Page 23 - Geology of Carbonate Reservoirs

4 INTRODUCTION

(a) (b) (c)
Calcite Crystal Forms

(d) (e)
Dolomite Crystal Forms

Figure 1.2 Typical calcite and dolomite crystal forms found in carbonate reservoir rocks:

(a) the scalenohedral form of calcite sometimes called “ dogtooth spar ” ; (b) a compound
rhombohedral form; (c) a hexagonal prism with rhombohedral faces, sometimes called “ nail-
head spar ” ; two common forms of dolomite crystals include (d) the ordinary rhombohedron,
typical of most low - temperature dolomites, and (e) the distorted, curved form called “ saddle
dolomite. ” Saddle dolomite is typically formed in the deep burial diagenetic environment and
is sometimes, perhaps confusingly, referred to as “ hydrothermal ” dolomite. (Adapted from
illustrations in Hurlbut and Klein (1977) .)

system, but it is different from calcite. The small size of Mg ions compared to calcium
ions causes a change in the dolomite lattice resulting in a loss of rotational symmetry.
Aragonite is common in the modern oceans but it is rare in the ancient rock record;
therefore it is safe to say that carbonate reservoirs and aquifers are composed of
calcite and dolomite — limestones and dolostones. Together, those rocks make up
about 90% of all naturally occurring carbonates (Reeder, 1983 ). Only a small frac-
tion of the remaining 10% of carbonate minerals includes azurite and malachite,
which are semiprecious stones and are commonly found in jewelry or other
ornaments.

1.1.2 Reservoirs

Reservoirs are usually defined as storage receptacles. To a petroleum geoscientist,
reservoirs are porous and permeable rock bodies that contain commercial amounts
of hydrocarbons. Reservoirs owe their porosity and permeability to processes of
deposition, diagenesis, or fracturing — individually or in combination. Although we
will focus on hydrocarbon reservoirs in carbonate rocks, many porous and perme-
able carbonates are groundwater aquifers. Reservoirs are three - dimensional bodies
composed of rock matrix and networks of interconnected pores. If the three -
dimensional geometry (size and shape) of a connected pore system is known, it is
possible to (1) determine drilling locations in exploration or development prospects,
(2) estimate the volume of the resource in the reservoir or aquifer, (3) achieve