DIFFERENTIATION AMONG DIFFERENT TYPES OF PORES ASSOCIATED WITH ORGANIC MATTER IN MUDROCKS USING SCANNING ELECTRON MICROSCOPE PETROGRAPHY

Pores associated with organic matter play an important role in the pore systems of organic-matter-bearing mudrocks. At least six distinct types of these pores exist in organic-matter-bearing mudrocks. Examination of Ar-ion-milled samples from more than 20 different rock units from a range of ages have shown the following organic-matter pore types. Inherited (predepositional) pores come in a variety of shapes and sizes and are common in organic matter derived from terrestrial sources. Subgrain (granular) pores are a generally rare pore type that also may have formed prior to deposition, but the pores have a distinctly different (three-sided) morphology from inherited pores. Modified mineral pores are original mineral pores that have been partly occluded by migrated bitumen leaving an organic-matter-hosted pore. Bubble pores are relatively large pores (up to 1 micrometer in diameter) that form in organic matter during thermal maturation in the oil window, commonly by amalgamation of smaller bubble pores. Nanometer-scale spongy pores also form during thermal maturation, but at higher temperatures (wet gas window) than bubble pores. Elongate dissolution pores surrounded by organic matter are thought not to be related to dissolution of the organic matter, but, instead, to dissolution of clay minerals intermixed in the organic matter. Very few of the microfractures seen in organic matter are thought to form naturally in the subsurface but are artifacts formed during coring or post coring. Those microfractures that do form in the subsurface are made of interconnected bubble pores.

Inherited and modified mineral pores are not thought to be connected to the overall pore system of the rock. Subgrain pores are connected within the organic-matter grain and may connect externally, but are generally not common enough to be important to the overall pore system. Dissolution pores in organic matter may contribute to storage, but not necessarily flow. Bubble and spongy pores do appear to be commonly connected and would then be important for flow and storage of hydrocarbons.