Evolution characteristics and model of nanopore structure and adsorption capacity in organic-rich shale during artificial thermal maturation: A pyrolysis study of the Mesoproterozoic Xiamaling marine shale with type II kerogen from Zhangjiakou, Hebei, China

Abstract

Thermal maturity has a considerable impact on hydrocarbon generation, mineral conversion, nanopore structure, and adsorption capacity evolution of shale, but that impact on organic-rich marine shales containing type II kerogen has been rarely subjected to explicit and quantitative characterization. This study aims to obtain information regarding the effects of thermal maturation on organic matter, mineral content, pore structure, and adsorption capacity evolution of marine shale. Mesoproterozoic Xiamaling immaturity marine oil shale with type II kerogen in Zhangjiakou of Hebei, China, was chosen for anhydrous pyrolysis to simulate the maturation process. With increasing simulation temperature, hydrocarbon generation and mineral transformation promote the formation, development, and evolution of pores in the shale. The original and simulated samples consist of closed microspores and one-end closed pores of the slit throat, all-opened wedge-shaped capillaries, and fractured or lamellar pores, which are related to the plate particles of clay. The increase in maturity can promote the formation and development of pores in the shale. Heating can also promote the accumulation, formation, and development of pores, leading to a large pore volume and surface area. The temperature increase can promote the development of pore volume and surface area of 1–10 and 40-nm diameter pores. The formation and development of pore volume and surface area of 1–10 nm diameter pores are more substantial than that of 40-nm diameter pores. The pore structure evolution of the sample can be divided into pore adjustment (T < 350°C, EqRo < 0.86%), development (350°C < T < 650°C, 0.86% < EqRo < 3.28%), and conversion or destruction stages (T > 650°C, EqRo > 3.28%). Along with the increase in maturity, the methane adsorption content decreases in the initial simulation stage, increases in the middle simulation stage, and reaches the maximum value at 650°C, after which it gradually decreases. A general evolution model is proposed by combining the nanopore structure and the adsorption capacity evolution characteristics of the oil shale.

Pore structural characteristics and methane adsorption capacity of transitional shale of different depositional and burial processes: a case study of shale from Taiyuan formation of the Southern North China basin

“…This is in line with the comparative study on pore structure of shale before and after extraction of soluble organic matter by Wang et al (2020). Xu et al (2019a) carried out thermal simulation experiments on the pore structure evolution of typical type II kerogen source rocks, and the results revealed different evolution stages of pore adjustment, development, transformation and destruction (Fig. 10).…”

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

“…However, pore networks evolve as a result of the combined influences of diagenesis and organic matter (OM) thermal maturation. ,,− Specifically, mineral-related pores generally decrease or disappear with increasing burial depth as a result of diagenesis and liquid hydrocarbon infilling. , In contrast, the formation and evolution of OM pores are thought to be associated with petroleum generation and expulsion during thermal transformation of kerogen. , Investigating the nanoscale pore network evolution is critical for understanding the gas accumulation processes and assessing the gas storage potential. Scientists have previously conducted studies on the pore network evolution via geologically matured shales ,− ,, or artificially matured shales. − However, most of these studies only focused on the structure parameter evolution using mercury intrusion capillary pressure (MICP) and gas physisorption (CO 2 , Ar, and N 2 ) and did not involve pore morphology evolution. Furthermore, these studies mainly involved mature (vitrinite reflectance ( R o ) from ∼0.5% to ∼1.2%) and postmature ( R o from ∼1.2% to ∼2.0%) shales, which makes it impossible to understand the complete pore structure evolution from an immature stage ( R o < ∼0.5%) to an overmature stage ( R o > ∼2.0%).…”

Nanostructure Effect on Methane Adsorption Capacity of Shale with Type III Kerogen

“…Currently, the most successful shale gas development is mainly in marine facies shales, such as the Longmaxi formation shale in South China [8,21,30,34], the Marcellus shale [8,21,30,34,36,37], and the Barnett shale in the US [38]. The organic matter type of marine shales is type I or type II [39][40][41]. Additionally, research on shale gas reservoirs has mainly focused on organic shale with type I or type II kerogen [42][43][44].…”