Abstract
In order to examine the pore structure and reveal the fractal geometric nature of shales, a series of laboratory experiments were conducted on lacustrine shale samples cored from the Kongdian Formation. Based on the low temperature nitrogen adsorption, fluorescent thin section and field emission scanning electronic microscope, a comprehensive pore structure classification and evaluation were conducted on shale samples. Fractal dimensions D1 and D2 (with relative pressure of 0–0.45 and 0.45–1.00, respectively) were obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) method. With additional means of X-ray diffraction analysis, total organic carbon content analysis and thermal maturity analysis, the relationships between pore structure parameters, fractal dimensions, TOC content and mineral composition are presented and discussed in this paper. The results show that interparticle pores and microfractures are predominant, whereas organic matter pores are rarely found. The pore morphology is primarily featured with wide-open ends and slit-shaped structures. In terms of pore scale, mesopores and macropores are predominant. The value of fractal dimension D1 representing small pores ranges from 2.0173 to 2.4642 with an average of 2.1735. The value of D2 which represents large pores ranges from 2.3616 to 2.5981 with an average of 2.4960. These low numbers are an indication of few pore types and relatively low heterogeneity. In addition, smaller D1 values reveal that large pores have more complicated spatial structures than smaller ones. The results of correlation analysis show that: 1) D2 is correlated positively with specific surface area but negatively with average pore diameter; 2) D1 and D2 literally show no obvious relationship with mineral composition, TOC content or vitrinite reflectance (Ro); 3) both total Barrett-Joyner-Halenda (BJH) volume and specific surface area show a positive relationship with dolomite content and a negative relationship with felsic minerals content. These results demonstrate that the pore types are relatively few and dominated by mesopores, and the content of brittle minerals such as dolomite and felsic minerals control the pore structure development whilst organic matter and clay minerals have less influence due to low thermal maturity and abundance of clay minerals.
Highlights
In response to rising petroleum costs at the turn of the 21st century, the exploration and exploitation of shale gas and shale oil have become more and more important in China (Lu et al, 2018; Wang et al, 2019)
The shale oil in United States is primarily light oil generated from widely-distributed and organic-rich marine shales with high maturity and low heterogeneity; while the majority of shale oil in China is relatively heavy oil generated from sparsely distributed and organic-rich lacustrine shale with low maturity and high heterogeneity (Lu et al, 2016)
The mineral composition of marine shale in the United States is dominated by biogenic quartz and carbonate, and the former consists of biogenic silica which is associated with organic matter enrichment
Summary
In response to rising petroleum costs at the turn of the 21st century, the exploration and exploitation of shale gas and shale oil have become more and more important in China (Lu et al, 2018; Wang et al, 2019). The United States, home to the Bakken formation, Eagle Ford Formation and Barnett Formation, is the world’s hotspot of shale oil exploration (Montgomery et al, 2005; Jarvie et al, 2007; Bustin et al, 2008; Chalmers et al, 2012). As their success story unfolds, shale oil has become one of the most attractive unconventional sources all over the world. The mineral composition of marine shale in the United States is dominated by biogenic quartz and carbonate, and the former consists of biogenic silica which is associated with organic matter enrichment. Detrital quartz is terrestrial, resulted from mechanical transportation in most cases, and usually shows no signs of organic origin (Nie et al, 2016)
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