Nanoscale Pore Network Evolution of Xiamaling Marine Shale during Organic Matter Maturation by Hydrous Pyrolysis

A suite of hydrous pyrolysis experiments was conducted on low-maturity organic-rich shale samples (with a total organic carbon (TOC) content of 6.9 wt % and marine-derived type I kerogen) from Xiamaling Formation to investigate the pore network evolution across a maturation gradient. Scanning electron microscopy and low-pressure gas physisorption (CO2 and Ar) were applied to observe the pore morphology and quantify the pore structure. On the basis of the geochemical properties and yields of the pyrolysis products, organic matter (OM) thermal maturation includes the following four stages: bitumen generation (unheated to 350 degrees C), oil window (350-410 degrees C), oil cracking (410-480 degrees C), and wet gas cracking (480-550 degrees C). The nanoscale pore network evolution shows a good correspondence to stages of hydrocarbon generation. Overall, the total pore volume increased in the bitumen generation stage and the oil window, followed by a decrease in the oil cracking stage, but then again increased in the wet gas cracking stage, while the total surface area progressively increased after an obvious decrease in the bitumen generation stage. The dominant pores at the bitumen generation stage are associated with minerals. The presence of shrinkage OM pores and microfractures contributes to increased volumes of meso- (diameter range of 2-50 nm) and macropores (diameter > 50 nm), while the decrease in micropore (diameter < 2 nm) volume is mainly related to bitumen infilling. During the oil window, bubble-like OM pores are greatly developed, which contributes to an increase in the total pore volume. A lower amount of modified mineral pores with relic OM is observed. However, the high expulsion efficiency causes a limited decline in the pore volume due to bitumen infilling. During the oil cracking stage, modified mineral pores progressively increase. Transformation of large-size bubble-like OM pores to small-size spongy OM pores leads to an increased micropore volume, as well as decreased meso- and macropore volumes. During the wet gas cracking stage, a large abundance of spongy OM pores is developed in highly transformed OM, leading to progressive increases in pore volume. Overall, mineral-related pores decrease, while OM pores change from nondevelopment, shrinkage pores, bubble-like to spongy pores during thermal maturation. Furthermore, OM thermal maturation primarily impacts pores less than 20 nm in size, since pore structure parameters for those pores exhibit the most change after pyrolysis. The pore evolution model revealed in this study will provide an analog for that of the other marine shales.