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Understanding
carbonate rocks: accomodation, depositional environments, and facies.
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The single
most important stratigraphic variable that controls the resultant
rock record in a shallow marine setting is depositional topography.
Simply put, the more varied the depositional topography the more
complex the resultant lithofacies, and the more difficult it will
be to interpret stratigraphy between wells. When building any scale
of stratigraphic model, it is critical to unravel the depositional
tapestry.
There are several fundamental data types that provide information
regarding depositional topography. For example: 1) cores combined
with wireline data logs 2) seismic images. 3) brothel image logs.
4) biostratigraphy 5) chemostratigraphy 6) magnetostratigraphy.
Sesimic images,
with the obvious caveats discussed in the prior section, can be
useful for interpretation of depositional geometry. Borehole image
logs can be quite valuable in terms of determining bedding dip at
the borehole scale, which can be a good proxy to depositional topography.
Biostratigraphy, chemostratigraphy and magnetostratigraphy can each
provide time-significant information that leads to improved sequence
stratigraphic interpretation, from which depositional topography
is inferred. Cores and logs, and the interpretation of facies from
these data, warrants further discussion.
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Depositional
topography represents a snapshot in time and is the single most
important variable controlling the depositional rock record in carbonates.
Changes in topography through time are represented by changes in
facies though time, and are a record of the long-term accomodation
history of a system.
In this sense, the available space is the 3-D volume bounded by
the water-sediment interface below, and base-level above.
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Definition:
Accomodation:
The
space available for sediment to accumulate (Jervey, 1988).
Accommodation is a composite of eustacy, subsidence, compaction,
tectonism, and erosion.
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In shallow-water
carbonate systems, sea-level is a reasonable proxy for base-level.
In some instances storm wave base can supplant sea level as the
upper limit of sediment accumulation, but in the big picture, the
two critical surfaces that define accommodation variation are the
water-sediment interface and air-water interface (sea level). There
is a systematic relationship between accommodation, systems tracts,
and stratigraphic styles. In general, the transgressive systems
tract is associated with higher accommodation during base-level
rise and the highstand systems tract is associated with low accommodation
settings during base-level fall. Therefore, interpreting the
position of the dataset within a depositional sequence can afford
substantial predictive information concerning stratigraphic style
and correlation methodology.
Periods of high accommodation (TST) are commonly characterized
by:
• thicker,
high-frequency cycles (the climbing limb of a Fischer Plot) (Read
and Goldhammer 1988; Goldhammer et al. 1990);
• greater vertical lithofacies diversity, including well-developed
transgressive "flooding surface" mudstones which can
be critical in vertical segmentation (Kerans et al. 1994);
• greater depositional topography, whether this be recorded
as biologic buildup "reefs" or grainstone shoals (cf.
Greenlee and Lehmann 1993);
• greater facies continuity in the dip direction (with the
exception of buildup relief);
• domination by tidally influenced deposition (Cross et al.
1992, Kerans et al. 1995; Barnaby and Ward 1995).
In contrast,
periods of low accommodation (HST) are characterized by:
• thinner
high-frequency cycles (the falling limb of a Fischer Plot);
• a less diverse vertical lithofacies succession within any
individual cycle, with a tendency towards cycle amalgamation;
• an abundance of grainstone- or tidal flat mudstone-capped
cycles
• downdip facies continuity is limited with a tendency towards
a more shingled offlapping correlation style;
• domination by wave influenced deposition.
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Definition:
Lithofacies
(Facies):
Basic descriptive rock element distinguished by lithology,
texture, grain components, pore types, sedimentary structures,
and bedding styles. Facies and facies successions provide
a record of depositional environment, and are used to infer
such physical parameters as water depth, depositional energy,
sediment supply, light, and temperature.
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| Lithofacies data
are critical for stratigraphic interpretation, construction of depositional
models, petrophysical analysis and prediction, and 3-D geologic modeling.
In order to quantify and use lithofacies data in 3-D modeling, the
dominant set of lithofacies (defined based on some combination of
lithology, texture, grain composition, and depositional environment)
can be given an ordinal numeric code. This can either be sequential
in terms of the ideal stratigraphic succession, or sequential in terms
of petrophysical quality (e.g. from lowest to highest porosity and/or
permeability). Fortunately, in carbonate rocks that have not been
extensively altered by diagenesis, reservoir quality often mimics
stratigraphic succession (e.g., a low-energy, subtidal carbonate mudstone
at the base of a cycle has the poorest reservoir quality, and a high-energy,
subtidal ooid grainstone at the top of a cycle has the best reservoir
quality), so the petrophysical succession can be similar to the depositional
succession. Numeric facies coding facilitates statistical analysis
and lithofacies prediction in wells without core control, which is
critical in all reservoirs that have limited core control (most reservoirs). |
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A
thin section photomicrograph example of mud supported carbonate
rock.
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| Grain-supported
fabrics from the Permian, including fusulinids and algae, are shown
in this thin section photomicrograph. Calcite is stained pink; porosity
is blue. |
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| This
outcrop reveals mound topography in a tidal-flat environment of algal
boundstones (invisible geologist glove for scale). |
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Grötsch
and Mercadier (1970)
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Permeability
in carbonate systems is constrained by the combination of lithology,
porosity, pore type, cementation, and degree of dolomitization.
The Lucia porosity/permeability classification system extends the
Dunham system by including grain-dominated packstone, a texture
class between grainstones and packstones, and is important in understanding
petrophysical properties. A series of photographs show ranges of
porosity and permeability that all fall within a single Lucia class,
but are of different rock types.
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after
Winkinson (1982)
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| Changes
in grain components through time are an important part of facies identification.
Most are predictable long term changes; others indicate episodic or
catastrophic extinction events. Grain component changes are primarily
the combined result of eustacy, plate tectonics, climate changes driven
by earth processes, and extraterrestrial causes. |
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| Pore type is
also an important component of facies identification. The Choquette
and Pray system (Fig. 8) classifies a variety of basic pore types.
Those in the left column are useful for stratigraphic analysis, while
those in the right column impact fluid flow in rock systems, and are
useful for both exploration and production problems. |
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after
James (1983)
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| Faunal
growth forms are an important key in understanding certain reef depositional
environments. Different growth forms indicate varying levels of wave
energy and sedimentation, and are therefore indicators of accommodation. |
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