The exploration of unconventional oil and gas, especially the exploration process of tight oil, is closely related to the evolution of tight reservoirs and the accumulation process. In order to investigate the densification and accumulation process of the Fuyu tight oil reservoir in the Sanzhao depression, Songliao Basin, through the new understanding of reservoir petrological characteristics, diagenesis and diagenetic sequence are combined with a large number of inclusions: temperature measurement, spectral energy measurement, and single-well burial history analysis, and then contrastive analysis with current reservoir conditions. The results prove that diagenesis is dominated by compaction and cementation, and the restoration of paleoporosity shows that its porosity reduction rate reached 67% and the densification process started in the early Nenjiang Formation and was finalized at the end of the Nenjiang Formation. The accumulation of the Fuyu oil layer generally has the characteristics of two stages and multiple episodes, and the main accumulation period is the end of the Mingshui Formation. The end of the Nenjiang Formation, where the main body of the reservoir is densified, is just a prelude to the massive expulsion of hydrocarbons in the Songliao Basin, which makes the Fuyu oil layer have the characteristics of first compacting and then accumulating. Through the above analysis, it can be seen that the accumulation of oil and gas in the Fuyu oil layer, Sanzhao depression, is more dependent on the fault-dominated transport system. In addition, it is believed that tight oil accumulation should have the characteristics of short-distance oil enrichment around the fault, and the development area of fracture deserts near the fault sand body should be the key area for further exploration.
Complex superimposed basins exhibit multi‐stage tectonic events and multi‐stage reservoir formation; hydrocarbon reservoirs formed in the early stage have generally late‐stage genesis characteristics after undergoing adjustment, reconstruction and destruction of later‐stage multiple tectonic events. In this paper, this phenomenon is called the late‐stage reservoir formation effect. The late‐stage reservoir formation effect is a basic feature of oil and gas‐forming reservoirs in complex superimposed basins, revealing not only multi‐stage character, relevance and complexity of oil and gas‐forming reservoirs in superimposed basins but also the importance of late‐stage reservoir formation. Late‐stage reservoir formation is not a basic feature of oil and gas forming reservoir in superimposed basins. Multi‐stage reservoir formation only characterizes one aspect of oil and gas‐forming reservoir in superimposed basins and does not represent fully the complexity of oil and gas‐forming reservoir in superimposed basins. We suggest using “late‐stage reservoir formation effect” to replace the “late‐stage reservoir formation” concept to guide the exploration of complex reservoirs in superimposed basins. Under current geologic conditions, the late‐stage reservoir formation effect is represented mainly by four basic forms: phase transformation, scale reconstruction, component variation and trap adjustment. The late‐stage reservoir formation effect is produced by two kinds of geologic processes: first, the oil and gas retention function of various geologic thresholds (hydrocarbon expulsion threshold, hydrocarbon migration threshold, and hydrocarbon accumulating threshold) causes the actual time of oil and gas reservoir formation to be later than the time of generation of large amounts of hydrocarbon in a conventional sense, producing the late‐stage reservoir formation effect; second, multiple types of tectonic events (continuously strong reconstruction, early‐stage strong reconstruction, middle‐stage strong reconstruction, late‐stage strong reconstruction and long‐term stable sedimentation) after oil and gas reservoir formation lead to adjustment, reconstruction and destruction of reservoirs formed earlier, and form new secondary hydrocarbon reservoirs due to the late‐stage reservoir formation effect.
Exploration practices show that the Jurassic System in the hinterland region of the Junggar Basin has a low degree of exploration but huge potential, however the oil/gas accumulation rule is very complicated, and it is difficult to predict hydrocarbon‐bearing properties. The research indicates that the oil and gas is controlled by structure fades belt and sedimentary system distribution macroscopically, and hydrocarbon‐bearing properties of sand bodies are controlled by lithofacies and petrophysical facies microscopically. Controlled by ancient and current tectonic frameworks, most of the discovered oil and gas are distributed in the delta front sedimentary system of a palaeo‐tectonic belt and an ancient slope belt. Subaqueous branch channels and estuary dams mainly with medium and fine sandstone are the main reservoirs and oil production layers, and sand bodies of high porosity and high permeability have good hydrocarbon‐bearing properties; the facies controlling effect shows a reservoir controlling geologic model of relatively high porosity and permeability. The hydrocarbon distribution is also controlled by relatively low potential energy at the high points of local structure macroscopically, while most of the successful wells are distributed at the high points of local structure, and the hydrocarbon‐bearing property is good at the place of relatively low potential energy; the hydrocarbon distribution is in close connection with faults, and the reservoirs near the fault in the region of relatively low pressure have good oil and gas shows; the distribution of lithologic reservoirs at the depression slope is controlled by the distribution of sand bodies at positions of relatively high porosity and permeability. The formation of the reservoir of the Jurassic in the Junggar Basin shows characteristics of favorable facies and low‐potential coupling control, and among the currently discovered reservoirs and industrial hydrocarbon production wells, more than 90% are developed within the scope of facies‐potential index FPI>0.5, while the FPI and oil saturation of the discovered reservoir and unascertained traps have relatively good linear correlation. By establishing the relation model between hydrocarbon‐bearing properties of traps and FPI, totally 43 favorable targets are predicted in four main target series of strata and mainly distributed in the Badaowan Formation and the Sangonghe Formation, and the most favorable targets include the north and east of the Shinan Sag, the middle and south of the Mobei Uplift, Cai‐35 well area of the Cainan Oilfield, and North‐74 well area of the Zhangbei fault‐fold zone.
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