Page 175 - Geology of Carbonate Reservoirs

156 DIAGENETIC CARBONATE RESERVOIRS

it is important to record each type of alteration, temporal relationships between
episodes of alteration, and correspondence between diagenesis and tectonosedimen-
tary history. Because diagenetic histories can be complex, a practical approach to
identifying and mapping diagenetic facies involves using the genetic classifi cation
scheme for carbonate porosity presented in Chapter 2 . In essence, reservoir pore
types are mapped on the basis of their depositional and diagenetic characteristics so
that pore facies maps can be constructed to pinpoint the spatial distribution of the
pore types with highest corresponding permeability and lowest resistance to fl uid
fl ow and that can be correlated stratigraphically at fi eld scale. Each episode of dia-
genesis leaves distinctive traces that can be compared with other microscopic traces
to reveal the timing of each event. This tracing of diagenetic events is done by iden-
tifying cross - cutting relationships in thin sections. The latest or last event cuts across

the previous one, and so on, until the first diagenetic event can be isolated. In this
way, episodes of dissolution, cementation, compaction, or other forms of diagenesis

can be identified and placed in chronological sequence to reveal the burial history
and the geological cause – effect system that modifi ed the reservoir rocks.

6.4 DIAGENETICALLY ENHANCED POROSITY

Diagenetic trends may or may not cut across depositional facies boundaries and
they may include a variety of pore types that represent more than one diagenetic
event. Knowing this, the traditional approach for many geoscientists is to focus on
case histories — catalogs of reservoir examples — in which cross - cutting relationships,
genetic categories of pore types, or chronologies of pore formation and modifi cation
are compiled. Case histories are usually used as models or analogs to help under-
stand other reservoirs by providing a similar appearance, a “ look - alike ” or a tem-
plate. However useful they may be as references for comparison, analogs cannot
explain how different reservoirs formed (cause – effect relationships) or the relative
timing of porosity formation, and analogs cannot provide reliable means with which
to predict the spatial distribution of reservoir properties. Analogs are commonly
used without doing additional studies of depositional and diagenetic histories that

influenced the formation of pore types, flow units, baffles, and barriers. Because

analogs and case histories have limited application in critical analysis of reservoir
origin and distribution, the focus of the following discussions is on how to recognize
and map diagenetic pore systems . Diagenetically enhanced porosity is considered
first. Diagenetically reduced porosity is discussed later. Once diagenetically enhanced

porosity is identified, geoscientists and engineers can focus directly on individual

pore types, their relationship to fundamental rock and reservoir properties, and their
times of origin relative to mappable depositional and diagenetic events. For example,
it is relatively common nowadays for specialists to identify the relative times of
origin and cause – effect relationships linked to specific diagenetic events during

burial history. Individual pore types can be distinguished even though multiple epi-
sodes of diagenesis may be present in the rock. With this information in place, it is

usually possible to identify reservoir flow units based on petrographic and petro-
physical attributes of pore types and on the cross - cutting relationships they display

within the rock framework. Finally, the spatial distribution of flow units can be
determined by correlating pore “ facies ” based on pore attributes and pore genesis