Mudrock Pore Systems: Ten Years of SEM-Based Research
Dr. Robert M. Reed
Bureau of Economic Geology
Researchers at the Bureau of Economic Geology were pioneers in the use of Ar-ion-milling to prepare mudrock samples for pore analysis using the scanning electron microscope (SEM). This milling technique has lead to a revolution in the understanding of mudrock pore systems in the past ten years.
Pores in mudrocks can be divided into three categories: interparticle pores, intraparticle pores, and organic matter pores. Interparticle pores and intraparticle pores in mudrocks are similar in many ways to pores in carbonates and sandstones, but at a much smaller size. Interparticle pores are common in mudrocks that have only undergone shallow burial and thus have had only limited compaction and cementation. Intraparticle pores are diverse, and in some cases widely developed, in mudrocks.
One of the main discoveries of our research was that pores are quite commonly hosted in the organic matter (OM) present in many mudrocks. These OM pores are the dominant pore type in many organic-rich mudrocks. OM pores can be subdivided into at least four categories. First, pores in some kerogen are inherited from the source OM. Research suggests these may be more common in terrestrial OM than marine OM. Second, a few of the µm-scale pores seem to be the result of bitumen only partially filling interparticle or intraparticle pores in the mudrock. Third, the majority of OM pores in thermally mature (<0.75 Ro) rocks seem to have formed during thermal maturation of OM. A fourth category of OM pores are rare in most units, but are related to spaces between spheroidal subparticles of OM within larger OM grains. This last pore type is present even at low thermal maturities and has several possible origins.
Although significant breakthroughs have been made, much research remains to be done. Determining whether OM pores are present in bitumen or kerogen or both remains controversial. Also of some contention is the role of naturally occurring microfractures (if any) in mudrock porosity and permeability. Other current research topics include: quantification of mudrock porosity, upscaling of results from the microscopic to the core scale, and heterogeneous development of OM pores.