Effect of Thermal Maturity on Pore Type and Size in Transitional Shale Reservoirs: An Example from the Upper Paleozoic Shanxi Formation, Ordos Basin, China

Abstract

The impacts of thermal maturity on the pore size, volume, and distribution in transitional shale are recognized. Combined with previous research results, Shanxi shale of the upper Paleozoic formation in the Ordos Basin, China, was selected to identify the evolution of shale pore size and types, and clarify the factors affecting the pore development. Samples of Shanxi shale from the northeastern Ordos Basin (NOB) were selected for experimental analysis, including total organic carbon (TOC) content, vitamin reflection (R o ), X-ray diffraction (XRD) analysis, argon ion polishing-scanning electron microscopy (AIP-SEM), energydispersive spectroscopy (EDS), and low-pressure carbon dioxide (CO 2 ) and nitrogen (N 2 ) adsorption analyses to illustrate the geochemistry of organic matter, mineral composition, pore morphology, size, and distribution. The data show that the Shanxi shale in the NOB is in the oil generation stage and has good conditions for gas generation. The interparticle (interP) and intraparticle (intraP) pores are related to mineral matrix rather than organic matter with a relatively high value of pore volume. In addition, combined with the characteristics of organic geochemistry and reservoir development of Shanxi shale in the southeastern Ordos Basin (SOB), the factors affecting shale reservoir development and the effect of thermal maturity on the evolution of the pore type and size can be explained. The presence of quartz inhibits the increase in pore volume in the low thermal maturity stage, but it is beneficial to pore preservation in the high thermal maturity stage, and different clay mineral components have different effects on pore development. Furthermore, the pore types of shale in the low maturity stage are mainly interP and intraP pores related to inorganic mineral, and organic pores are rarely developed. With the increase in thermal maturity, the number and volume of organic matter pores increase, but inorganic pores are still the main pore type. The effect of thermal maturity on the evolution of the pore type and size will enrich a basis of understanding of gas storage, transport mechanics, and fracturability that can be beneficial to the evaluation of shale reservoirs.

Pore Development Characteristics of Marine–Continental Transitional Shale of Benxi Formation from the Ordos Basin, China

“…Generally, when the TOC content of shale samples is higher than ∼5%, the meso- and macropore volumes of shale samples decrease with an increasing TOC content (panels b and c of Figure ). The decrease of meso- and macropore volumes with an increasing TOC content might be due to the infilling of organic matter in mineral pores and compaction . Shale samples with a high TOC content are more ductile and could be more easily compacted, reducing the pore volume.…”

“…The nanopore structure of shales is of great significance for the storage, enrichment, and transport of shale gas. , The characterization and controlling factors for the development of nanopores in transitional shale samples have been extensively studied and reported by previous studies. , Clays are the principal minerals in most transitional shales, followed by quartz, ,,− and the clay mineral content is one of the most important factors that influences the pore volume of transitional shale samples. ,,,,,,− For transitional shale samples in Shanxi Formation, Xue et al reported that compaction was the main factor that reduced the pore volume of shale samples with different burial depths . The clay mineral content of shale samples from Benxi Formation is much higher than that of Shanxi and Taiyuan Formation shales, which might be related to the sedimentary environments and diagenesis processes. , The effect of compaction on the pore volume of Benxi Formation shale samples might be more evident than that on the pore volume of Shanxi Formation shale, because clay mineral is more ductile than quartz, while the gas content of Benxi Formation shale samples is much higher than that of Shanxi Formation shale, which might make the effect of compaction on the pore volume less evident.…”

“…There is controversy about organic pore development in transitional shales. On the basis of field emission scanning electron microscopy (FE-SEM) observations, many studies have shown that organic pores in transitional shales (of type III organic matter) with thermal maturity up to 3.6% are frequently poorly developed and isolated. ,,, However, other studies have shown that abundant organic pores developed in migrated solid organic matter in transitional shale samples with type III organic matter at relatively high thermal maturity ( R o = 2.3–2.68%). , The organic matter in marine–continental transitional shales is composed of mainly type III kerogen, ,,,,,,, although some areas contain type II kerogen, of which the occurrence is related to the sedimentary environment. − For shale with high to overmaturity, it is difficult to identify the organic matter type based on T max versus hydrogen index (HI) and organic maceral composition. The arguments mentioned above about the organic pore development in transitional shales might be related with the differences in the organic matter composition of the shale samples.…”

Paleoenvironment, Geochemistry, and Pore Characteristics of the Postmature to Overmature Organic-Rich Devonian Shales in Guizhong Depression, Southwestern China

“…Contributors to Pore Development. Researchers have concluded that the pore structure of organic-rich shale is generally governed by diagenesis, organic matter, mineral components, thermal maturity, kerogen type, etc., depending on specific geological conditions [14,23,[53][54][55][56][57]. Shale lithofacies have important controls on porosity and pore structure due to different sedimentary environments and mineralogical variations [19,51,[58][59][60][61].…”

“…Organic matter-hosted pores have been identified as an important pore system in gas shales [9,[14][15][16][17][18]. Thermal maturity has been considered by previous studies as one of the critical controllers of organic pore growth [1,13,[19][20][21][22][23]. Researchers have tried to restore the hydrocarbon generation process and associated organic pore growth through pyrolysis, which can cover the weakness of measurements mentioned above in pore system characterization and prediction [24][25][26][27][28][29].…”