Abstract: The Carboniferous volcanic rock oil and gas reservoirs have abundant reserves in Shixi area, Junggar Basin, showing good exploration and development potential. However, there is a lack of clear understanding of stratigraphic framework system of the Carboniferous volcanic rocks, and lithofacies characteristics of various rhythmic layers, which restricts the optimal selection of vertical exploration layers in this area and the implementation of horizontal wells. The cores, thin sections, logging, and seismic data are comprehensively studied to construct stratigraphic framework and analyze the vertical lithofacies distribution in the Carboniferous in Shixi area. The seismic random simulation inversion of sensitivity curves (GR and CNL) and fusion technology are used to predict the distribution of volcanic rock lithofacies. The study results indicate that based on the principle of “cycle–stage–rhythm–lithofacies”, volcanic rocks in the Carboniferous Batamayineishan Formation are divided into three cycles (C₂b₁, C₂b₂, C₂b₃), and the upper two cycles are subdivided into four stages and eight rhythmic layers. From stage A1 to stage C1, the lithology shows a transition from intermediate basic rock–intermediate rock–intermediate acidic rock. The lithofacies of the Carboniferous volcanic rocks is divided into two types, i.e., explosive facies and overflow facies. The explosive facies is subdivided into thermal debris flow subfacies and air-fall flow subfacies, while the overflow facies is subdivided into intermediate basic overflow facies, intermediate overflow facies, and acidic overflow facies based on rock types. Vertically, the volcanic rocks of eruptive and overflow facies are superimposed from bottom to top, with a larger thickness of overflow facies than the eruptive facies, and a weakly continuous and dispersed lens-shaped eruptive facies within the overflow facies. The overflow facies in B1 and B2 stages were widely developed, and volcanic activity was dominated by overflow. In C1 stage, the overflow facies weakened and explosive facies strengthened, with explosive facies dominant. On the plane, the overflow facies in each stage has a relatively large thickness near the fault, gradually thinning towards the interior of the structure. The physical property analysis shows that the predominant lithofacies for reservoir development includes explosive facies and intermediate overflow facies. The study of plane lithofacies distribution in the framework of volcanic rhythmic layers provides a geological basis for the high-efficiency development of the Carboniferous volcanic oil reservoirs and well implementation in Shixi area, and serves as practical exploration and development case of similar oil and gas reservoirs.

Key words: Junggar Basin, Shixi area, Carboniferous, volcanic rocks, stratigraphic framework, volcanic lithofacies distribution