Microstructure Characterization of a Biogenic Shale Gas Formation—Insights from the Antrim Shale, Michigan Basin

Abstract

Biogenic gas shales, predominantly microbial in origin, form an important class of organicrich shale reservoirs with a significant economic potential. Yet large gaps remain in the understanding of their gas generation, storage, and transport mechanisms, as previous studies have been largely focused on mature thermogenic shale reservoirs. In this study, the pore structure of 18 Antrim Shale samples was characterized using gas adsorption (CO 2 and N 2 ). The results show that most of the Antrim Shale samples are rich in organic matter content (0.58 wt.% to 14.15 wt.%), with highest values found in the Lachine and Norwood members. Samples from the Paxton Member, characterized by lower organic content, have smaller micropore surface area and micropore volume but larger meso-macro pore surface area and volume. The deconvolution results of the pore size distribution from the N 2 adsorption indicate that all of the tested Antrim Shale samples have similar pore groups. Organic matter in the Antrim Shale hosts micro pores instead of meso-macro pores, while clay minerals host both micro and meso-macro pores. Mineralrelated pores play a primary role in the total porosity. The biogenic Antrim Shale, therefore, has different pore structures from other well-studied thermogenic gas shales worldwide. KEY WORDS: gas adsorption, Antrim Shale, biogenic shale, organic matter. INTRODUCTIONThe recent boom in hydrocarbon production from unconventional reservoirs has led to significant advances in understanding of the microstructure and petrophysical properties of organic-rich shales. However, most of the shale gas formations being developed and extensively studied in the United States are thermogenic systems, such as Bakken Shale (in the Williston Basin), Barnett Shale (in the Fort Worth Basin), Marcellus Shale (in the Appalachian Basin), and Woodford Shale (in the Arkoma Basin) (

Supercritical High-Pressure Methane Adsorption on the Lower Cambrian Shuijingtuo Shale in the Huangling Anticline Area, South China: Adsorption Behavior, Storage Characteristics, and Geological Implications

“…Low-pressure carbon dioxide (CO 2 ) and nitrogen (N 2 ) adsorption analyses are widely used to characterize micropore (<2 nm) and meso-/macropore (>2 nm) structures and were conducted on a Quantachrome Autosorb iQ3 at temperatures of 0 and −196 °C. − The pores are classified based on pore size according to the IUPAC . Shale samples were manually ground into particles with a size of 60–80 mesh (0.18–0.25 mm in diameter) and degassed at 110 °C for 12 h under vacuum before gas adsorption tests.…”

Proper Experimental Parameters in N2 Adsorption: The Effects of Data Points and Equilibrium Interval Time

“…Assessing pore structures of reservoir rocks is a key factor to better understand their storage capacity and flow capabilities . In this regard, the gas adsorption method, which measures adsorption quantity at varying pressure ranges, is a well-known tool for evaluating the pore structures particularly in shale plays such as the Longmaxi, , the Barnett, , the Permian Carynginia, the Bakken, and the Antrim . It is well understood that shale plays are typical unconventional reservoirs with low permeability, low porosity, and complicated pore networks.…”

“…1 In this regard, the gas adsorption method, which measures adsorption quantity at varying pressure ranges, is a well-known tool for evaluating the pore structures 2 particularly in shale plays such as the Longmaxi, 3,4 the Barnett, 5,6 the Permian Carynginia, 7 the Bakken, 8 and the Antrim. 9 It is well understood that shale plays are typical unconventional reservoirs with low permeability, low porosity, and complicated pore networks. Therefore, performing adsorption analysis to reveal their pore structures on intact samples would be very difficult due to slow diffusion and equilibration of gases in the sample.…”

Determination of Clay Bound Water in Shales from NMR Signals: The Fractal Theory

“…The N 2 adsorption in their study was aimed to derive and compare the pore structure information under different dehydration temperatures, thus making the determine of critical temperature possible. As it is well understood, the transverse relaxation time, T 2 , itself can be used to represent the pore structure. − Therefore, if we take full advantage of this feature of the NMR T 2 relaxation time and analyze it in detail, in conjunction with the information, it can provide us information about the CBW, we will be independent from running gas adsorption or other complementary pore structure experimental techniques.…”

“…Fractal dimensions can represent the complexity of the pore structures in shale samples. − Hence, if we can realize the relationship between the total number of pores and the pore sizes, the fractal dimension can be calculated based on the NMR data. Considering the conventional fractal dimension calculations from the NMR data conducted by many scholars, the entire pore size range can be described by two fractal dimensions: one under the short T 2 time range and the other one via the long T 2 time range. ,, Liu et al studied the fractal dimensions of the T 2 spectrum of the oil shale samples and found that the fractal dimensions of the T 2 spectrum have a negative correlation with BVI and T 2cutoff .…”