Laboratory Simulation of Moldic Porosity Formation

Amy B. Herhold, Stephen D. Cameron, Hubert E. King, John H. Dunsmuir, Deniz Ertas, Chris E. Kliewer, and Doug L. Dorset
ExxonMobil Research and Engineering Co, Annandale, NJ

Moldic porosity is an important rock fabric in carbonate reservoirs. To study the microscopic formation mechanism, we examine pore-scale and sub-grain-scale changes that occur during early diagenesis simulated in the laboratory. Packed beds of aragonitic ooid grains from the Caicos platform are heated to 180C for days to months, causing in-place dissolution of the ooids and precipitation of calcite cements in the original interparticle pore space. Fresh water is used to mimic phreatic conditions. Morphology is examined with SEM, TEM, and x-ray tomography.

Like many ooids from high-energy environments, these ooids are made of concentric laminae of submicron tangentially-aligned aragonite needles. During the experiments, each grain dissolves with a front that moves inward from the outer surface to form a porous rim. Examination of this rim shows a scaffolding of undissolved needles. Closer inspection of the starting grains reveals that the laminae contain a significant fraction of nm-scale aragonite particles. These nano-particles dissolve more readily than the larger needles, and it is this differential dissolution rate that gives rise to the partially-dissolved outer rim. This highly-porous aragonite scaffolding allows simultaneous blocky calcite growth on the outside of original grain surfaces, causing the pore-space inversion of moldic porosity.

The laboratory-generated diagenetic features closely resemble that observed in natural phreatic environments, suggesting that the observed mechanism pertains to the natural process. The simultaneous aragonite dissolution and calcite precipitation indicates that moldic porosity forms in a local, pore-scale process. In addition, the ooid internal microscopic structure is key to the formation mechanism.