Shiyou shiyan dizhi (Jul 2023)
Characteristics of volcanic-sedimentary formations and developmental patterns of source and reservoir rocks in an island arc environment of Shibei Sag, Junggar Basin: taking the Carboniferous Jiangbasitao Formation as an example
Abstract
The Shibei Sag is located in the north of Luliang Uplift in the eastern Junggar Basin. There has been no breakthrough in the exploration of Carboniferous volcanic traps in the Shibei Sag in the past years. Recent drilling of well Zhunbei 6 reveals that the self-generating and self-preserving oil and gas reservoirs in the Carboniferous Jiangbasitao Formation have great exploration potential. However, there is scarce study on this type of reservoir. In order to study the characteristics of volcanic-sedimentary formations and developmental patterns of source and reservoir rocks in an island arc environment of Shibei Sag, the volcanic-sedimentary formations and source rock characteristics of the Jiangbasitao Formation were systematically studied based on previous study, drilling cores, seismic and test data, as well as the analysis of geochemical characteristics of source rocks, and the developmental pattern of volcanic, pyroclastic and source rocks under the control of island arc was established, with the aim of providing theory basis for oil and gas exploration in areas with the same sedimentary background. The results show that the basic volcanic rocks of the Jiangbasitao Formation in the Shibei Sag were formed in an island arc environment related to oceanic subduction. The volcanic formations are mainly composed of near-crater explosive facies, near-crater overflow facies and far-crater explosive facies. The sedimentary formations are dominated by fan delta facies tuffaceous glutenite and sandstone in the franking regions of the island arc and littoral neritic facies tuffaceous mudstone in the low-lying area of the far island arc. The tuffaceous mudstone of the Jiangbasitao Formation is a set of medium source rocks in the mature and high-maturity stages, which have a banding distribution along the northeast-southwest direction with a thickness of 100-500 m. During the sedimentary period of the Jiangbasitao Formation, the volcanic island arc erupted intermittently, and the tuff in the low-lying area of the far island arc and the littoral neritic facies tuffaceous mudstone were interbedded. The tuffaceous mudstones have a good source-reservoir configuration relationship with fan delta glutenite and island arc volcanic rocks. These two types of reservoirs are the key exploration directions in the next step.
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