Page 163 - Geology of Carbonate Reservoirs

CHAPTER SIX

DIAGENETIC CARBONATE

RESERVOIRS

6.1 DIAGENESIS AND DIAGENETIC PROCESSES

This chapter covers processes and products of diagenesis and how they affect car-
bonate reservoirs. Emphasis is on recognition of diagenetic and highly altered
hybrid pore types and on methods for analyzing and exploiting diagenetic reservoirs.
While diagenetic porosity and permeability are created or modifi ed by postdeposi-
tional processes, depositional porosity and permeability are formed at the time of
deposition. The key to understanding and exploiting diagenetic reservoirs lies in the
ability to identify and correctly classify pore types by origin. A further challenge is
to determine whether the pore systems are purely diagenetic or some hybrid of
diagenetic plus depositional or diagenetic plus fracture porosity. The economic
bottom line is to assess the extent to which diagenesis has modifi ed depositional

porosity and permeability and how those modifications have infl uenced reservoir
performance. Purely diagenetic porosity such as intercrystalline porosity in dolos-
tones requires exploration and development strategies built around a geological
concept that enables one to map and predict the spatial distribution of the porous
and permeable dolostone. That is, one has to understand the origin and spatial dis-
tribution of diagenetic changes — not the distribution of depositional facies. As with
depositional reservoirs, the main materials needed to analyze diagenetic reservoirs
are borehole cores, cuttings, and subsurface cross sections and maps. Depositional
facies maps are commonly used as proxies for maps for porosity in reservoirs with
purely depositional or lightly altered hybrid pore types. Most wireline logs and
seismic records are not useful for identifying diagenetic porosity, but that informa-
tion is helpful in identifying structural or stratigraphic trends that may have infl u-
enced diagenetic patterns. Borehole logs and other petrophysical characteristics

Geology of Carbonate Reservoirs: The Identification, Description, and Characterization of Hydrocarbon
Reservoirs in Carbonate Rocks
By Wayne M. Ahr Copyright © 2008 John Wiley & Sons, Inc.
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