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.