Geology Reference
In-Depth Information
by the differentiation of aragonitic and calcitic bioclasts
(Fig. 4.9) and the course of skeletal diagenesis (Sect.
4.2.1; Pl. 29/1; Pl. 30/1). Depositional frameworks and
primary porosity are controlled by the abundance and
distribution of calcareous algae (Sect. 10.2.1), Larger
foraminifera (Sect. 10.2.2), benthic sessile organisms
(Sect. 10.2.3) and various shells (Sect. 10.2.4). These
fossils can form specific biofabrics that control poros-
ity and permeability development (Sect. 5.1.2). Small-
scale heterogeneity can be related to different micro-
structural types of skeletal grains, e.g. rudists (Pl. 88;
Russell et al. 2002).
Peloids and cortoids.
These grains (Sects. 4.2.2 and
4.2.3) are susceptible to microborings and selective do-
lomitization. Both processes contribute to micro-
porosity.
Oncoids and rhodoids.
Oncoids (Sect. 4.2.4.1) are
common in lagoonal reservoir rocks, particularly in
microbial carbonates. Rhodoids (Sect. 4.2.4.2) are
major constituents of Tertiary reservoir rocks. Morpho-
logical and systematic differentiation enables paleo-
depths and subenvironments to be recognized.
Ooids.
Depending on their primary mineralogy and
the composition of their nuclei and cortices, ooids (Sect.
4.2.5) are susceptible to complete or partial leaching,
resulting in oomolds (Pl. 13/7). Micritic ooids contrib-
ute to microporosity and are susceptible to dolomitiza-
tion (Pl. 13/8). Many oolitic reservoirs are controlled
by paleoenvironmental conditions producing shoal- or
sheetlike geometries (Fig. 4.26) and deposition and/or
mixing of aragonitic and calcitic grains.
Pisoids
. Pisolitic limestones (Sect. 4.2.6) can act as
reservoirs if the interparticle porosity is enhanced by
vadose dissolution of interparticle cements.
Aggregate grains
(Sect. 4.2.7) associated with oo-
ids and peloids are common constituents of lagoonal
and open-platform reservoirs.
Lithoclasts.
Lithified rock particles contribute to the
formation of debris and talus accumulated in proximal
or distal positions from the source area. A thin-section
based differentiation of clast composition in cores is
necessary for distinguishing forereef talus from litho-
clasts of debris flows or turbidites.
birdseyes may facilitate fluid and gas migration. Key-
stone vugs (Pl. 29/9) may be responsible for high po-
rosities.
Burrowing and bioturbation
(Sect. 5.1.4).
Intercon-
nected burrows forming networks may have a distinct
influence on cementation, permeability and porosity.
Nodular fabrics
(Sect. 5.1.6). Nodular limestones
can exhibit strongly varying porosity values due to the
inhomogeneous composition of the rock. Variations de-
pend on the proportion of nodules and matrix, and the
mode of origin of the nodules.
Fissures
(Sect. 5.3.1). Depositional and tectonic fis-
sures related to block-faulting can act as important mi-
gration paths for hydrocarbons. The history of fissure
networks can be deciphered from the relations of inter-
nal sediment, cements and multiple rupturing.
Fractures and veins
(Sect. 5.3.2). Microfractures
associated with shear zones, extensional movements
or hydraulic fracturing can be used to predict macro-
fracture patterns acting as hydrocarbon migration paths
and reservoirs.
Breccias
(Sect. 5.3.3). Fractured talus breccias as
well as solution breccias may yield high porosities, de-
pending on the mode of breccia origin.
• Pores
Evaluation of visible porosity in thin sections is
commonly based on thin sections impregnated with
blue-dye resin (Yanguas and Dravis 1985) and must
consider the following questions: Abundance and dis-
tribution of open and closed pores. Are open pores re-
stricted to specific areas? Are the pores occluded by
early or late diagenetic cement? Frequency of fabric-
selective and non-fabric selective pore types using the
terminology and classification by Choquette and Pray
(1970; Fig. 7.5; Box 7.4). Which pore type dominates?
Are primary pores, e.g. interparticle pores, enlarged by
solution grading into vugs and enhancing effective po-
rosity (Pl. 29/9)? Are intraskeletal pores of importance
(see Bachman 1984)? Are moldic pores restricted to
specific grain types (e.g. oomoldic, biomoldic)? Are
vugs connected or disconnected (McNamara et al.
1992)? Are pores collapsed (Mowar et al. 1998)? The
3D pore system and dynamic properties of the pore
network are measured by special core analysis as input
for dynamic modeling. The pore network forms the link
between static characterization of the reservoir and its
dynamic behavior.
• Fabrics
Sedimentary and diagenetic fabrics enhance and/or
reduce porosity and permeability.
Lamination and bedding
(Sect. 5.1.3). Lamination
caused by finely distributed clay may reduce perme-
ability. Interbedding of millimeter- to centimeter-thick
layers consisting of wackestone or mudstone with grain-
stone layers can result in different porosity values.
Fenestral fabrics
(Sect. 5.1.5). Interconnected
• Cement
Cementation patterns must be described in regard
to cement mineralogy (carbonate, anhydrite, silica etc.),
cement types (Sect. 7.4.2.1), cement fabrics (Sect.
