Abstract Primary stratigraphic traps : in clastic rocks – in chemical rocks. Secondary stratigraphic traps. Fluid traps. STRATIGRAPHIC TRAP 1 is a general term for traps that are chiefly the result of a lateral variation in the lithology of the reservoir rock, or a break in its continuity. A permeable reservoir rock changes to a less permeable or to an impermeable rock; it is truncated by an unconformity, and overlapped; or it changes along its bedding; and the boundary between the two kinds of rocks chiefly determines the extent of the reservoir. This boundary may be sharp, or it may be gradational; the condition may be either local or regional in extent; and the change in permeability may be wholly responsible for the trap or only partly so. Nearly all stratigraphic traps, indeed, have some structural elements, the only exceptions being some of those in isolated lenses and organic reefs, which generally are traps without regard to the regional dip or to any arching or deformation. There is no sharp demarcation between structural traps and stratigraphic traps, and some traps are determined in about equal measure by stratigraphic and structural causes; they might be classed as stratigraphic by some geologists and as structural by others. For that reason, it is useful to set up an intermediate class of combination traps. Stratigraphic traps may be conveniently divided into two general classes. Primary stratigraphic traps , formed during the deposition or diagenesis of the rock, include those formed by lenses, facies changes, shoestring sands, and reefs. Secondary stratigraphic traps have resulted from later causes, such as solution and cementation, but chiefly from unconformities.