PETROGRAPHIC IMAGE-ANALYSIS AND PETROPHYSICS - ANALYSIS OF CRYSTALLINE CARBONATES FROM THE PERMIAN BASIN, WEST TEXAS

Classification of porosity in carbonate rocks from thin section can be performed quickly and objectively using computer-based image acquisit ion and classification procedures. Pore type information (size, shape and volumetric abundance) is determined with high precision, equivalen t to millions of point counts. Such pore type information is always re levant because each pore type has a different distribution of associat ed pore throat sizes. Five pore types occur in a sequence of highly re crystallized dolomites from the Reinecke Field in the Late Carbonifero us Horseshoe Atoll Complex in the northern Midland basin. These pore t ypes represent an expanded version of conventional classification, rep resenting two kinds of intercrystalline porosity and three kinds of ch annel porosity. The channel pore Lypes represent secondary porosity wh ich survived cementation by dolomite, and can be classified into chann el pores of three distinctly different sizes. When combined with capil lary pressure data the image-based pore type data reveals that the int ercrystalline pores have very small throats while the channel pores ha ve throats which are closer in size to the pore body. The product of t he 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 con tribution of each pore type to discharge. In the Reinecke Field rocks, the smallest type of channel pore is the major control on flow. Its r elatively small throat size is compensated by its great numerical abun dance. The preference of throat size for pore type in these highly rec rystallized dolomites is just as strong as previously observed in detr ital sandstones. Relatively tight limestones found with the dolomites also have a strong relationship between pore type and throat size, tho ugh 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 al l 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 physica l properties is tied to variation in pore type abundance.