Page 187 - Geology of Carbonate Reservoirs

168 DIAGENETIC CARBONATE RESERVOIRS

and Mg - calcite. Freshwater vadose and phreatic cements crystallize as calcite with
blocky crystals and bladed rhomb crystal habits. Vadose cements typically exhibit
the distinctive meniscus patterns where cementation occurs only at grain contacts

where a meniscus film held liquid from which the cement precipitated. Phreatic
cements typically occur as isopachous rim and pore - lining cements. As more micro-
layers of cement form, a kind of chronology or microstratigraphy of cementation
can be recognized. This cement “ stratigraphy ” displays the sequence of events
during burial diagenesis (Figures 6.6 and 6.9 ). Some of the early work that defi ned
cement stratigraphy, as it has come to be known, was done by Meyers (1974) , who

recognized that the successive layers of pore - filling calcite cements in Mississippian
limestones from New Mexico had recorded a kind of microstratigraphy of cementa-
tion events. Successive episodes of cementation are most easily recognized with
cathode luminescence (CL). This technique involves placing thin sections in a
vacuum and bombarding them with electrons to induce luminescence in some min-
erals. The principle is somewhat similar to the way a television CRT works. If car-
bonate minerals contain the right mixture of trace elements, they luminesce in
different colors and intensities. Trace element compositions and their effects on CL
have been described by many authors, notably by Machel (1987a) . Generally, each
cement microlayer luminesces (or does not) uniformly, enabling the petrographer
to identify the number of cement layers, their relative times of origin, and something
about their trace element compositions.

1

3
2

1. MARINE PHREATIC – ISOPACHOUS RIM CEMENT
2. VADOSE – MENISCUS CEMENT
3. BLOCKY CALCITE – LATE BURIAL CEMENT
Figure 6.9 A sketch illustrating “ cement stratigraphy ” that can reveal different times and

styles of cementation before and after burial. The first stage marine phreatic cement is in the
form of an acicular, isopachous rim around the grains. The next stage cement is meniscus
cement that forms only in the vadose diagenetic environment, indicating that the marine -
cemented grain was exposed to the vadose environment after initial cementation. Finally, two
stages of blocky, burial calcite cement mark the last of the “ cement stratigraphic ” episodes.
(Adapted from an illustration in McIlreath and Morrow (1990) .)