A note on the differences between the sandstone and carbonate tutorials

The sandstone tutorial (Milliken, Choh, and McBride, 2007) and the carbonate tutorial use dramatically different programming. If you have experience with the sandstone tutorial, this contrast was immediately obvious to you upon launching its carbonate companion. Why this difference? It relates to the fundamental difference between sandstones and carbonates: Sandstones are made dominantly of mostly equant inorganic particles in the size range of 62 to 1000 µm. In fact, most sandstones of importance in subsurface reservoirs fall into an even lesser range of sizes, say, 62 to 350 µm, and are well-sorted. This means that the key petrographic features of sandstones can be displayed nicely in static images across a fairly limited range of magnifications (mostly 40x to 400x). A few sedimentary structures can be seen in lower magnification images (some burrows, for instance), but sedimentary structures are not, in general, studied in thin sections. Very low-magnification views of most sandstones, are, frankly, kind of boring.

In carbonate rocks, the particles are dominantly biogenic, have a tremendous range of size, and can have shapes that are far from equant. Weirdly shaped biogenic particles can stack up to yield an amazing array of different pore-space arrangements. Thin-section-scale textures of limestones have more important variability than do textures at this scale in sandstones. So, it's simply not enough to look very closely at some fossil bit and say, "yep, that's an oyster!" You also need to step back and see what's going on around the oyster. Is the particle a part of a rounded and well-sorted grain assemblage, or is it part of a messy pile of other oysters, some whole and some broken, all encased in mud? Is it a beach rock or an oyster bank? In carbonate rocks you need to be able to see the grains in the proper context of the whole rock in order to really get the benefit of from your grain identification skills. Thus, the carbonate tutorial adopts an approach in which the images can be zoomed from a whole-thin section view, all the way down to individual grains.

"How to use this DVD".