Two kinds of natural hydraulic fracturing were found in laminated argillaceous dolomite and homogeneous argillaceous dolomite of the Upper Xiaganchaigou Formation in the Shizigou Structure in the southwestern Qaidam Basin (SW Qaidam Basin). To explore the geometric characteristics of networks of natural hydraulic fracturing, the temperature-salinity conditions when hydraulic fracturing occured, the interaction between fluid in fractures and surrounding rocks, and controlling factors of natural hydraulic fracturing under the background of multiple overpressure factors, the two kinds of multilevel fracture networks and inclusions in veins were described at the core scale and thin section scale under single polarized light, orthogonal light and cathodoluminescence. Types, distribution, and contents of minerals in veins and surrounding rocks were further characterized by TESCAN integrated mineral analyzer (TIMA). In addition, the homogenization temperature and salinity of aqueous inclusions in veins were tested. Results show that natural hydraulic fracturing in laminated argillaceous dolomite and homogeneous argillaceous dolomite has similarities and differences. Regarding similarity, both fracture networks consist of three-level veins, including the main vein, the first-level hydraulic veins, and the second-level hydraulic veins, showing orthogonal nets in cores. In addition, anhydrite is the main filling mineral in veins. The differences are as follows. Firstly, the first-level hydraulic veins develop along the lamina directions that intersect the main vein with an angle in laminated argillaceous dolomite, while the first-level hydraulic veins are perpendicular to the main vein in homogeneous argillaceous dolomite. The thicker vein inhibits the development of thinner veins which are nearly parallel to the thicker one. Moreover, thinner first-level and second-level hydraulic veins in homogeneous argillaceous dolomite are mostly filled by calcite and celestite. Secondly, only low-mature heavy oil inclusions develop in the main vein in laminated argillaceous dolomite, while in homogeneous argillaceous dolomite, only aqueous inclusions develop in the main vein, and heavy oil traces exist in intergranular cleavage and secondary fissures. The average homogenization temperature of aqueous inclusions (215℃) is greater than the paleogeothermal range (35–150℃) controlled by the burial depth of the sample. Considering multiple overpressure backgrounds, the tectonic evolution history, thermal history, burial history, and hydrocarbon accumulation history, we propose that natural hydraulic fracturing in the laminated argillaceous dolomite may be attributed to the superposition of tectonic compression, gypsum-anhydrite transformation, and hydrocarbon generation. Hydraulic fracturing in homogeneous argillaceous dolomite is attributed to overpressure caused by upwelling of deep thermal fluid. During the reaction between hydrothermal solution and surrounding dolomite, celestite in thin veins is beneficial to enrichment by extracting Sr2+ from surrounding dolomite. Formation process and geometrical patterns of two kinds of hydraulic fracturing have enlightening significance for enrichment of celestite deposits and the current artificial hydraulic fracturing of shale oil reservoirs in the upper Xiaganchaigou Formation in the SW Qaidam Basin.

在柴达木盆地西南部狮子沟构造下干柴沟组上段纹层状泥质白云岩和均质泥质白云岩中发现天然流体压裂现象。为探究两类天然流体压裂缝网络的几何学特征、超压破裂发生时的温度–盐度条件、缝内流体与围岩作用过程以及多种超压因素背景下天然流体压裂的控制因素, 在岩芯尺度和单偏光–正交偏光–阴极发光下薄片尺度下, 对两类多级裂缝网络和脉体包裹体进行描述, 运用矿物分析仪(TIMA)对脉体及围岩的矿物类型、含量和分布进行表征, 测试脉体盐水包裹体的均一温度和盐度。结果表明, 纹层状和均质泥质白云岩中的天然流体压裂缝具有相似性和差异性。相似性在于压裂缝网络均由主脉、一级压裂脉和二级压裂脉这3级脉体组成, 呈现“丰”字型特征, 硬石膏是主要脉体矿物类型。差异性表现在两方面: 1) 纹层状泥质白云岩中一级压裂脉沿与主脉呈锐角斜交的陆源碎屑纹层方向延伸, 均质泥质白云岩中一级压裂脉沿近似垂直于主脉的方向扩展, 一级粗脉抑制周围平行脉体的发育, 方解石和天青石充填一级和二级压裂细脉; 2) 纹层状泥质白云岩主脉中发育低成熟度原生重质油包裹体, 均质泥质白云岩主脉中仅发育原生盐水包裹体, 晶间解理和次生裂隙中存在重质油油迹, 盐水包裹体平均均一温度(215℃)大于样品埋藏深度控制的地温范围(35~150℃)。结合构造演化史、热演化史、埋藏史和油气成藏史, 提出在多种超压因素背景下, 纹层状泥质白云岩中天然流体压裂受控于构造挤压、石膏–硬石膏转化脱水以及生烃增压等多种超压因素的叠加; 均质泥质白云岩中天然流体压裂主要由下部热流体上涌产生的超压导致, 在热液与白云岩围岩作用过程中有利于萃取细脉中的Sr²⁺而富集天青石。天然流体压裂的形成过程和发育样式对现今柴达木盆地西南部天青石矿的富集和下干柴沟组上段页岩油储层的人工水力压裂具有启示意义。