Petrographic image analysis and petrophysics: Analysis of crystalline carbonates from the Permian basin, west Texas

Abstract

Classification of porosity in carbonate rocks from thin section can be performed quickly and objectively using computer-based image acquisition and classification procedures. Pore type information (size, shape and volumetric abundance) is determined with high precision, equivalent to millions of point counts. Such pore type information is always relevant because each pore type has a different distribution of associated pore throat sizes. Five pore types occur in a sequence of highly recrystallized dolomites from the Reinecke Field in the Late Carboniferous Horseshoe Atoll Complex in the northern Midland basin. These pore types represent an expanded version of conventional classification, representing two kinds of intercrystalline porosity and three kinds of channel porosity. The channel pore types represent secondary porosity which survived cementation by dolomite, and can be classified into channel pores of three distinctly different sizes. When combined with capillary pressure data the image-based pore type data reveals that the intercrystalline pores have very small throats while the channel pores have throats which are closer in size to the pore body. The product of the numbers of pores per unit volume of each type and the fourth power of the associated mean throat diameter is an index of the relative contribution of each pore type to discharge. In the Reinecke Field rocks, the smallest type of channel pore is the major control on flow. Its relatively small throat size is compensated by its great numerical abundance. The preference of throat size for pore type in these highly recrystallized dolomites, is just as strong as previously observed in detrital sandstones. Relatively tight limestones found with the dolomites also have a strong relationship between pore type and throat size, though their maximum throat radius is 7 microns and some pore types have mean throat sizes less than 1 micron. The Reinecke Field data, coupled with data from studies on sandstones, suggests that throat size in all sedimentary rocks can be expected to be non-randomly associated with pore type. The non-random pore/throat association means that the pore types defined by image analysis represent fundamental elements of the porous microstructure and that variability in a wide range of physical properties is tied to variation in pore type abundance.