Page 269 - Geology of Carbonate Reservoirs

250 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

of the reservoir rocks as cores or cuttings. As of now, there are no indirect methods
of determining pore origin in carbonates, although NMR logs do show promise. If
pore origins cannot be determined, then reservoir size, shape, and connectivity
cannot be predicted with enough certainty to provide realistic economic assess-

ments of drilling and development prospects. If porosity can be classified by direct
observation of rock samples, then reservoir characterization, economic predictions,
and reservoir simulation can be done with much greater accuracy. The next step in

reservoir analysis, after recognizing genetic pore types, is the identification of fl ow

units, baffles, and barriers.
We defi ne flow units as high - connectivity zones within reservoirs. High connectiv-

ity means high porosity and permeability and low capillary resistance to fl uid fl ow.

Baffles are zones with low connectivity but they are limited in lateral and vertical

extent so that fluids can still flow at low rates around, over, through, or under them.

Baffles can be compared to islands in a stream. Barriers are zones that do not allow

fluids to flow at reasonable rates and barriers may be laterally or vertically extensive,

or both. Barriers can be compared to dams on a stream. Some barriers may even
be seals that prevent fluid escape from reservoirs. We examined ways in which to

test seal capacity based on capillary pressure differences between the seal rock and
the reservoir rock. Rocks with high seal capacities are barriers. Another way of

defining seal capacity is the maximum height of hydrocarbon column that can be
supported by a given reservoir rock, or h max . We search for reservoirs with high con-
nectivity but that are closed by tight seals. Almost all stratigraphic and most struc-

tural settings have some internal baffles, but the baffles only interfere with fl ow to

various degrees. If the baffles are comparatively small in size and allow some fl uid
flow through them, then they are more a nuisance than a real problem. When baffl es

are large and have much higher capillary resistance to flow than the reservoir rock,

they become problems because they may create reservoir compartments that can

be difficult to exploit. Commonly encountered baffles are strata with irregular dis-
tribution of muddy depositional textures mixed with porous and permeable grain -
supported textures. Other types include rocks with irregular distribution of diagenetic
cements or compaction - reduced pores and pore throats.

We can estimate the degree of connectivity within and between flow units by
constructing a data matrix of porosity and permeability values, preferably measured
porosity and permeability from core analyses. This ranking method is one way to

assess the relative quality of reservoir flow units and identify the ones that are best,
intermediate, or poor in quality. In order to identify the best, intermediate, and poor
connectivity zones from the matrix of porosity and permeability data, a system of
slices or layers of arbitrary thickness can be constructed to trace the quality - ranked
segments from the top to the bottom of the reservoir interval. This is especially
useful in hybrid reservoirs where the distribution of diagenetic effects is diffi cult to

map or predict. The spatial distribution of ranked flow units can be identifi ed and
visualized at field scale with slice maps. In practice, stratigraphic slices about 10 feet

thick are the limit in practical applications because that is about the lower limit of
resolution on most wireline logs. Extreme highs and lows of porosity and permeabil-
ity in the reservoir can be excluded to simplify the results, providing that the
extremes also have low frequency of occurrence. The same is done with permeabil-
ity; that is, the most representative range of values is chosen.