Diagenetic evolution of deep sandstones and multiple-stage oil entrapment: A case study from the Lower Jurassic Sangonghe Formation in the Fukang Sag, central Junggar Basin (NW China)

The deposition and preservation of high-quality rocks in a heterogeneous petroleum reservoir and associated mechanisms for hydrocarbon emplacement and migration are one of the most critical issues in deep exploration. Applying an integrated approach of petrography, SEM, stable carbon and oxygen isotopes, and fluid inclusion analyses, this study was designed to examine the relationship between diagenetic evolution and oil emplacement in the Lower Jurassic Sangonghe Formation in the Fukang Sag, central Junggar Basin (NW China). The sandstones consist mainly of feldspathic litharenite, and litharenite. Primary sandstone texture and compositions (grain size, ductile lithic sand grains) determine reservoir diagenetic heterogeneity. Grain size controls the overall abundances of cement and porosity, and reservoir properties through its effect on ductile lithic grains and hence on mechanical compaction. Ductile lithic-rich, very fine- to fine-grained sandstones had a limited diagenetic process in which compaction of easily deformed, clay-rich lithic grains predominated, resulting in a very rapid loss of porosity during burial. They achieved a high capillary entry pressure before the first oil arrival and were not charged later. In contrast, diagenetic events in the relatively coarser-grained sandstones with less abundant ductile lithic grains included dissolution and cementation as well as ductile compaction. Diagenesis progressed alternately with oil emplacement and in some cases, they occurred synchronously. Late diagenetic Fe-calcite, ankerite, and barite may be good mineralogical signatures of oil charge and migration. The nonreservoir, ductile lithic-rich, tightly compacted sandstones can constitute impermeable barrier interbeds embedded in permeable reservoir rocks, probably resulting in heterogeneous flow.