Structural Properties of Kerogens with Different Maturities

Abstract

The thermogenic transformation of kerogen into hydrocarbons accompanies the development of a pore network within the kerogen that serves as gas storage locations both in pore space and the surface area for adsorbed gas with source rocks. Therefore, the successful recovery of gas from these rocks depends on the accessible surface area, surface properties, and interconnectivity of the pore system. These parameters can be difficult to determine because of the nanoscale of the structures within source rocks. This study seeks to investigate the pore structure, surface heterogeneity, and composition of isolated kerogens with progressively increasing thermogenic maturities from source rocks at a middle-east reservoir. Prompt gamma-ray activation analysis (PGAA), nitrogen and methane volumetric gas sorption, and small-angle neutron scattering (SANS) are combined to explore the relationship between the chemical composition, pore structure, surface roughness, surface heterogeneity, and maturity. PGAA results indicate that more mature kerogens have lower hydrogen/carbon ratios. Nitrogen gas adsorption indicates that the pore volume and accessible specific surface area are higher for more mature kerogens. The methane isosteric heat at different methane uptakes in the kerogens is determined by methane isotherms and shows that approximately two types of binding sites are present in less mature kerogens while the binding sites are relatively homogeneous in the most mature kerogen. The hysteresis effects of the structure during the adsorption and desorption processes at different CD 4 gas pressures are studied. An extended generalized Porod's scattering law method (GPSLM) is further developed here to analyze kerogens with fractal surfaces. This extended GPSLM quantifies the surface heterogeneity of the kerogens with a fractal surface and shows that more mature kerogen is chemically more homogeneous, consistent with the results from methane isosteric heat. SANS analysis also suggests a pronounced surface roughness in the more mature kerogens. A microporous region circling around the nanopores, which contributes to high surface roughness and methane storage, is shown to develop with maturity.

Gas Adsorption Capacity of Type-II Kerogen at a Varying Burial Depth

“…22−24 Bazilevskaya et al 25 found that over 50% pores in shale are smaller than 20 nm in size. Chiang et al 26 revealed that kerogen with higher maturity has a larger pore volume and a larger contactable specific surface area. Garum et al 27 reconstructed a microscopic three-dimensional shale structure and found that the small and large pores form a highly connected pore network together.…”

“…The pore structures have been the focus in many studies. − Nitrogen adsorption, , mercury intrusion porosimetry, , and helium ion microscopy have been used to characterize the pore structure and connectivity of kerogen, revealing that kerogen is abundant of micro- (<2 nm), meso- (2–50 nm), and macro-pores (>50 nm). − Bazilevskaya et al found that over 50% pores in shale are smaller than 20 nm in size. Chiang et al revealed that kerogen with higher maturity has a larger pore volume and a larger contactable specific surface area. Garum et al reconstructed a microscopic three-dimensional shale structure and found that the small and large pores form a highly connected pore network together.…”

Technical Aspects of the Pore Structure in Shale Measured by Small-Angle and Ultrasmall-Angle Neutron Scattering: A Mini Review

“…Recently, Liu et al developed a method to quantify surface heterogeneity of porous materials. , This method also assumes that all of the pore are accessible to external fluids because it relies on contrast variation. By variation of the SLDs of the fluid, such as increasing the pressure of CD 4 , several scattering curves are obtained.…”

“…When the experimental data are fitted with IR (ρ f ), a surface-averaged SLD and a normalized mean square variation of the SLD (Δ H 2 ) are obtained. A smaller Δ H means that surface chemistry over the entire interface in the material is more homogeneous. , …”

“…A smaller Δ H means that surface chemistry over the entire interface in the material is more homogeneous. 65,66 4. CHALLENGES AND PERSPECTIVES SANS and USANS are considered as a special method as a result of the limited accessibility and required expertise in data interpretation.…”