Calcite cementation is often an important factor in the evolution of reservoir pore systems. Although petrographically obvious, the effect that cementation has had on the petrophysical properties of a pore system may be difficult to evaluate quantitatively. To this end, a computer-assisted petrographic image analysis (PIA) technique was developed to quantify porosity and permeability reduction due to calcite cementation.

With this technique, pore area and specific surface of the extant pore system are measured from digital images of core-plug thin sections. Porosity is estimated from the measurements of pore area, and an empirical equation relating pore area and specific surface to core permeability is derived using the Kozeny-Carman expression. In this manner, a permeability model is developed for the pore system in question, thus providing a means of estimating permeability from PIA measurements.

To estimate the porosity and permeability of the precalcite pore system, calcite cement is discriminated from the same digital images and analyzed as pore space. This effectively backstrips calcite from the extant pore network to yield the precursor pore network. A comparison of the porosity and permeability of the extant and precalcite pore networks shows the quantitative significance of calcite cementation.

This technique is demonstrated using two dolomite reservoirs that exhibit varying amounts of late-stage calcite cements: Little Sand Draw field, Wyoming, and Bindley field, Kansas. Calcite cement was found to be minor and restricted to moldic pores in Little Sand Draw dolomites, resulting in less than a twofold change in permeability. In contrast, late calcite cements are somewhat more abundant in Bindley dolomites, but more importantly, they occupy intercrystalline pores as well as moldic pores. The net effect was a 10- to 1000-fold decrease in permeability and the localized destruction of reservoir-quality rocks in Bindley field.