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.
Highlights
Organic-rich shale is important as a source rock for conventional and unconventional oil and gas and as a reservoir for shale gas
The geological control factors of pore structure in gas shales include total organic carbon (TOC) content, thermal maturity, and mineralogy, which have been preliminarily discussed in previous works(Chalmers and Bustin, 2008; Ross and Bustin, 2009; Tian et al, 2013; Valenza et al, 2013)
The TOC value of the initial shale is 14.85%, and the TOC content of the solid residuals ranges between 14.05% and 14.42%
Summary
Organic-rich shale is important as a source rock for conventional and unconventional oil and gas and as a reservoir for shale gas. A gas shale reservoir is characterized by abundant pores, with sizes ranging from several to several hundreds of nanopores (Chalmers et al, 2012; Loucks et al, 2009, 2012). Elucidating the complex pore networks in gas shales has become a strategic subject because the shale pore structure is one of the most important factors controlling the gas adsorption capacity. The geological control factors of pore structure in gas shales include total organic carbon (TOC) content, thermal maturity, and mineralogy, which have been preliminarily discussed in previous works(Chalmers and Bustin, 2008; Ross and Bustin, 2009; Tian et al, 2013; Valenza et al, 2013)
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