SOFIA - MGS-Winston-v1 1995, The Boulder Zone Dolomites of Florida, Volume 1, Paleogene & Upper Cretaceous Zones of the Southeastern Peninsula and the Keys

The Boulder Zone Dolomites of Florida, Volume 1, Paleogene & Upper Cretaceous Zones of the Southeastern Peninsula and the Keys

George O. Winston

ABSTRACT

Boulder Zone boulders are man-made. They are produced by chunks of dolomite falling to the bottom of the drill hole where they are rolled around by the drill bit. If not drilled up, they must be fished out. Boulder-producing dolomites are characterized by the presence of wall features such as cavities and wall-collapse zones. Wall features are unrelated to dolomite textures, referred to herein as the Dolomite Triad consisting of euhedral, anhedral and cryptocrystalline types. Lithologic data in the southeastern Florida Peninsula are from injection wells drilled by the reverse air method.

The Delray Dolomite, averaging 250 feet in thickness, consists of a lensing or interfingering of the three textural varieties, anhedral being the most common. The Delray Dolomite occurs everywhere, but thins over the Rebecca Shoal Dolomite reef, suggesting that the basal Delray elsewhere is a lagoonal equivalent of the reef. In the Keys, limited data show the same Dolomite Triad to be present, but here there is a limestone bed in the middle of the unit. In one well this limestone directly overlies the Rebecca Shoal reef, further demonstrating the equivalence of the lower Delray Dolomite with the uppermost reef section.

Cedar Keys A, averaging 250 feet in thickness, consists of the same Dolomite Triad found in the Delray Dolomite. Cedar Keys A occurs everywhere except where the main body of the Rebecca Shoal reef is present. This absence of Cedar Keys A demonstrates that it is the lagoonal equivalent of the reef. The age of the Delray Dolomite and Cedar Keys A is indeterminate, but lies somewhere within the Lower Eocene-Upper Paleocene interval.

Continuity of the Dolomite Triads in the Delray Dolomite and the Cedar Keys A between closely spaced wells is virtually nonexistent, but gross lithologic assemblages do extend laterally at multi-well sites. Regionally there is no continuity of the Dolomite Triad members.

Cedar Keys B consists mostly of fine microcrystalline euhedral dolomite. Only in Brevard County does it contain enough Boulder Zone wall features to be included in the Boulder Zone. Avon Park dolomites, when thick, also contain some Boulder Zone wall features.

The Rebecca Shoal reef is present in the Keys and in the area adjacent to St. Lucie County in the Peninsula. Sparse lithologic data in the Keys shows the reef and its Tavernier and Plantation Tongues to consist of euhedral and anhedral dolomite. Porosity is mainly intercrystalline with some vugs. Large caverns have been reported in the lower reef section in several wells. In the St. Lucie County area, injection wells penetrated several hundred feet of Rebecca Shoal reef. Here good lithologic data show the same reef lithology as in the Keys. By regional correlation, the age of the upper Rebecca Shoal (Tavernier) is essentially Paleocene and the middle Rebecca Shoal (Plantation) is Upper Cretaceous.

In the southeastern peninsula, regional structure on top of the Delray Dolomite or Cedar Keys A dips southward from Brevard County into a large low area occupying most of Broward County. Dip reverses southward into Dade County. Faulting is present in Brevard, Palm Beach and Dade Counties, as shown by missing section or by structural anomalies between closely spaced wells.

Video tapes of injection well boreholes into Boulder Zones reveal the presence of cavities and vugs of various sizes, wall-collapse zones and wide vertical channels, all of which contribute to the reservoir permeability and capacity. Wall-collapse zones are created by intersecting open fractures, which when exposed by drilling, collapse into the hole due to the lack of support. Thick wall-collapse sections appear to be associated with faults. Cavities, large horizontal openings which frequently encompass the entire wall of the hole, may be several feet in diameter. There are no valid explanations for their origin, only speculations. Large vugs and vuggy zones occur sporadically. In the study area, true caverns have been identified only in the Rebecca Shoal reef. In the main body of the reef in the Keys they all occur some 2,000 feet below the top. This suggests a possible subaerial origin, but the absence of a significant sea level fall in the middle of the Upper Cretaceous mitigates against this explanation. The uncommon vertical channels are obviously solution-enlarged fractures. Continuity of Boulder Zone wall features between closely spaced injection wells is limited to wall-collapse zones.

(The entire report is available below)