Method for control of geometry of fragmented mass in an situ oil shale

Abstract

A method for forming an in situ oil shale retort in a subterranean formation containing oil shale is provided. The in situ retort contains a fragmented permeable mass of formation particles within top, bottom, and generally vertically extending side boundaries of unfragmented formation. A lower portion of the fragmented permeable mass of formation particles having a nonlevel top surface is initially formed in the retort. A void space is left within the retort boundaries extending between the nonlevel top surface of the fragmented mass lower portion and a generally horizontally extending free face of an overlying layer of unfragmented formation. Thereafter, the overlying layer of unfragmented formation is explosively expanded into the void space to thereby form the remaining portion of the fragmented mass in the retort. The overlying layer is expanded in a plurality of separate horizontally spaced regions with a time delay between explosive expansion of each successive region. The average vertical distance from the generally horizontal free face of each such region of the layer expanded earlier in the sequence to the nonlevel top surface of the lower portion of the fragmented mass is greater than the average vertical distance from the generally horizontal free face of each such region expanded later in the sequence to the nonlevel top surface of the lower portion of the fragmented mass.