Page 245 - Geology of Carbonate Reservoirs

226 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

particularly to variations in Mn and Fe. Metallic elements in oxygenated water will
generally be in oxidized states and less likely to be incorporated in calcite or dolo-
mite cements that precipitate from those waters. Reducing waters, on the other
hand, contain reduced metallic cations that are readily incorporated in calcite and
dolomite lattices so that iron - rich, or ferroan, carbonates can be used as indicators

of reducing conditions in deeper burial waters. Goldstein ’ s (1988) study of cement
stratigraphy in Pennsylvanian carbonates from New Mexico includes an extensive
bibliography on cement stratigraphy.

8.4.2 Field Examples of Diagenetic Reservoirs

Nonfractured, diagenetically modified carbonate reservoirs exhibit either hybrid or
purely diagenetic porosity and account for so many examples that it is beyond the
scope of this book to review all of them. Also note that the literature does not
include a category for hybrid pore types. Previously, writers described reservoirs
with diagenetic, fractured, or “ primary ” porosity, if they described the pore system
characteristics at all. A sampling of the literature reveals the overwhelming number
of fields that produce from either hybrid or purely diagenetic pore systems. Scott et

al. ( 1993 ) note that of the 509 giant fields having over 500 MMbbls of oil equivalents,

59 produce from Cretaceous carbonate reservoirs, including 8 of the 36 supergiant
fields that alone account for 12.9% of the world ’ s estimated ultimate recoverable

reserves. Of the 16 Cretaceous carbonate oil and gas fields tabulated by Scott et al.
( 1993 ), 13 reservoirs, or 80% of the total number, produce from either moldic or
vuggy porosity, or both. Scott ’ s description does not give enough information to

determine how many of those fields produce from hybrids as opposed to purely
diagenetic porosity. Three of the 16 fields produce at least in part from fracture

porosity in addition to diagenetically altered porosity. Assuming that these examples
are reasonably representative of carbonate reservoirs in general, a conservative
estimate is that 7 - in - 10 carbonate reservoirs produce from strongly altered
porosity.
An extensive compilation of carbonate reservoir case histories by Roehl and
Choquette ( 1985 ) included a review of the structure, stratigraphy, and reservoir
characteristics of 35 carbonate reservoirs of different geological ages and locations.
Of the 35 examples, 8 are described as having depositional porosity with little altera-
tion except pore space reduction by cementation (hybrid: depositional attributes
dominant). Nine reservoir examples are interpreted to be largely dependent on
fracture porosity, and all others exhibit pore systems in which porosity enhancement
by diagenesis is in those authors ’ words “ strongly predominant ” (hybrid: diagenetic
attributes dominant). Karst and paleocave reservoirs are, in general, classifi ed as
purely diagenetic and independent of depositional characteristics, although they

may have been influenced by fractures and joints. Studies by Whitaker and Smart
( 1998 ), Purdy and Waltham ( 1999 ), Loucks ( 1999 ), Kosa et al. ( 2003 ), and Loucks
et al. ( 2004 ) present extensive discussions on the origins and characteristics of karst
and paleocave reservoirs. Detailed illustrations of paleocaves in the Guadalupe
Mountains of New Mexico by Kosa et al. ( 2003 ) are instructive because they show
the scale and complex architecture of paleocaves that developed along syndeposi-
tional faults. Along the same line, Whitaker and Smart ( 1998 ) describe the hydro-
logical and diagenetic mechanisms that formed fracture - related caves along the east