Print this pageHow Predictable Are Patterns of Calcite Cement in Sandstones?
McBRIDE, EARLE F.
The abundance and geometry of calcite cement in sandstone is one of the least predictable factors influencing reservoir quality. Calcite cement can form at the depositional surface (beachrock, caliche), at burial depths of 6 km, and all depths in between. Thus, it is not depth-predictable. Availability of calcium generally is the limiting factor in calcite cementation. Calcium sources include intraformational fossils, carbonate rock fragments, and, at depth, albitized calcic plagioclase. Adjacent limestone beds are another important source of both calcium and carbonate.
The initial stage of cementation by calcite forms tightly cemented bodies (concretions) of various sizes and shapes scattered throughout otherwise uncemented host sands (or sands cemented by other minerals). Cementation commonly begins as pea-size concretions that grow in clusters of variable size from closely spaced nucleation cites when the rate of calcite precipitation is relatively fast. For undetermined reasons, certain cites become preferred growth cites. Concretions at these sites grow to decimeters or meters in diameter and are spaced from 1 to 4 m apart. Concretions may grow in size, coalesce, and develop into completely cemented, tight, non-reservoir sandstone. Sandstone beds intercalated with limestone or calcitic shale are those most likely to be completely cemented, especially if they are older than Mesozoic age.
Although locally concretions are selective to shell placers, burrows, day clasts faults, or the top, bottom, or middle of sandstone beds, the pattern of concretionary calcite cement is generally unpredictable. In Mesozoic and younger rocks, concretions rarely make make up more than 30 percent of a sandstone body. Concretions that are spherical or spheroidal grow chiefly by diffusive supply of intraformationally derived cement components. Concretions that are tabular or elongate grow chiefly by advective supply of cement components. Highly elongate concretions (aspect ratios >2:1) tend to have remarkably uniform axial trends that can be used to map ancient paleogroundwater flow paths; these concretions most commonly form from near-surface meteoric water.