Evidence from a Miocene Reaction Front (Netherland Antilles) Demonstrates Formation of Lateral Cyclic Patterns in Rock Properties by Dolomitization
Prior work has shown that petrophysical and geochemical properties of dolomites may exhibit a nested set of lateral patterns, typically at the scales of a few meters to ~20 m. This study assesses whether those lateral patterns are inherited from the limestone precursor or formed during dolomitization, and if lateral patterns are controlled by fabric or textural properties of the rock. Sampling was done across a preserved dolomite-to-limestone reaction front in fore-reef and upper slope grainstones of the Miocene Seroe Domi Formation, Bonaire, Netherland Antilles. Two ~200 ft lateral transects were drilled at 1-ft spacing in two separate beds and extend from rocks of 100% dolomite through the dolomitization front into pure limestone. Porosity, permeability, and geochemical (δ18O, δ13C, Fe, Mn, Sr, and Na) analyses were performed on all 287 recovered samples and 287 thin sections were point-counted for petrographic attributes. Stable isotope data indicate that the limestones underwent diagenesis to low-Mg calcites in a meteoric environment, and subsequently dolomitization occurred in the more saline third of a freshwater-seawater mixing zone. Subsequent diagenetic overprinting in the two rock types is absent or minimal.
Variography of the resultant data sets show that dolomite porosity, permeability, and trace-element attributes contain three scales of lateral variability: ~60% of the total spatial variance occurs at 1-ft spacing, a short-scale correlation length of 10-36 ft in all attributes, and a long-range oscillatory pattern in porosity (hole effect) at 62 ft that is equivalent to ~30% of the total variance in porosity. In contrast, none of the variograms of limestone attributes show evidence for any short-range correlation or long-range oscillatory patterns. Additionally, no dolomite petrographic feature appears to control lateral patterns.
Lateral patterns in the varied geochemical and petrophysical properties of the dolomite could not have been inherited from the limestone precursor, as the limestone contains no patterns in those attributes. The process of dolomitization is interpreted to be the cause of the observed spatial patterns as that is the only geologic process that has affected the Seroe Domi dolomites but not the limestones on the other side of the dolomitization front. It is also the only process that could have affected these petrophysical and geochemical attributes in the same spatial context.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California