36 serves for the location of piping 42 required for injection of a viscosity reducing agent into the petroleum bearing strata 12 and also the piping can serve for removal of produced fluids the central tunnel 36 also serves a return air conduit tunnels 14 and 40 are used for traffic and development it can be seen that the arrangement would as well permit the use of the central tunnel 36 for development and for intake air while the third main tunnel 30 serves as a return air entry the selection of a development plan whether that illustrated in fig1 and 5 or any similar such system will be determined by circumstances governing a particular site which will include the mining rules of the jurisdiction controlling the cost of mining and the strength of the rock surrounding the petroleum bearing strata 12 and the nature of the production bearing strata referring again to fig9 and 10 as previously stated each well 24 should penetrate the production bearing formation 12 in substantially vertical orientation to assist in accomplishing this each well except vertical well 24a will be deviated from the vertical perhaps at an angle of approximately 30 u00b0 from the horizontal after drilling about 35 feet a joint of casing will be grouted into the hole to serve as a collar casing after the first casing is set and the grout has hardened all further operations will be conducted through a rotating blow out preventer not shown this will serve two useful purposes one of which is to accumulate all.