The Otter Park Formation, Horn River Basin, northeastern British Columbia, Canada, holds a significant amount of self-sourced gas trapped in a tight shale lithofacies. The shale succession has slow lateral variations in terms of thickness and rock properties. Some wells have encountered marly laminated mudstone deposits encased in the shales. These marl deposits constitute lean, non-reservoir zones that reduce anticipated reserves. The individual thicknesses of the marl units are below seismic resolution, so no individual seismic event can be uniquely attributed to the presence of marl. However, the elastic properties of the marls are sufficiently different from the encasing shales to cause measurable effects in the composite seismic reflection response. By comparing the real seismic data and synthetic seismic data modeled from real wells, a trace matching process can resolve sub-seismic resolution lithofacies' signatures. The method is geology driven and takes into account non-unique solutions. It outputs rock property probability volumes with associated uncertainties. It may be extrapolated to other unconventional reservoirs where baffle and barrier lithofacies variations have a measurable seismic response as modeled from well data.