Page 168 - Geology of Carbonate Reservoirs

DIAGENESIS AND DIAGENETIC PROCESSES 149

A C

B D

Figure 6.2 (a) Photomicrograph of a modern skeletal grainstone with isopachous aragonite

rim cement typical of the marine phreatic environment. The grains are foraminiferal and red

algal fragments. The width of the photo is 2.0 mm. (b) An outcrop photo of botryoidal ara-
gonite cement filling a cavity in the Guadalupian (Permian) reef of West Texas. The dark

brown cement is presently calcite but it retains the original aragonite crystal structure visible
as fan - shaped crystal arrays, two of which are to the right of the coin and one is to the left.
The bright (white) marks are hammer scars left by careless students. (c) Rhombohedral
dolomite crystals typically indicative of early diagenetic replacement. The width of this photo

is 2 mm. (d) Saddle dolomite crystals filling a large vug in a Mississippian limestone. Saddle

dolomite is typical of late burial replacement and is formed at temperatures of 60 ° C and

above. (Photo (c) courtesy of David Kopaska - Merkel; the others are by the author.)