Seepage laws for heavy oils in a compound well group, combining a single horizontal well and several vertical wells and exploited by SAGD (steam-assisted gravity drainage) technologies, are different from the laws for conventional reservoir exploitation. In order to determine the potential function for a reservoir exploited according to such a scheme, we use a model for the potential field distribution for horizontal and vertical wells, based on the principle of superposition of the potential in three-dimensional space. We have determined the areas affected for three-dimensional seepage, and also the boundary conditions ensuring movement of the fluid only under the action of gravity. We have compared SAGD technologies taking into account gravity and neglecting gravity, and we have analyzed the influence of the gravitational field on the pressure gradient in the formation. Based on the results obtained, we present a theoretical model for studying and predicting seepage mechanisms for heavy (high-viscosity) crude oil in a compound well group.