Page 21 - Geology of Carbonate Reservoirs

2 INTRODUCTION

spatial distribution of reservoir flow units at field scale. In other words, the funda-
mental rock properties that correspond to good, fair, and poor combined values of

porosity and permeability can be identified and put in larger stratigraphic context,
or “ scaled - up. ” Then the temporal and genetic characteristics of the large - scale
petrologic and stratigraphic properties (proxies) are used for reservoir prediction

and flow unit mapping.
Carbonate reservoir porosity usually represents the combined effects of more

than one geological process. Sometimes it reflects multiple episodes of change
during burial history; therefore particular care must be given to identification of the

sequence of events that led to the final array of rock properties and pore charac-
teristics. Usually it is possible to identify cross - cutting relationships between rock
properties so that their relative times of origin are distinguishable. Reservoir poros-
ity governed only by depositional rock properties, a rather uncommon occurrence,
will not exhibit cross - cutting relationships because rock texture, fabric, porosity,
and permeability share a single mode and time of origin. In that case, reservoir
architecture and spatial distribution conform to depositional facies boundaries.
These reservoirs are referred to as stratabound , and porosity is facies - selective,
fabric - selective , or both. Diagenesis and fracturing do not always follow depositional
unit boundaries. Although carbonate reservoirs exist in which diagenetic porosity
corresponds with depositional rock properties (fabric - selective or facies - selective
diagenesis), in many instances it does not. In the latter case, it is especially important
to identify the type of alteration, how it was formed, when it was formed, and what
cross - cutting relationships it shares with other diagenetic and fracture attributes.
Fractures cut across most rock boundaries but there are some fundamental rock
properties that dictate how and where fractures will form. Fractures happen as a
result of brittle failure under differential stress, usually in conjunction with faulting
or folding. Fault and fold geometry can be determined; therefore it follows that
associated fracture patterns can also be determined. In short, there are many rock
and petrophysical characteristics in carbonates that expose a wealth of information
about the origin and architecture of carbonate reservoirs.

1.1 DEFINITION OF CARBONATE RESERVOIRS

1.1.1 Carbonates
2−
Carbonates are anionic complexes of (CO 3 ) and divalent metallic cations such as
Ca, Mg, Fe, Mn, Zn, Ba, Sr, and Cu, along with a few less common others. The bond
between the metallic cation and the carbonate group is not as strong as the internal
bonds in the CO 3 structure, which in turn are not as strong as the covalent bond in
carbon dioxide (CO 2 ). In the presence of hydrogen ions, the carbonate group breaks
down to produce CO 2 and water. This breakdown reaction, commonly experienced
when acid is placed on limestone, is the chemical basis for the fizz test that distin-

guishes carbonates from noncarbonates. It is also used to distinguish dolostones,

which fizz slowly, from limestones, which fizz rapidly. Carbonates occur naturally as

sediments and reefs in modern tropical and temperate oceans, as ancient rocks, and
as economically important mineral deposits. The common carbonates are grouped
into families on the basis of their crystal lattice structure, or the internal arrange-