Effect of the properties of Silurian shales from the Barrandian Basin on their methane sorption potential

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 °C), oil window (350–410 °C), oil cracking (410–480 °C), and wet gas cracking (480–550 °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.

Section: Introductionmentioning

confidence: 99%

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

Wang

Liu

et al. 2020

“…This indicates that slit-shaped pores are dominant. All the N 2 desorption branches persist down to very low pressures ( P / P 0 < 0.42), indicating that low-pressure hysteresis (LPH) is taking place rather than the “forced closure” that has been widely reported for N 2 isotherms from shales. ,,− The BET surface areas for the four samples tested range from 14.07 to 23.61 m 2 /g, the total pore volumes range from 20.246 to 31.292 cm 3 /kg, and the average pore diameters are between 4.995 and 5.755 nm. The pore average diameter is in the mesopore range.…”

Section: Resultsmentioning

confidence: 83%

High-Pressure Methane Adsorption and Desorption in Shales from the Sichuan Basin, Southwestern China

Mischo

2020

Adsorbed methane is an important component of shale gas. Methane desorption, commonly ignored, can affect gas production. Gaining a better understanding of methane sorption in shale is important for evaluating gas reserve and predicting gas production. This paper presents the results from a study of the methane adsorption and desorption of four shale samples from the Sichuan Basin, southwestern China. Adsorption experiments were performed on dry samples at temperatures of 40−100 °C to a maximum equilibrium pressure of 30 MPa. The desorption measurements were conducted at a temperature of 60 °C. Slight sorption hysteresis was observed over the entire pressure range for every sample. Both the three-parameter Langmuir model and the supercritical Dubinin−Radushkevich (sD-R) model were applied to describe the adsorption data. Model-fitted methane maximum absolute adsorption capacities (n 0 ) ranged from 1.83 to 3.04 mg/g at 40 °C, and n 0 increased linearly with increasing total organic carbon content. The decrease in n 0 with increasing temperature can also be represented by a linear model. The isosteric heat of adsorption ranged from 16.54 to 22.03 kJ/mol. A possible mechanism for sorption hysteresis is the irreversible swelling of micropore throats caused by methane adsorption. The swelling reduces the size of the pore throats and makes them smaller than the kinetic diameter of methane molecules so that when the pressure is reduced, more energy is required for the methane molecules in the pores to escape. Thus, fewer methane molecules can desorb from the material than were previously adsorbed at the same pressure.

“…The different varieties of clay minerals found in shales are a result of the sediment source and diagenetic history, , and clay minerals are mainly distinguished as stratigraphic markers and environmental indicators. − The depositional environment has a significant impact on clay mineralogy due to differential flocculation properties in different environments, and diagenetic/catagenetic mineral transformations also have a significant impact on clay mineral configuration . Clay mineral variety and abundance have a significant impact on gas sorption capacity. ,, Wang et al , documented organic matter (type, amount, and maturity), mineralogy, and petrophysical controls on sorption capacity to show that lacustrine facies shale samples have higher sorption capacities than fluviatile facies samples. This observation is significant for this study as the Barakar Formation was deposited via cyclic fluvial sedimentation. ,, Thus, the depositional facies of Auranga shales (Barakar Formation) may have a significant influence for its organic matter (type and amount) and mineralogy, which could affect sorption capacities.…”

Section: Resultsmentioning

confidence: 99%

“…116 Clay mineral variety and abundance have a significant impact on gas sorption capacity. 111,117,118 Wang et al 119,120 documented organic matter (type, amount, and maturity), mineralogy, and petrophysical controls on sorption capacity to show that lacustrine facies shale samples have higher sorption capacities than fluviatile facies samples. This observation is significant for this study as the Barakar Formation was deposited via cyclic fluvial sedimentation.…”

Section: Analytical Proceduresmentioning

confidence: 99%

Organo-Facies and Mineral Effects on Sorption Capacity of Low-Maturity Permian Barakar Shales from the Auranga Basin, Jharkhand, India

Mishra¹,

Varma

Mendhe

et al. 2021

Shales associated with the Lower Permian (Barakar Formation) sediments of the Auranga Coalfield, India, occur in the immature−early mature stage. The sorption capacity of Barakar shale samples has been studied through high-pressure methane (CH 4 ) adsorption and low-pressure N 2 gas adsorption (LPN 2 GA) methods, supported with proximate analyses, programmed pyrolysis, optical petrography, and with energy-dispersive spectroscopy, X-ray diffraction, and inductively coupled plasma mass spectrometry. The sorption capacity is a function of the organic and inorganic constituents present in the shale samples. The methane sorption capacity (MSC) and Langmuir volume of the shale samples vary from 0.217 to 0.314 and 0.315 to 0.429 mmol/g rock, respectively. The BET-calculated surface area of the studied shales varies from 8.12 to 30.36 m 2 /g. The sorption capacities show the importance of the total organic content (TOC) through weak but positive correlations with MSC (r 2 = 0.45) and S1 values (mg hydrocarbons/g rock from programmed pyrolysis; r 2 = 0.40). Moreover, apparent inverse relationships were observed between MSC and clay mineral abundances, suggesting that individual clay mineral types may influence MSC, although more work is needed. The TOC-normalized MSC (MSC*) of shale samples shows a positive trend with quartz plus clay mineral content and ash yield of r 2 = 0.64 for both. In addition, MSC* shows a negative logarithmic relationship with S1 + S2 (r 2 = 0.63) and a positive linear relationship with TOC-normalized total organic matter (TOM*) (r 2 = 0.88, when 5 low TOM* samples are excluded) indicating complex relationships possibly including bitumen retention in the sample pore spaces. The micropore study of the samples through LPN 2 GA, applying Dubinin−Radushkevich, Dubinin−Astakhov, and density functional theory models, shows the dominance of micro-mesopore concentrations in the shale matrix of ∼2 nm pore diameter. However, these pores might be present as blind or closed pores. The presence of thorium and zirconium is reflective of terrigenous detrital matter, i.e., moderately to strongly recycled sediments. The fluviatile facies of deposited shales in the Auranga Coalfield are noted by the significant presence of kaolinite (32.5−78.3%), which suggests the importance of its effect on the sorption capacity of proximal terrigenous shales.