Page 162 - The Petroleum System From Source to Trap

7. Carbonate Reservoir Rocks 155

Figure 7.15. Profile of the distrib­
ution of subsurface fluids
showing diagenetic environ­
ments in an idealized shelf to
basin profile. The single middle
shelf high through which the
columnar section of Figure 7.16
is drilled is exaggerated in height
to show the freshwater lens
typical of most islands.

MARINE PHREATIC M-PH

- -- -- - --- ---- ----- ----- --
- - - - - - -- - -- - -- -
SUBSURFACE BRINES

phreatic into shallow burial and finally deep burial First, inner shelf, outer shelf, and slope lithofacies belts
diagenetic realms, or (2) uplift from the marine phreatic are prime exploration fairways that are relatively
to be exposed to meteoric diagenesis, then subsidence predictable. Second, middle shelf prospects are variable
back into the marine realm and finally into burial diagen­ in their size and distribution and present more difficult
esis. Carbonate sediments tend to build upward to sea exploration problems. Third, slope facies may exist as a
level, thus meteoric exposure commonly affects at least porous downslope extension of an outer shelf fairway,
inner shelf deposits. It also strongly affects emergent formed as debris flow deposits, and may host belts of
shoals or reefs of the middle and outer shelf. The cause of porous pinnacle reefs. Finally, basinal or oceanic settings
dissolution porosity remains problematic in that several may produce porous chalk facies or may have shallow
modes of origin exist: (1) subaerial exposure, (2) regional water carbonate facies deposited as atolls on horst blocks
freshwater aquifers extending out below the sea floor, or or volcanic pedestals, producing rimmed margins of
(3) deep burial reactions involving weak acids produced outer shelf facies that encircle a central lagoonal area
at depth from the dewatering of shales or from the with numerous middle shelf patch reefs.
formation of weak organic acids associated with the
maturation of kerogen. Of these possibilities, the first
two-occurring near or at the surface-appear to be
most likely because fluids there are exchanged relatively Acknowledgments This paper was improved by reviews
rapidly and contain relatively high concentrations of from William A. Morgan of Conoco, Inc., and Perry 0. Roehl
unspent reactants. of Trinity University. Thanks for contributions to Figure 7.1
Carbonate diagenesis is greatly limited by the and Table 7.1 by Mark Longman (Consultant, Denver,
presence of migrating hydrocarbons. As pores become Colorado), Ian Russell of Mobil Exploration and Producing,
filled with less reactive substances, rock-water reactions Australia, and Mateu Esteban (ERICO-Petroleum Infonna­
are restricted to residual water saturations that coat pore tion, London, England). Computer drafting was done by Ceth
walls as thin films (Feazel and Schatzinger, 1985). Jordan.

CONCLUSIONS
References Cited
The lithology and types of porosity that characterize
carbonate reservoirs are summarized in Figure 7.17,
which also shows geographic positions favoring porosity Bebout, D. B., and R. G. Loucks, 1977, Cretaceous carbonates
development on shelf to basin profiles. Wilson (1980a,b) of Texas and New Mexico, applications to subsurface
summarized the occurrence of carbonate reservoirs as exploration: The University of Texas at Austin, Bureau of
Economic Geology Report of Investigations 89, 332 p.
seven recurrent settings. Table 7.3 lists these generically, Choquette, P. W., and L. C. Pray, 1970, Geologic nomenclature
whereas Table 7.4 presents a summary based on 39 field and classification of porosity in sedimentary carbonates:
studies of carbonate fields by Roehl and Choquette AAPG Bulletin, v. 54, p. 207-250.
(1985). Dunham, R. J., 1962, Classification of carbonate rocks
From these data and the basic carbonate lithofacies according to depositional texture, in W. E. Ham, ed., Clas­
patterns discussed and portrayed, certain trends emerge. sification of carbonate rocks: AAPG Memoir 1, p. lOS--121.