Microstructure and pore systems of shallow-buried fluvial mudstone caprocks in Zhanhua Depression, east China inferred from SEM and MICP

Abstract

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 103 nm2 are 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 106 nm2. 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.