Observations on the origin of micrite crystals

The transformation of depositional lime mud to microporous micrite has been the subject of several important papers recently in response to the recognition that micropores are important to the production of carbonate hydrocarbon reservoirs. The origin of micrite has generally been described as a micro dissolution and reprecipitation process (often referred to as Ostwald ripening), with nanometer-sized depositional particles being dissolved and larger crystals (micrite) averaging 2 μm in diameter being precipitated. These conclusions were tested by detailed scanning electron microscope (SEM) observations of micrite and minimicrite crystals found in the mid-Pliocene to upper Miocene section of the Clino core taken on the western edge of the Bahama Bank. Minimicrite in this data set is shown to be composed of both aragonite and calcite with calcite being the dominate mineral. In addition, micrite crystals are composed of cemented nanometer-sized calcite crystals and are not individually precipitated calcite crystals. As a result, the faces of the micrite are anhedral and knobby in appearance rather than being smooth, euhedral crystal faces. Extensive size measurements could find no size selectivity in the dissolution of the calcite minimicrite, indicating that the Ostwald ripening process is not active. Aragonite and calcite crystals are dissolved and some calcite crystals act as nuclei for the precipitation of the dissolved carbonate. The origin of the micropores is also debated. Some authors call upon a dissolution event to increase porosity of a partially cemented micrite, based largely on the anhedral nature of the micrite. Others conclude that porosity is originally inherited from the deposition lime mud and that porosity is lost by overgrowth cementation, which results in euhedral micrite crystals. The observations presented here show that anhedral crystals are due to the inclusion of nanometer calcite crystals rather than to dissolution of euhedral crystals. Lastly, these micrite crystals are formed by calcite and aragonite sediment precipitated from an aragonitic sea, disproving the assertion of some authors that porous micrite only forms from sediments deposited in calcitic seas.