Determining the locus of a processing zone in an in situ oil shale retort

Abstract

A processing zone advances through a fragmented permeable mass of particles containing oil shale in an in situ oil shale retort in a subterranean formation containing oil shale. The fragmented mass has layers of formation particles of differing composition, such as kerogen content, corresponding to strata of differing composition in the formation. The processing zone advances in a direction substantially perpendicular to such layers in the fragmented mass. Kerogen in oil shale is decomposed to produce gaseous and liquid products including shale oil, and shale oil is withdrawn from the retort. At least one characteristic of the shale oil withdrawn from the retort varies in response to differences in composition of such layers of formation particles through which the processing zone advances. Such a characteristic can be a physical property of the shale oil such as viscosity or specific gravity, or a chemical property such as sulfur content or trace metal content. To determine the locus of the processing zone with respect to such layers in the fragmented mass, formation is analyzed for defining the locus of at least one such layer in the fragmented mass before retorting, and shale oil withdrawn from the retort is monitored for variation of such a characteristic corresponding to advancement of the processing zone through such a layer in the fragmented mass.