I'm putting in depletion information in section 20-T on my K-1 for royalties on an oil well but it is not having an effect on the tax owed. Is depletion not deductable?

You are entitled to a deduction that is equal to the greater of percentage depletion or cost depletion (the greater amount is shown as “sustained depletion” in Line 20T1). However, the deduction for percentage depletion may be limited depending on your taxable income and other limiting factors. Please refer to IRS Publication 535. 

Additionally, your ability to take a deduction in the current year may be restricted if you have a net passive activity loss from the Partnership for the year.

Amounts reported in Box 20 on Schedule K-1 are typically explained by Supplemental Information on another page of the K-1.  Code T designates "depletion", which may be either cost or percentage depletion. You need more information in order to choose the correct type. 

Contact the source of the K-1 for help if necessary.