Abstract: Sequence Boundary Indicators and Porosity Development at the Top of the Floridan Aquifer: A High Resolution Quantitative Core Analysis

Robinson S. Noble, David A. Budd

The objectives of this study were: (1) to quantitatively document what subaerial exposure criteria are preferentially associated with a type-I sequence boundary as opposed to those associated with intraformational unconformities; and (2) to document the effects a type-I sequence boundary, as opposed to facies and intraformational exposures, has had on porosity development.

Cores were analyzed from the lower Oligocene Suwannee Formation in west-central Florida. Deposition during the lower Oligocene occurred on a carbonate ramp. Shallow- and outer-ramp rocks (grainstones and packstones) are volumetrically dominant and occur in shallowing-upward cycles, many of which terminate in subaerial exposure. The major type-I unconformity above the Suwannee represents up to 5 m.y. of subaerial exposure. The known exposure indicators in the Suwannee include: caliche, pedobreccia, collapse breccia, red stain, rooting, vugs and infiltrated sediment. Quantitative documentation of these features was accomplished by point counting each feature's abundance over every foot of core in twelve wells. Whole-rock porosity was measured at 2-foot intervals in five wells.

The results indicate that distinguishing a type-I sequence boundary from an intraformational exposure surface in cores may not be an easy process. Coring needs to be initiated within 10 feet below a sequence boundary in order for features associated with the sequence boundary (collapse breccia and infiltrated sediment) to be recognized in abundance. Subaerial exposure features preferentially associated with intraformational exposure surfaces (caliche, pedobreccia, rooting and red stain) should not be taken as evidence for a sequence boundary. In addition, porosity with depth trends show no obvious relationships with the type-I sequence boundary or the intraformational exposure surfaces. All the samples are very porous (20% to 50%). The porosity results indicate that up to 5 m.y. of su aerial exposure neither created nor destroyed reservoir potential.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994