Over the last decade, the picture that has emerged from several geologic disciplines is that the Earth's crust is permeable to great depths and that fluids are more or less constantly moving, transporting both heat and mass and affecting virtually every geologic process. In contrast, workers in the petroleum industry have maintained that sections of the crust contain impermeable pressure seals and hydraulically sealed compartments. We show that these starkly different conceptions can be reconciled by a theory of self-sealing through gas generation in sedimentary basins. The model especially applies to older sedimentary basins where overpressuring cannot be readily explained as the result of ongoing sedimentation and compaction disequilibrium. In our conceptual model, overpressuring is created by catagenic gas generation and maintained by a combination of vertical and horizontal gas capillary seals. Data from the Anadarko Basin in Oklahoma provide support for the self-sealing hypothesis. Well logs reveal the presence of 9 or 10 gas-water interfaces over a 30-m overpressured interval. Capillary pressure measurements show that the force necessary for gas to displace water from a shale is ~3 ¿ 106 Pa, a pressure equivalent to that exerted by a column of water 300 m high. A theory of self-sealing explains the anomalous overpressuring observed in some older sedimentary basins by invoking known mechanisms and forces; it is both parsimonious and falsifiable.