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Distinguishing Organic Matter Pores Associated With Depositional Organic Matter versus Migrated Organic Matter in Mudrocks

Abstract

Organic-matter (OM) pores are an important constituent of mudrocks and comprise the dominant or subsidiary pore network of many shale-gas and shale-oil systems. New research suggests that OM pores form not only in kerogen, as originally proposed, but also in solid bitumen and pyrobitumen. Identifying the type of nanometer- to micrometer-sized organic matter that is present in mudrocks is extremely difficult, if not impossible, using a scanning electron microscope (SEM). However, distinguishing whether the OM-pore hosted organic material exists in place or has migrated would allow the determination to be made whether the original organic material was kerogen or migrated bitumen. There are several SEM-based petrographic criteria that can be used to separate depositional versus migrated organic matter. These criteria include: (1) organic matter occurring after cementation in mineral pores, (2) fossil body-cavity voids filled with organic matter, (3) dense, spongy pore texture of the organic matter, (4) abundant contiguous pores filled with organic matter having a spongy pore network, (5) no alignment of pores in organic matter (aligned OM pores are present in kerogen), (6) cracks in organic matter related to devolatilization, and (7) anomalously larger bubbles associated with development of two hydrocarbon phases. It is important to recognize the difference between deposition organic matter versus migrated organic matter associated nanopores because their distribution is different and this has a profound effect on reservoir quality. Original depositional organic material is composed of kerogen, which can be transformed to bitumen and then oil, gas, solid bitumen, and pyrobitumen (char) during thermal maturation. When bitumen is produced from the kerogen, it can migrate into the mineral pore network and later transform to solid bitumen or pyrobitumen. The final pore network and associated reservoir quality within the mudrock is dependent on the proportions of the distribution of these two organic matter states. OM pores in isolated depositional organic matter may not be well connected and not form a continuous permeability pathway for the hydrocarbons. Migrated organic-matter-hosted pores mimic the three-dimensional distribution of the original mudrock mineral pore network and provide more extensive contiguous permeability pathways than isolated organic matter, thus providing a higher reservoir-quality mudstone system.