Abstract: To determine the pore-throat structures and the lower limits of fluid-movable pore-throat radii in reservoirs are critical for oil and gas exploration. Based on the experimental results including cast thin section, low-temperature nitrogen adsorption, high-pressure mercury injection, transverse relaxation time (T₂) spectra of nuclear magnetic resonance, contact angle, and oil-water interfacial tension, the study determines pore-throat structures and the lower limits of fluid-movable pore-throat radii in various types of conventional and unconventional lithologic reservoirs of Lower Permian Fengcheng Formation in Mahu sag, Junggar Basin. The results show as follows. (1) In Fengcheng Formation of Mahu sag, the primary distribution range and peak value range of the throats and pore radii of various lithologic reservoirs shift to smaller sizes with the decreasing particle sizes of sediments. Meanwhile, throat and pore radius distribution transitions from multimodal to unimodal, and the pore connectivity gradually decreases. (2) As the pore-throat radii increase, the ratio of the cumulative movable fluid volume to the pore volume of Fengcheng Formation reservoir exhibits a stepwise combination from low-value plateau segment to ascending segment to high-value plateau segment. The reservoir samples with multimodal distributions of throats and pore radii, demonstrate multiple stepwise combinations from plateau segment to ascending segment to plateau segment, indicating that the pore structures are characterized by large pores/fractures or micro fractures coupled with small pores. Both the lower limits of fluid-movable and apparent-movable pore-throat radii decrease with the decreasing of sediment particle size. (3) The mechanical equilibrium analysis reveals that the oil displacement pressure in Fengcheng Formation reservoirs show three stages of slow increase, relatively rapid increase and rapid increase as the pore throat radius decreases. The lower limit of movable pore-throat radii of oil decreases with the decrease of reservoir particle size and the increase of temperature. These results are of important guiding significance in the evaluation of the available potential of conventional and unconventional oil and gas resources in Fengcheng Formation of Mahu sag.

Key words: Junggar Basin, Mahu sag, Fengcheng Formation, pore-throat structure, movable fluid

摘要： 储层孔喉结构和流体可动孔喉半径下限的确定是油气勘探领域的关键问题。基于铸体薄片、低温氮气吸附、高压压汞、核磁共振横向弛豫时间(T₂)谱、接触角和油水两相界面张力等实验分析结果,揭示了准噶尔盆地玛湖凹陷下二叠统风城组常规—非常规各类岩性储层的孔喉结构和流体可动孔喉半径下限。研究结果表明：①玛湖凹陷风城组各类岩性储层样品的喉道、孔隙半径的主要分布范围和峰值范围均随着沉积物颗粒粒径的减小而有序地向小尺寸偏移,喉道、孔隙半径由多峰分布向单峰过渡,孔隙的连通性逐渐变差。2风城组储层样品的累积流体可动体积与孔体积的比值随着孔喉半径的增加依次呈现出低值平稳段、上升段和高值平稳段的阶状组合;当储层样品的喉道、孔隙半径呈多峰分布时,具有多个平稳段、上升段和平稳段阶状组合,指示具有大孔缝-小孔或者微裂缝-小孔组合的孔隙结构特征;流体可动孔喉半径下限和流体明显可动孔喉半径均随沉积物颗粒粒径的减小而减小。3力学平衡分析揭示,风城组储层中原油的驱动压力随孔喉半径的减小而呈现出缓慢增加、较快增加和快速增加共3个阶段,原油可动孔喉半径下限随着储层颗粒粒径的减小而减小,随温度升高而减小。研究成果对玛湖凹陷风城组常规和非常规油气资源的可动用潜力评价具有重要指导作用。

关键词: 准噶尔盆地, 玛湖凹陷, 风城组, 孔喉结构, 可动流体