Page 146 - The Petroleum System From Source to Trap

Magoon, L. B, and W. G. Dow, eds., 1994, The petroleum
s y stem-from source to trap: AAPG Memoir 60.

Chapter 7

v
Carbonate Reser o ir Rocks

Clifton F. Jordan, Jr. James Lee Wilson
l11tegrated Data Services New Braullfels, Texas, U.S.A.
Bonne Terre, Missouri, U.S.A.

Abstract
The main factors in evaluating carbonate reservoirs are lithofacies, pore type, shelf setting,
sequence stratigraphy, and diagenetic overprint. Several patterns are evident based on a review of
carbnate reseoirs from around the world. First, dolomites, graitones, and boundstones are the
most common carbonate reseoir rock type, but any carbnate lithofacies can be modifed by
diagenesis to form porous rock Second, secondary pore type tend to dominate carbnate reseoir
facies, as opposed to prima pore type. Thir, inner shelf, outer shelf, and slope lithofacies belts
are prime exploration fairwys that are relatively predictable, with middle shelf prospect being
less so. Fourth, sequence stratigraphy descrbes the shelf-building and basin-filling pattern of
carbnate sediments and provides usef models for exploration and production. Fmally, carbnate
sediments are subjected to two main type of diagenec overprinting: steady subsidence into deep
buri real of diagenesis or subsidence interrupted by one or more periods of uplift and associ­
ated porosity-producing diagenetic reactions.

INTRODUCTION emerge from this data: (1) the number of basins that
produce hydrocarbons from carbonate rocks is lowest
Carbonate reservoirs with their seemingly endless for South America, Africa, and Australia and highest for
variety of textures, fossil components, and diagenetic North America and Eurasia; and (2) Tertiary carbonate
overprints present a challenge to categorize (Owen, 1964; reservoir rocks are found mainly in Southeast Asia,
Schmoker et a!., 1985). Nonetheless, there are generalized whereas the rest of the world's production is primarily
trends and patterns that account for oil and gas produc­ from Paleozoic and Mesozoic carbonate reservoirs,
tion from carbonate reservoir rocks on a worldwide which includes the giant and supergiant Jurassic and
basis. These patterns are the subject of this review and Cretaceous fields of the Middle East.
are discussed here with regard to five considerations:
lithofacies, pore type, shelf setting, sequence stratig­
raphy, and diagenetic overprint. This review concen­ LITHOFACIES
trates on reservoir facies development and not on source
rocks, seals, or traps involving carbonates. It was Many schemes of classifying carbonate rocks have
necessary to avoid the diversity of rock descriptions used been proposed, but that of Dunham (1 %2) has been used
in the literature by standardizing lithofacies terminology in more studies involving carbonate rocks than any
for purposes of comparison. Hence, throughout this other. The reasons for this are its simplicity and direct­
d i scussion, a scheme of "symbol logic" is used to ness, as well as its effectiveness in accurately describing
describe the texture, composition, sedimentary struc­ reservoir facies in carbonate rocks. The reader is referred
tures, and diagenetic overprints of carbonate rocks to Dunham's (1%2) classic paper in which he described
(Jordan, 1985). This review is directed toward two main grain versus matrix support, the effects of particle shape
audiences: exploration geologists, who need to under­ on grain packing, and the spectrum of carbonate rock
stand carbonate reservoirs in a regional framework, and types observed from mudstones through grainstones
production geologists, who need to know details and boundstones.
regarding the distribution of reservoir facies in a partic­ Dunham's attention to grain- versus matrix-sup­
ular field. ported framework is more in accord with the principles
Figure 7.1 shows the worldwide distribution of basins of carbonate sedimentation than classifications that
that produce hydrocarbons from carbonate reservoirs. A consider primarily particle size or amounts of matrix.
list of these basins and the ages of their carbonate reser­ Particulate carbonate sediment, unless w i nnowed
voirs are shown in Table 7.1 . The following trends completely to form a grainstone deposit, is a mixture of
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