Microscopic pore structure characteristics and mobility of shale oil reservoirs in Liushagang Formation, Weixinan Sag, Beibu Gulf Basin
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摘要: 页岩油储层致密且非均质性较强，微观孔隙结构影响着页岩油在储层中的储集与流动，但常规单一手段往往难以准确表征微观孔隙结构。为揭示页岩油储层的微观孔隙结构与可动性特征，指导后续海上页岩油的高效勘探开发，以北部湾盆地涠西南凹陷流沙港组基质型、纹层型和夹层型3种类型的页岩油储层为研究对象，综合运用铸体薄片、扫描电镜、高压压汞、氮气吸附、核磁共振等分析测试方法，对孔隙结构参数、压汞形态、吸附曲线特征等进行了分析。结果表明：基质型与纹层型储层粒度较细，孔隙相对不发育，孔隙形态多以平板狭缝形为主，发育层理缝、有机孔、黏土矿物片间孔、黄铁矿晶间孔等；夹层型储层孔隙形态多以墨水瓶形为主，发育矿物粒间孔、溶蚀孔、网状缝等，孔径分布及储层连通性较好。荧光薄片、核磁共振对页岩油可动性分析表明，基质型和纹层型储层可动性相对较差，可动孔隙度分别为0.72%和4.62%，可动油含量相对较低；夹层型储层可动孔隙度为6.37%，烃类组分更轻，可动性更好，可动油含量更高，是涠西南凹陷最大的页岩油勘探有利储层。Abstract: Shale oil reservoirs are characterized by tightness and strong heterogeneity, and the microscopic pore structures affect the storage and flow of shale oil in reservoirs. However, conventional single analytical methods often fail to accurately characterize these microscopic pore structures. This study aims to reveal the microscopic pore structure and mobility characteristics of shale oil reservoirs, thereby guiding efficient exploration and development of offshore shale oil. Three types of shale oil reservoirs—matrix-type, laminated-type and interbedded-type—in the Liushagang Formation, Weixinan Sag, Beibu Gulf Basin were selected as the research objects. Integrated analytical and testing methods were employed, including cast thin-sections, scanning electron microscopy, high-pressure mercury intrusion, nitrogen adsorption, and nuclear magnetic resonance, to analyze pore structure parameters, mercury intrusion morphology, and adsorption curve characteristics. The results showed that the matrix-type and laminated-type reservoirs exhibited finer grain sizes and relatively underdeveloped pores, dominated by slit-shaped pore morphologies. These reservoirs commonly featured bedding fractures, organic pores, interlayer pores within clay minerals, and intercrystalline pores within pyrite. The interbedded-type reservoirs mainly had ink-bottle-shaped pore morphologies, along with intergranular pores within mineral particles, dissolution pores, and fracture networks, showing good pore size distribution and reservoir connectivity. Analysis of shale oil mobility through fluorescence thin sections and nuclear magnetic resonance revealed that the matrix-type and laminated-type reservoirs exhibited relatively poorer mobility, with movable porosities of 0.72% and 4.62%, respectively, along with lower movable oil content. The interbedded-type reservoir exhibited a movable porosity of 6.37%, with lighter hydrocarbon components, better mobility, and higher movable oil content, making it the most favorable reservoir type for shale oil exploration in the Weixinan Sag.
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Key words:
- reservoir pore structure /
- shale oil mobility /
- Liushagang Formation /
- Paleogene /
- Weixinan Sag /
- Beibu Gulf Basin
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图 2 北部湾盆地涠西南凹陷流沙港组三段上层序(a)和流沙港组二段下层序(b)沉积相
Figure 2. Sedimentary facies of upper sequence in the third member (a) and lower sequence in the second member (b) of Liushagang Formation, Weixinan Sag, Beibu Gulf Basin
