Page 164 - Geology of Carbonate Reservoirs

DIAGENESIS AND DIAGENETIC PROCESSES 145

such as capillary pressure and NMR measurements are very important for identify-

ing flow units, baffles, and barriers within reservoirs and for establishing a quality

ranking system for reservoir flow units, but logs alone cannot distinguish between
genetic pore types.
When diagenetic porosity and permeability are intimately related to depositional
rock properties, reservoir boundaries conform to depositional facies boundaries. If
diagenetic changes follow fracture or joint patterns, then determining the size and
shape of the reservoir may be a job of interpreting the fracture distribution pattern
rather than one of interpreting patterns of diagenesis. In purely diagenetic pore
systems that do not conform to fracture or depositional trends, the techniques for
analyzing reservoir performance are defined by the type and extent of diagenesis

that created the porosity. Reservoir size and shape may depend on the mechanism
of diagenesis, the environment of diagenesis, and the size and shape of the zones
that were exposed to diagenesis. For example, reservoirs formed by replacement
diagenesis — as in replacement dolostones after limestones — could have boundaries
roughly related to the size and shape of ancient coastal salinas, where evaporative
brines reacted with metastable carbonates to form new minerals, fabrics, pores, and
pore throats. Diagenetic reservoir architecture may correspond to the geometry of
an ancient water table, to an unconformity or exposure surface, to a paleosol horizon,
or to a karst surface. In short, diagenetic porosity may or may not correspond to
depositional or structural trends. The challenge lies in doing the geological detective
work to determine relationships between diagenetic porosity and other geological
attributes of reservoir zones.

6.1.1 Definition of Diagenesis
The term diagenesis derives from ancient Greek dia — across or through — and
genesis — origin or generation. In today ’ s literature, diagenesis is generally inter-

preted to mean “ across generations ” in the sense that diagenetic changes cut across
(modify) different generations of minerals or rock properties. Most current refer-
ence books more or less have the same defi nition for diagenesis; namely, it is all of
the changes that happen to sedimentary rocks after deposition and before metamor-
phism . All changes in size, shape, volume, chemical composition, or crystalline struc-
ture of a sedimentary rock after its detrital, biogenic, or crystalline constituents have
been deposited. It is easy to imagine changes happening to sedimentary particles
but the boundary between diagenesis and metamorphism is not as easy to recognize.
Because that is a gradational boundary and it is not easy to identify one side as
purely sedimentary and the other side as purely metamorphic, most modern workers
accept that the boundary is gradational and focus instead on the key rock properties

that can be identified as low - temperature and low - pressure changes that occurred
in the burial diagenetic domain. For those wanting more precise defi nitions of this
boundary region, sedimentary geochemists have devised a scheme to measure
“ organic metamorphism ” based on the degree to which kerogen and other organic
constituents are heated during burial. This scheme is used to describe the maturity
of kerogen - rich source rocks that yield petroleum hydrocarbons when heated to
high enough temperatures. Kerogen releases petroleum hydrocarbons at burial
depths characterized by temperatures ranging between about 65 and 150 ° C (Selley,

1985 ). This temperature range is sometimes called the “ oil window ” if the principal