Observations and Implications of Organic Shale Microstructure in the Oil Window
M. Curtis
Organic shales have emerged as important reservoirs of hydrocarbons; however, they remain poorly understood compared to conventional reservoirs. Many of the important properties of shales such as porosity and permeability are controlled by their microstructures which exhibit significant variations between plays and even within a play. Nuclear magnetic resonance (NMR) spectroscopy and mercury injection capillary pressure (MICP) measurements indicate, without direct imaging, that a large population of pores and pore throats in organic shales are on the nanometer scale. Because such features are below the wavelength of visible light, electron microscopy has been utilized, confirming that such small pores exist. These nanoscale pores have been observed in the organic matter and associated with the inorganic grains, which has consequences for the wettability, structural integrity, and possibly the connectivity of the pore system. In addition, the physics of flow is expected to change in pores on the nanoscale due to fluid-surface interactions.
Initial work on the microstructure of shales focused primarily on the gas window. Due to economic pressure, the focus has now been shifted towards liquids-rich shales. It has been observed that thermal maturity and the generation of hydrocarbons appears linked to the formation of secondary nanoporosity in organic shales. While pores appear to dominate the organic matter in gas shales, shales in the oil window exhibit a more varied and less predictable microstructure. Initial imaging indicated that organic matter in the oil window lacks porosity. As more shales have been investigated, it has been observed that the microstructure of organic matter in oil window shales is complicated. Some samples exhibit no porosity in the organic matter while others have organic matter which is primarily porous. Some oil window shales have organic matter that is dominated by fractures with no porosity. Determining what gives rise to these differences in microstructure has important implications for storage and transport of hydrocarbons in liquids rich shale reservoirs.
AAPG Search and Discovery Article #90186 © AAPG Geoscience Technology Workshop, Hydrocarbon Charge Considerations in Liquid-Rich Unconventional Petroleum Systems, November 5, 2013, Vancouver, BC, Canada