--> Abstract: Bahaman Patch Reefs: Reservoirs Under Construction, by Paul Enos, Robert N. Ginsburg, Adam Harrison, and Mark S. Palmer; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Bahaman Patch Reefs: Reservoirs Under Construction

Paul Enos1; Robert N. Ginsburg2; Adam Harrison2; Mark S. Palmer3

(1) Geology, University of Kansas, Lawrence, Kansas, KS.

(2) RSMAS, University of Miami, Miami, FL.

(3) St. Anselm Exploration, Denver, CO.

Reefs have long fascinated geologists, especially petroleum geologists. Most studies have been on platform-margin reefs. Patch reefs, generally smaller and less glamorous, have languished in the shadows.

Patch reefs abound in the eastern Great Bahama Bank. The Exuma lobe hosts at least 2193 large patch reefs, readily identifiable with existing imagery. Less conservative counts suggest the number might be 70,000 patch reefs larger than 10 m diameter, the approximate limit of resolution. Patch reefs are locally common in the Bight of Eleuthera and on the platform south of Andros Island, and are abundant over large areas of the southeastern GBB, which we are currently studying with higher resolution images.

Patch reefs are virtually absent in platform interiors of the western Bahamas, i.e. the Andros lobe and Little Bahama Bank. Possible explanations are lack of suitable substrates (e.g. hardgrounds) and poor water circulation. Hardgrounds provide optimal substrates for colonization by corals to nucleate patch reefs. Other environmental parameters must be satisfied, but once these thresholds are crossed, the most rapid and most dense development of patch reefs is to be expected where hardgrounds exist at the surface.

Bahaman patch-reef clusters are up to 1200 m in diameter and 2150 m long. Collectively they cover a much larger area and have a larger volume than the platform-margin reefs. They appear capable, in time, of producing economically significant biostromes several meters thick, with static sea level. With rising sea level, bioherms with seismic-exploration dimensions should develop. Relevant examples are in the Belize lagoon (Holocene) and Onondaga Limestone (Devonian). Patch-reef reservoirs could have initial grain-supported porosities of 65% (30% BP; 35% WP). This reservoir potential provides targets in carbonate-platform interiors, considered wastelands in many exploration plays.