Abstract
To better understanding the evolutionary characteristics of pore structure in marine shale with high thermal maturity, a natural Longmaxi marine shale sample from south China with a high equivalent vitrinite reflectance value (Ro = 2.03%) was selected to conduct an anhydrous pyrolysis experiment (500–750 °C), and six artificial shale samples (pyrolysis products) spanning a maturity range from Ro = 2.47% to 4.87% were obtained. Experimental procedures included mercury intrusion, nitrogen adsorption, and carbon dioxide adsorption, and were used to characterize the pore structure. In addition, fractal theory was applied to analyze the heterogeneous pore structure. The results showed that this sample suite had large differences in macropore, mesopore, and micropore volume (PV), as well as specific surface area (SSA) and pore size distributions (PSD), at different temperatures. Micropore, mesopore, and macropore content increased, from being unheated to 600 °C, which caused the pore structure to become more complex. The content of small diameter pores (micropores and fine mesopores, <10 nm) decreased and pores with large diameters (large mesopores and macropores, >10 nm) slightly increased from 600 to 750 °C. Fractal analysis showed that larger pore sizes had more complicated pore structure in this stage. The variance in pore structure for samples during pyrolysis was related to the further transformation of organic matter and PSD rearrangement. According to the data in this study, two stages were proposed for the pore evolution for marine shale with high thermal maturity.
Highlights
Inspired by the success of the exploration and development of shale gas in North America [1], China began to study shale gas in 2005 and has made great breakthroughs in recent years, especially for marine shale gas in south China [2]
The Ro value of the initial sample M-0 was 2.03%, which corresponded to the high over-mature stage, and the total organic carbon (TOC) content was 3.84%, indicating that the shale had fair to good source potential [23]
The preset temperature and Ro values of the synthetic Longmaxi marine shale samples had a good linear relationship, and pore structure parameters changed during pyrolysis, indicating the temperature can represent maturity and promote the evolution of pore structure in the samples
Summary
Inspired by the success of the exploration and development of shale gas in North America [1], China began to study shale gas in 2005 and has made great breakthroughs in recent years, especially for marine shale gas in south China [2]. High thermal maturity is a major feature of marine shale in south China (vitrinite reflectance (Ro) values typically range between 2.0 and 4.5%), which is much higher than shale from. Mastalerz et al selected five natural shale samples that ranged in maturity from 0.35 to 1.41% to quantitatively characterize the evolutionary characteristics of pore structure based on mercury intrusion and gas adsorption, and proposed a model [15]. There were two shortcomings in their study: (1) Pore evolution of higher maturity was not addressed in their work and (2) the five natural shale samples had different Ro values, the quartz, clay, and TOC content were different. Pore evolution characteristics of shale in the Longmaxi Formation from south China were investigated by Liu et al [20], and the effects of TOC, thermal maturity, diagenesis and tectonism on shale porosity were discussed. The evolutionary characteristics of pores in marine shale with high thermal maturity were described
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