The Jurassic petroleum system plays an increasingly important role in petroleum explorations in the southern Junggar Basin. To assess the hydrocarbon distribution and the accumulation history of the Jurassic reservoirs in this area, we carried out reservoir petrographic, fluorescence spectroscopic, and fluid inclusion petrographic analyses on the core and cutting samples from the QQ‐2 well in the Qigu Anticline. The oil accumulation has the partiality for high‐quality sandstone reservoirs that have the high porosity and permeability with values of 14–18% and (1–450) × 10−2 μm2, respectively, in the Middle Jurassic Toutunhe Formation (J2t) reservoirs. The pores are big with average diameters of 60–100 μm, and the pore throats are wide with values of 3.45–6.92 μm in the oil reservoirs. The intergranular pores survive thanks to the poor compaction and the poor cementation, and the dissolved and mould pores develop attributed to the strong dissolution in the oil reservoirs. Two episodes of oil charging and one episode of gas invasion were identified in the Middle Jurassic reservoirs in the southern Junggar Basin. The low‐maturity paleo‐oil charged the sandstone reservoirs at about 35 Ma when the paleo‐traps took shape during the Cretaceous–Eocene. A large number of faults and anticlinal traps was developed in the southern Junggar Basin at about 7 Ma, caused by the reactivation of the Tianshan Mountains in the late Cenozoic. Simultaneously, the leakage of the paleo‐oil, the accumulation of late oil, and the gas invasion all took place. Fault activities led to the leakage of paleo‐oil and helped the oil and gas accumulate in the Middle Jurassic reservoirs in the anticlinal trap shaping at about 7–8 Ma in the southern Junggar Basin.
The Silurian Kepingtage Formation of Tarim Basin is an important deep clastic reservoir. Exploration practice shows that the reservoirs are generally characterized by extremely low porosity and permeability, exhibiting strong mineral composition and physical property heterogeneities. The study of heterogeneity and its effect on oil charge is of great significance to the understanding of oil migration and accumulation in such reservoirs and thus is crucial to oil exploration. Therefore, based on identification and quantitative statistics of thin sections, SEM, and homogenization temperature measurements, this study was designed to examine the relationship between diagenetic evolution and oil emplacement in the Silurian Kepingtage Formation in S9 Block in Tarim Basin. The control of differential diagenesis and physical property evolution on oil charge process are discussed. It is found that the sandstones with fine grain size and high content of ductile grains (>20%) have experienced strong mechanical compaction, the porosity decreases rapidly in the early stage of burial. The densification process has been completed before the first period of oil charge so that no oil was found in the reservoir. In contrast, sandstones with relatively coarse grain size and low ductile grain content have better porosity and permeability properties, which are generally oil-bearing. This kind of sandstone generally experienced two periods of oil charge. Apart from moderate compaction, multiple diagenesis events were also observed, including quartz cementation, calcite cementation, and dissolution. Before the early period of oil charge, the diagenesis events are dominated by moderate compaction, chlorite cementation, and dissolution, and the reservoir property is still high. It seems that the time of late period oil charge is relatively long. With the growth of multiple types of authigenic minerals such as quartz, calcite, and illite after or sometimes synchronously occurred with the late period oil charge, the porosity gradually decreases.
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