The effect of strong heterogeneity on the onset of convection induced by a vertical density gradient in a saturated porous medium governed by Darcy's law is investigated. The general case, where there is heterogeneity in both the vertical and horizontal directions, and where there is heterogeneity in permeability, thermal conductivity, and applied temperature gradient, is considered. A computer package has been developed to implement an algorithm giving a criterion for instability, and this is now employed to investigate the case where there is two-dimensional variation in a horizontal plane and the case where the variation is generated by a log-normal distribution. In the latter case, spatially correlated fields with known stochastic properties are generated, and the results are analyzed in a statistical framework. We now test cases that are representative of natural, field-scale, geologic conditions-both in terms of the correlated structure and the much larger standard deviation of the permeability distribution.