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Pores Observed in Organic Matter in Mudrocks: A Ten-Year Retrospective

Abstract

Nanometer- to micrometer-scale pores in organic matter (OM) contribute significantly to the total porosity of many mudrocks. Research into the development, distribution, and origins of these pores has been ongoing for more than ten years. Although much has been learned, much remains unknown or in dispute. OM pores can be divided 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. Evidence shows the third category of OM pores form when the OM reaches a threshold thermal maturity, apparently around 0.75% Ro. Above that threshold, OM pores seem to develop rapidly over a narrow thermal window. But with further heating towards the gas window, there does not seem to be a steady increase in pore number or percentage. There is debate over whether thermally-driven OM pores are present in just bitumen or in both kerogen and bitumen. This debate is driven by several factors, including the difficulty in telling kerogen from bitumen in the SEM, the difficulty in recognizing inherited pores in kerogen, and the possibility that bitumen can pseudomorph kerogen (rendering petrographic distinctions more difficult). However, the presence of pores in large (> several µm) grains of OM, where the pores commonly show a pattern influenced by underlying structure in the OM, suggests that pores do form in kerogen. Another area of interest is the heterogeneity of the development of OM pores at scales ranging from micrometers to meters. Single SEM images can show variable pore development both in size, number and shape between adjacent particles of OM. Heterogeneity within the limited area of an Ar-ion-milled surface is ubiquitous. In some units, OM pores in samples can vary from almost nonexistent to common within a single core. Ongoing research targets whether heterogeneity may be related to differences in kerogen type.