More than half of the world's petroleum is found in carbonate rocks -- for example, in the Middle East, the former USSR and in North America. These rocks show a bewildering diversity of grains and textures, due in part to the wealth of different fossil organisms that have contributed to carbonate sedimentation, and in part to a wide variety of diagenetic processes that can radically modify textures and obscure the depositional fabric. Careful petrographic...