Sequences during low-amplitude, high-frequency cycles

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting reservoir system quality and performance
Author	Dan J. Hartmann, Edward A. Beaumont
Link	Web page
Store	AAPG Store

General characteristics

During greenhouse times, third- through fifth-order cycles of sea level change because of little or no polar ice, and they tend to have high frequency and low amplitude. Read (1996) lists the general characteristics of carbonate platforms deposited during these cycles:

• Cycles of 20 k.y. or less sometimes superimposed on 100- and 400-k.y. cycles
• Carbonate platforms aggraded and flat topped to gently sloping
• Peritidal parasequences with regionally extensive tidal flat caps
• High-relief buildups absent from the platform top
• Parasequences with layer cake stacking patterns
• Reef and/or grainstone facies of platform margins with limited lateral migration, thick and poorly partitioned or highly compartmentalized
• Relatively poorly developed cycle-capping disconformities
• Small sea level changes limiting groundwater table vertical migration and consequently diagenesis

Arid zone characteristics

Carbonates deposited in arid zones during greenhouse times generally have the following characteristics (Read, 1996):

• Oolitic and cryptalgal mound facies with intertidal laminite caps
• Completely to partly dolomitized parasequences
• Reservoirs less than length::3 m thick
• Reservoir-quality facies that are
  • Dolomitized intertidal laminites (intercrystalline porosity)
  • Siliciclastic supratidal caps (intergranular porosity)
  • Variably dolomitized subtidal grainstone/packstone shoals (intergranular and intercrystalline porosity)
• Regional top and lateral seals composed of sabkha evaporites, making traps strongly stratified with potential for multiple pay zones
• Sulfate minerals plugging the pores in grainstones (calcite plugging rarely significant but hard to leach if present)

Examples are the Cambrian–Early Ordovician of the U.S., Early Silurian Interlake Formation of the Williston basin, and Upper Pennsylvanian San Andres–Grayburg Formations of the Permian basin.

Humid zone characteristics

Carbonates deposited in humid zones during greenhouse times generally have the following characteristics:

• Parasequence facies of very fossiliferous subtidal wackestones to grainstones with fenestral or rare supratidal caps
• Planar to microkarsted cycle tops
• Leached aragonite fossils
• Fibrous marine and vadose–phreatic sparry calcite cements plugging fenestral porosity
• Best original porosity in subtidal facies; cycle tops more porous than arid counterparts
• Good reservoir facies in downdip, noncyclic, subtidal grainstone complexes
• Poor internal top and updip seals; traps form only as a result of later events such as being sealed by overlying transgressive systems tract organic muds or shales

Examples of low-amplitude cycles in humid zones include Middle Ordovician peritidal sequences, Middle to Late Devonian Swan Hills–Judy Creek Formations in Canada, and Mississippian sequences in Virginia.

Contrasting arid and humid zone cycles

The diagram below contrasts low-amplitude carbonate arid zones sequences with humid zone sequences.

Figure 9-81 Modified. Copyright: Read, 1995; courtesy SEPM.

See also

• Predicting carbonate porosity and permeability
• Carbonate facies
• 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 moderate-amplitude, high-frequency cycles
• Sequences during high-amplitude, high-frequency cycles
• Predicting carbonate reservoir location and quality

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