UNDERSTANDING EXPULSION CAPACITY AND ORGANIC POROSITY IN UNCONVENTIONAL PETROLEUM SYSTEMS

With the increasing importance of unconventional resource plays, efforts have been made to better estimate the storage capacity of unconventional reservoirs. Unlike conventional systems, economical tight plays require a significant percentage of generated hydrocarbons be retained within the source interval. In self-sourced unconventional petroleum systems (e.g., Marcellus Shale), source and reservoir occur within the same unit, hydrocarbons are largely retained within the source and limited amounts of resource “bleed” from the unit’s edge. Limited expulsion efficiency (EE) leads to the development of excellent shale plays and somewhat reduced conventional resources. Thickness of the source interval is a major control on the EE. Two additional expulsion models in unconventional plays include (1) “sandwiched reservoirs,” where a reservoir lays between organic-rich source intervals with the upper and lower source intervals feeding into the common reservoir (e.g., Bakken Fm.) and secondary migration is possible, and (2) “interbedded source-reservoir couplets” (e.g., Wolfcamp Fm.), which has the highest EE and a difficult-to-define net reservoir interval.

Porosity is another parameter critical for hydrocarbon in-place estimates. Organic porosity potentially plays a key role on hydrocarbon storage, migration, and production, particularly in massive source-reservoirs. Organic pores provide space for hydrocarbon storage and increase surface area resulting in higher absorption capacity. The connectivity of these pores may be somewhat limited and dependent on the nature of the organic network. Available literature on organic porosity reveals contradictory information regarding where (e.g., in kerogen macerals, bitumen, and/or pyrobitumen), when (prior to diagenesis, within the oil window, or beyond), and how (e.g., inherited or authigenic) organic pores form. The influence of organic richness on organic porosity development is also up for debate. Many of these contradictions are a function of the nature of the data sets upon which the studies are based. Some of the key issues that need to be clarified include terminology and differences in pore morphology (can indicate multiple mechanisms for formation and/or growth of organic pores). Finally, it is important to evaluate the extent by which the acts of obtaining and observing the samples may alter the rocks and the pores themselves, potentially producing “organic pores” not reflective of native conditions.