Diagenetic characteristics of ultra-shallow reservoirs and influences on hydrocarbon accumulations: case study of Chunfeng Oilfield, Junggar Basin

Located in the northeast of the Chepaizi Uplift of the Junggar Basin, the Chunfeng Oilfield is regarded as a typical ultra-shallow reservoir since the main producing strata are about 500 m deep. Core and thin section observations show that the reservoir can be divided into three structural layers: basal conglomerates, gritstones and gravel sandstones in the middle, and fine sandstones on the top. Carbonate cementation is the most typical diagenetic effect, which took place mainly early during deposition, so that clastic grains are usually floating or have point contacting. Accordingly, compaction and dissolution are weak. The oil storage ability of the top and the bottom layers is poor due to strong cementation and poor sorting respectively. The oil and water distribution of the middle reservoir is complex that oil/water inversion phenomenon is a serious concern. The studies of hydrocarbon accumulation show that the unconformities at the bottom of the Shawan Formation, the Hongche Fault Zone and the thick sandstones widespread in the study area provide good pathways for hydrocarbon migration. Influenced by paleo tectonics and hydrocarbon filling, large-scale calcium carbonate deposition took place in the north, northwest and west of the Chunfeng area during the early stage, which blocked later oil migration and resulted in the oil/water inversion phenomenon. The special diage-nesis and reservoir distribution made it clear that there is not only stratigraphic or lithologic reservoirs developed in the shallow region, but also a certain size of diagenetic traps developed under the influence of carbonate cementation. This special trap, different from the conventional dissolution diagenetic traps, provides for useful exploration potential in the northwestern margin of the Junggar Basin.