Page 48 - Geology of Carbonate Reservoirs

CLASSIFICATION OF CARBONATE ROCKS 29

have distinct seaward and leeward sides because they are shaped or “ polarized ” by
the prevailing wind – wave direction. Windward sides of reefs are characterized by
massive and encrusted organic growth with boulder - to - gravel - sized particles as
rudstones. Leeward sides of reefs are characterized by higher percentages of detrital

sediments such as floatstones, grainstones, and more delicate growth forms of reef
organisms. Shallow - water patch reefs and shelf - edge reefs tend to be streamlined in
plan view with buttress - like structures and more massive skeletal frameworks on
the windward side. In modern coral – algal reefs, structures called spur and groove
or buttress and chute develop on the windward sides of reefs (Shinn, 1963 ; James,
1983 ). Deep - water buildups, or those that grew in protected shallows, do not exhibit
windward and leeward sides, streamlining, or polarized facies geometry. In short,
reservoir characteristics of reef rocks are related to variables different from those

that form the basis for detrital rock classifications. Embry and Klovan (1971) found
it hard to map reef reservoir porosity zones with nothing more defi nitive than
Dunham ’ s boundstone or Folk ’ s biolithite terminology. They cited papers from the
1960s focusing on, if not lamenting, the problem. As an improvement, they adapted

Dunham ’ s classification to reef reservoirs and developed a more detailed scheme
to account for different organic growth forms and for the associated detrital carbon-
ates that surround and fill - in open spaces within reefs.

The Embry – Klovan terms framestone , bindstone , and baffl estone refer to growth
patterns of reef organisms. That is, a reef constructed of stout coral skeletons in a
girder - like frame arrangement is called a framestone. One in which the reef is con-
structed by pavement - builders or encrusters is known as a bindstone, and reefs that
exhibit detrital carbonate accumulations in the midst of organic thickets such as
seagrass beds are known as bafflestones. There is some controversy over whether

reefs can truly be formed by baffl ing action of sessile benthonic organisms because

sessile benthonic animals are mainly filter feeders that would be smothered by a
rain of carbonate sediment. If the bafflers were plants, true grasses can be eliminated

for much of geological history, as they have existed only since the Mesozoic Era.

Bafflestones tend to have high proportions of lime mud, and there is evidence,
especially in mud mounds of Lower Carboniferous age, that muds are formed in
place by biological or biochemical processes rather than having been trapped by

organisms. The term bafflestone is considered by Tucker and Wright (1990) to be
“ rather subjective. ” Those authors also point out, that reef rocks are subject to
extensive diagenesis and bioerosion that may dramatically alter the original rock
fabric. Diagenetic micritization of reef rocks is common and may account for the
loss of 20 – 70% of the original reef framework (Tucker and Wright, 1990 ). Finally,
the Embry – Klovan terms rudstone and fl oatstone refer to detrital rocks associated
with reefs. Rudstones are the reef - derived, gravel equivalent of grainstones and
packstones; floatstones are the gravel and sand equivalent of wackestones.

Riding (2002) developed an alternative classification for reef rocks. He defi nes
reef as “ in - place calcareous deposits with topographic relief, created by sessile
organisms. ” The different types of reef rocks are classified on the basis of whether

there is matrix (mud) support (the carbonate mound category), skeletal support (the
frame reef category), or cement support (the cement reef category). Terms such as
sparse and dense are used to describe the three - dimensional fabric of skeletal ele-
ments in matrix supported reefs, and open, tight, and solid describe the architecture
of the constructor assemblages in frame reefs. These terms refer to spatial patterns