Carbonate Sequence Stratigraphy and Field Examples:

Carbonate Sequence Stratigraphy and Field Examples:
Fundamentals of Carbonate Sequence Stratigraphy

Charles Kerans
Scott W. Tinker
Bureau of Economic Geology

Design Objectives

Understanding carbonate rocks: accomodation, depositional environments, and facies.

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 useful in the interpretation of depositional topography. For example: 1) cores combined with wireline data logs 2) seismic images. 3) borehole 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.

Topography and Accomodation

After James (1988)

Depositional topography represents a snapshot in time. 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.

Definition:

Accomodation: The space available for sediment to accumulate (Jervey, 1988). Accommodation is a composite of eustacy, subsidence, compaction, tectonism, and erosion.

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.

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.

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).

A thin section photomicrograph example of mud supported carbonate rock.

Grain-supported fabrics from the Permian, including fusulinids and algae, are shown in this thin section photomicrograph. Calcite is stained pink; porosity is blue.

This outcrop reveals mound topography in a tidal-flat environment of algal boundstones (invisible geologist glove for scale).

Grötsch and Mercadier (1970)

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.

after Winkinson (1982)

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.

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.

after James (1983)

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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