Page 28 - Geology of Carbonate Reservoirs

UNIQUE ATTRIBUTES OF CARBONATES 9

maps. Diagenesis can be a pore - forming and a pore - destroying process. Diagenetic
patterns are related to environments at the surface, in the shallow subsurface, and
in the deep burial environment. Shallow diagenetic environments are commonly
linked to depositional or paleostructural trends and these relationships can be seen
with map overlays. Deep - burial diagenetic trends record the burial and thermal
history of basin evolution, although the odd tectonic or geothermal event may also

influence late diagenesis. Finally, natural fracture trends can be detected with a
combination of special core analyses and acoustic or electronically generated bore-
hole images. These combined types of data provide information on the size, intensity,
and orientation of natural fractures. Fracture data plotted on structure maps can
sometimes be correlated with reservoir geometry, because in most cases fracture
trends correspond with geometrical orientation and position of the borehole with
respect to the geometry of subsurface structures such as faults and folds. Cavernous
and connected - vug pore systems could be mistaken for fractures, especially during
drilling; therefore borehole log responses, digital imaging logs, drilling history char-
acteristics, microscopic examination of drill cuttings, and the behavior of the drilling
mud system are commonly used together to test for the presence of natural frac-
tures. Pressure transient tests provide very important information to help confi rm
fracture behavior.

1.3 UNIQUE ATTRIBUTES OF CARBONATES

Texture is defined as the size, shape, and arrangement of detrital grains in a sedi-

mentary rock. In siliciclastic rocks, it is strongly influenced by parent rock type,
weathering, and transportation history. Most sandstones are classified on the basis

of how much quartz, feldspar, rock fragments, and matrix they contain. Fossils are
generally ignored. Sandstone diagenesis is usually treated as a process that alters
depositional texture, fabric, and porosity only after extensive burial. Sandstone
porosity and permeability are nearly always described as facies - specifi c; that is, the
rock properties of the depositional facies determine reservoir characteristics. Frac-
tured reservoirs in terrigenous clastics are less commonly reported in the literature
than are fractured reservoirs in carbonates, although fractures are certainly not
exclusive to one rock type or the other.
In contrast, carbonates have unique attributes that distinguish them from silici-
clastics and that require different methods of study. Some of these attributes were
recognized decades ago by Ham and Pray (1962) . First, carbonates form within the
basin of deposition by biological, chemical, and detrital processes. They do not owe
their mineralogical composition to weathered, parent rocks and their textures do
not result from transport down streams and rivers. Carbonates are largely made up
of skeletal remains and other biological constituents that include fecal pellets, lime
mud (skeletal), and microbially mediated cements and lime muds. Chemical con-
stituents, including coated grains such as ooids and pisoids, cements, and lime mud,
are common in carbonates but are absent in most siliciclastics. Clastic grains exist
in carbonates, as they do in siliciclastics. In carbonates, however, these grains are
mainly clasts of intraformational, lithified sediment (intraclasts) or of reworked,

older rock (lithoclasts). The second major difference between carbonates and silici-
clastics is that carbonates depend greatly on biological activity. They are composed