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

Tools of sequence stratigraphy

Symbology

The process of depositional fill is illustrated schematically in the figure below. Blue triangles indicate transgressive deposition (deepening up) and green triangles indicate highstand deposition (shallowing up). In this example, three High Frequency Sequences comprise a Composite Sequence. The triangles help to visualize the changing proportion of facies from highstand dominated successions on the landward (left) side of the illustration, to transgressive dominated successions on the seaward (right) side of the succession. The proportion of highstand-dominated facies increases upward in any vertical succession, indicating an overall prograding sequence. It is these types of rock-based observations from 1-D well bore data that lead to a successful 2-D and 3-D sequence stratigraphic interpretation.

COMPOSITE SEQUENCE: PROGRADING HFS

There are certain rules and tools of sequence stratigraphic analysis that are useful for interpreting 1-D well bore data. These are illustrated schematically in the figure below. A set of Walther's Law depositional models is first created based on core observations, regional knowledge, outcrop analogs and any other information. These models will guide 1-D cycle analysis. An analysis of facies proportions, thickness variations, and cycle symmetry guide the interpretation of a vertical succession into cycles, cycle sets and high frequency sequences. Cycle symmetry describes the proportion of overall deepening versus overall shallowing facies present in a given cycle, and provides information about the longer-term prograding, aggrading, or backstepping signal. For example, thickening upward cycles composed of an increasing proportion of transgressive facies indicate an overall backstepping succession, whereas thinning upward cycles composed of a increasing proportion of highstand facies indicate an overall prograding succession.

Stratal dip from borehole image logs, dipmeter data, or high-frequency seismic data can be useful for interpreting the stratigraphic architecture of a system. Facies offset is an indicator that there has been a significant change in what the Walther's Law model s would predict, and can help to rank the significance of surfaces. Indications of exposure surfaces in core or logs can be useful for interpreting unconformities and significant gaps in rock record. Prediction of key indicator facies from logs is vital because most wells in a field are not cored. Seismic data can be useful, but as illustrated in earlier sections can also be quite misleading in prograding systems because seismic reflectors tend to follow similar lithofacies and actually cross depositional surfaces at the reservoir scale. Finally, biostratigraphic resolution is rarely adequate for reservoir scale interpretation. However, increase understanding of paleoecology and associated water depth is an area of reach that has significant potential for improving sequence stratigraphic understanding.

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