Search

Purpose: Land subsidence has caused serious geological damage in many countries, including China. Soil pore number, size, shape, and pore size distribution affect soil deformation and thus land subsidence. Quantification of complex pore structures in different soil types and at various soil depths, however, remains cumbersome. The objective of this study was to quantify pore characteristics of three different soil types using computed tomography (CT) technique, investigate the effects of soil depth, examine the relationships between various parameters, and preliminarily determine the pore deformation pattern. Materials and methods: Undisturbed soil samples (70 mm diameter × 50 mm height) of clay, silt, and silty clay soils were collected from two 300-m-deep engineering geological boreholes on both sides of the Xi'an D7 ground fissure. One hundred CT images were acquired for each soil sample. Soil pore characteristics and morphological parameters, including porosity, macroporosity, coarse mesoporosity, microporosity, number of pores, number of macropores (diameter > 1000 μm), number of coarse mesopores (diameter 200–1000 μm), number of micropores (diameter < 200 μm), circularity, structure model index, degree of anisotropy, and the Euler number, were analyzed. Results and discussion: Micropores were generally regularly shaped and the main contributors to the total number of pores, whereas macropores were mostly elongated and contributed to total porosity. With increasing depth, the most noticeable changes appeared in silt soil and the least noticeable in clay soil. The alternate pore deformation pattern was significant in silt and silty clay soils. Conclusions: Soil depth significantly influenced all soil pore parameters, whereas soil type significantly affected the remaining parameters, except circularity and degree of anisotropy. The elongated macropores in silt and silty clay soils favor land subsidence in the Xi'an area. The present results can help optimize the groundwater exploitation layer, reducing soil compaction and land subsidence in this area. [ABSTRACT FROM AUTHOR]

In this paper, in a case study of Santanghu Basin in China, the morphological characteristics and size distribution of nanoscale pores in the volcanic rocks of the Haerjiawu Formation were investigated using the results of low temperature nitrogen adsorption experiments. This research showed that within the target layer, a large number of nanoscale, eroded pores showed an 'ink bottle' morphology with narrow pore mouths and wide bodies. The fractal dimension of pores increases gradually with increasing depth. Moreover, as fractal dimension increases, BET-specific surface area gradually increases, average pore diameter decreases and total pore volume gradually increases. The deeper burial of the Haerjiawu volcanic rocks in the Santanghu Basin leads to more intense erosion by organic acids derived from the basin's source rocks. Furthermore, the internal surface roughness of these corrosion pores results in poor connectivity. As stated above, the corrosion process is directly related to the organic acids generated by the source rock of the interbedded volcanic rocks. The deeper the reservoir, the more the organic acids being released from the source rock. However, due to the fact that the Haerjiawu volcanic rocks are tight reservoirs and have complicated pore-throat systems, while organic acids dissolve unstable minerals such as feldspars which improve the effective reservoir space; the dissolution of feldspars results in the formation of new minerals, which cannot be expelled from the tight reservoirs. They are instead precipitated in the fine pore throats, thereby reducing pore connectivity, while enhancing reservoir micro-preservation conditions. [ABSTRACT FROM AUTHOR]

The occurrence and mobility of oil obtained from shales are primarily controlled by the pore–fracture networks of the shales. To clarify the distributions and characteristics of the type, morphology, and size in lacustrine shale pores, shale samples obtained from the Shahejie Formation, Dongying Sag, Bohai Bay Basin in China were examined using argon ion milling and scanning electron microscopy. In this study, pores were mainly classified as interparticle pores, intraparticle pores, and fractures and further classified into 14 types, comprising intraparticle pores (7 types), interparticle pores (6 types), and fractures, based on the differences in their components. Interparticle pores have more complex morphologies than intraparticle pores and fractures. Complex shapes characterize the interparticle pores between quartz granules and at the edges of feldspar granules. Intraparticle pores mainly contribute to mesopores and then to macropores, while interparticle pores behave in the opposite manner. Micropores are primarily derived from the intraparticle pores present in clay aggregates and dissolution pores present in calcite or dolomite granules. However, macropores mainly come from the interparticle pores present within or at the edges of quartz granules; consequently, siliceous-rich shales may be the favorable shale oil reservoirs called interlayer shale oil. Moreover, organic-rich calcareous shales with numerous large dissolution pores can become optimal self-generation and self-storage shale oil reservoirs. This study gives insight into the evaluation of shale oil exploration and development in the Dongying Sag in the Bohai Bay Basin. • Fracture, 7 types of intra- and 6 types of interparticle pores were observed. • The morphology and size of each type of pore were well revealed. • Micropores mainly come from intraparticle pores in clay and carbonate minerals. • Macropores primarily come from interparticle pores associated with quartz granules. [ABSTRACT FROM AUTHOR]