Page 192 - Geology of Carbonate Reservoirs

DIAGNOSING AND MAPPING DIAGENETIC RESERVOIRS 173

porosity values are high on isopach thins, it indicates that ancient structural
highs and lows influenced diagenesis and resulting pore — pore throat attri-

butes. Be aware when doing volumetric calculations or planning future drilling

locations that paleostructurally influenced reservoir geometry does not follow
present structure.

6. Purely diagenetic porosity (e.g., dolostones in which all traces of original lime-

stone properties have been replaced) may correspond to position in strati-
graphic cycles (e.g., top, middle, or bottom of the cycle) or to horizons where

evaporites, ordinary dolomites, silicates and sulfides, or saddle dolomites occur
together. Dolomite forms in a wide variety of geological settings including
deep subsurface environments where saddle dolomite is typical. Although
saddle dolomite is found in fractures and solution - enlarged vugs, it is not
always an indicator of enhanced porosity. On the negative side, replacement
by exotic minerals and dolomite may reduce porosity instead of enhancing it.
Use the same methods that are used to fi nd flow units (this checklist) to avoid

baffles and barriers.
7. Develop geological concepts to explain reservoir characteristics by incorporat-

ing all available data on rock and petrophysical characteristics. Compare wire-
line log and seismic data with rock and pore characteristics observed in
full - diameter cores to determine whether borehole log or seismic “ signatures ”
can be used to identify porous zones in the borehole. Caliper logs, porosity
logs, image logs, and NMR logs are first choices. Gamma ray logs may help

identify concentrations of insoluble organic matter at unconformities, which
in turn may be proximity indicators to porous zones. Once characteristic sig-
natures are found that correspond to porous zones, the locations of genetic
pore types that govern reservoir quality can be pinpointed and correlated from
well to well.

Mapping flow units in diagenetically altered pore systems can be done as with

depositional pore systems, but instead of focusing on depositional rock characteris-
tics, cycles, and sequences, mapping flow units with diagenetic porosity focuses

almost entirely on classification of pore types, determining their modes and times
of origin, and their spatial distribution within the reservoir. If diagenesis has not

removed all traces of depositional influence and porosity still conforms closely to

depositional textures and fabrics, then identification of flow units is simply the iden-

tification of depositional rock properties that have the highest combined values of
porosity and permeability. In such cases, individual flow units can be ranked by

identifying pore categories that exhibit highest poroperm paired values (highest
porosity paired with highest permeability for any given stratigraphic horizon within
the reservoir), the intermediate poroperm values, and the lowest poroperm values.
Mapping flow units in purely diagenetic pore systems that show no relationship

to depositional rock properties requires identifying the following sequence of events:
(1) the types of diagenesis that influenced porosity; (2) the direction of that infl uence

(pore creation, pore enhancement, or pore reduction); (3) the relative timing of the
different diagenetic events with respect to each other; and finally, (4) the geological

conditions or events that caused the different episodes of diagenesis to occur. Dis-
tinguishing different types of diagenesis enables one to isolate reservoir zones