Natural convection, caused by nonhorizontal isolines for water density in sloping sandstone layers, is studied. The flow field is calculated for a fluid-filled three-layer model composed of two identical sandstone strata separated by a low-permeability layer (shale). The condition for recirculation within the sandstone is found. It is shown that for layers of sufficiently small aspect ratio, the shale layer has negligible effect on the flow. With the assumption that the fluid density is a function only of temperature, the effect of the flow on the change of porosity in the sandstone layers is examined. The spatial change of porosity is calculated for small vlaues of time. It is shown that the maximum change as takes place at the midpoints of the end walls if the angle between the isotherms and the slab is less than 36.5¿ and at the midpoints of the lateral boundaries if this angle is greater than 36.5¿. This result is independent of the angle the slab makes with the gravity vector. Also discussed is how the angle the isotherms make with the horizontal depends on the thermal conductivities of the sandstone layers and the bounding rock. ¿ American Geophysical Union 1992