Lateral variations in petrophysical, geochemical and petrographic properties of an Avon Park (Middle Eocene) dolograinstone, Gulf Hammock Quarry, Florida

Carbonates have very complex pore networks and are very susceptible to post-depositional diagenesis, which can generate considerable heterogeneity in rock properties at the scale of 3 to 330 ft. Detailed outcrop studies of lateral petrophysical and geochemical variability is essential to the understanding of subsurface reservoir properties because subsurface data only exists from widely spaced wells.;An Avon Park (Middle Eocene) dolograinstone outcrop in the Gulf Hammock Quarry, Florida, was sampled to evaluate lateral petrophysical, geochemical and petrographic properties within this unit. A 462 ft lateral transect was drilled at a 1-foot sample spacing with 397 samples recovered. Unrecovered footages were too badly weathered for drilling. Petrophysical (porosity and permeability) measurements were made for all 397 core samples, and 305 samples were also analyzed for their whole-rock geochemical (delta 18O, delta13C, Sr, Na, Fe, and Mn) attributes. Thin sections of those 305 samples were point counted to quantify petrographic characteristics.;Using all samples, variography of the three different sets of rock properties show three scales of lateral variability: random noise (nugget effect) from spatial variations at the 1 foot sampling interval, correlation lengths (ranges) reflecting sample-to-sample similarities at short scales, and longer-scale patterning (hole effects). For the entire data set, petrophysical and geochemical variables exhibit large nugget effects (>57% of total variance), short-scale correlation lengths of 12 to 30 ft, and small hole effects (<7% of total variance) at wavelengths ranging from 42 to 46 ft. Petrographic data exhibit similar nugget effects and short correlation lengths, but only petrographic features associated with weathering exhibit hole effects. The magnitudes of the observed nuggets and various pattern lengths are very similar to those documented in Paleozoic dolomites by prior workers.;Sorting of the entire data set to remove samples that are either not dolograinstones or exhibit evidence of weathering (iron oxide staining, rootlets, partial dissolution of dolomite rhombs) reduces the nugget effect (i.e., less sample-to-sample variance) and eliminates the hole effects. These results indicate that the origin of the longer-scale patterns in these Avon Park dolograinstones is a result of the large weathering overprint present at this site. They are not the product of dolomitization itself, as has been speculated by other case studies.