图 3 北部湾盆地涠西南凹陷不同类型页岩油储层XRD全岩衍射分析
Figure 3. Whole-rock XRD analysis of different shale oil reservoir types in Weixinan Sag, Beibu Gulf Basin
图 4 北部湾盆地涠西南凹陷不同类型页岩油储层孔隙度与渗透率关系
Figure 4. Relationship between porosity and permeability of different shale oil reservoir types in Weixinan Sag, Beibu Gulf Basin
图 5 北部湾盆地涠西南凹陷流沙港组不同类型页岩油储层铸体薄片与扫描电镜照片
a.WY-X井，2 959 m，基质型铸体薄片，微裂缝；b.WY-X井，2 959 m，基质型铸体薄片，团状黄铁矿顺层发育；c.W12-Z井，3 161.1 m，基质型扫描电镜，黄铁矿晶间孔、黄铁矿伴生有机孔；d.WY-1井，3 164.9 m，基质型扫描电镜，黏土矿物顺层微裂缝；e.WY-X井，3 016.3 m，纹层型铸体薄片，微裂缝；f.WY-X井，2 998.3 m，纹层型铸体薄片，见铁方解石；g.WY-X井，3 556.9 m，纹层型扫描电镜，矿物粒间孔；h.WY-X井，3 556.9 m，纹层型扫描电镜，方解石与微晶石英；i.WY-Z井，3 496.52 m，夹层型铸体薄片网状缝；j.WY-X井，3 059.8 m，夹层型铸体薄片，溶蚀孔、铸模孔；k.WY-X井，3 126.8 m，夹层型扫描电镜，见微裂缝、碎屑矿物溶蚀；l.WY-X井，3 233 m，夹层型，碎屑矿物溶蚀。
Figure 5. Cast thin-sections and scanning electron microscopy images of different shale oil reservoir types in Liushagang Formation, Weixinan Sag, Beibu Gulf Basin
图 6 北部湾盆地涠西南凹陷不同类型储层毛管压力与汞饱和度关系(a)以及孔隙半径分布(b)
Figure 6. Relationship between capillary pressure and mercury saturation (a), and pore radius distribution (b) of different reservoir types in Weixinan Sag, Beibu Gulf Basin
图 7 北部湾盆地涠西南凹陷不同类型页岩油储层氮气吸附特征曲线
Figure 7. Nitrogen adsorption characteristic curves of different shale oil reservoir types in Weixinan Sag, Beibu Gulf Basin
图 8 北部湾盆地涠西南凹陷流沙港组不同类型页岩油储层荧光薄片尺度下的发育特征
a.W12-Z井，3 164.9 m，基质型，层理缝；b.W12-Z井，3 159.7 m，基质型，层理缝；c.W12-Z井，3 159.7 m，基质型，网状缝；d.WY-X井，3 561.7 m，纹层型，层理缝；e.WY-X井，3 560.25 m，纹层型，网状缝；f.WY-X井，3 556.9 m，纹层型，层理缝；g.WY-Z井，3 496.52 m井，夹层型，网状缝；h.WY-Z井，3 397.5 m，夹层型，高角度构造缝；i.WY-Z井，3 413.69 m井，层理缝与网状缝。
Figure 8. Development characteristics of different shale oil reservoir types at fluorescence thin-section scale in Liushagang Formation, Weixinan Sag, Beibu Gulf Basin
图 9 北部湾盆地涠西南凹陷流沙港组不同类型页岩油储层核磁共振T2谱分布
a.WY-1井，2 973.4 m，基质型；b.WY-Y井，2 896.5 m，纹层型；c.WY-1井，3 254 m，夹层型。
Figure 9. Nuclear magnetic resonance T2 spectrum distributions of different shale oil reservoir types in Liushagang Formation, Weixinan Sag, Beibu Gulf Basin
图 10 北部湾盆地涠西南凹陷页岩油储层含油饱和度指数(a)、孔隙度与可动流体饱和度分布(b)
Figure 10. Oil saturation index (a) and distribution of porosity and movable fluid saturation (b) in shale oil reservoirs of Weixinan Sag, Beibu Gulf Basin
表 1 北部湾盆地涠西南凹陷不同类型页岩油储层孔隙结构及可动性发育特征
Table 1. Pore structure and mobility development characteristics of different shale oil reservoir types in Weixinan Sag, Beibu Gulf Basin
类型 主要孔缝类型 孔隙度/% 渗透率/10-3 μm2 孔喉连通性 回滞环形态 ω(TOC)/% S1/(mg/g) 荧光薄片 可动孔隙度/% OSI/(mg/g) 基质型 矿物颗粒粒间孔、层理缝、黏土矿物层间缝、黄铁矿晶间孔、有机孔 $\frac{0.49 \sim 4.75}{2.68}$ $\frac{0.022 \sim 1.700}{0.25}$ 较差 H3型 $\frac{2.71 \sim 7.49}{5.19}$ $\frac{5.92 \sim 8.96}{7.51}$ 淡蓝色 0.72 $\frac{86.60 \sim 297.04}{154}$ 纹层型 层理缝、黄铁矿晶间孔、矿物颗粒粒间孔、碳酸盐胶结孔 $\frac{1.70 \sim 16.66}{7.47}$ $\frac{0.011 \sim 2.750}{0.36}$ 中等 H3型 $\frac{2.55 \sim 6.31}{4.67}$ $\frac{4.61 \sim 9.49}{7.12}$ 淡蓝色 4.62 $\frac{97.57 \sim 200.28}{142}$ 夹层型 杂基微孔、黏土矿物晶间孔、铸模孔、网状裂缝 $\frac{0.21 \sim 20.79}{14.61}$ $\frac{0.080 \sim 36.000}{20.79}$ 较好 H2型 $\frac{1.26 \sim 4.50}{2.40}$ $\frac{4.08 \sim 4.48}{4.34}$ 蓝色 6.37 $\frac{99.56 \sim 323.81}{243}$ 注：表中分式意义为$\frac{\text { 最小值~最大值}}{\text { 平均值}}$。 
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