BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
called “bio”; (2) fragments of previously deposited limestone, called “intraclasts”, (3) small round
pellets, the excreta of worms and other small burrowing organisms; and (4) ooliths, spheres formed by
rolling and coating lime particles along the bottom.
The matrix is lime of clay particle size (lime mud). It is called micrite. The clear secondary calcite
cement is called sparite.
Figure 2-8 Carbonate Rock Types
Thus, a rock consisting mainly of clear secondary calcite with intraclast grains would be called a
“intrasparite”. A rock consisting mainly of micrite (lime mud) with grains of broken shell fragments
would be called “biomicrite”. Biomicrite and pelmicrite are the most common limestone types. These
eight types are shown in diagrammatically below.
Besides these eight combinations, there are limestones consisting only of micrite and some consisting
of the remains of upstanding reef-building organisms. So there are ten types of limestone in all.
Diagenesis of Carbonates
Lithification. Calcium carbonate (CaCO 3 ) is slightly soluble in water, but calcium bicarbonate
(CaHCO 3 ) is very soluble. When carbon dioxide gas (CO 2 ) is dissolved in water, it forms carbonic
acid (H 2 CO 3 ), which changes calcium carbonate to calcium bicarbonate. The reactions are
complicated but may be summarized in the following equation:
++
−
+ H
2O
+ CO2
↔ Ca + •
3
CaCO3 2 HCO
The CaCO 3 is crystalline, the Ca ++ and 2 HCO 3¯ are ions in solution. The reaction is reversible, so that
crystalline calcium carbonate may be either dissolved or precipitated, depending on conditions in the
water solution. Of these, the most important is pH. When carbon dioxide dissolves in water it makes
the water more acid, that is, it lowers the pH. Carbon dioxide is dissolved out of the air and is also
produced by bacteria that decompose organic matter and by animals in their respiration. It is taken out
of water by plants such as algae that use it to form organic carbonic compounds. Slight changes in pH
(and also in Eh, which is oxidation-reduction potential) thus cause solutions to dissolve or precipitate
calcium carbonate.
Reference Manual
Baker Hughes Drilling Fluids
2-12 Revised 2006