Page 126 - Geology of Carbonate Reservoirs

DEPOSITIONAL CARBONATE RESERVOIRS 107

rock properties, one has to examine rock samples — borehole cores or cuttings —

first - hand. There is no indirect method of measurement or logging yet known that
can distinguish between depositional and diagenetic porosity. Once the key deposi-

tional attributes are identified from rock descriptions, they should be incorporated
into subsurface structural and stratigraphic cross sections and maps that include
facies descriptions so that the spatial distribution of reservoir attributes can be
predicted. Ordinary wireline logs do not measure rock properties that discriminate
between pore types in carbonate reservoirs. Neutron, density, and acoustic logs can
be used to calculate total porosity but those logs cannot discriminate between car-
bonate pore types or pore origins. Conventional wireline logs do not distinguish
between depositional facies in carbonates either, because logs cannot detect differ-
ences in carbonate grain types, depositional textures, fossil content and diversity, or
most sedimentary structures. It therefore follows that one cannot make electrofacies
maps of depositional or diagenetic facies in carbonate reservoirs. As mentioned
earlier, when enough well control is available in an established fi eld, log signatures
of depositional carbonate rock and pore characteristics can be identified with some

success, especially if reservoir porosity is facies selective and relatively free of com-
plications or partitioning by diagenesis. In such cases, so - called electrofacies can be

identified and mapped with reasonable success. The “ rock typing ” methods of Lucia
(1995) , Gunter et al. (1997) , and Martin et al. (1997) for distinguishing between fl ow
units, baffles, and barriers in carbonate reservoirs have also been comparatively

successful but the method does not distinguish between depositional, hybrid, and
diagenetic pore types — the key information needed to predict the distribution of
reservoir zones in stratigraphic space. Large - scale sedimentary structures such as

eolian crossbeds and talus beds that dip off the flanks of carbonate buildups can be
detected with dipmeter and with imaging logs. Imaging logs can also detect large
vuggy pores in carbonate reservoirs. The borehole log that has great undeveloped
potential to discriminate between carbonate pore types is the NMR log. Much
research is being done on this subject as this book is written and there is strong
evidence that the NMR log may become a powerful application for identifying pore
types in carbonate reservoirs both by origin and by pore geometry (Genty et al.,
2007 ).
Depositional facies maps made from sample or core examination can be used as
proxies for maps of effective porosity if reservoir porosity is depositional in origin.
Seismic data alone is not useful to discriminate between depositional and diagenetic
reservoirs, but seismologists can identify structural features that may have infl u-
enced depositional or diagenetic patterns and in a few cases where impedance con-
trasts are sufficient, they can extract seismic attributes that can distinguish between

reservoir and nonreservoir rocks. Borehole logs and other petrophysical measure-
ments such as capillary pressure and NMR measurements are very important for

identifying flow units, baffles, and barriers within reservoirs when used in conjunc-
tion with sample or core descriptions. Such integrated data is especially useful for
establishing a quality ranking system for reservoir flow units. Using the different

types of data, one can determine a relative quality or rank of the flow units based

on their combined porosity and permeability values, their capillary pressure char-
acteristics, including median pore throat diameters, NMR values for pore geometry,
and their dominant genetic pore types identified by direct observation. The genetic

classification of porosity links pore types to geological events during depositional