The lateral seals of straigraphic traps located on monoclines offer some interesting perspectives on problems of hydrocarbon entrapment. On the eastern flank of the Denver basin, oil reservoirs in Cretaceous valley-fill sandstone bodies afford a particularly instructive example. There the updip seal is provided by sandy, silty, and clayey marine facies into which were cut the slightly younger valleys. A subsequent marine transgression covered the entire sequence with clay shale, providing a top seal.

The lateral seal can be visualized as a maze of pipes and capillaries through which the oil phase finds its way, until blocked at each potential escape route by entry pressures that exceed that generated by the buoyancy of the oil column. Cores of the sealing facies show that burrowed silty sandstone is a small-scale maze, where silty laminae shortly block oil invasion. Wave-rippled shallow-shelf sandstone beds offer more continuous conduits, but are lenticular on a scale of a few hundred meters and are separated by shale beds that ultimately baffle oil escape. Cross-stratified shoreline sandstone is also a sealing facies by diagenetic kaolin; stained sets of cross-laminae are enclosed in unstained rock, a testimony to slightly different capillary pressure characteristics.

Careful examination of cores of rock facies can give vital information on the geometric nature of a seal; on how to convert laboratory measurements of capillary pressure using mercury and air to natural oil-brine systems; on critical oil saturation levels that distinguish seal and non-seal rocks; and on the height of hydrocarbon column than may be trapped by a seal.

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