260
6 DEPOSITIONAL SYSTEMS
pock marks and cold seeps, as described elsewhere (see Section 5.3.5.7). The petroleum
may have fed methanogenic bacteria, which formed the base of a higher food chain of
invertebrates. The occurrence of stromatactis structure (see Section 3.2.2.3.3) in mud
mounds suggests that gas bubbles were trapped in rapidly cemented lime mud, and the
resultant voids then infilled by carbonate cement.
6.3.2.8.3 Economic aspects of carbonates in general and reefs
in particular
Carbonate rocks, especially reefs, are of great economic importance. They may be quarried for building stone or aggregate, they may serve as aquifers, and they host many
ores and nearly 50% of the world's oil. In carbonates, like sandstones, the amount and
distribution of porosity and permeability control their suitability as aquifers, petroleum
reservoirs, and sites of mineralization. There is, however, a major difference between
the distribution of porosity and permeability in carbonates, when compared with sandstones. In sandstones the effects of diagenesis are normally not too extreme. Porosity is
largely primary and related to facies. As Chapter 9 shows, however, porosity in carbonates is largely secondary due to extensive diagenesis. The porosity and permeability of
ancient carbonates is therefore often unrelated to their original facies-related primary
porosity distribution. Thus many limestone aquifers owe their capacity to tectonically
controlled fracture pore systems, or to leached paleokarst horizons where unconformities cross-cut facies and stratigraphy.
Limestones and dolomites often make excellent petroleum reservoirs. This is because
they are commonly developed along basin margins, acting as natural traps for petroleum
migrating up from basinal source beds. Reefs have been aptly termed "sedimentary
anticlines." They are often transgressed by organic-rich muds that serve both as source
and seal (Fig. 6.64). There are problems, however. The eccentric distribution of porosity and permeability in carbonate petroleum reservoirs has mystified geologists and engineers for many years. Numerous excellent studies, mostly unpublished, describe the
diagenesis and porosity evolution of carbonate reservoirs in the most intimate detail
(e.g., Reeckman and Friedman, 1982). Sadly, most of these studies are of limited predictive value, and are unable to guide the drilling and development program of a field.
In many carbonate provinces the distribution of petroleum often appears to be random, and within the fields themselves porosity is unrelated to depositional environment.
Nonetheless, Wilson (1975, 1981) has described a series of characteristic petroleum reservoir models (Fig. 6.65).
Carbonates in general, and reefs in particular, are hosts to ore bodies. The most characteristic variety is lead-zinc sulfide mineralization. This is often referred to as being
of Mississippi Valley type, because that is one region noted for such ores (Brown, 1968;
Ohle, 1980; Gustafson and Williams, 1981; Wolf, 1981; Anderson and McQueen, 1982;
Briskey, 1982; Clemmey, 1985). The Mississippi Valley mining district extends through
Oklahoma, Kansas, and Missouri. Other similar North American examples are known
in Wisconsin, and at Pine Point in the Northwest Territories of Canada (Fig. 6.66). There
are many examples in the Carboniferous limestones of Europe, ranging from Poland to
Yugoslavia, England and Ireland (Hitzman and Large, 1986). Fluid inclusion studies of
6 DEPOSITIONAL SYSTEMS
pock marks and cold seeps, as described elsewhere (see Section 5.3.5.7). The petroleum
may have fed methanogenic bacteria, which formed the base of a higher food chain of
invertebrates. The occurrence of stromatactis structure (see Section 3.2.2.3.3) in mud
mounds suggests that gas bubbles were trapped in rapidly cemented lime mud, and the
resultant voids then infilled by carbonate cement.
6.3.2.8.3 Economic aspects of carbonates in general and reefs
in particular
Carbonate rocks, especially reefs, are of great economic importance. They may be quarried for building stone or aggregate, they may serve as aquifers, and they host many
ores and nearly 50% of the world's oil. In carbonates, like sandstones, the amount and
distribution of porosity and permeability control their suitability as aquifers, petroleum
reservoirs, and sites of mineralization. There is, however, a major difference between
the distribution of porosity and permeability in carbonates, when compared with sandstones. In sandstones the effects of diagenesis are normally not too extreme. Porosity is
largely primary and related to facies. As Chapter 9 shows, however, porosity in carbonates is largely secondary due to extensive diagenesis. The porosity and permeability of
ancient carbonates is therefore often unrelated to their original facies-related primary
porosity distribution. Thus many limestone aquifers owe their capacity to tectonically
controlled fracture pore systems, or to leached paleokarst horizons where unconformities cross-cut facies and stratigraphy.
Limestones and dolomites often make excellent petroleum reservoirs. This is because
they are commonly developed along basin margins, acting as natural traps for petroleum
migrating up from basinal source beds. Reefs have been aptly termed "sedimentary
anticlines." They are often transgressed by organic-rich muds that serve both as source
and seal (Fig. 6.64). There are problems, however. The eccentric distribution of porosity and permeability in carbonate petroleum reservoirs has mystified geologists and engineers for many years. Numerous excellent studies, mostly unpublished, describe the
diagenesis and porosity evolution of carbonate reservoirs in the most intimate detail
(e.g., Reeckman and Friedman, 1982). Sadly, most of these studies are of limited predictive value, and are unable to guide the drilling and development program of a field.
In many carbonate provinces the distribution of petroleum often appears to be random, and within the fields themselves porosity is unrelated to depositional environment.
Nonetheless, Wilson (1975, 1981) has described a series of characteristic petroleum reservoir models (Fig. 6.65).
Carbonates in general, and reefs in particular, are hosts to ore bodies. The most characteristic variety is lead-zinc sulfide mineralization. This is often referred to as being
of Mississippi Valley type, because that is one region noted for such ores (Brown, 1968;
Ohle, 1980; Gustafson and Williams, 1981; Wolf, 1981; Anderson and McQueen, 1982;
Briskey, 1982; Clemmey, 1985). The Mississippi Valley mining district extends through
Oklahoma, Kansas, and Missouri. Other similar North American examples are known
in Wisconsin, and at Pine Point in the Northwest Territories of Canada (Fig. 6.66). There
are many examples in the Carboniferous limestones of Europe, ranging from Poland to
Yugoslavia, England and Ireland (Hitzman and Large, 1986). Fluid inclusion studies of