Monitoring the changes in the microstructure and the elastic and transport properties of Eagle Ford marl during maturation

Abstract

Organic-rich marl is one of the best unconventional reservoirs because of its high calcite and low clay content leading to relatively high permeability and fracability. However, how its stiff pores and relatively high permeability affect the changes in its microstructure and elastic and transport properties during maturation remains a research interest. We have induced ex situ maturation of organic-rich marl core plugs by conducting confined pyrolysis in fine steps across the maturity windows from immature through the early-peak oil, late oil, wet gas, and finally, the dry gas window. This was performed under high and low confining pressures on different samples to investigate the role of confining pressure during maturation. After each pyrolysis, we monitored the changes in microstructure, porosity, velocity, permeability, and geochemical properties. The results indicate increasing porosity, decreasing velocity, and increasing permeability as the maturation progresses. The time-lapse scanning electron microcopy images reveal the progressive development of secondary organic porosity at the expense of kerogen volume. Most of the changes in the acoustic velocity and permeability occur in the late oil window and are concurrent with the generation of connected secondary organic porosity. The total organic carbon (TOC) and Rock-Eval results indicate that most of the generated hydrocarbons immediately exit the samples during pyrolysis so that the generation of microcracks from pore-pressure buildup is unlikely. Rather, secondary organic porosity is the main microstructural change, and the amount of depleted TOC can be used as a proxy to predict the increase in porosity and the changes in the velocity and permeability. Finally, confining pressure plays a minor role in the evolution of the elastic and transport properties of organic-rich marl.

Effect of Diagenesis on Geomechanical Properties of Organic‐Rich Calcareous Shale: A Multiscale Investigation

“…to identify a shale-invariant relationship. This may be due to the complex biological origins and amorphous nature of organic matter and the range of physical and chemical processes that can occur with burial, such as the development of secondary organic porosity (related to gasification) that could reduce the effective stiffness measured on the organic phase by AFM towards the gas window (Mathia et al, 2016;Suwannasri et al, 2018).…”

The Assessment of Organic Matter Young's Modulus Distribution With Depositional Environment and Maturity

“…Several studies have identified that organic matter develops secondary porosity with maturity, which can range to nanoscale intraparticle circular pores (Bernard et al, 2012) or interparticle elongate fissures (Allan et al, 2014(Allan et al, , 2016(Allan et al, , 2018. This secondary porosity accounts for the majority of porosity increase within shale samples with maturity and can comprise up to 6-7% volume (Suwannasri et al, 2018). The onset of this porosity increase occurs during the oil window, with a large proportion already generated by the wet gas window.…”

“…Kerogen undergoes both chemical and physical changes with maturity, becoming increasingly aromatic, losing a large proportion of its thiophenes, amines, and other nitrogen, sulphur, oxygen (NSO) compounds. This is coupled with kerogen becoming increasingly porous while also reducing in overall volume (Curtis et al, 2012; Okiongbo et al, 2005; Suwannasri et al, 2018). Pores within the kerogen are primarily nanometer intraparticle pores, with secondary porosity created where the kerogen volume has decreased (Bernard et al, 2012).…”

“…Pores within the kerogen are primarily nanometer intraparticle pores, with secondary porosity created where the kerogen volume has decreased (Bernard et al, 2012). Several studies have identified that organic matter develops secondary porosity with maturity, which can range to nanoscale intraparticle circular pores (Bernard et al, 2012) or interparticle elongate fissures (Allan et al, 2014, 2016, 2018). This secondary porosity accounts for the majority of porosity increase within shale samples with maturity and can comprise up to 6–7% volume (Suwannasri et al, 2018).…”

“…Effective elasticity is affected directly by porosity in studies using both acoustic wave velocity (Allan et al, 2014, 2016; Suwannasri et al, 2018) and nanoindentation, whereby derived organic matter Young's modulus decreases from 20 to 7–12 GPa (Zargari et al, 2016). However, in higher‐resolution studies using AFM, an increase in organic matter Young's modulus is observed with maturity (Emmanuel et al, 2016a; Goodarzi et al, 2017; Li et al, 2018).…”