A borehole core sample from a Mississippian (Chappel Formation) carbonate buildup in North Texas. The mound produces oil from fractures that are partly filled with late-burial, saddle dolomite. Saddle dolomite is commonly found in oil reservoirs, and with MVT lead-zinc deposits. It can be a good indicator of fractures, and it can be identified in small samples such as rotary cuttings. These fractured mounds filled with saddle dolomite were part of Ahr student Sheila Ross' MS thesis project. Similar fractures with saddle dolomite are present in the Mississippian mounds currently under study by Ahr and students. Photo by W. Ahr.