Cation Charge Controls on Calcite Crystal Size and Morphology: Potential Impact on Reservoir Rock Cementation, Porosity, and Permeability.

Laura M. Moore and Callum J. Hetherington
Department of Geosciences, Texas Tech University

Crystal shape, surface structure, nucleation density, and crystal growth rate during lithification and cementation are some of the important factors that control porosity and permeability of oil and gas reservoirs. Calcite is a common cement in sedimentary rocks and is also recognized to have one of the greatest diversities of crystal morphologies. Diversity in calcite crystal shape, growth direction, and crystal size during reservoir diagenesis may impact many decisions regarding reservoir viability as well as how a reservoir will behave during development (e.g. fracking), exploitation and extraction. A series of calcite precipitation and growth experiments have been conducted and the solution chemistry was doped with a range of 1, 2, and 3+ charged cations. The products of the experiments have been imaged by optical and electron microscopy, compositionally analyzed by LA-ICP-MS, and their crystallographic properties refined by X-ray diffraction methods. Fifteen morphologies found in Goldschmidt’s Atlas of calcite morphology have been identified and crystallite size was observed to vary significantly. In experiments without a dopant cation the average calcite crystal size produced is ~0.7mm. In solutions doped with sodium the average size distribution profile is skewed towards 0.02mm, with a few larger crystals up to 0.05mm. Solutions doped with lanthanum produced larger, dendritic crystals visible to the naked eye with size variances of 0.01mm – 3.0mm. The experiments demonstrate that fluid composition plays a first order control over calcite surface structures and crystal size, and by extension, may significantly impact the development of cement and its control over porosity and permeability.

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012