Pore-System Facies: Pore Throats and Pore Bodies

When Archie (1950) first introduced the term “petrophysics,” he outlined a tentative petrophysical system “. . . which revolves mainly around pore-size distribution which defines the capillary pressure curve, permeability, and porosity.” As such, a pore distribution does not necessarily coincide with a specific rock type. Different lithologies might contain similar pore distributions and a single lithology might be characterized by several distinctive pore distributions. In the latter case, these differences could be used as the basis for a lithofacies subdivision, where the criteria were defined by pore-network properties rather than more conventional fabric observations. Often, there will be a substantial commonality between the two approaches, because the pore network and rock framework are complementary. The term “petrofacies” (which comes from “petrophysical facies”) extends the facies concept to pore networks. Although this name is commonly (but not exclusively) used for this purpose, the range of published definitions is fairly broad, as pointed out by Sullivan et al. (2003). Some authors intermingle notions of petrofacies with electrofacies and lithofacies, which is understandable, because in many reservoirs there are strong intercorrelations between them. In this text, we distinguish between electrofacies, either seemingly natural petrophysical log associations found by unsupervised methods, or those determined from lithofacies by supervised methods. Lithofacies are generally recognized by standard visual observations of a core, although they may be defined by reference to distinctive porosity-permeability associations (petrofacies) in core measurements. The two fundamental reservoir components of pore microarchitecture are essentially the same as the spatial elements of conventional architecture: the relative sizes and arrangement of the pore bodies (rooms) and the pore throats (doors between rooms). In an oil or gas reservoir, the volume of pore space contained in the pore bodies dictates the total storage capacity, while the access of hydrocarbon to the pore bodies is regulated by the size of the linking pore throats. Realistic pore-network models for characterizing hydrocarbon recovery from reservoirs are, appropriately, labyrinthine in their intricacy. However, the key pore attributes that are the focus of petrophysical applications are the size distributions of the pore bodies and pore throats, together with the aspect ratio of pore-body size to pore-throat size.