Geology Reference
In-Depth Information
Fig. 4.8.
Implication of carbonate grain studies. A statistically-
based microfacies analysis of core slabs of gas-producing
carbonate reservoir rocks demonstrates the relationships
between grain types, depositional subfacies, and porosity
distribution. The subtypes can be recognized in cuttings as
well.
A
: Presence and composition of grains are indicative of
subfacies types that reflect specific environmental conditions
and settings on a tidal to subtidal carbonate platform. Peloids
and coated grains are the most reliable indicators of subtypes.
Peloids are differentiated according to shape and size, ooids
according to size, number of laminae and shape, and aggregate
grains (including grapestones) according to internal structure
and size.
B
: Intergranular and moldic porosity increases distinctly with
increasing ooid size and abundance. The amount of algal
content is negatively correlated with porosity. Late Permian
(Zechstein Ca2 platform): Northwest Germany. Modified
from Steinhoff and Strohmenger (1996).
organic phases (Mann 2001; Cuif and Sorauf 2001).
Both phases are involved in biomineralization and af-
fected by destructive processes. Today more than 60
biominerals occurring in bacteria, plants, protists and
metazoans are known. About 50% of the more com-
mon skeleton-building biominerals are represented by
carbonate phases. Other more common skeleton-
building minerals are phosphates and silica (Fig. 4.9).
Biomineralization follows two basic lines, charac-
terized by biologically controlled or biologically in-
duced processes.
Bio-controlled
deposition of miner-
als takes place in isolated spaces and in close associa-
tion with an organic matrix (Degens 1979, 1989;
Lowenstam and Weiner 1989; Simkiss and Wilbur
1989; Addadi and Weiner 1992). According to this
model, a matrix of acidic macromolecules rich in as-
partic and glutamic aminoacids acts as a binding sur-
A
B
