Dissolution experiments and geological implications of tight sandstones in the Xujiahe Formation of Upper Triassic, Sichuan Basin
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摘要: 四川盆地上三叠统须家河组为致密砂岩储层，溶蚀孔隙发育，但其形成机理仍不十分明确。选取须家河组长石岩屑砂岩、岩屑长石砂岩和长石石英砂岩等3块样品，通过配制与地层有机酸组分相近的反应液，设计5种温压条件进行溶蚀模拟实验，得到以下4点认识：（1）随温压条件升高，反应溶液中K、Na离子浓度不断增加，Ca、Mg离子浓度基本保持不变，Al离子浓度明显降低；（2）反应产生少量石英和大量高岭石等新矿物，长石颗粒和碳酸盐胶结物溶蚀产生大量溶蚀孔隙，增加孔隙度，改善了孔隙结构；（3）相同温压条件下，长石岩屑砂岩比长石石英砂岩溶蚀率高，高温压条件下（180 ℃，53 MPa），砂岩样品的溶蚀率大幅增加；（4）溶蚀实验结果为重建须家河组储层埋藏、成岩、孔隙演化序列，预测有利储层的分布提供了依据，长石等在高温压条件下快速溶蚀是深层碎屑岩有效储层的成因机理之一。Abstract: The Upper Triassic Xujiahe Formation in the Sichuan Basin is a tight sandstone reservoir with developed dissolution pores, but its formation mechanism is still unclear. In this paper, three samples, including feldspathic lithic, lithic arkose and feldspathic quartz sandstones of the Xujiahe Formation, were taken as examples. Some reaction fluid similar to the organic acid component of the formation was prepared. Five kinds of temperature and pressure conditions were applied in the corrosion simulation experiments, and four understandings were obtained. Firstly, with the increase of temperature and pressure, the concentrations of K+ and Na+ ions in the reaction solution increased, the concentrations of Ca2+ and Mg2+ ions remained stable, but the concentration of Al3+ ions decreased significantly. Secondly, the reaction produced a small amount of quartz and a large number of kaolinite and other new minerals, and the dissolution of feldspar particles and carbonate cement produced a large number of dissolution pores, the porosity was increased and the pore structure was then improved. Thirdly, with the same temperature and pressure, the dissolution rate of feldspathic lithic sandstone was relatively higher than that of feldspathic quartz sandstone. With higher temperature and pressure (e.g. 180℃, 53 MPa), the dissolution rate of sandstone samples were significantly increased. Lastly, the results of dissolution experiments provided a reference for the reconstructing of burial sequence, diagenesis as well as pore evolution of the Xujiahe Formation and the distribution of favorable reservoirs can be predicted. The rapid dissolution of feldspar under higher temperature pressure is one of the genetic mechanisms of effective reservoir of deep clastic rock.
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图 2 溶蚀模拟实验装置示意
Figure 2. Schematic diagram of experimental device for dissolution simulation
图 3 不同温压条件下溶蚀模拟实验的实验参数的变化
Figure 3. Variation of experimental parameters of dissolution simulation under different temperatures and pressures
图 4 溶蚀模拟实验中溶蚀作用生成的新物质
Figure 4. New substances formed by dissolution in simulation experiments
图 5 溶蚀模拟实验样品反应前后扫描电镜照片对比
a, b.样品2，90 ℃、23 MPa条件下，长石颗粒明显被溶蚀前后对比；c, d.样品1，180 ℃、53 MPa条件下长石颗粒被溶蚀前后对比；e, f.样品3，90 ℃、23 MPa条件下长石颗粒明显被溶蚀前后对比；g, h.样品3，150 ℃、43 MPa条件下原裂缝溶蚀扩大、且产生新裂缝
Figure 5. Comparison of scanning electron microscope photos of samples before and after dissolution simulation experiment
图 6 四川盆地上三叠统须家河组储层埋藏、成岩、孔隙演化序列
Figure 6. Burial, diagenesis and pore evolution sequences of reservoirs in Upper Triassic Xujiahe Formation, Sichuan Basin
图 7 川西北上三叠统须家河组储层孔隙度与深度关系
Figure 7. Correlation between porosity and burial depth of reservoirs in Upper Triassic Xujiahe Formation, Sichuan Basin
表 1 溶蚀模拟实验的样品岩石学特征
Table 1. Petrological characteristics of samples for dissolution simulation
样号 井号 层位 井深/m 岩性 石英/% 长石/% 岩屑/% 孔隙度/% 渗透率/(10-3μm2) 1 平落2 须二段 3 491.0 粗粒长石石英砂岩 84 11 5 5.22 0.07 2 潼1 须二段 2 208.5 中粒长石岩屑砂岩 74 11 15 8.68 0.55 3 营21 须四段 2 306.0 中粒岩屑长石砂岩 70 16 14 4.23 0.56 
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