Lithofacies assemblages and differential productivity of volcanic rocks in the Jiamuhe Formation, Jinlong oilfield, Junggar Basin

Volcanic rocks in the Permian Jiamuhe Formation of the Jinlong oilfield in the Junggar Basin exhibit unclear lithofacies types and dominant reservoir distribution, which hinder the exploitation of the oil and gas resources therein. To address these challenges, we systematically analyze the volcanic facies types, reservoir physical properties, and storage space characteristics in the study area using data from core observations, log analysis, and laboratory tests. Accordingly, we establish lithofacies assemblage models, while clarifying their controlling effects on productivity. The results indicate that the volcanic rocks in the Jinlong oilfield in the Junggar Basin can be categorized into three facies:explosive, overflow, and volcanic sedimentary facies. The explosive facies, among others, consists primarily of welded volcaniclastic rocks and andesitic volcanic breccias, and exhibits an average porosity exceeding 10 %, forming dominant reservoirs. While the overflow facies is dominated by lavas, featuring an average porosity of below 6 %, and the volcanic sedimentary facies exhibits poor physical properties. In the volcanic reservoirs in the study area, vesicles, intraamygdale pores, and dissolution pores predominate, with a minor presence of primary intergranular pores. Additionally,dissolution vugs are most developed at the top of the explosive and overflow facies. Fractures in the volcanic reservoirs are dominated by structural and dissolution fractures, with developmental degrees closely related to their distance from faults. The intermediate-acidic overflow facies mainly exhibits oblique and reticulate fractures, while the mafic overflow facies is dominated by high-angle, straight-split fractures. The study area exhibits four lithofacies assemblages: interbedded intermediate-acidic pyroclastic flow subfacies and intermediate-acidic overflow subfacies, frequently interbedded mafic pyroclastic flow subfacies and mafic overflow subfacies, interbedded neutral air-fall subfacies and neutral overflow subfacies, and interbedded neutral air-fall subfacies and intermediate-acidic pyroclastic flow subfacies. The interbedded neutral air-fall subfacies and intermediate-acidic pyroclastic flow subfacies exhibits the highest productivity, followed by interbedded intermediate-acidic pyroclastic flow subfacies and intermediate-acidic overflow subfacies and interbedded neutral air-fall subfacies and neutral overflow subfacies, and with the frequently interbedded mafic pyroclastic flow subfacies and mafic overflow subfacies coming at last in daily production. The productivity of the volcanic reservoirs is governed most significantly by the effective reservoir thickness and oil saturation, followed by porosity and formation pressure.