Estate Valuation - North Robertson Unit

The North Robertson Unitized Trust was formed in October 1986 from over 140 producers dating back to at least 1956. Can anyone tell me if the basis of valuation for purposes of calculating estate taxes is the value of the individual wells and/or mineral rights when they were first conveyed to the current owner in 1956 or if that valuation changed as a result of establishing the North Robertson Unitized Trust in 1986?