SIMULATING CARBONATE EARLY DIAGENESIS IN THE LABORATORY

Early diagenesis has a significant impact on the porosity, permeability, and mechanical strength of carbonate rocks. However, deciphering early diagenesis mechanisms is complicated by the superposition of different diagenetic environments over time due to overprinting associated with sea level fluctuations and tectonics. Also, few samples of early diagenetic rocks are available for a systematic comparison of physical properties as a function of diagenetic evolution.

In order to enable a systematic and quantitative investigation of early diagenetic processes, we have developed a method to synthesize early diagenetic rocks in the laboratory. Loose grains are packed together and subjected to a prescribed time-temperature profile in a water-filled autoclave, usually resulting in intact rocks due to cementation. The advantage of this technique is that the resulting morphology and amount of dissolution and precipitation can be directly related to the diagenetic conditions.

Our first experiments focus on the formation of moldic porosity from aragonitic ooid grain packings. The ooid grains are obtained from the Caicos platform and are made up mainly of layers of submicron aragonite needles. With high-purity water as the starting fluid, batch reactions are run for days to months at 180 C and lower, causing in-place dissolution of the aragonite ooids and precipitation of calcite cements in the original interparticle pore space. X-ray diffraction, x-ray computed tomography, optical imaging, and SEM are used to determine the rate of conversion and the morphology as a function of conversion.

The original grains dissolve with a front that moves inward from the outside edge to form a porous rim, consisting of undissolved aragonite remnants of the original layered structure. Blocky calcite grows on the outside of this rim. The morphology closely resembles those observed in a variety of natural phreatic environments. This simultaneous dissolution of aragonite and precipitation of calcite indicates that the formation of moldic porosity is a local, pore-scale process. Although this initial work mimics phreatic conditions, this technique can be applied to study diagenesis in other environments by varying the water chemistry and starting material.