AUTHOR=Song Zezhang , Zhao Junyi , Zhang Yuanyin , Yang Dailin , Wang Yunlong , Hu Li , Li Chenghai , Liu Wenlei 

TITLE=Bi-Fractal Characterization of the Pore Network of Tight Sandstone

JOURNAL=Frontiers in Earth Science

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2021.751944

DOI=10.3389/feart.2021.751944

ISSN=2296-6463

ABSTRACT=<p>Fluid seepage performance and accumulation in tight sandstone is a critical research topic for in-depth exploration and development, closely related to the heterogeneity of the pore network. The fractal characterization is one of the most compelling and direct ways for quantitative investigation of heterogeneity. However, only one kind of fractal is used in most studies, and the differences and relations between different fractal dimensions are rarely discussed. This paper chose one of the most representative tight sandstone formations in China, the second member of the Xujiahe Formation, as the research object. First, based on physical analysis and XRD analysis, we carried out a qualitative investigation on pore structure utilizing thin-section and scanning electron microscopy. Then, detailed pore structure parameters were obtained using high-pressure mercury intrusion (HPMI). Lastly, we combined two-dimensional fractal analysis on thin-section images and three-dimensional fractal analysis on HPMI data to characterize the pore network heterogeneity quantitatively. The Xu2 tight sandstone is mainly medium- to fine-grained lithic feldspathic sandstone or feldspathic lithic sandstone with low porosity and permeability. Also, the Xujiahe tight sandstone is mainly composed of quartz, feldspar, and clay. The pore types of Xu2 tight sandstones are primarily intergranular pores, micro-fractures, and intra- and intergranular dissolution pores. Moreover, most of the micro-fractures in gas-bearing formation are open-ended, while most are filled by clay minerals in the dry formation. The <italic>r</italic><sub><italic>50</italic></sub> (median pore radius) is the most sensitive parameter to seepage capability (permeability) and gas-bearing status. The 2D fractal dimension (<italic>D</italic><sub><italic>s</italic></sub>) of gas-bearing samples is significantly larger than that of dry samples, while the 3D fractal dimension (<italic>D</italic><sub><italic>1</italic></sub>, <italic>D</italic><sub><italic>2</italic></sub>) of gas-bearing samples is lower than that of dry samples. There is a strong negative correlation between <italic>D</italic><sub><italic>2</italic></sub> and gas-bearing status, permeability, quartz content, and <italic>r</italic><sub><italic>50</italic></sub>, but a positive correlation between <italic>D</italic><sub><italic>s</italic></sub> and these parameters. <italic>D</italic><sub><italic>2</italic></sub> represents the heterogeneity of pore space, while the <italic>D</italic><sub><italic>s</italic></sub> indicates the development of the pore network. Tectonic movements that generate micro-fractures and clay cementation that blocks the seepage channels are the two main controlling factors on fractal dimensions. Combining 2D and 3D fractal analysis could give a more in-depth investigation of pore structure.</p>