later. By contrast, diagenetic events in the relatively coarser-grained sandstones with less abundant ductile-lithic grains included dissolution and cementation as well as ductile compaction. At least two episodes of oil charge occurred. Diagenesis progressed alternately with oil emplacement. The nonreservoir, ductile lithic-rich, very fine-to fine-grained sandstones can constitute impermeable barrier interbeds embedded in permeable reservoir rocks, probably resulting in heterogeneous oil migration.

Key words: Fukang sag, sandstone, tight reservoirs, hydrocarbon charge, Sangonghe Formation, oil and gas, Junggar basin