Sequences during high-amplitude, high-frequency cycles
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| Series | Treatise in Petroleum Geology |
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| Chapter | Predicting reservoir system quality and performance |
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High-amplitude (60 to over length::100 m), high-frequency (fourth- and fifth-order; see Hierarchy of sequences) sea level fluctuations occurred during periods of global continental glaciation.[1] These icehouse periods were late Precambrian, Pennsylvanian to Early Permian, and Pleistocene (see International Chronostratigraphic Chart).
General characteristics
The following are general characteristics of carbonate sequences deposited during icehouse conditions:[1]
- Flat-topped platforms with layer cake, 1–10-m-thick fourth-order sequences bounded by regional disconformities; sequences on platform margins shingled
- Ramp sequences highly shingled and erosionally bounded
- Pinnacle reefs and high-relief banks common on tropical platforms
- Tidal flat facies absent except adjacent to shorelines
- Deeper water facies juxtaposed with shallow-water facies and emergence features (except on the shallowest parts of platforms) because of large sea level changes
- Karstic sinkholes and cave systems extend down through several sequences
- Intense leaching/cementation (and sometimes dolomitization) possible because stillstands repeatedly localize paleowater tables
- Diagenesis usually complex because large sea level changes repeatedly cause large vertical and lateral migration of diagenetic zones
Arid zone characteristics
Carbonate sequences deposited in arid zones tend to have the following characteristics:[1]
- Porosity plugged by caliche at sequence caps
- Below caliche cap porosity is intergranular in shoals and in nondolomitized build-ups
- Porosity is intercrystalline and remnant intergranular in dolomitized sequences that consist of inner platform highstand sabkha facies or late highstand to low stand evaporite basin facies
Humid zone characteristics
Carbonate sequences deposited in humid zones tend to have the following characteristics:[1]
- Single to multiple caliche zones at sequence tops if wet–dry seasons
- In a sequence package that lacks internal seals, the uppermost sequences generally have preserved intergranular porosity in contrast to lower sequences, which generally have moldic, vuggy, and cavernous porosity
- In a sequence package that contains internal seals, the upper part of each sequence can have preserved primary porosity because the seals protect the sediments from diagenesis
High-amplitude sequences schematic
Figure 1 shows (1) a typical succession of carbonate lithofacies sequences that formed during high-amplitude, high-frequency sea level fluctuations and (2) a corresponding sea level curve related to diagenesis. In this example, sequences lack internal barriers or seals to inhibit the vertical and lateral migration of the paleowater table; therefore, sequence sediments are subjected to a complex sequence of diagenetic events due to large-scale sea level fluctuations. If internal barriers were present, diagenesis would be lessened.
See also
- Predicting carbonate porosity and permeability
- Carbonate facies
- Carbonate diagenesis
- Carbonate diagenetic stages
- Early carbonate diagenesis
- Basics of carbonate porosity formation and preservation
- Sea level cycles and carbonate sequences
- Sea level cycles and carbonate diagenesis
- Sea level cycles and climate
- Sequences during low-amplitude, high-frequency cycles
- Sequences during moderate-amplitude, high-frequency cycles
- Predicting carbonate reservoir location and quality
References
- ↑ 1.0 1.1 1.2 1.3 Read, J., F., 1995, Overview of carbonate platform sequences, cycle stratigraphy and reservoirs in greenhouse and ice-house worlds, in Read, J., F., Kerans, C., Webber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-house Worlds: SEPM Short Course 35, 183 p. Good summary of concepts of climatic effect on sea level cycles, carbonate deposition, and reservoir development.
- ↑ Read, J. F., and A. D. Horbury, 1993, Eustatic and tectonic controls on porosity evolution beneath sequence-bounding unconformities and parasequence disconformities on carbonate platforms, in A. D. Horbury and A. G. Robinson, eds., Diagenesis and basin development: AAPG Studies in Geology 36, p. 155-197
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