Microstructure and pore systems of shallow-buried fluvial mudstone caprocks in Zhanhua depression, east China inferred from SEM and MICP
Microstructure and pore systems of shallow-buried fluvial mudstone caprocks in Zhanhua depression, east China inferred from SEM and MICP
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DOI：
10.1016/j.marpetgeo.2021.105189
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发表时间：
2021-10
影响因子：
4.2
通讯作者：
Yuyuan Li;M. Zha;Rong-cai Song;A. Aplin;L. Bowen;Xingmou Wang;Yunyin Zhang
中科院分区：
文献类型：
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作者：
Yuyuan Li;M. Zha;Rong-cai Song;A. Aplin;L. Bowen;Xingmou Wang;Yunyin Zhang
沾化凹陷浅层河流相泥岩是天然气成藏的重要盖层。四个样品被选择来代表细粒微相的范围，从粘土丰富的粉砂丰富的胶结，压汞毛细管压力（MICP）孔隙度仪结合扫描电子显微镜（SEM）定量表征微观结构和孔隙系统。还估计了允许毛细管突破并因此渗漏的孔隙系统的性质。从代表性单元面积（REAs）获得的SEM数据推断的孔隙面积在特定范围内遵循与本体样品MICP相似的幂律分布，表明面积大于103 nm 2的孔隙连接良好。在没有碳酸盐胶结物的样品中，粘土基质内的孔隙在粗粒、粉质样品中较大，并且在非粘土矿物的边缘处有更多的孔隙;这是由于大颗粒的力链承担更有效的应力。随着粉粒含量的增加，孔隙度增大，非粘土矿物间孔隙贡献率显著增大，粘土基质内孔隙贡献率减小。在更多的粘土丰富的样品，毛细管突破估计发生在孔隙与粘土基质;在粉质样品，突破将发生在较低的入口压力与较大的颗粒间孔隙。碳酸盐胶结物通过部分填充（a）粘土和非粘土矿物之间的界面和（B）大于106 nm 2的非粘土矿物区域之间的孔隙，在减少某些粉砂质样品的孔隙空间方面发挥着关键作用。通过填充较大的孔隙，在相对高的进入压力下，通过粘土基质中的孔隙发生碳酸盐胶结样品中的毛细管突破。然而，碳酸盐胶结物（通常小于20%）不足以使富粉砂泥岩成为有效的屏障。粘土含量是控制泥岩封闭能力的最关键因素。
Shallow-buried fluvial mudstones are of great significance as potential top seals for natural gas accumulations in the Zhanhua depression. Four samples were chosen to represent the range of fine-grained microfacies, from clay-rich to silt-rich to cemented, and mercury injection capillary pressure (MICP) porosimetry is combined with Scanning Electron Microscopy (SEM) to characterize microstructure and pore systems quantitatively. The nature of the pore systems that allow capillary breakthrough and thus leakage, were also estimated. Pore areas inferred from SEM data, obtained from representative elementary areas (REAs), follow a similar power law distribution to bulk sample MICP within a specific range, indicating that pores with areas larger than 103nm2are well connected. In samples without carbonate cement, pores within the clay matrix are larger in coarser-grained, siltier samples, and there are more pores at the edges of non-clay minerals; this results from force chains of large grains shouldering more effective stress. With increasing silt content, SEM-visible porosity increases and the contribution of pores between non-clay minerals grows significantly, while the contribution of pores within clay matrix reduces. In more clay-rich samples, capillary breakthrough is estimated to occur in pores associated with the clay matrix; in siltier samples, breakthrough will occur at lower entry pressures associated with larger interparticle pores. Carbonate cements play a key role in reducing pore space in some siltier samples by partially filling interparticle pores at (a) the interfaces between clay and non-clay minerals, and (b) pores between non-clay minerals areas larger than 106nm2. By filling larger pores, capillary breakthrough in carbonate-cemented samples occurs at relative high entry pressures through pores in the clay matrix. However, carbonate cements, generally less than 20%, are not sufficient to enable silt-rich mudstones to become effective barriers. Clay content is the most critical control on mudstone seal capacity.