En échelon structures which may trap oil and gas develop in a systematic pattern along wrench zones in sedimentary basins. Laboratory clay models simulate the formation of en échelon folds and faults caused by wrenching. Folds form early in the deformation and are accompanied or followed by conjugate strike-slip, reverse, or normal faulting. Deformation may cease at any stage or may continue until strike slip along the wrench zone produces a wrench fault and separation of the severed parts of early structures. Oblique movements of fault blocks on opposite sides of a wrench fault cause divergence or convergence and enhancement, respectively, of extensional or compressional structures. Basins form in areas of extension and are filled with sediment, whereas upthrust blocks emerge in areas of compression and become sediment sources. The combined effects of wrenching in a petroliferous basin are to increase its prospectiveness for major hydrocartwn reserves.