Here's what the stratigraphy looks like in the area where the famous Drake Well was drilled in Titusville, Pennsylvania. In actuality, these formations are very hard to trace because the geology can change rapidly from place to place. This is very typical of highly crossbedded sandstone deposits formed near an ancient shoreline, the kind that form stratigraphic traps.

To illustrate the point of highly changeable geology, the Drake well actually struck oil within the Riceville Shale. If we've learned anything about reservoir rocks, we know that they generally are not shales. As it turns out, even shales sometimes contain thin horizons that are more sand-rich, which can act as a very local reservoir rock.