--> Organic Porosity: A Geochemist's View of the Current State

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Organic Porosity: A Geochemist's View of the Current State

Abstract

With the interest in unconventional resources, there have been a number of changes in how reservoirs and their attributes are assessed and characterized. Possibly of greatest importance to unconventional gas plays has been the role and development of organic porosity. Organic pore size limits the role organic porosity in liquid-rich plays. The literature reveals contradictions with respect to where organic porosity developments (kerogen or pyrobitumen), when it develops with respect to generation and cracking (within the oil-window or beyond), mode of formation (inherited or authigenic), influence of carbon content, and importance with respect to gas storage and production. Many of these contradictions appear to result from the database upon which the studies are based. From the available information it has become clear that there are number of issues. One issue is terminology. There appears to be confusion in the usage of the terms kerogen, bitumen, and pyrobitumen. This confusion exists in the literature and reports prepared by service providers. Another issue is differences in the morphology of organic pores – spongy, bubbles, and fractures. Differences in morphology suggest multiple mechanisms for porosity development. For example, the spongy appearance present in some systems could be a reflection of the amorphous character of some oil-prone macerals, whereas the bubbles could be a reflection of degassing during transition from the oil-window into the gas-window, and fractures could be a result of the conversion to gas and the inability of gas to escape. Organic pores increase space for free gas storage and increase surface area resulting in an increase in gas absorption capacity. However, the connectivity of these pores may be limited. A further complication when studying organic porosity and its formation is observed variability within an individual reservoir. Adjacent “macerals” may display very different pore distributions and individual “macerals” may also display varied distribution of pores. It has been suggested that this could be, in part, a result of organic-inorganic interactions. The lack of a clear understanding of organic porosity by no means is a surprise. Porosity and its development in conventional reservoirs have been studied since Sorby began the examination of thin sections and is still under examination, while organic porosity has been studied for less than a decade.