A surface-area-based theoretical approach to the analysis of relative stability of various shapes of liquid-liquid interfaces observed in binary immiscible liquid systems in slit-like pores is proposed. The theoretical estimates produced by this approach are compared to the results of systematic molecular dynamics simulations for n-decane/water mixtures in pores of hydrophilic siliceous and hydrophobic carbonaceous materials. The limitations of the proposed approach and the dependence of the accuracy of theoretical predictions on system size and the composition of the liquid mixture are discussed.