--> Abstract: Using Quaternary Reefs and Platforms as a Comparative Tool, by William F. Precht and Paul "Mitch" Harris; #90039 (2005)

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Using Quaternary Reefs and Platforms as a Comparative Tool

William F. Precht1 and Paul "Mitch" Harris2
1 PBS&J, Miami, FL
2 ChevronTexaco, San Ramon, CA

Exposures of Quaternary reefal limestones provide an excellent opportunity to evaluate carbonate platform history relative to high-frequency sea-level change. Using reef examples from the Eemian (Marine Isotope Stage 5e), we have identified globally correlative para-sequences.

These Eemian reefs consist of at least two discrete shallowing-upward sedimentary units that can be correlated within and between complexes. The individual stages are marked by abrupt deepening and lateral shifts in facies. The bases of the sequences are characterized by a transgressive surface. This surface reveals an initial rapid deepening of the platform. This “start-up” phase is followed by a progressive shallowing of reef facies or “catch-up” style of reef development. As the accommodation space across the platform becomes more restricted the community structure changes to a “keep-up” style reef.

The stacking patterns observed between the individual cycles put facies with very different petrophysical properties in both temporal and spatial contact. Understanding the spatial context of facies within the sequences is essential to predicting porosity and permeability distributions across ancient carbonate platforms. Further analysis indicates that the diagenetic potential of the various facies and their position relative to sequence boundaries also control the observed heterogeneous reservoir properties.

The exploitation history of many hydrocarbon bearing reef complexes of various ages from around the world underscores the significance of discerning reservoir continuity relative to individual cycles of deposition. Because of the ability to unravel relative sea level history in Quaternary reefs, their use as a counterpart for comparative studies makes them an invaluable tool for developing sound models for reefs through time and space.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005