Fluvial sandstones and conglomerates of the Sergi Formation, a Jurassic unit of Reconcavo Basin, northeastern Brazil, contain significant amounts of interstitial detrital clays, which are interpreted as the product of early mechanical infiltration developed in an arid/semi-arid environment. Within such an environment, the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium, concentrating the clays in the upper phreatic zone near the sources of influent seepage. Petrographic textures considered to be typical of mechanically infiltrated (MI) clays are herein presented, using examples from the Sergi Formation. These textures include 1) ridges and bridges, 2) geopetal fabrics, 3) loose aggregates, 4) anisopachous coatings of tangentially-accreted lamellae (cutans), 5) massive aggregates, and 6) shrinkage patterns developed during diagenesis. In addition, their detrital aspect as revealed by SEM observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays. In most hydrocarbon reservoirs in the Sergi Formation, the clays described in this paper are among the main controls on porosity and permeability distribution. Commonly, zones with maximum clay concentration are the main internal permeability barriers within the Sergi Formation reservoirs.