communication with respectively the upper and lower portions of the leached zone as shown by the arrows in fig1 in an initial stage steam is injected near the bottom of zone 1 around well 2 while fluid is produced from near the bottom of that zone around well 3 the injected steam tends to rise towards the upper part of the zone since the steam has a relatively low density and since an injected fluid tends to flow preferentially into regions of greatest permeability as the steam transfers heat to the components of the leached zone and condenses the heat pyrolyzes and removes the organic components of the oil shale and the condensate dissolves soluble minerals the resulting removal of solids from the upper portions of the formation tends to further increase the permeability in that region it also further weakens the rock matrix causing more settling and releases undissolved fine solids which tend to move into and plug the initial production location in the lower portion of the leached zone as indicated by the cross hatched region 11 fig2 shows a subsequent stage of the process in which the production of fluid through conduit 6 from the initial production zone has been significantly diminished by the increased plugging effect of fines deposited within an expanded zone 11 production has been initiated.