AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Reconsidering the Distinction Between Matrix and Porosity in Light of Molecular Structural Models of Coal and Petroleum Source Rocks


Studies of the organic structural chemistry of coal and petroleum source rocks over the past few decades can provide useful insights into the characteristics of carbonaceous low permeability (CLP) reservoirs. Rather than using these models to provide a valid foundation to describe the reservoir at molecular dimensions, then up-scaling, however, the petroleum industry has doggedly applied the same conceptual model developed for conventional reservoirs at orders of magnitude larger scale, then down-sizing to nanometer scale, while largely neglecting sub-nanometer molecular features. The conventional reservoir model is based on the binary distinction between matrix and porosity, where matrix broadly represents the solid fraction, and porosity represents the fluid fraction (liquid and gas). These simple terms are difficult to apply in practice, however, which can lead to oversimplified or erroneous description and analysis results. Even classification of constituents as solid, liquid, and gas is problematic, as the dispersion of low molecular weight species through a high molecular weight medium is better described in terms of colloids or clathrates than the classic states of matter. In reality, organic materials span a continuum of molecular sizes and weights, ranging from methane at one extreme, to complex, cross-linked macromolecules at the other. Setting a boundary between matrix and porosity is somewhat arbitrary, misleading, and dependent on specific analysis method(s), P-V-T conditions, and changes to the physical structure and chemical composition occurring during sampling and analysis. During the 1970s, coal chemists introduced the term “mobile phase” to describe the fraction of organic matter exhibiting vibrational mobility in 1H NMR, representing non-bonded molecular species occluded within a macromolecular matrix, where the density of cross linkages is dependent on organic matter rank and type. Mobile phase is largely immobile, however, in the context of expulsion and migration, and is only partially accessible to common organic solvents at low temperature. Petroleum geochemists have made an analogous distinction between bitumen (soluble) and kerogen (insoluble), based upon solubility. The relevancy of these binary classes to the distinction between porosity and matrix in reservoir engineering context is dubious. It would be better to start with new conceptual models and analysis methods, rather than try to force a square peg into a round hole.